Imide-Based Molten Salts As Solvents for Homogeneous Catalysis

Bis(trifluoromethylsulfonyl)imide-based Molten Salts as Solvents for homogeneous Catalysis

Bis(trifluoromethylsulfonyl)imid-basierte Salzschmelzen als Lösungsmittel für homogene Katalyse

Technische Fakultät

der Friedrich-Alexander-Universität Erlangen-Nürnberg

zur

Erlangung des Doktorgrades Dr.-Ing.

vorgelegt von Marlene Scheuermeyer

aus Erlangen

Als Dissertation genehmigt von der Technischen Fakultät der Friedrich-Alexander-Universität Erlangen-Nürnberg.

Tag der mündlichen Prüfung: 15.6.2018

Vorsitzender des Promotionsorgans: Prof. Dr.-Ing. Reinhard Lerch

Gutachter: Prof. Dr. Peter Wasserscheid

Prof. Dr.-Ing. Andreas Paul Fröba

Acknowledgements

The results of this PhD thesis are based on my research carried out at the Institute of Chemical Reaction Engineering (Chemische Reaktionstechnik, CRT) at Friedrich-Alexander-Universität Erlangen- Nürnberg between 1.10.2013 and 31.3.2017.

First and above all I would like to thank my supervisor Peter Wasserscheid, for the inspiring conversations and the guidance trough the time I spend at his chair. I greatly appreciate his trust and encouragement. He always could motivate me with his never dying enthusiasm and optimism. It wasn’t always simple but without him I would not have come this far.

The financial support of the project “H2-SMS-CAT – Engineering of Supported Molten Salt Catalysts for Dehydrogenation Reactions and Hydrogen Production Technologies” within the program “ERC Advanced Grant” is gratefully acknowledged.

Also I would like to thank Friederike Agel and the other group leaders of CRT for all the practical advice, the ideas and the assistance.

In addition, a warm thank you goes to Peter Schulz who takes care that all GCs, especially the “Kleine Klaus” and the NMR do what they should, Nicola Taccardi for the ICP measurements, Dirk Lüdke for the XRD measurements, Christina Wronna for the elemental analysis and all the other staff at CRT especially the mechanical and electrical workshops and the secretary which makes the everyday work possible.

I also would like to thank my supervisor at DTU during my short term scientific mission, Rolf W. Berg, for the help with the Raman measurements, for the wonderful time I had in Denmark and for all the scientific anecdotes. In addition I cannot thank Anders Riisager, Rasmus Fehrmann, Lotte Skafte Jespersen, Nanette Zahrtmann and all the other members of the group at DTU enough for the warm integration and the great time. I also would like to thank Irene Shim for her advice with the calculations.

I would like to gratefully acknowledge my cooperation partners: Patrick Schreiber, Florian Maier and Hans-Peter Steinrück from the Chair of Physical Chemistry 2 in Erlangen for the XPS measurements. Florian Heym and Andreas Jess from the University of Bayreuth for support with the TG measurements and the group of James Davis, Jr. from the University of South Alabama for the synthesis of an interesting molten salt.

I would like to thank all my colleagues at CRT for the wonderful time in the lab and after work; especially the fellows in my office. Even if I hated you guys sometimes, when you were discussing about acrylic acid for hours, days, months… I had a great time in the office, nice conversations and always open ears, good advice and help. It really helps to have so nice people around you every day. Also the boys from the third floor for all the wonderful sushi evenings when I couldn’t stop laughing. Johannes Schwegler for being the best lab buddy ever. Kaija Pohako-Esko for all the coffee breaks and wonderful time at our dancing lessons. Patrick, Holger, Christina, Julian and the other wonderful people who I just can’t name all here…

I also would like to acknowledge my students, Dominic Hecht, Anja Goblirsch, Ying Zhou, Sebastian Raquet, and especially Peter Weißhaupt. iii

There are some people outside of the institute who made my life so wonderful and helped me also through the hard times. Caspar and Claudia, Hanna, Max, Mario and Jana, Kerstin, thank you all so much for your friendship.

And last but not least, I thank my family for the emotional support and always being there when I need them.

Declaration

I declare that the work presented within this PhD thesis, except where otherwise stated, is based on my own research carried out during my time at the Institute of Chemical Reaction Engineering (Chemische Reaktionstechnik CRT) at Friedrich-Alexander-Universität Erlangen-Nürnberg between 1.10.2013 and 31.3.2017.

______

Parts of this work have already been published

M. Scheuermeyer, M. Kusche, F. Agel, P. Schreiber, F. Maier, H.-P. Steinrück, J.H. Davis, Jr., F. Heym, A. Jess, P. Wasserscheid: Thermally stable bis(trifluoromethylsulfonyl)imide salts and their mixtures, New J. Chem. 40 (2016) 7157-7161.

M. Scheuermeyer, F. Agel, P. Wasserscheid: Thermally stable bis(trifluoromethylsulfonyl)imide salts and their mixtures, Poster presented at the Conference on Molten Salts and Ionic Liquids (EUCHEM), Vienna 3.-8.7.2016.

M. Scheuermeyer, R.W. Berg, P. Wasserscheid: Raman spectroscopy and DFT calculations of bis(trifluoromethylsulfonyl)imide-based molten salts, Oral report given at the Conference on Molten Salts and Ionic Liquids (EUCHEM), Vienna 3.-8.7.2016.

M. Schwarz, P. Bachmann, T. N. Silva, S. Mohr, M. Scheuermeyer, F. Späth, U. Bauer, F. Düll, J. Steinhauer, C. Hohner, T. Döpper, H. Noei, A. Stierle, C. Papp, H.-P. Steinrück, P. Wasserscheid, A. Görling, J. Libuda: Model Catalytic Studies of Novel Liquid Organic Hydrogen Carriers: Indole, Indoline and Octahydroindole on Pt(111), Chem. Eur. J. 23 (2017) 14806-14818.

Abstract

The work presented in this thesis can be divided in two parts. The first part covers the physico-chemical investigation of chosen molten salts, all based on the bis(trifluoromethylsulfonyl)imide anion, one of the most common anions in ionic liquids (ILs). The Cs[NTf2] and [PPh4][NTf2] molten salts were of special interest. With melting points of 125 and 134 °C, respectively, they cannot be defined as ionic liquids, but they are highly thermally stable. The high thermal stability makes these molten salts very interesting as solvents for reactions at higher temperatures (between 150 and 350 °C) where common ionic liquids already decompose. Also, binary mixtures of these salts were investigated and a eutectic mixture containing 32 mol% [PPh4][NTf2] with a melting point of 98 °C was found. The density and viscosity of the mixtures lie between the ones of the pure salts.

The molten salts were further investigated with vibrational spectroscopy which was extended by DFT - calculations. The [NTf2] anions of both salts were found to be in cis conformation where both CF3 groups are on the same side of the S-N-S plane. With high temperature Raman spectroscopy and mass spectrometry investigations all volatile decomposition products after prolonged heating of the salts could be identified.

In order to use the molten salts as solvents for homogeneous catalysis the solvation of transition metal bis(trifluoromethylsulfonyl)imide compounds with the general formula M(NTf2)2 was studied. Raman spectroscopy, powder X-ray diffraction and mass spectrometry lead to the conclusion Co(NTf2)2 in - [PPh4][NTf2] is octahedrally coordinated and the anion [Co(NTf2)3] is formed. The mixtures of metal compounds and molten salts were investigated regarding their melting points, viscosity and thermal stability.

The second part of the thesis deals with the question if the molten salts can serve as solvents for homogeneous catalysis. Thus, the M(NTf2)2 compounds (with M = Mn, Co, Ni, Cu, Zn) were used as catalyst dissolved in the IL [PMeBu3][NTf2] for the Friedel-Crafts acylation of toluene with benzoylchloride. The yields of 4-methylbenzophenone were moderate and the catalyst was found to be not stable under reaction conditions.

In the context of renewable energy the storage of excess energy in the form of hydrogen has recently received a great deal of interest. One very advantageous storage method for H2 is by covalently binding it to liquid organic hydrogen carriers (LOHC). In this thesis hydrogenation and dehydrogenation were tested with homogeneous Ir catalysts immobilized in a molten salt to realize a liquid-liquid biphasic reaction with the substrates residing in a second phase of extracting agent. The dehydrogenation of indoline to indole was optimized and the extracting agent diphenyl ether was superior to dibutyl ether due to the different solubilites of the substrates. Also, the molten salts with aromatic cations showed slightly higher activity. The most active catalyst was the commercially available Crabtree catalyst

[Ir(cod)(Py)(PCy3)][PF6]. Also, Co(NTf2)2(PPh3)2 was found to be active in the dehydrogenation of indoline.

The biphasic approach was extended to the homogeneous Ir catalyzed hydrogenation of indole, which was found to be rather slow. However, a one-pot pressure swing reversible hydrogenation- dehydrogenation of two LOHC pairs, namely indole/indoline and quinaldine/tetrahydroquinaldine was possible. This is the first example of an ultra-low temperature hydrogen battery. vii

Deutsche Zusammenfassung

Die hier präsentierte Arbeit kann in zwei Abschnitte geteilt werden. Der erste Teil beschäftigt sich mit der physiko-chemischen Untersuchung von einigen ausgesuchten Salzschmelzen, die alle das für ionische Flüssigkeit häufig verwendete Bis(trifluoromethylsulfonyl)imid-Anion gemeinsam haben.

Dabei waren die Cs[NTf2] und [PPh4][NTf2] Salzschmelzen besonders interessant. Mit Schmelzpunkten von jeweils 125 und 134 °C fallen sie nicht mehr unter die Definition der ionischen Flüssigkeiten, allerdings sind sie sehr thermisch stabil. Diese hohe thermische Stabilität macht diese Salzschmelzen sehr interessant als Lösungsmittel für Reaktionen bei höheren Temperaturen (zwischen 150 und 350 °C), bei der herkömmliche ionische Flüssigkeiten sich bereits zersetzen. Außerdem wurden

Mischungen der Salze untersucht und eine eutektische Mischung bestehend aus 32 mol% [PPh4][NTf2] und einem Schmelzpunkt von 98°C wurde gefunden. Die Dichte und die Viskosität der binären Mischungen liegen zwischen denen der reinen Salze.

Die Salzschmelzen wurden außerdem mit Vibrationsspektroskopie untersucht, die mit DFT- - Rechnungen unterstützt wurde. Das [NTf2] Anion ordnet sich in der cis-Konformation an, in der sich beide CF3-Gruppen auf derselben Seite der S-N-S-Ebene befinden. Mit Hilfe von Hochtemperatur- Raman-Spektroskopie und Massenspektrometrie konnten alle flüchtigen Zersetzungsprodukte nach längerem Heizen der Salze identifiziert werden.

Da die Salzschmelzen als Lösungsmittel für Katalysen Verwendung finden sollten, wurde die Löslichkeit von Übergangsmetall-bis(trifluoromethylsulfonyl)imid-Verbindungen mit der

Summenformel M(NTf2)2 untersucht. Raman-Spektroskopie, Röntgendiffraktometrie und

Massenspektrometrie führten zu dem Ergebnis, dass Co(NTf2)2 in [PPh4][NTf2] oktaedrisch koordiniert - ist und als [Co(NTf2)3] Anion vorliegt. Die Mischungen der Metallverbindungen und Salzschmelzen wurden auch bezüglich ihres Schmelzpunktes, Viskosität und thermischen Stabilität untersucht.

Der zweite Teil der Arbeit beschäftigt sich mit der Frage, ob die Salzschmelzen als Lösungsmittel für homogene Katalyse geeignet sind. Eine der untersuchten Testreaktionen ist die Friedel-Crafts-

Acylierung von Toluol mit Benzoylchlorid, bei der die M(NTf2)2 Verbindungen (mit M = Mn, Co, Ni,

Cu, Zn) gelöst in der ionischen Flüssigkeit [PMeBu3][NTf2] verwendet wurden. Die Ausbeuten von 4- Methylbenzophenon waren moderat und es wurde festgestellt, dass der Katalysator unter Reaktionsbedingungen nicht stabil ist.

Im Zusammenhang mit erneuerbaren Energien ist die Speicherung von Überschussenergie in Form von Wasserstoff kürzlich stark in den Fokus gerückt. Eine sehr vorteilhafte Speichermethode für Wasserstoff ist, ihn kovalent an flüssige, organische Wasserstoffträger (LOHC; englisch: Liquid Organic Hydrogen Carrier) zu binden. In der vorliegenden Arbeit wurden Hydrierung und Dehydrierung mit homogenen Ir-Katalysatoren immobilisiert in Salzschmelzen getestet, um eine flüssig-flüssig zweiphasige Reaktion zu realisieren mit den Substraten in einer zweiten Phase aus Extraktionsmittel. Die Dehydrierung von Indolin zu Indol wurde optimiert und das Extraktionsmittel Diphenylether war wegen der anderen Löslichkeit der Substrate besser geeignet als Dibutylether. Ebenso zeigten die Salzschmelzen mit aromatischen Kationen eine bessere Aktivität. Der aktivste Katalysator war der kommerziell erhältliche

Crabtree-Katalysator [Ir(cod)(Py)(PCy3)][PF6]. Auch mit Co(NTf2)2(PPh3)2 als Katalysator konnte Indolin dehydriert werden. viii

Die zweiphasige Versuchsführung wurde auch auf die homogene, Ir katalysierte Hydrierung von Indol erweitert, die allerdings sehr langsam war. Trotzdem wurden reversible Hydrier-Dehydrier-Versuche von zwei verschiedenen LOHC Paaren, nämlich Indol/Indolin und Chinaldin/Tetrahydrochinaldin, nur durch Druckänderung durchgeführt und waren erfolgreich. Das ist das erste Beispiel einer Tieftemperatur-Wasserstoff-Batterie.

Table of Contents

Acknowledgements ______ii Declaration ______iv Parts of this work have already been published ______v Abstract ______vi Deutsche Zusammenfassung ______vii Table of Contents ______ix List of Symbols and Abbreviations ______xii 1. Introduction ______1 2. Theoretical Background ______4 2.1 Ionic Liquids towards Catalytic Applications______4 2.1.1 Properties of Ionic Liquids ______5 2.1.1.1 Volatility of Ionic Liquids ______5 2.1.1.2 Thermal Stability of Ionic Liquids ______5 2.1.1.3 Decomposition Products of Ionic Liquids ______7 2.1.2 Raman Spectroscopy and Ionic Liquids ______7 2.1.3 Solubility and Coordination of Metal Compounds in Ionic Liquids ______10 2.1.4 Catalysis in Melts ______13 2.1.4.1 Biphasic Systems with IL ______14 2.1.4.2 Supported Ionic Liquid Phase Catalysis ______15 2.2 Model Reactions ______17 2.2.1 Friedel-Crafts Acylation ______17 2.2.2 Hydrogenation and Dehydrogenation ______19 2.2.2.1 Hydrogen and Energy Storage ______20 2.2.2.2 Heterogeneous Hydrogenation and Dehydrogenation ______22 2.2.2.3 Homogeneous Hydrogenation and Dehydrogenation Catalysis ______26 2.2.2.3.1 Homogeneous Hydrogenation ______26 2.2.2.3.2 Homogeneous Dehydrogenation ______27 2.2.2.3.3 Reversible homogeneous Hydrogenation-Dehydrogenation ______29 2.2.2.4 Homogeneous biphasic Systems______32 3. Materials and Methods ______35 3.1 Inert Gas Techniques ______35 3.2 Analytical Methods ______35 3.2.1 Gas Chromatography ______35 3.2.2 Elemental Analysis ______35 3.2.3 Nuclear Magnetic Resonance ______35 3.2.4 Infra-Red Spectroscopy ______36 3.2.5 Raman Spectroscopy ______36 3.2.6 Thermal gravimetric analysis-Mass Spectrometry ______36 3.2.7 Mass Spectrometry ______37 x

3.2.8 X-Ray Diffraction ______37 3.2.9 ICP-AES ______37 3.2.10 Karl-Fischer Coulometry ______37 3.2.11 Viscosity ______37 3.2.12 Thermal Stability ______37 3.2.13 Melting Points ______38 3.2.14 Density ______38 3.2.15 Partition Coefficient ______38 3.3 Chemicals ______38 3.4 Syntheses ______41

3.4.1 Synthesis of Cs[NTf2] ______41

3.4.2 Synthesis of [PPh4][NTf2] ______41

3.4.3 Synthesis of [PMeBu3][NTf2] ______42 3.4.4 Synthesis of Transition Metal Compounds ______42

3.4.4.1 Co(NTf2)2 ______42

3.4.4.2 Ni(NTf2)2 ______42

3.4.4.3 Other M(NTf2)n ______43 3.4.5 Synthesis of Hydrogenation/Dehydrogenation Catalysts ______43 3.4.5.1 [Ir(cod)(NHC)]I ______43

3.4.5.2 [Ir(cod)(NHC)(PPh3)][PF6] ______44

3.4.5.3 [Ir(cod)(NHC)(PPh3)][NTf2] ______44 3.5 Test Reactions ______45 3.5.1 Friedel-Crafts Acylation ______45 3.5.2 Dehydrogenation ______45 3.5.3 Hydrogenation ______46 3.5.3.1 Reaction Setup ______46 3.5.3.2 Hydrogenation Procedure ______47 3.5.3.3 Reversible Hydrogenation-Dehydrogenation Procedure ______47 3.6 DFT Calculations ______48 3.7 Calculations ______48 4. Results and Discussion ______49 4.1 Characterization of the Melts ______49 4.1.1 Physico-chemical Properties ______49 4.1.1.1 Mixtures of Molten Salts – Eutectica ______49 4.1.1.2 Viscosity and Density ______54 4.1.2 Thermal Stability ______59 4.1.3 Vibrational Spectroscopy on Molten Salts ______61 4.1.3.1 Vibrational Spectroscopy and DFT Calculations of the neat Salts ______61 - 4.1.3.1.1 The [NTf2] Anion ______61 + 4.1.3.1.2 DFT calculations and experimental Spectra of the [PPh4] Cation ______64

4.1.3.1.3 [PPh4][NTf2] Molten Salt ______66

4.1.3.1.1 Raman Spectroscopy of the binary Mixtures of Cs[NTf2] and [PPh4][NTf2] ______69 4.1.3.2 Raman Spectroscopy at elevated Temperatures ______70 4.1.3.2.1 Raman Spectroscopy of the eutectic Mixture at elevated Temperatures ______70

4.1.3.2.2 Raman Spectroscopy of Cs[NTf2] at elevated Temperatures ______73 4.1.4 Solubility of Transition Metal Compounds ______77

4.1.4.1 Investigations on neat M(NTf2)2 ______77 4.1.4.2 Physico-chemical Properties of the Mixtures ______80

4.1.4.3 Investigation of the Solvation Structure of M(NTf2)2 in [PPh4][NTf2] ______84 xi

4.2 Melts as Solvent for Catalysis ______89 4.2.1 Friedel-Crafts Acylation ______89

4.2.1.1 Comparison with other M(NTf2)2 Catalysts ______90 4.2.1.2 Recycling and Catalyst Decomposition ______91 4.2.2 Biphasic homogeneous Dehydrogenation ______93 4.2.2.1 The Catalysts and the Molten Salts ______93 4.2.2.2 Comparison of different Melts and Extracting Agents ______96 4.2.2.3 Temperature variation ______98 4.2.2.4 Comparison of different Substrates ______99 4.2.2.5 Alternative Catalysts ______100 4.2.2.6 Recycling Experiments ______101 4.2.2.7 Comparison with Literature ______103 4.2.3 Biphasic homogeneous Hydrogenation ______104 4.2.4 Pressure Swing Experiments ______105 5. Summary and Outlook ______108 6. Appendix ______111 6.1 Raman Spectroscopy ______111 6.1.1 Raman Instruments and Measurement Setup ______111 6.1.2 Supplementary Raman Spectra ______113 + 6.1.3 Calculated Values for [PPh4] ______116 6.2 Supplementary IR Spectrum ______121

6.3 Investigation of the Temperature dependent Viscosity of M(NTf2)2 Mixtures with

[PPh4][NTf2] ______121 6.4 Gas Chromatography ______122

6.5 Literature Values of the Viscosity of Cs[NTf2] ______123 6.6 Mass Spectrometry ______124

6.7 Heat of Fusion for M(NTf2)2-[PPh4][NTf2] Mixtures (M = Co, Ni) ______125 6.8 Turnover Frequencies and Hydrogen Productivites of the Dehydrogenation Experiments ______126 7. References ______127

xii

List of Symbols and Abbreviations

Å Ångström: 1 Å = 0.1 nm = 10-10 m Ac acetyl AES Atom Emission Spectroscopy aq aqueous Ar aryl ATR Attenuated Total Reflectance a.u. arbitrary units B3LYP Becke’s three-parameter hybrid exchange functional, with Lee-Young-Parr correlation and exchange functional b.p. boiling point BASF Badische Anilin- & Soda-Fabrik BASIL Biphasic Acid Scavenging utilizing Ionic Liquids BMIM 1-butyl-3-methyl-imidazolium Bu butyl BzOCl benzoylchloride cat. catalyst CCD Charge Coupled Device cod cyclooctadienyl const. constant Cp* pentamethylcyclopentadienyl CRT Chair of Chemical Reaction Engineering (Lehrstuhl für Chemische Reaktionstechnik) Cy cyclohexyl DART Direct Analysis in Real Time DBT dibenzyltoluene DCHM dicyclohexamethane DFT Density Functional Theory DPM diphenylmethane DRIFT Diffuse reflection infrared Fourier transform spectroscopy DSC Differential Scanning Calorimetry DTU Danish University of Technology E Energy EA Elemental analysis Et ethyl et al. et alii (Latin for “and others”) EMIM 1-ethyl-3-methyl-imidazolium eq. equivalents ESI Electrospray Ionization EXAFS Extended X-ray Absorption Fine Structure FAU Friedrich-Alexander-University FID Flame Ionization Detector FT Fourier transformation GC Gas Chromatography He hexyl xiii

HPLC High Performance Liquid Chromatography HR High Resolution ICP Inductive Coupled Plasma IFP Institute Français du Pétrole IL Ionic Liquid iPr iso-propyl IR Infrared IRAS Infrared reflection-absorption spectroscopy KF Karl Fischer liq. liquid LOHC Liquid Organic Hydrogen Carrier LTMS Low Temperature Molten Salt M metal (depending on context) M molar = mol L-1, unit for concentration (depending on context) MBP methylbenzophenone Me methyl MIM 1-methylimidazolium MMIMBz 1,3-diemethyl-1H-benzimidazolium MO Molecular Orbital MOF Metal-Organic Framework m.p. melting point MS Molten Salt (depending on context) MS Mass Spectrometry (depending on context) m/z mass-to-charge ratio n.a. not available nbd norbornadiene NEC N-etyhlcarbazole NHC N-Heterocyclic Carbene NHC N,N-dimethyl-benzimidazol-2-ylidene (in chemical formulas) NIR Near Infrared NMR Nuclear Magnetic Resonance

NTf2 bis(trifluoromethylsulfonyl)imide or bistriflimide OTf trifluoromethylsulfonate p para P Productivity PEM proton exchange membrane Ph phenyl PNP bis(phosphino)amine pincer ligand Py pyridinium Pyr pyrrolidinium ® registered in U.S. patent and Trademark Office R organic substituent, also known as rest Ref. Reference rpm rounds per minute S Selectivity SCILL Solid Catalyst with Ionic Liquid Layer SILP Supported Ionic Liquid Phase STSM Short-Term Scientific Mission tbe tert-butylethylene xiv

tBu tert-butyl Temp. temperature tert tertiary TG Thermogravimetry TGA Thermogravimetrical Analysis THF tetrahydrofuran TM as Trade Mark notified, but not registered tmgH 1,1,3,3,-tetramethylguanidinium TOF turnover frequency TRIPHOS 1,1,1-Tris(diphenylphosphinomethyl)ethane UHV Ultra-high vacuum UV/vis Ultra Violet and visible light VFT Vogel-Fulcher-Tammann x Mole fraction X Conversion X Halide anion XPS X-ray Photoelectron Spectroscopy XRD X-ray Diffraction Y Yield

YAG Yttrium-Aluminum-Garnet (Nd:Y3Al5O12)

For better visualization liter was abbreviated with a capital letter L, and not the SI symbol l.

It’s still magic even if you know how it’s done.

from A Hat Full of Sky by Terry Pratchett

1 1. Introduction

1. Introduction

Salt – white gold. Not too long ago salt was very valuable and of considerable importance. Trading salt made a lot of cities rich and influential. Wars have been fought over it or were ended by salt famines. There are numerous fables and sayings revolving around salt. Salt is a major ingredient for all sorts of food, irreplaceable for life itself.1

When people use the word “salt”, they are often talking about table salt, which consists mainly of sodium chloride. But for the chemist salt in the most general sense means a chemical compound composed of ions.2 There are innumerable different salts. This thesis will deal with one special class of salts – molten salts; however, not just any molten salt, because if you heat NaCl to 801 °C, you also get a molten salt, but Low-Temperature Molten Salts or Ionic Liquids.

In 1914 the first report of such a salt was published by Paul Walden. Ethylammonium nitrate melts at 12 °C.3 The term ionic liquid was established as soon as 1943.4 In 1997 Kenneth Seddon wrote: “To use the term molten salts to describe these novel systems (meaning ionic liquids) is as archaic as describing a car as a horseless carriage.”5 He referred to the distinction of ionic liquids with high-melting salts, which have melting points well above 400 °C and are highly viscous and corrosive.

Nowadays, the accepted definition of an ionic liquid is: it consists solely of ions; usually bulky organic cations and inorganic, polyatomic anions and the somewhat arbitrarily set melting point below 100 °C.6 Often some more properties are mentioned: the salts are liquid at or near room temperature and have a negligible vapor pressure. Also, they have relatively low viscosity and high conductivity.7-9

Since their “discovery”, interest in ionic liquids increased, as easily deduced from the rising number of publications from 3 till 1950, over 182 till 2000 to 35658 in the end of 2017 (numbers determined by SciFinder).10

Ionic liquids were described as solvent and catalysts for organic synthesis in the 1980s for the first time.11,12 It took another ten years before the first reports of transition metal catalysts dissolved in ILs were published. Chauvin et al. dissolved nickel catalysts in chloroaluminate melts to dimerize alkenes.13 Until then the work in this field focused on these acidic chloroaluminate ionic liquids, which are not stable under moisture, have only little tolerance to functional groups of substrates and catalysts, and thus, only limited application possibilities.14

Even after decades of catalytic developments improvements on selectivity and productivity are still of major importance. Not only the efficiency but also the ease of processing are under intensive investigation. There is also a strongly rising interest in “green” processes. Ionic liquids are acknowledged to solve several of these requirements in one go. For example, the environmental impact will be lowered due to their very low vapor pressure compared to conventional organic solvents. ILs are also referred to as “designer solvents”, meaning their physical and chemical properties can be tuned by an intelligent combination of suitable cations and anions. Thus, they can serve as solvent and ligand or even catalyst at the same time.8,15,16 2 1. Introduction

The biggest problem with conventional ionic liquids is their lack of thermal stability.16-18 The focus of this thesis lies on the “Low-Temperature Molten Salts” (LTMS). This group bridges the gap between ionic liquids and classical high temperature molten salts. The LTMS should open a novel class of reaction media for reactions at temperatures between 100 °C and 350 °C. LTMS use to enhance reactivity due to better thermal stability, better selectivity and catalyst stabilization due to the tunable physical and chemical properties.

The first part of this thesis covers the physico-chemical investigation of some selected low-temperature molten salts and the second part deals with the application of these salt systems as solvents in model reactions.

All chosen low-temperature molten salts are based on the bis(trifluoromethylsulfonyl)imide anion. This is one of the most common anions in ionic liquids. It is acknowledged to have high conformational flexibility, high charge distribution and low coordination power. These salts are often characterized by low melting points, low viscosities and high thermal stability. The chosen salts, namely M[NTf2] (with

M = Li, Na, Cs) and [PR4][NTf2] (with R = Ph, Me, Bu), and binary mixtures of these are investigated regarding their melting points, the viscosity, density, and the thermal stability. Structural aspects of the salts and gas phase components from evaporation or decomposition are studied with vibrational spectroscopy, which is often combined with Molecular Orbital (MO) model calculations helping to better understand and interpret experimental results.

Also the solvation of first-row transition metal compounds with the general formula M(NTf2)2 (M = Mn, Co, Ni, Cu, Zn) in the molten salts is of interest due to the possible application of these salts as catalysts.

For the second, catalytic part, two relevant reactions were chosen and carried out in LTMS as new solvent concept with homogeneously dissolved catalysts.

The Friedel-Crafts acylation is a representative for C-C bond formation reactions of synthetic value. The focus of literature is still on chloroaluminate melts as catalyst, which have the disadvantage of sensitivity to air and water, the need for stoichiometric salt amounts. Hydrolysis is required for product isolation after the reaction and thus destruction of the “catalyst”.19 First-row transition metal compounds are rarely used as catalyst but have tremendous advantages especially in combination with ionic liquid solvents.20 For this thesis the commonly applied chloroaluminate melts were excluded and the work was focused on the previous presented air and moisture stable neutral salt melts with dissolved transition metal compounds M(NTf2)2 as catalysts. Not only the performance of the reaction, but also the stability of solvent and catalyst shall be investigated.

In the context of renewable energy a method for storing excess energy at times or in places with much wind or sun is urgently needed. This energy can be stored in form of hydrogen, since it can be released later by combustion or conversion to electricity. But storing hydrogen is a challenging topic of its own. Recently, the storage of hydrogen by its covalently binding to Liquid Organic Hydrogen Carriers (LOHC) received a great amount of attention.21,22 The reversible hydrogenation/dehydrogenation reaction is usually performed with heterogeneous catalysts, precious metals supported on solids.21

The great advantage of homogeneous over heterogeneous catalysis with respect to activity, selectivity and efficiency is well documented. However, homogeneous catalysis has the major drawback that catalyst-product separation can be difficult. One possibility to overcome this is to immobilize the catalyst in a second phase, especially in form of liquid-liquid biphasic catalysis. Immobilizing an ionic homogeneous catalyst in a molten salt combines the advantages of high activity and selectivity inherent for homogeneous catalysis with the ease of processing. However, homogeneous biphasic hydrogenation and dehydrogenation reactions have rarely been investigated so far.7,23 3 1. Introduction

The second model reaction is thus the dehydrogenation of a model LOHC compound. The utilization of the applied precious metal should be enhanced by using molecular, homogeneous catalysts. To tackle the drawback of homogeneous catalysis, namely the problems of separation of products and catalyst, a liquid-liquid biphasic approach will be applied by dissolving the homogeneous catalyst in the molten salt. As LOHC molecules an indole-based system shall be investigated. The LOHC concept with molten salts as solvents should be extended to homogeneous biphasic hydrogenations.

In summary, new low melting bis(trifluoromethylsulfonyl)imde-based salts should be developed and characterized and used in the described test reactions as novel solvents for the catalysts.

4 2. Theoretical Background

2. Theoretical Background

2.1 Ionic Liquids towards Catalytic Applications

Since the scientific interest in ionic liquids has strongly increased during the last decades, it is not surprising, that there are numerous publications about the properties and applications of ILs. As mentioned before, ionic liquids consist entirely of ionic species. The nomenclature “ionic liquid” is bound to a low melting point (arbitrarily set to be below 100 °C). In general, the cations are bulky, organic compounds with low symmetry (see Figure 2.1), such as dialkylimidazolium24, N- alkylpyridinium25 or tetraalkyl-phosphonium26 compounds to name just a few, in combination with a wide variety of anions, ranging from simple inorganic ions (e.g. halides) over polyatomic anions like hexafluorophosphate to more complex, organic species (e.g. lactates27). Simple changes in the cation and anion combinations, or the nature of the moieties attached to each ion, allow the physical properties of ionic liquids to be tailored for specific applications. Their structural diversity results into nearly unlimited possibilities. There are at least 106 potential simple ILs and many times more binary and ternary systems.15 By suitable combinations of cations and anions the physical and chemical properties of ionic liquids can be tuned, therefore ILs are also referred to as “Designer Solvents.8,15,28

Figure 2.1-Top: Common cations for ionic liquids with R,R’=alkyl chains; -Bottom: common anions for ionic liquids.

Ionic liquids are considered to be environmental-friendly alternatives to volatile organic compounds and suitable solvents for “green” chemistry.8 Often the thermodynamics and kinetics of reactions carried out in ILs are different to those in conventional molecular solvents; sometimes with unpredictable outcome.28 One way to explain some of these unexpected effects is to consider ionic liquids as originally described: as molten salt.29

Numerous reviews were published over the last two decades. The interested reader is referred to7,8,30,31. 5 2. Theoretical Background

2.1.1 Properties of Ionic Liquids Some desirable properties beside the low melting points, which ILs have in common by definition5,7, are especially their negligible vapor pressure at elevated temperatures. But also their high intrinsic electrical conductivity, good chemical and thermal stabilities, their ability to dissolve many organic and inorganic materials and to remain liquid in a wide range of temperature (sometimes over 200 °C) are very interesting. Some other properties are important for specific applications of ILs; e.g. the viscosity for transport phenomena and the density for the speed of phase separations.32

The properties of ionic liquids are difficult to predict. Nevertheless, some trends and tools have been found. The viscosity of ILs is usually much higher than that of conventional solvents like water. Most commonly, ionic liquids can be classified as Newtonian fluids, although non-Newtonian behavior has been observed as well. The temperature dependent viscosity can often be fitted with the Vogel-Fulcher- Tammann (VFT) equation. The viscosity of ILs is mainly attributed to the van der Waals forces, thus - - the viscosity of [RMIM]X ionic liquids with X = [PF6] and [NTf2] increases with an increasing number of carbon atoms in the alkyl chain of R. By branching the alkyl chain the viscosity can be further lowered. Also, the viscosity is dependent on the hydrogen bonding between the counter ions.33 The viscosity of ionic liquids is strongly dependent upon the temperature. It usually decreases with increasing temperature.6

In general, ionic liquids are denser than water. The molar mass of the anions significantly affects the overall density of ILs: comparing ionic liquids with the [BMIM]+ cations and different anions the higher the molar volume of the anion the smaller the density. This is attributed to the more compact packing of the ions if they are similar in size.33

2.1.1.1 Volatility of Ionic Liquids After the discovery of the volatility of ionic liquids34, the vapor phase above these melts have become a topic of interest. Knowing the composition of the gas phase is vital for designing new concepts and processes. Different approaches to determine evaporated compounds have been applied, e.g. mass spectrometry35,36 or spectroscopic methods like Raman37,38. The volatility of Ionic Liquids is not considered a major disadvantage but a new possibility for purification.39

Protic and aprotic ILs have different vaporization mechanisms. Aprotic ILs like dialkylimidazolium hexafluorophosphates, bistriflimides or triflates or tetraalkylphosphonium salts vaporize under pressure and temperature conditions similar to those of a reduced-pressure distillation. The gas phase is composed of discrete anion-cation pairs, with no detectable concentration of either free ions or larger clusters. On the other hand, protic ILs (e.g. 1-methylimidazolium ethanoate) dissociate fully during evaporation and exist as separated neutral molecules in the gas phase.36

The properties of M[NTf2] compounds with Group 1 metal cations (M = Li, Na, K, Cs, Rb) in the gas phase lie in between the conventional ILs (only neutral ion pairs) and Group 1 metal halides (mainly higher aggregates). The vapor phase of M[NTf2] molten salts primarily consists of neutral ion pairs, but + - also higher aggregates like [Mn+1(NTf2)n] or [Mn(NTf2)n+1] were observed. These are generated through + - a sequence of ion-molecule reactions initiated by the M and (NTf2) ions formed by electron ionization + - and the evaporated neutral species. No parent radical ions [M(NTf2)]• or [M(NTf2)]• were detected in the mass spectrometry experiments.35

2.1.1.2 Thermal Stability of Ionic Liquids Another upper operation limit for ionic liquids is the thermal decomposition. Classical thermogravimetric analysis yields a decomposition onset temperature which has to be handled with care. 6 2. Theoretical Background

This temperature Tonset is dependent on the size and material of the sample pan, the amount of sample, the heating rate, the flow rate and the carrier gas itself and even the geometric construction of intercepts in the resulting diagram of mass loss versus temperature.16,40

Thus, the reported onset temperatures should be considered as estimation, which might be useful as a comparative value, but the experimental conditions have to be reported and still then the scientific significance of thermal gravimetric analysis should be treated with care. Usually Tonset gives a overestimation of the thermal stability, since the actual critical temperature is passed through quickly and often a lack of parameter knowledge reduces the significance of the value.16,18

The mass loss of evaporation is often superimposed by the mass loss of thermal degradation. The discrimination of the two effects is not easy, but is highly important to determine the maximum allowable operation temperature of IL applications. Heym et al. developed a method to overcome this drawback of TGA and to distinguish between decomposition and evaporation of the sample. This approach also allows the calculation of vapor pressures. The TG experiments are conducted with different carrier gases. The mass loss by decomposition does not depend on the nature of the gas. On the other hand the mass loss by evaporation is faster in He than in N2, since the diffusion in He is about three times faster.18,41 The TGA approach of Heym et al. to distinguish between evaporation and decomposition was extended to high vacuum experiments with a magnetic suspension balance to measure even the low vapor pressure of ILs, which show no contribution of evaporation under ambient pressure conditions and high temperatures.18

Comparing the thermally relatively stable [EMIM][NTf2] with [EMIM][MeSO3] in the above described approach shows the mass loss of [EMIM][NTf2] at ambient pressure and moderate temperatures is dominated by evaporation, while for [EMIM][MeSO3] almost only thermal decomposition takes place. From TG experiments and simulations, the maximum operation temperature (1 % mass loss per month) of [EMIM][NTf2] and [EMIM][MeSO3] were determined to be 308 °C and 181 °C, respectively. Even at very small heating rates the Tonset of [EMIM][NTf2] underestimates the thermal stability because of the strong evaporation, quite in contrast to [EMIM][MeSO3] where Tonset overrates the thermal stability.42

A criterion of the maximum operation temperature of ILs in a technical application, e.g. as solvent for two-phase catalysis or as extraction agent, could be the annual decomposition of 1 % (Tmax, 1%/a). In this case the evaporation is irrelevant and only thermal degradation limits the operation temperature. For applications where a gas flows through an IL, e.g. for regeneration of ILs used for extraction or absorption, or for solid catalyst supports coated with IL (like in the SILP or SCILL concept, see section 2.1.4.2), the evaporation has to be taken into account and might become the limiting factor for the maximal operation temperature.18

By immobilizing the IL as thin film on a solid support like silica or the surface of a Ni catalyst, the IL has a lowered vapor pressure compared to pure ILs. But the measurement of the thermal stability of coated ILs is as well quite elaborate. It is dependent on several factors, especially the sample mass, the surface of the support, the gas flow and the heating rates. On the other hand the evaporation of the coated IL is independent on the initial filling degree of the pores. In some cases the thermal stability of ILs has been found to alter by about 150 K due to the presence of a solid surface.43 This is in accordance with the observations made by Kosmulski et al. who reported the accelerated thermal degradation of dialkylimidazolium phosphates from the addition of silica, even though this effect is less pronounced with titania or alumina ceramics.40 In contrast, Riisager et al. found the thermal stability of ILs is enhanced by mesoporous silica (pores between 20 and 200 Å) if the ILs is composed of anions that can take part in hydrogen bonding like oxyanions. The effect was not significant for other anions like 7 2. Theoretical Background chloride.44 Kinetic and thermodynamic effects play an important role for the decomposition or evaporation of supported ionic liquids. By strongly increasing the surface of the IL by the porous support the evaporation in a continuous gas flow is enhanced. But the lowered vapor pressure leads to pore 43 condensation in smaller pores which can increase the Tonset.

2.1.1.3 Decomposition Products of Ionic Liquids Similar to the volatile components of ionic liquids the thermal decomposition products are of vital importance to design new processes. In IL research the degradation is one of the most complex topics. A broad variety of factors can lead to decomposition of ionic liquids. Some efforts were made to define the stability and decomposition products of ionic liquids in the literature. Next to thermal degradation the decomposition in electrochemical processes45,46 or upon friction47 was investigated. Also, the chemical stability (e.g. hydrolysis or reaction with bases)48,49 is a topic of interest and even the decomposition by special treatments like ultra-sound pyrolyis50, photolysis51 or the radiolysis52-54 of ILs has been investigated with numerous different experimental and computational methods.

The analysis of the liquid phase was carried out by different spectroscopic techniques, with NMR being the most frequently applied one. Also, UV spectroscopy, vibrational spectroscopy (like IR and Raman) have been used, next to mass spectrometry. These methods analyze all components contained in the entire liquid phase simultaneously. The resulting information is a superposition of signals of the impurities with those of the neat IL. Often the signals of the decomposition products are small or not visible at all. Chromatographic methods coupled with suitable detectors overcome this problem by separating the different species. A rarely applied yet very promising method is capillary electrophoresis coupled with mass spectrometry to give even lower limits of detection than ionic chromatography. The experimental data are often extended and fitted with DFT calculations of possible decomposition mechanisms.55-59

Comprehensive data on the solid and insoluble decomposition products of IL do not exist. The reason may be the difficulty to separate these products from the non-volatile ionic liquids.60

The gaseous decomposition products were investigated by Raman spectroscopy (see also section 2.1.2) and in situ FTIR studies. The main volatile decomposition products of the N-butyl-N- methylpyrrolidinium bistriflimide IL from cathodic polarization experiments at Li metal and graphite electrodes as determined by in situ FTIR studies are trifluoromethane CHF3, tertiary amines and hydrocarbons, like methane, ethylene and butene.60 Fluorinated anions decompose in contact with water at elevated temperatures and liberate HF.55

A typical decomposition of a dialkylimidazolium cations is the reverse reaction of the quarternization. The formed products are imidazole derivates and alkylated anions. Also, the deprotonation of the C2- atom of the fused ring by strong nucleophiles was observed. The resulting carbene reacts with other IL cations or any dissolved decomposition product present. Addition of electrolyte salts like Li[ClO4] did not change the decomposition behavior of the IL, whereas the different anions showed an influence on the decomposition products of several dialkylpyrrolidinium and dialkylimidazolium ILs.55,56,59

Ammonium and phosphonium salts decompose via a reverse Menschutkin reaction or Hofmann elimination reaction. Aryl phosphonium cations lacking the alpha-hydrogen atom will degrade differently.48

2.1.2 Raman Spectroscopy and Ionic Liquids Raman spectroscopy is an important analytical technique for the study of local environments and intermolecular interactions of a sample. It studies the vibrational modes of a system, which are based on 8 2. Theoretical Background the inelastic scattering of monochromatic light from the sample. The effect was first observed in 1928 by Indian physicists Raman and Krishnan,61 and brought Raman the Nobel Prize in Physics in 1930.62 The Raman effect may be defined as instantaneous inelastic scattering of electromagnetic radiation (light). The relationship between infrared absorption, Rayleigh and Raman scattering is shown in Figure 2.2. When light is passed through a sample, photons from the beam can interact with the electrons of the molecules. Depending on the energy of the incident light, the interaction might result in a change in the electronic, vibrational or rotational state of the sample molecule. Infrared (IR) and Raman spectroscopy are both referred to as vibrational spectroscopy, but the mechanism with which the light interacts with the sample is different and the obtained data from these techniques are complementary.63

During the IR absorption process, a photon of particular energy E and with a frequency ν (E = hν) is absorbed. This excites the molecular system to undergo a transition from the ground state (quantum number n = 0) to a higher vibrational state (n = 1). The sample absorbs only certain frequencies which match its vibrational transitions. The measured spectra show the difference in the intensity of the incident and detected light, caused by the absorption.63

Figure 2.2: Schematic illustration of Infrared absorption, Rayleigh and Raman scattering, and the side effect of fluorescence.

In contrast to this, during Rayleigh and Raman scattering, an exciting photon of much higher energy (about three orders of magnitude) hits the molecular system and raises it to a virtual state, from where it “immediately” falls back. There are two possibilities: in Stokes Raman scattering, the system falls back to the n = 1 state, or in Rayleigh scattering to the n = 0 ground state. If the system is starting from the n = 1 state, similar transitions can happen and are referred to as the anti-Stokes Raman scattering. The Rayleigh scattering is an elastic process, and due to its much higher probability it is much more intense. Typically only 10-6–10-8 of the photons hitting the sample are scattered in Raman. Especially the intensity of the anti-Stokes Raman scattering depends on the amount of molecules in an excited vibrational state, however, at room temperature the majority of the molecules are in the ground state (Boltzman distribution), which means the anti-Stokes scattering is ca. two orders of magnitude weaker than the Stokes scattering.63 9 2. Theoretical Background

The spectra are obtained as a result of a shift in the wavelength between the incident and emitted light, usually the reported data corresponds to Stokes Raman transition. This shift is characteristic for the measured system, enabling the identification of compounds.63,64

Some samples (or impurities in the sample) having energy states near the “virtual” ones may absorb photons from the incident light and later re-emit the light as a broad intensive background called fluorescence. This is one of the major drawbacks of the Raman technique. This can be overcome by e.g. decreasing the exposure time and/or the laser power, or increasing the laser power to burn the fluorescence molecules out, or using a laser at a wavelength which most materials do not absorb (near- IR (NIR) Fourier Transform (FT) Raman spectroscopy). The advantage of Raman spectroscopy over IR and other analytical techniques (where fluorescence problems can be circumvented) arises from the ability of Raman spectroscopy to identify discrete species in situ. Raman spectra can be obtained from any phase, e.g. from samples inside ampoules or vials, because of the transparency of most glasses and window materials.64

During the last decade Raman spectroscopy was realized to be a powerful investigation technique of ionic liquids which provides insights on ionic interactions, molecular conformations in the liquid phase or during phase transitions. The spectroscopic data are often extended with quantum chemistry methods, with density functional theory (DFT) is one of the most often used technique for the calculation of vibrational frequencies with a good compromise of accuracy and computational expense. There are plenty of studies dealing with all kinds of cations like imidazolium, pyridinium, pyrrolidinium, or ammonium derivatives or different anions, recently reviewed by Pascoal et al..65

Especially bis(trifluoromethylsulfonyl)imide salts have been investigated, since these often are low temperature melting salts. Assignments of the vibrational frequencies and identification of the molecular conformers were carried out by Herstedt et al.66,67 and others68,69 for different ILs, and mixtures. There - are two conformers of [NTf2] of C2 and C1 point group symmetry (a two-fold rotational axis and no symmetry), respectively. The C2 conformer, often referred to as transoid, has the CF3 groups on opposite sides of the S-N-S plane and is calculated to be slightly more favorable compared to the C1 also known as cisoid conformer (with the CF3 groups on the same side of the S-N-S plane). Also, a pronounced delocalization of the negative charge over the nitrogen and oxygen atoms and a strong double bond character of the S-N-S moiety is found. By comparing the different reports it can be concluded that the vibrational frequencies are strongly dependent on the counter ion and the physical state. Combining the Raman spectroscopic studies with IR investigations a deeper understanding can be obtained. For example, the longer the alkyl chain in ammonium ionic liquids the more likely is the cisoid - conformation, because the strong interactions between the [NTf2] anion and the polar head of the cation stabilize this conformation.70 Sometimes there is even a coexistence of the transiod and the cisoid - conformer in the normal liquid phase. By phase transitions the conformation of the [NTf2] anion might change. Also, kinetic effects are found to play a role in phase transitions. The crystal phase of -1 [EMIM][NTf2] at -160 °C obtained after a slow cooling process (~ 1 K min ) contained the cisoid conformer, while the glassy phase at the same temperature obtained by fast cooling (~ 20 K min-1) was composed mainly of the transoid conformer.71 A similar behavior was found for tetralkylammonium bistriflimide ILs.72

For the [BMIM]+ cation a rotational isomerism was found. Formerly concluded from X-ray crystal structures it could also be proven by Raman spectroscopy. The two conformers, namely the anti-anti and the gauche-anti (shown in Figure 2.3) coexist in liquid state. The two conformers were also found for the 1-hexyl-3-methyl-imidazolium cation.73 10 2. Theoretical Background

Ionic liquids are already considered to be designer solvents, but by mixing different ILs this effect can be further maximized. For mixtures of ionic liquids the additivity was found, this means the spectrum of the mixture is the sum of the spectra of the neat liquids weighted by the concentration. For mixtures of ionic liquids with the [BMIM]+ cation and different halides or tetrafluoroborate anions it was found that the cation-anion interaction does not change upon mixing.74

Berg et al. investigated molten salts with the 1,1,3,3-tetramethylguanidiunium cation (abbreviated as 37,75,76 [tmgH]) and different anions. The [tmgH]Cl forms a solid state adduct of the ionic liquid and SO2 gas. The absorption capacity of [tmgH]Cl is very high, it can bind up to 3 mol of SO2 per mol IL at 1 bar 75 SO2 and room temperature. This might be useful in the absorptive purification of flue gas. Upon heating the pure [tmgH]+ based ILs with Cl- and Br- form a vapor phase which gives almost identical Raman spectra. Supported by DFT calculation this was explained by a decomposition of the ILs to 1,1- dimethylcyanamide in the gas phase leaving dimethylammonium halogenide behind.76

The power of Raman spectroscopy on gas phase samples was also useful to determine the evaporated species at elevated temperatures above the protic 1-methylimidazolium ethaonate (abbreviated with

[MIM][O2CCH3]) ionic liquid. The vapor phase consists of neutral molecules, namely ethanoic acid and methylimidazol.38

The combination of FT-Raman spectroscopy with complementary electrical conductivity and NMR measurements proved to be very useful in the investigation of a thermomorphic ionic liquid-organic liquid system. The 1,3-bis[2-(2-methoxyethoxy)ethyl]imidazolium bis(trifluoromethylsulfonyl)- imide/1-hexanol mixture changes its phase separation behavior with temperature (one homogeneous phase at slightly above 50 °C, and two separate phases at room temperature). This is very interesting for possible applications in biphasic reaction systems.77

2.1.3 Solubility and Coordination of Metal Compounds in Ionic Liquids If ionic liquids should be used as solvents for catalysis, the solubility of the catalyst is quite important. One of the most commonly used properties to describe solvents is the solvent polarity, and even for molecular solvents it is poorly understood and often confused. General demonstrations are missing that ionic liquids are highly polar solvents, following the simplest qualitative definition: “a polar solvent is one that will dissolve and stabilize dipolar or charged solutes.”8

The choice of cation and anion can influence the solvent properties. The solubility of 1-octene in a tosylate-based IL increases with increasing nonpolar character of the cation, until a single-phase mixture with methyl-tri-n-octylamminum tosylate is obtained. The [BMIM]+ ionic liquids with anions like Br-, 11 2. Theoretical Background

- - - - [CF3COO] or [OTf] are water soluble, while [PF6] or [NTf2] anions form biphasic mixtures with water.7

Depending on the coordinative properties of the anion the IL can be regarded as “innocent solvent” or as cocatalyst (often for chloroaluminate melts). Also great care has to be taken, that the catalyst is immobilized in the IL and no leaching occurs during the reaction. This can be achieved by ionic catalyst complexes or catalysts with highly polar ligands.7 Since the solubility of metal salts is not so good, a promising approach to obtain highly concentrated solutions is by simply dissolving metal salts in ILs with the same anion.78

Following this procedure, Chiappe et al.79 investigated mixtures of metal salts and ionic liquids with the same anion with X-ray photoelectron spectroscopy (XPS) and electrospray ionization mass - - spectrometry. The ionic liquids used in this study were imidazolium-based with [NTf2] , [OTf] or - -1 [NO3] anions. They reported solubilities of a molar fraction up to 0.3 (= 1.14 mol L ) of Ni(NTf2)2 in

[BMIM][NTf2]. The solubility of metal nitrates in nitrate-based ILs is much lower, around 0.55 M. The high solubility for the homoanionic solutions of Ni(NTf2)2 in [BMIM][NTf2] is not a result of the same solvation structure as for Ag(NTf2) or Al(NTf2)3 in the same IL (where only concentrations around 0.4 mol L-1 are possible). The XPS investigations of the solutions also showed very different near- surface chemical compositions: while the Al3+ and Ag+ ions were found in all parts of the ILs, the Cr3+ and Ni2+ are depleted from the surface. This was interpreted as a result of the coordination spheres. Al3+ and Ag+ having no strong coordination spheres stand alone in these solutions as single ions. On the other hand, the strong coordination spheres of Cr3+ and Ni2+ do not allow the metal ion to reach the physical surface of the liquid. This assumption was further confirmed by electrospray ionization mass spectrometry of the solutions of Ag(NTf2), Al(NTf2)3 and Ni(NTf2)2 in [BMIM][NTf2]. While a clearly - detectable peak of charged [Ni(NTf2)3] was found, the corresponding charged complexes of - - 79 [Ag(NTf2)2] and [Al(NTf2)4] were significantly smaller.

Other groups investigated similar mixtures. Fujii et al. used the IL [EMIM][NTf2] as solvent and 80 81 investigated its mixtures with Li[NTf2] and two-valent first-row transition metals , namely

Mn(NTf2)2, Co(NTf2)2, Ni(NTf2)2 and Zn(NTf2)2 with UV/vis spectroscopy and Raman spectroscopy.

By Raman spectroscopic measurements the solvation number of alkali metal salts in [BMIM][NTf2] were evaluated to be 1.95 for Li+, 2.88 for Na+, 3.2 for K+ and 3.9 for Cs+, respectively. By taking into - account that [NTf2] acts as a bidentate ligand, the atomic coordination numbers are proposed to be 4, 6, 6 and 8 for Li+, Na+, K+ and Cs+, respectively. The Raman spectra in the lower wavenumber region - + suggest that cis [NTf2] is favored in the first solvation sphere of the smaller alkaline metal ions like Li , Na+ and K+, while for Cs+ the trans conformation is preferred.82

UV/vis investigation revealed an octahedral ligand field for the solution of Ni(NTf2)2 in [EMIM][NTf2]. This means that three of the bidentate bistriflimide ligands coordinate to the metal center. The excitation wavelength used in the Raman studies was 1064 nm (Nd:YAG laser). Of special interest was the Raman -1 - band at 744 cm which is ascribed to the coupled ν(SNS) + δ(CF3) vibrations of the free [NTf2] anion, which shifts to higher frequency upon binding to the metal ion. This binding process was observed as a weakening of the band at 744 cm-1 and an increase in the band at 750 cm-1 for the solutions of all four studied transition metal compounds Mn(NTf2)2, Co(NTf2)2, Ni(NTf2)2 and Zn(NTf2)2 in [EMIM][NTf2].

In Figure 2.4 (left hand side) the Raman spectra of [EMIM][NTf2] solutions containing Mn(NTf2)2 at - various concentrations and the deconvoluted bands of the [NTf2] species are shown. Plotting the ratio of integrated intensity of the free bistriflimide ion If to the concentration of the metal ion cM against the ratio of the total concentration of bistriflimide ions cT to the concentration of the metal ion cM results in a linear graph and from this the solvation number can be deduced to be approximately three concluding 12 2. Theoretical Background

- the presence of [M(NTf2)3] molecules in solution. The resulting If/cM vs. cT/cM plot is shown in the right part of Figure 2.4.81

Figure 2.4-Left: Raman spectra of (a) [EMIM][NTf2] solutions containing Mn(NTf2)2 of various concentrations, and (b) the - + deconvoluted bands of [NTf2] (solid line) and [EMIM] (dotted line). -Right: The If/cM vs. cT/cM plot of the Mn(II) and Zn(II) systems. Both Figures adapted from reference81. Copyright © 2008 the Japan Society for Analytical Chemistry.

Matsumaiya et al. used the IL triethylpentylphosphonium bis(trifluoromethylsulfonyl)imide

[P2225][NTf2] as solvent and investigated its mixtures with Fe(NTf2)2, Co(NTf2)2 and Ni(NTf2)2 with UV/vis spectroscopy, Raman spectroscopy and DFT calculations. Here, the UV/vis spectroscopy analysis indicated an octahedral ligand field. Thus, the metal ion forms a six-coordinated cluster with - three [NTf2] anions. The Raman spectroscopy of the solutions and the before described analysis method80,81 gave solvation numbers of 2.56, 3.13 and 3.31 for Ni2+, Fe2+ and Co2+, respectively, which is in good agreement with the UV/vis results. DFT calculations were carried out to distinguish the conformation of the coordinated bistriflimide ion. The evaluation of the optimized geometries showed, - - -1 that [Fe(transNTf2)3] is slightly more stable than [Fe(cisNTf2)3] by 3.1 kJ mol , which led to the conclusion that the trans conformer would be preferred in the vicinity of the Fe2+ ion, although no experiments were carried out to undermine this.83

Even more unambiguous demonstrations of the solvation structure can be obtained by X-ray 84 crystallography. Nockemann et al. dissolved Co(NTf2)2∙6H2O in 1-butyl-1-methyl-pyrrolidinium bistriflimide. Upon heating the deep purple solution to 120 °C the complex was dehydrated. Single crystals of [C1C4Pyr]2[Co(NTf2)4] were obtained and the crystal structure is shown in Figure 2.5. The - cobalt(II) ion is octahedrally surrounded by two bidentately and two monodentately coordinating [NTf2] anions. By dissolving the Co(II) precursor in a series of specially synthesized nitrile-functionalized pyrrolidinium ionic liquids (1-cyanoalkyl-1-methylpyrrolidinium bistriflimide, with alkyl = methyl, ethyl, propyl and butyl) slightly different coordination structures were found. On the right hand side of Figure 2.5 the crystal structure of cobalt(II) bis(1-cyanomethyl-1-methylpyrrolidinium)- tetrakis(bis(trifluoromethyl-sulfonyl)-imide) is shown. The cobalt(II) ion is octahedrally surrounded by four monodentately coordinating bistriflimide anions and the nitrile function of two axially coordinating 13 2. Theoretical Background

+ [C1C1CNPyr] cations. Dissolving Co(NTf2)2∙6H2O in the 1-cyanoethyl-1-methylpyrrolidinium derivate of the IL gave compounds with the composition [Co(C1C2CNPyr)6][NTf2]8 with six nitrile groups of the 1-cyanoethyl-1-methylpyrrolidinium cation coordinated to the cobalt(II) ion and heavily disordered, noncoordinating bistriflimide anions (crystal structure not shown here, but in ref.84).

Figure 2.5-Left: Crystall structure of [C1C4Pyr]2[Co(NTf2)4]. One cation is omitted for clarity. -Right: Crystal structure of 84 [Co(C1C1CNPyr)2(NTf2)4]. Adapted from reference and the Cambridge Crystallographic Data Center (CCDC). Color code: deep blue: Co; light blue: N; red: O; grey: C; yellow: S; green: F; white: H.

In another study several copper imidazole complexes could be crystallized from ILs. One complex has the general formula [Cu(RIM)4][NTf2]2 with R = Me, Et, Bu, He and contains square planar copper(II) centers. The other type of metal complex found in this study has the composition [M(MIM)6][[NTf2]2 (M = Cu, Ni, Co). In the latter the metal center is octahedrally coordinated.85

Other reports deal with metal containing ILs to add magnetic, photophysical/optical or catalytic properties that originate from the metal incorporated in the complex anion to the “green” properties of ionic liquids. Also the extraction of valuable or toxic metal ions from downstream processes is investigated. The solvation of f-block metals like europium(III)86,87, lanthanide(III)88 and other trivalent lanthanides and actinides (Am3+, Nd3+)89 in ILs was of special interest and solved by crystal structures, Raman spectroscopy, UV/vis spectroscopy or Extended X-ray Absorption Fine Structure (EXAFS).

2.1.4 Catalysis in Melts As previously mentioned, one of the major interests in ionic liquids is their use in “green” chemistry. Their negligible vapor pressure minimizes the risk of atmospheric contamination and reduces associated health concerns. They can also lower the amount of solvent or catalyst in a chemical reaction. The great advantage of homogeneous over heterogeneous catalysis is well documented with respect to activity, selectivity and efficiency. However, it has the major drawback for industrial application that it can be difficult to separate the catalyst from the products of the reaction. This leads to a waste of precious resources and the danger of releasing highly reactive chemicals into the environment. A possible solution to this problem includes biphasic catalysis; this represents a method to heterogenize catalyst and products into two separate immiscible phases without losing the selectivity and efficiency inherent in homogeneous catalysis. The catalyst resides in one of the two phases, and the substrates reside in the other. Once the reaction has reached an appropriate stage of conversion, the two immiscible phases can be easily separated. The catalyst dissolved in the ionic liquid is recovered and available for subsequent runs, thus overcoming the main drawback of homogeneous catalysis.8,15,28 14 2. Theoretical Background

2.1.4.1 Biphasic Systems with IL Advantages of ionic liquids as catalyst phase are the usually inert solvent properties and the possibility to immobilize a catalyst either because of its ionic character or by using polar modified ligands. Also, the solubility of substrates and products can be tuned by an intelligent combination of cations and anions.7,23

Till 1987 most biphasic reactions utilizing melts dealt with alkali halide salts or their eutectic mixtures, often very corrosive and with high melting points. Pyridinium, ammonium or phosphonium halides already have a much lower melting point (60–150 °C).90 Also, aluminum chlorides found application in a broad variety of acid catalyzed, organic reactions. The aluminum chlorides will not be discussed here in more detail, they can usually be regarded as cocatalyst not as inert solvent.

One of the first successful applications of biphasic catalysis with inert ILs were hydrogenations as published by Chauvin et al.14 and de Souza et al.91 in 1995 (see section 2.2.2.3.1 for more detail). Several other biphasic hydrogenations were reported since then. Especially, asymmetric hydrogenations have a certain appeal since the stereoselectivity can be influenced not only by the (chiral) ligand system of the catalyst92, but also by the IL solvent93.

The [BMIM][PF6] IL found application in the stereoselective hydrogenation of sorbic acid. To realize a two-phasic system the organic phase was methyl-tert-butyl ether in which the substrate was dissolved 4 while the [Cp*Ru(η -CH3–CH=CH–CH=CH–COOH][CF3SO3] catalyst was immobilized in the IL. At 60 °C the sorbic acid was converted in 40 % yield and a selectivity of 90 % to cis-3-hexenoic acid at

10 bar H2. Even though the high viscosity of the IL led to lower activity due to mass transport limitations, at the elevated temperature of 60 °C this drawback could be overcome and a better activity was found than for the reaction with glycol. The recycling experiments showed almost identical activity and selectivity in the first two runs, but a slow decomposition of the catalyst lowered the activity for the subsequent runs. Again, a dependency of the anion of the IL was found. The stronger coordinating [Tf] - anion blocks the reaction.94

Over the last decades several examples of biphasic reactions like hydroformylation95,96, isomerization97, and polymerizations98 were published to name just a few. Also, the biphasic approach could be used in extractions.99 The interested reader is referred to reviews by e.g. Steinrück and Wasserscheid23, Olivier- Bourbigou et al.31 or Jutz et al.100.

Some biphasic reactions have already reached the industrial level and are performed in multi-ton scale. Two of the first, very prominent examples are the BASILTM process101 and the Difasol process102. The BASIL process (Biphasic Acid Scavenging utilizing Ionic Liquids) was introduced at the BASF site in Ludwigshafen, Germany in 2002. It is schematically depicted in Scheme 2.1. In the synthesis of the photoinitiator precursor alkoxyphenylphosphine (for printing inks, glass fibers and wood coatings) hydrochloric acid has to be scavenged. Formerly, this was done with trimethylamine, the salt resulting from that reaction formed a solid and made the mixture difficult to handle. Replacing the trimethylamine with 1-methylimidazole results in the formation of 1-methylimidazolium chloride which is liquid at reaction conditions and forms a separate phase. The biphasic process increased the yields from 50 % to 98 % and a much smaller reactor can be used.30,101 The Difasol process superseded the Dimersol process of the IFP (Institute Français du Pétrole), the existing Dimersol plants were retro-fitted to run the new process (see Figure 2.6). The biphasic reaction conditions of the Difasol process are clearly superior in the dimerization of alkenes (usually propene or butane). With one phase containing the cationic nickel catalyst dissolved in a chloroaluminate IL (a mixture of AlCl3 or dichloroethylaluminum or chlorodiethylaluminum with [BMIM]Cl), the products form a second phase, enhancing both activity and selectivity to dimers and lowering the amount of catalyst needed.102 15 2. Theoretical Background

Scheme 2.1: BASILTM process (adapted from ref.15).

Performing reactions in ionic liquids showed huge improvements. However, compared to water, ILs will always be significantly more expensive (even though they are commercially available now) and the use of biphasic reaction systems requires large amounts of ionic liquid. In addition to that, the intrinsically high viscosity of molten salts leads to slow transport between the two liquid phases. Due to these drawbacks, industry still prefers heterogeneous catalysis.104

2.1.4.2 Supported Ionic Liquid Phase Catalysis A concept developed to overcome the limitations of biphasic catalysis and to combine the advantages of highly product- and stereoselective homogeneous catalysis and the stationary catalyst phase, as known for heterogeneous catalysts is the “Supported Ionic Liquid Phase” (SILP). It involves the large inner surface of a porous support that is covered with a thin film of IL (typically a couple of nanometers thick) containing the homogeneously dissolved catalyst as schematically shown in Figure 2.7. The first monolayer of IL is immobilized on the surface of the support material either by physisorption or chemisorption. Addition of more ionic liquid results in the formation of a multiple layer of free ionic liquid on the support, which serves as reaction phase for the catalyst. The resulting material is a free flowing powder, but the active species is dissolved in the IL phase and performs like a homogeneous catalyst. The resulting catalyst system exhibits high activity, outstanding stability, the ease of separation and enables the usage of the fixed-bed technology preferred for industrial applications. Despite all these 16 2. Theoretical Background advantages SILP catalysis is often limited to gas-phase reactions due to problems with leaching of catalyst or IL into a liquid phase.104-107

One example of a successful application of SILP is the reaction from n-butane to n-pentanal in a two- step process as applied by Walter et al..108 The first step is the dehydrogenation of the alkane feed with a heterogeneous Cr/Al2O3 catalyst, followed by the gas-phase hydroformylation using a SILP catalyst consisting of a Rh complex modified with diphosphite ligands and the IL [EMIM][NTf2] on silica support. The SILP catalyst is extremely stable (over 900 hours of time on stream) and highly efficient to yield linear pentanals (S = 98 %) because it is not only active in the hydroformylation but also in the isomerization of 2-butene to 1-butene.108 The kinetics and mechanism of this hydroformylation were recently investigated in more detail and the experimental data were fitted with DFT calculations.109

The properties of the support (like particle size, pore size distribution and texture) show a close relation to the catalytic activity of a Brønsted acidic SILP catalyzed oligomerization of iso-butene.110 By modifying the support, an interesting increase in activity of the hydroformylation of propene to butyraldehydes was observed. A carbon support was functionalized with amines, thus raising the nitrogen content and the point of zero charge of the activated carbon. This increased the activity of the SILP catalyst by an order of magnitude compared to SILPs with unfunctinalized carbon or oxidic supports.111

SILPs have also been used in Friedel-Craft alkylation112, ultra-low temperature water-gas shift reaction113,114, gas-phase oxycarbonylation of methanol115, C-C coupling and cross-coupling reactions116 or acetylene hydrochlorination117.

118 119 In the purification of gases and also liquids SILP systems have proofed to be highly efficient. NH3 120 could be removed from a nitrogen gas flow (wet and dry) , CO2 and SO2 could be absorbed by a chitosan ionogel also known as inverse supported ionic liquid phase from flue gas.121,122 By using a water-insoluble polyoxometalate-ionic liquid supported on porous silica water could be purified from organic, inorganic and microbial impurities123, flue gas could be decontaminated from elemental mercury with task-specific Ionic Liquids on silica124.

The interactions of ionic liquid and support have been studied with surface science approaches like operando Diffuse reflection infrared Fourier transform spectroscopy (DRIFT)125 or X-ray photoelectron spectroscopy (XPS)126

In the similar concept known as SCILL (Solid Catalyst with Ionic Liquid Layer) a solid catalyst, usually a precious metal on a porous support, is covered with a layer of ionic liquid to enhance the selectivity of the heterogeneous catalyst. This particular topic goes beyond the scope of this thesis and the interested 17 2. Theoretical Background reader is referred to the excellent review127 and reports of catalysis with ionic liquids128-131 and surface science investigations of IL-coated materials132,133.

2.2 Model Reactions

For the investigation of the potential of molten salts as solvents for homogeneous catalysis in this thesis, two different reaction types were chosen and the state of the art as reported in literature is outlined in this chapter.

2.2.1 Friedel-Crafts Acylation There are several reactions to form C-C bonds. Beside examples like the Suzuki crosscoupling, Heck and Stille reaction, Sonogashira coupling or different forms of metathesis (all catalyzed by precious metals), Lewis-acid-catalyzed Friedel-Crafts reactions are another possibility. They are, in principle, electrophilic aromatic substitutions where a benzene molecule is attacked by an alkyl halide in the case of the Friedel-Crafts alkylation or an acyl halide in case of the Friedel-Craft acylation. The reaction proceeds always in the presence of a Lewis acid like AlCl3. However, the alkylation reaction has many drawbacks like polyalkylations due to the higher electrophilicity of the alkylated aromatic ring or isomeric products due to rearrangement reactions of the carbocation.134

The Friedel-Crafts acylation does not share these disadvantages. The general reaction is depicted in Scheme 2.2. The acyl cation is formed by the dissociation of the adduct of the acyl halide with the Lewis acid catalyst (see Scheme 2.3). Sometimes also acid anhydrides are used. Then the arene attacks the acylium. The aromaticity is reinstalled by the deprotonation by the halide (see Scheme 2.4).134 The resulting ketone forms a complex with the Lewis acid catalyst, removing it from the reaction. This means at least equimolar amounts of “catalyst” are necessary for full conversions and after the reaction an aqueous workup destroys the catalyst producing extensive amounts of waste.19

Scheme 2.2 Friedel-Crafts acylation; with X = halide.

Scheme 2.3: Formation of the acyl cation, by dissociation of the adduct of the acyl halide with the Lewis acid catalyst.

Scheme 2.4: Attack of the arene to the acyl cation, followed by reinstallation of the aromaticity by deprotonation. 18 2. Theoretical Background

Still these extremely wasteful Friedel-Craft reactions based on over 100 year old chemistry are used to synthesize aromatic ketones which are often important intermediates of pharmaceuticals, fine chemicals, polymers, dyes, fragrances, cosmetics and agrochemicals (like fungicides, herbicides or insectizides).19,20,135-137

Problems of reusability and instability of the catalyst are the biggest issuses of the Friedel-Crafts reactions. Especially, high excess amounts of Lewis acid are necessary and the catalyst cannot be reused.138-140

The introduction of chloroaluminate ionic liquids brought some improvements for Friedel-Craft reactions. One of the first reports of catalysis in molten salts was in the 1980s and deals with Friedel- 11 Craft reactions. The group of Wilkes used AlCl3 mixed with [BMIM]Cl. Since then, the most common catalyst system is AlCl3 or FeCl3 reacted with an imidazolium halide, but also pyridinium-based ionic liquids were reported.135 The rate of the formation of the carbonium cation in the acetylation of benzene with acetyl chloride was found to be dependent on the Lewis acidity as shown by cyclic voltammetry in 11,141 AlCl3/n-butylpyridinium chloride. Already, in 1995 BP filed a patent for the alkylation of benzene 142 with alkenes using [EMIM][Al2Cl7] as catalyst. In situ IR spectroscopy studies confirmed that the mechanism of the Friedel-Craft acylation in ionic liquids is the same as in ordinary organic solvents. + - 143,144 They found the acetyl ions [RCO] [MCl4] as key intermediate. Also, acidic ionic liquids immobilized on solid supports were found to be active in the gas phase Friedel-Crafts acylation. The

[BMIM]Cl-MCl3 (with M = Al or Fe) supported on charcoal gave moderate yields. Used in contact with a liquid phase these systems suffered from leaching.145 Even more complex organic and organometallic compounds can be acylated with [EMIM]X-AlCl3 systems (X = halide). Some examples are depicted in Scheme 2.5.

Scheme 2.5: Acylation of a) naphthalene from reference146; b) ferrocene from reference147; and c) pyrene from reference148. Numbers given under the products correspond to yields under optimized conditions. 19 2. Theoretical Background

For the acylation of naphthalene (Scheme 2.5 a), the thermodynamically unfavored acylation to the 1- isomer in IL was explained by the size of the acylium ion which is much smaller in IL than the adduct 146 of acylating agent-AlCl3-solvent of Friedel-Crafts acylation in common organic solvents. The organometallic ferrocene complex (see Scheme 2.5 b) could also be monoacylated.147 The acylation of pyrene was most successful with [EMIM]Cl-AlCl3 and gave high yields and selectivity towards the 1- isomer. The optimization of the reaction conditions led to 30 °C and 5 hours as shown in Scheme 2.5 c.148

For these chloroaluminate systems there are still several drawbacks. The melts are moisture unstable, highly corrosive and oxophilic. The catalyst often has to be destroyed after the reaction.149

The reports on other catalysts than chloroaluminates are rare. Metal triflates (with scandium, yttrium or lanthanides) were found to catalyze the Friedel-Crafts acylation in nitromethane with excellent yields. Only catalytic amounts (0.2 eq.) were necessary and the big advantage of this system is the recyclability. The catalyst could simply be extracted with water. After drying of the catalyst no loss of activity was found for the following runs.150 It did not last long until metal triflates were used in ionic liquids.

Bismuth seems to be a favorable catalyst. Bi(OTf)3 and other Bismuth derivates were used in 151 152 [BMIM][PF6] or [BMIM][NTf2] . These water and air insensitive systems showed no activity in the absence of IL, additional microwave irradiation improved the results further on. Especially triflates are good catalysts for the Friedel-Crafts acylation. A comprehensive study of the catalytic activity and catalyst deactivation of Cu(OTf)2 was published by Ross et al.. These authors did not only find higher yields for reactions in [BMIM][BF4] than in organic solvents, but also a dependency of the catalytic activity from the counter ion of the metal. CuCl2 and Cu(OAc)2 failed to reproduce the conversion of 20 Cu(OTf)2 due to their lower solubility in the IL. Zayed et al. used among others In(OTf)3 in a continuous biphasic system with supercritical CO2 and a pyridinium-based bistriflimide IL. The biphasic reaction system gave higher yields and was better reusable.153 For Tran et al. indium(III) triflate (among others) was also the catalyst of choice for their acylations with a very broad substrate scope. They used a Brønsted acidic ionic liquid, namely 1-isobutyl-3-methylimidazolium dihydrogenphosphate, which shows a catalytic activity without the addition of another catalyst.154 On a slightly related subject also chloroindate(III) ionic liquids as prepared by Earle et al. are effective Friedel-Crafts acylation catalysts. This novel catalyst system is water stable and could be extracted by water after the reaction and reused after drying.155

The M(NTf2)3 compounds (with M = Al, Ti, Yb) were first synthesized and used as Friedel-Crafts acylation catalysts in organic solvents by Mikami et al.. Catalytic amounts (5–20 mol%) were necessary to acylate anisole in a couple of minutes at room temperature. There are no remarks about recycling or stability in this report.156 The group around Earle inspired by their reports about the catalytic activity of transition metal compounds with the bistriflimide ligand in the Friedel-Crafts acylation157 some of the work of this thesis. These M(NTf2)n were active catalysts in solvent-free systems as well as in ionic liquids. The highest yields were found for unexpected metals like manganese, cobalt and nickel used in catalytic amounts of 1 mol%. The catalyst could be separated after the reaction by distillation or extraction and reused with similar activities.157

2.2.2 Hydrogenation and Dehydrogenation Beside some synthetically applications for the production of fine and bulk chemicals (see e.g.158-160), hydrogenation and dehydrogenation reactions became more and more important in the context of energy storage. 20 2. Theoretical Background

2.2.2.1 Hydrogen and Energy Storage The prediction how long the oil reserves will last are vague and change every few years, but no one questions that they are limited and sooner or later run dry.161,162 Beside the lack of sustainability, the burning of fossil fuels releases extensive amounts of CO2 into the atmosphere. This is a severe environmental problem leading to global warming.163

Bearing this in mind, 195 countries agreed at the 21st Conference of the Parties in Paris in 2015 to limit global warming to below 2 °C, compared to pre-industrialized levels.164

Latest after the nuclear disaster in Fukushima, Japan, in March 2011, it is widely acknowledged that nuclear power holds far too high risks for its benefits (like steady output and low emission of greenhouse gases). Germany shut down its eight old nuclear power plants the same year and decided to shut down the remaining nine nuclear power plants till 2022.165

Now the energy demand has to be satisfied from other sources, but the burning of coal is obviously no alternative. Thus, the renewable energies like solar and wind power are to be expanded. Germany plans to increase the share of renewables in electricity production to become the mainstay of energy supply to be as high as 80 % in 2050 against 1990 levels.166

The renewables have the main disadvantage to not be available at all times. They suffer from seasonal and daily fluctuations. While the renewables cannot yet satisfy the energy demand alone, there is already an energy excess on sunny and windy days. Storing this energy for the night or times with low energy production would solve this problem of negative energy costs and the unpredictable fluctuations. Storage technologies need to be developed and installed. They have to be able to compensate over- and underproductions of energy on short, but also on longer time scales (e.g. to compensate for seasonal differences).21

Pumped hydroelectric storage facilities would be one possibility, but this is not accepted in the population and restricted to certain areas of the planet. Using hydrogen as an energy carrier was strengthened after the global energy crisis of 1974. Hydrogen production from fossil fuels by breaking the C-H bonds is in absolute contradiction with policies towards a green and sustainable energy cycle. But hydrogen can also be produced by splitting water by means of electrolysis with excess energy. This process is completely independent and can be performed everywhere on the globe as long as water and electricity are present. This energy stored in hydrogen can be released later by combustion or conversion to electricity with a fuel cell.167

Hydrogen has a unique energy density of 33.3 kWh kg-1 (= 120 MJ kg-1). However, the volumetric storage density of hydrogen is very low (0,003 kWh L-1). For a better volumetric storage density the hydrogen can be stored with physical methods. Though developed and applied for decades the cryogenic storage of hydrogen below its boiling point of -253 °C consumes massive amounts of energy (around 30 % of the overall hydrogen energy content) and needs highly sophisticated tanks. Still, the loss of hydrogen by boil-off has to be taken into account. Compressed gaseous hydrogen between 350 to 700 bar has slightly better storage conditions, but costly cylindrical high pressure tanks are needed with still unsatisfying volumetric storage densities.168 The storage of pure hydrogen is not feasible and to circumvent these drawbacks researchers everywhere are looking for alternatives.

Another method is to store the hydrogen via physisorption. Currently, none of the different storage solid state materials can reach the required storage densities for e.g. a hydrogen powered vehicle.

Storing hydrogen via a liquid phase condensed by capillary forces inside pores of materials with big specific surfaces (often > 1000 m2 g-1) is quasi impossible at ambient temperature and pressure. 21 2. Theoretical Background

Pysisorption results in formation of only a monolayer at the adsorbent surface. Small pores (< 1 nm) were found to be most efficient for hydrogen storage. Possible adsorbent materials are narrow single wall carbon nanotubes, graphitic nanofibers, porous silica, alumina and zeolites or MOFs. With physisorption hydrogen storage densities between 0.5 and 5 wt% are possible at low temperatures (77 K) and elevated pressures (1–70 bar).169

Metal hydrides exhibit an impressive volumetric hydrogen density on a material basis. However, their solid form makes the handling more difficult and often the storage is not reversible.170

In recent years there was an increasing interest in ammonia-borane as hydrogen storage media. H3NBH3 has a hydrogen content of 19.6 wt%, if all hydrogen could be extracted. The release of hydrogen from ammonia-borane is very straightforward, but the regeneration proves to be problematic. Combinations of different amine-borane derivatives with IL have also been investigated.171-173

Hydrogen can also be stored by covalent binding to an energy-lean molecule by catalytic hydrogenation and reversibly released by dehydrogenation. As energy-lean molecules liquid organic, often aromatic compounds are particularly advantageous. These molecules are also known as Liquid Organic Hydrogen Carriers (LOHC).21 The process is schematically depicted in Figure 2.8.

Figure 2.8: Illustration of the LOHC concept, adapted from reference174.

When bound to the carrier molecule, hydrogen can be handled easily and stored at ambient conditions for long periods; it can be safely transported, and since LOHC are, as the name suggest, liquid the existing infrastructure for fossil fuels (tanker ships, road tankers, pipelines and tank farms) can be used. No high-pressure or super insulated tanks are required. Depending on the LOHC system different storage densities up to 2.2 kWh L-1 are possible. After dehydrogenation the carrier molecule is not consumed but can be loaded with hydrogen again, that is why some people talk about recyclable gasoline. The storage and transportation of the hydrogenated and dehydrogenated LOHC materials decouple the energy production and the energy consumption.22,175

Several different compounds have been suggested as LOHC molecules. Already in 1980 the first LOHC pair was suggested to be toluene/methylcyclohexane (with a hydrogen storage density of 6.1 wt%)176. Other examples are benzene/cyclohexane (7.2 wt%)177, naphthalene/decalin (7.3 wt%)178,179 or isomeric mixtures like benzyltoluene or dibenzyltoluene (each 6.2 wt%)174,180, which are commercially available as heat transfer oils. 22 2. Theoretical Background

The hydrogen release from pure hydrocarbon systems suffer from quite high enthalpies of dehydrogenation and thus high temperatures are needed, but the incorporation of a heteroatom into the aromatic ring has a favorable effect and lowers the ΔH of dehydrogenation.163,181,182 This extends the list of possible LOHC molecules to N-heterocyclic aromatics like indoles, quinolines, and carbazoles to name just a few.183-187

Less favorable but also possible hydrogen storage systems are for example methanol188, formic acid189 190 or ammonia . But after dehydrogenation the resulting energy-lean molecules are CO2 or N2 which are gaseous and the ease of processing of a liquid system does not apply.

2.2.2.2 Heterogeneous Hydrogenation and Dehydrogenation In the context of hydrogen storage and release the majority of the research is focused on heterogeneous catalysis for the respective hydrogenation and dehydrogenation reaction.

One of the best investigated systems is N-ethylcarbazol (H0-NEC)/dodecahydro-N-ethylcarbazole (H12- NEC) as first proposed as LOHC system by Air Products and Chemicals, Inc. in 2003. It has a hydrogen capacity of 5.8 wt% and due to the heteroatom a rather low heat of hydrogenation/dehydrogenation. A

Pd catalyst fully hydrogenates H0-NEC with 72 bar at 160 °C. The dehydrogenation with Ru at 50–

197 °C gave H0-NEC. These catalytic cycles were performed in the same reactor with both catalysts present and can be run for at least five times without degradation (see Scheme 2.6).191,192 The main drawback of this system is the melting point of H0-NEC, which is 68 °C. Thus, it is solid at room temperature, resulting in a high effort to heat not only the reactor but also the complete periphery of the reaction rig or refrain from complete dehydrogenation, which limits the usable H2 capacity to 5.2 wt% to stay in the liquid regime.22

Scheme 2.6: Dehydrogenation of dodecahydro-N-ethylcarbazol and hydrogenation N-ethylcarbazol.

Already some studies were performed to solve the mechanism of the dehydrogenation. In several model studies a detailed picture of the microkinetics of the heteroatom-containing LOHC perhydro-N- ethylcarbazole was obtained. The dehydrogenation mechanism of H12-NEC was solved by combining different surface science techniques like High Resolution X-ray photoelectron spectroscopy (HR-XPS) and Infrared reflection-absorption spectroscopy (IRAS) under ultra-high vacuum conditions.193-196 After the desorption of the multilayer the dehydrogenation starts around -50 °C with a C-H activation in the direct neighborhood of the N atom yielding the first stable intermediate octahydro-N-ethylcarbazole. The following stepwise dehydrogenation is much faster above 0 °C. Above 115 °C the dealkylation of the carbazole on a Pt(111) surface is observed, which is an undesired side reaction.193-195 This degradation already starts at lower temperature (around 100 °C) for a Pd(111) surface.196 Also, small 23 2. Theoretical Background defect-rich particles enhance the degradation and shift it to lower temperatures.193 Comparing the two metals, the dehydrogenation mechanism is very similar.194 The alkyl chain length of the carbazole does not change the dehydrogenation mechanism, but the longer the chain (e.g. H12-N-butylcarbazole) the less pronounced is the C-N bond scission.195

Surface science approaches are powerful tools to resolve elementary steps of reactions, though the ultra- high vacuum conditions and the resulting evaporation of compounds might be a problem. This was circumvented by Matsuda et al. by covalently linking the perhydrocarbazole to an imidazolium cation and thus forming a non-volatile ionic liquid. The X-ray photoelectron spectroscopy of the macroscopically thick film found very similar temperature onset for the start of the dehydrogenation reaction for a Pt foil or Pd particles dispersed in the perhydrocarbzole-IL. The found surface science temperatures match very well to conventional experiments under realistic reactor conditions.197

The tendency of the hydrogen-lean as well as the hydrogen-rich species to split off the alkyl group even at temperatures below the hydrogen release reaction is a real problem. The dealkylated carbazoles can still be used as LOHC molecules but the alkyl substituent is important for the melting point and the fragments might poison the catalyst or the fuel cell. These disadvantages are not shared with the LOHC molecules proposed by Brückner et al.. These authors used dibenzyltoluene (H0-DBT). It is available at low cost in multi-ton scale as heat-transfer oil. It is thus industrially well established and accepted (e.g. low toxicity) and is sold under trade names like Marlotherm. It is a mixture of isomers with a melting point around -36 °C and a high thermal stability and boiling point of 390 °C. With 6.2 wt% it even has a higher hydrogen capacity than NEC or toluene. The full hydrogenation proceeds smoothly at 150 °C and 50 bar H2 in the presence of a Ru/Al2O3 catalyst in 4 hours. Even though, H18-DBT showed slower dehydrogenation kinetics than H12-NEC, a reversible dehydrogenation of H18-DBT could be achieved with a Pt/C catalyst at 310 °C in 2 hours (see Scheme 2.7). 174

Scheme 2.7: Dehydrogenation of H18-DBT and hydrogenation H0-DBT.

Amende198 and Gleichweit199 studied the dehydrogenation mechanisms of heteroatom-free LOHCs (i.e. diphenylmethane DPM and dicyclohexalmethane DCHM as isomer-pure model compounds for DBT) with surface science techniques on Pd(111)198 and Pt(111)199 single crystals.

The reaction mechanism of Pd and Pt are in general similar, but the stability of adsorbed intermediates is quite different on the two surfaces. Starting at -83 °C, the first stable dehydrogenation products are found for Pd(111), which could be observed for Pt(111) at slightly higher temperatures of -73 °C. The C-H bond scission of the methylene bridge is more rapid on Pd(111) than on Pt(111): Beginning at around 0 °C on the Pd(111) surface the dehydrogenation of the methylene bridge is observed. The same is found at 77 °C for Pt(111). It was concluded that no intact DPM was formed at any time neither on Pt(111) nor on Pd(111).198,199 24 2. Theoretical Background

The bond breakages of DCHM to smaller fragments which reside on the surface of Pt(111) single crystals might be a severe problem. Thus, the regeneration of catalysts poisoned by degradation products of energy-rich LOHC molecules (e.g. H18-DBT and DCHM) was investigated. Different ultra-high vacuum to near-ambient pressure IR methods using CO as probe molecule were used. Pt(111) model catalysts as well as real Pt/Al2O3 catalyst pellets were examined. The catalyst structure and particle size strongly influences the cleaning procedure of heating the catalyst under oxidative conditions (“coke burn-off”). While the surface of the Pt(111) single crystal could be completely restored at around 500 °C, the real Pt/Al2O3 pellets showed major regeneration at 300 °C already, but could not be cleaned completely even after employing harsh conditions.200

Of course, the hydrogenation process is difficult to investigate with ultra-high vacuum techniques. Thus, 1 the reaction pathway of catalytic hydrogenation of H0-DBT was studied with H-NMR spectroscopy.

The hydrogenation was conducted between 120 and 200 °C and 50 bar H2 in the presence of a Ru/Al2O3 catalyst. It was found that the reaction proceeds via a side-ring preference. Additional HPLC investigations confirmed three independent steps. One side ring is fully hydrogenated before the second side ring reacts. The resulting H12-DBT species with the two side-rings hydrogenated accumulates, 201 before the full hydrogenation to H18-DBT occurs with the middle-ring hydrogenation as final step.

Also recently, derivatives of the indole/indoline/octahydroindole system were identified to be suitable LOHC molecules. The reaction steps are shown in Scheme 2.8. The dehydrogenation of indoline is thermodynamically more favorable than of most other hydrogen-rich LOHC molecules. This means that the hydrogen can be released at temperatures around 100 °C, which is close to the operating temperature of PEM fuel cells. It was found that by increasing the temperature between 100 and 180°C the rate of dehydrogenation gets faster. But this also means more undesired side reactions. Pd/SiO2 was the best catalyst for indoline dehydrogenation (in 1 hour conversion of 81 %), while other metals like Ir, Co, Cu, 202 Fe and Ni supported on SiO2 showed no reaction at 100 °C. Similar results were already reported by the Crabtree group. Indoline can be fully dehydrogenated to indole at 110 °C with a Pd/C in only 30 min. They also support their findings with DFT calculations. Incorporation of a heteroatom, especially nitrogen, lowers the enthalpy of dehydrogenation.182

Scheme 2.8: Dehydrogenation of Hx-indole and hydrogenation of H0-indole.

Rh on charcoal was the catalyst of choice for Cui et al. to fully hydrogenate 1-methylindole under 70 bar

H2 and 125 °C in one hour. Slightly lower conversions were obtained with RuO2 or Pd/SiO2. They also were able to convert indole completely in 5 hours under the same conditions. For the dehydrogenation 25 2. Theoretical Background of 1-methyloctahydroindole at 200 °C with Pd/Al2O3 100 % conversion was found, but a rather bad selectivity of only 43 % with unknown products was observed. The selectivity gets better (around 60 %) at lower dehydrogenation temperatures of 165 or 180 °C, but also the conversion drops.181

The N-ethylindole has a hydrogen storage capacity of 5.23 wt% and a melting point of -17.8 °C. It could 183 be fully hydrogenated at 160–190 °C and 90 bar of H2 with a Ru/Al2O3 catalyst in 80 min. The same catalyst was used to hydrogenate 2-methylindole (melting point: 58 °C; hydrogen capacity: 5.76 wt%) 184 at 120–170 °C and 70 bar H2. Using a Pd/Al2O3 catalyst both systems could be dehydrogenated at 190 °C with 100 % selectivity and moderate reaction rates.183,184

The dehydrogenation of octahydroindole and indoline was investigated by Schwarz et al. with surface science techniques on a Pt(111) single crystal. The deprotonation of the N atom of octahydroindole occurs already at 0 °C in UHV. This reaction is observed for the indoline molecule at 110 °C, also the dehydrogenation of the five-membered ring takes place. The dehydrogenation of the (deprotonated) ocathydroindole does not start at the five-membered ring but at the six-membered ring. The five- membered ring is first dehydrogenated at 120 °C yielding the same surface species as found for indole and indoline at this temperature. Above 150 °C the same decomposition compounds are found for all three molecules on the surface.203

Perhydrogenation of quinolines and their methylated derivatives is a stepwise process which depends on the catalyst. For both Raney nickel (10 bar H2, 200 °C) and ruthenium on carbon (100 bar H2, 150 °C) the first step is the hydrogenation of the heteroatom ring to 1,2,3,4-tetrahydroquinoline. While Ru/C directly converted the intermediate product to perhydroquinoline, the R-Ni hydrogenated the 1,2,3,4- tetrahydrospecies directly or via a isomerization step to 5,6,7,8-tetrahydroquinoline followed by complete hydrogenation. The observed reaction mechanism was further supported with calculations.204

One of the major concerns regarding applicability of LOHC in the “real world”, namely the quick response to variations in power demand, was recently tackled by Fikrt and Brehmer et al.. They showed that the dehydrogenation of H18-DBT can be handled dynamically, tuned with the free volume in the LOHC release unit by pressure changes. While the increase of hydrogen output by heating the reactor is too slow to react to fluctuations of the electrical grid, the hydrogen production rate can also be increased by a higher volume flow of LOHC through the reactor. As the pumping performance can be adapted quickly this approach can release more hydrogen, but at the cost of conversion, thus lowering the efficiency and effective storage density of the system. The free volume of the reactor (and purification unit and the connecting pipes) can be used as pressure buffer where the hydrogen accumulates before the fuel cell. By changing the pressure from 1.5 bar to 1.1 bar an increase in the power output from 2.32 kWth to 2.70 kWth can be realized, which can bridge the delay time for reaching the required, increased hydrogen production in the dehydrogenation reactor by adjusting the LOHC flow rate and reactor temperature.205

To even more simplify the process of loading and unloading of the LOHC a new concept has just been published recently by Jorschick et al., namely the hot hydrogen battery. Both reactions, hydrogenation and dehydrogenation are carried out in the same reactor with the same Pt/Al2O3 catalyst. The pressure of H2 (30 bar for hydrogenation) is the only parameter which is changed between LOHC charging and hydrogen release. The LOHC pair of choice is H0-DBT/H18-DBT due to the very high thermal stability and high hydrogen capacity of 6.2 wt%. The hydrogenation is performed at unusual high temperatures of 290-300 °C to utilize the heat of hydrogenation for the subsequent dehydrogenation step, if an appropriate heat storage medium is applied. Four hydrogenation/dehydrogenation cycles proved the concept and showed only a very mild deactivation of the catalyst.206 26 2. Theoretical Background

2.2.2.3 Homogeneous Hydrogenation and Dehydrogenation Catalysis Since the advantages of homogeneous catalysis are widely accepted, a number of catalysts were developed which effectively hydrogenate aromatic compounds or heteroatomic aromatic compounds. Also, there are catalysts which dehydrogenate the LOHC counterparts or even catalyze both reactions. However, most of these reactions seek application in synthesis and not in hydrogen storage and release.

2.2.2.3.1 Homogeneous Hydrogenation

A classical active metal in hydrogenation is Ru. [RuH2(H2)2(PCy3)2] is one of these catalysts. It converts benzene, naphthalene or anthracene under 3 or 20 bar of H2 and 80 °C to cyclohexane, tetralin or a mixture of 1,2,3,4-tetrahydroanthracene and 1,2,3,4,5,6,7,8-octahydroanthracene, respectively. When

Borowski et al. discovered that the increase of H2 pressure lowers the yield of hydrogenation products the investigations were extended by mechanistic studies and it was found that the dissociation of the H2 ligand provides the free coordination side for substrates and this is hindered at higher pressures.207

Another very effective active metal for hydrogenation is Rh. Already in 1977 the very stereoselective

[Rh(Cp*)Cl2]2 catalyst was tested for the hydrogenation of arenes to cyclohexanes. Under 50 °C and

50 bar of H2 the arenes yielded only all-cis cyclohexanes with no partly hydrogenated products.

However, the addition of great excess of base (Et3N) is necessary as co-catalyst, at least 15 eq. per mole catalyst. The base ties up the HCl generated from the heterolytic cleavage of hydrogen and thus, promotes the formation of the active Rh-hydrido species.208

More unconventional active metals were investigated by Rothwell. Hydrido derivatives of Nb and Ta with bulky ancillary aryloxide ligands (see Figure 2.9) showed high regio- and stereoselectivity in the hydrogenation of naphthalene and anthracene. The reactions were conducted in benzene as solvent which is also hydrogenated but extremely slowly. However, the catalysts are deactivated in the presence of free phosphine and very sensitive to oxygen and water impurities, resulting in high efforts for solvent purification and special requirements for instruments. The Nb catalyst is more active than the Ta based one. It was found that when heated under H2 pressure in absence of substrate the catalysts hydrogenate the phosphine ligand. This is rarely observed and a very interesting synthetic pathway to produce PCy3 compounds.209

Figure 2.9: Ta and Nb catalysts as reported in reference209.

Even though the rates are usually much lower there are several reports of homogeneous hydrogenation 3 reactions with cobalt catalysts. [(η -C3H5)Co(P(OCH3)3)3] was reported to hydrogenate benzene at 25 °C and 1 bar H2. The reaction rate is faster in polar solvents like alcohols and nitriles. The catalyst is stereoselective and only the all-cis isomer is found after the hydrogenation.210,211

Many hydrogenations are also performed with Ir based catalysts. Cyclometalated Ir(III) catalysts were investigated by Wu et al. for the hydrogenation of N-heterocycles. The catalysts (for some examples see

Figure 2.10) were very active under mild conditions (room temperature and only 1 bar of H2). The solvent was trifluoroethanol. Non-fluorinated alcohols or other polar solvents (e.g. H2O, THF) and 27 2. Theoretical Background unpolar solvents (e.g. toluene) gave no conversion. A broad substrate scope of quinoline derivatives could be hydrogenated to the corresponding 1,2,3,4-tetrahydroquinolines; also for several substituents like halides, esters, and free hydroxyl groups good to excellent yields were found. Other N-heterocycles like quinoxalines, indoles and cyclic and acyclic imines were also reduced with high conversions. However, the reaction is chemoselective to C=N bonds, since no reaction was observed for acetophenone or styrene.212

Figure 2.10: Cyclometalated Ir(III) complexes, adapted from the publication of Wu et al.212

2.2.2.3.2 Homogeneous Dehydrogenation The dehydrogenation of cyclic alkanes is very challenging. These inert compounds have strong localized C-C and C-H bonds and no empty orbitals of low energy or filled orbitals of high energy that could readily participate in a chemical reaction. The dehydrogenation is a thermodynamic uphill process.213 Incorporation of a heteroatom in the ring lowers the reaction enthalpy drastically, and with that milder reaction conditions get possible.

Thus, high temperatures (225–265 °C) and long reaction times (60 hours) were necessary to dehydrogenate dihydroanthacene to anthracene in 97 % yield with the Wilkinson catalyst [RhCl(PPh3)3]. Other Rh, Ru and Ir based catalyst with chloride and phosphine ligands showed far less conversion (around 45 %). The results of the homogeneous dehydrogenation were compared to heterogeneous ones with Pd/C and almost identical rates and but higher selectivity was found for the homogeneous reaction, while much more decomposition of the substrates was observed for the heterogeneous catalysts.214

At much lower temperature operates the Ir pincer complex 1 as reported by the group of Jensen (see left hand side of Figure 2.11). The transfer dehydrogenation of cyclooctane with the hydrogen acceptor tert- butylethylene (tbe) proceeds at a turnover rate of 82 h-1 at 150 °C (see Scheme 2.9). The catalyst is highly thermally stable, no decomposition was found after 1 week at 150 °C. The raise of dehydrogenation temperature to 200 °C gave lower rates due to the starting decomposition of the catalyst. The Rh analog 2 of the complex showed a decreased activity with only 0.8 turnovers in 1 hour. However, for both metals an inhibition of the transfer dehydrogenation reaction was found for higher concentrations of tbe.215 The Ir pincer catalyst was also active in the acceptorless dehydrogenation of n- octane, yielding mostly internal octenes at 150 °C, with a selectivity to e.g. oct-1-ene of 20 % and to trans-oct-2-ene of 40 %. The transfer dehydrogenation with the addition of tbe of methylcyclohexane at 200 °C to the different methylcyclohexene isomers is much faster (with a TOF of 41 h-1 to 4- methylcyclohexene, 20 h-1 to 3-methylcyclohexene and 8 h-1 to 1-methylcyclohexene) than to toluene (TOF 11 h-1) (see Scheme 2.10).216 28 2. Theoretical Background

Figure 2.11: Pincer catalysts as synthesized by the Jensen group.215-218

Scheme 2.9: Transfer-dehydrogenation of cyclooctane.215

Scheme 2.10: Transfer-dehydrogenation of methylcyclohexane.216

Scheme 2.11: Acceptroless dehydrogenation of perhydro-N-ethylcarbazole.

t Some years later the same group used a similar catalyst, IrH2[2,6-C6H3-(OP( Bu)2)2] 3 (see right hand side of Figure 2.11), of for the dehydrogenation N-ethyl-dodecahydrocarbazole, which is depicted in Scheme 2.11. The hydrogen-rich LOHC molecule could be dehydrogenated in 9 hours at 200 °C with 100 % conversion. The yield of N-ethyl-octahydrocarbazole was 70 %, the yield of N-ethyl- tetrahydrocarbazole 30 %. 18 % of the completely dehydrogenated H0-NEC could only be found after 216 hours. At lower temperature the reaction is significantly slower and the product distribution different. After 48 hours at 150 °C the yield of H8-NEC is 90 % and to H4-NEC is 10 %. This corresponds to a turnover frequency of 2.3 h-1 while at 200 °C a TOF of 28 h-1 was found. Catalyst 1 was also tested.

Full conversion of H12-NEC was obtained after 96 hours, yielding 69 % H8-NEC and 31 % H4-NEC. In conclusion, with catalyst 3 a liberation of 2.2 wt% and 2.7 wt% of hydrogen upon heating to 150 or 200 °C, respectively, is possible.217

Other N-heterocyclic LOHC molecules like perhydroindole, perhydro-N-methylindole, perhydro- bipiperidine were also investigated with the Ir pincer catalyst 3. However, the yields were rather low and some problems with decomposition of catalyst and products were reported: perhydroindole showed

100 % conversion at 200 °C in 24 hours, but only the liberation of 1 eq. of H2 was observed. It was 29 2. Theoretical Background found that only the C-N bond was dehydrogenated. The perhydro-N-methylindole gave yields of 90 % for the reduction of the C-N bond and 7 % of fully dehydrogenated N-methylindole at 150 °C in 1 week. If the transfer reagent tert-butylethylene was added to this reaction even 62 % of the fully dehydrated product were found. The bipiperidine compounds polymerized under reaction conditions of 150 or 200 °C.218

Another example of an acceptorless dehydrogenation with a broad variety of neutral Ir catalysts (for examples see Figure 2.12) was published in 2008 by Taubmann et al.. The reaction temperature was 300 °C. The dehydrogenation of cyclooctane gave usually 100 % selectivity and turnover numbers between 10 and 75. They also remarked that at such a high reaction temperature the catalysts may not have the original composition, but are still catalytically active. The activity, however, decreased for longer reaction times and the homogeneity of the catalyst was not tested as well as no recycling experiments were performed. Thus, it is arguable if this example shows really a homogeneous catalyzed dehydrogenation rather than a dehydrogenation catalyzed with dispersed nanoparticles.219

Figure 2.12: Examples of Ir catalyst as reported in reference219.

Hydrogen release is also possible by electrocatalytical dehydrogenation from N-heteroatomic organic compounds, also the recharging of the fuel by applying an electrical current is possible. This will go beyond the scope of this thesis, thus the interested reader is referred to e.g.220,221.

2.2.2.3.3 Reversible homogeneous Hydrogenation-Dehydrogenation Reports about the reversible hydrogenation-dehydrogenation (or vice versa) are relatively rare. In 2009 Yamaguchi et al. found a homogeneous Cp*Ir complex with 2-hydroxypyridine 4 and 2-pyridonate (see top of Scheme 2.12) ligands for the hydrogenation/dehydrogenation of LOHC molecules. If the latter ligand is also substituted with an electron donating trifluoromethyl group in position 5 (cat. 5) superior activity was found for the dehydrogenation of 1,2,3,4-tetrahydroquinaldine as depicted in the lower part of Scheme 2.12. 2 mol% of catalyst showed full conversion after 20 hours in p-xylene reflux (b.p.

138 °C). The same catalyst (4 mol%) fully hydrogenated the quinaldine at 1 bar H2, 110 °C in 20 hours. By increasing the hydrogen pressure to 10 bar the time necessary for full conversion was only 2 hours.

Since the Cp*Ir complex with 5-CF3-pyridonate 5 catalyzed both the dehydrogenation of tetrahydroquinaldine and hydrogenation of quinaldine the reversible one-flask dehydrogenation- hydrogenation reactions were investigated. The dehydrogenation of tetrahydroquinaldine was conducted under reflux in p-xylene under ambient argon atmosphere, yielding quinaldine quantitatively in 20 hours. For the hydrogenation only the atmosphere in the flask was replaced with hydrogen (1 bar) and the temperature was decreased to 110 °C. This procedure gave back tetrahydroquinaldine quantitatively. The dehydrogenation-hydrogenation cycle could be repeated for another four times with nearly no loss of efficiency. This is the first example of an ultra-low temperature hydrogen battery even though the authors didn’t use this terminology. Based on their experimental investigations they proposed that the reversible dehydrogenation-hydrogenation mechanism proceeds via a reversible interconversion of catalytic species between the Cp*Ir complex 5 and a hydride bridged Cp*Ir dinuclear complex.222 This mechanistic studies were further refined by independent calculations of Li et al.223 and Zhang and Xi.224 30 2. Theoretical Background

Scheme 2.12-Top: Cp*Ir catalysts as synthesized in reference222; -Bottom: Reversible Dehydrogenation of tetrahydroquinaldine and hydrogenation of quinaldine with cat. 5 as reported in reference222.

By using 2,6-dimethyldecahydro-1,5-naphthyridine as substrate the perhydrogenation and perdehydrogenation (see right hand side of Scheme 2.13) with the Cp*Ir catalyst 5 was successful. A similar catalyst 6 (see left hand side of Scheme 2.13) showed even better conversion and selectivities at the same reaction conditions except the higher H2 pressure. Again one-flask experiments were performed with very promising results.225

Scheme 2.13-Left: Cp*Ir catalysts with a new ligand system. -Right: Perhydrogenation and perdehydrogenation of 2,6- dimethyldecahydro-1,5-naphthyridine; both illustrations adapted from reference225.

It is very desirable to perform hydrogenation and dehydrogenation reactions with abundant, sustainable and cheaper transition metals. This is unfortunately very challenging. Nonetheless, there are some examples.

The motivation for the investigation of first-row transition metal catalyzed hydrogenation and dehydrogenation reactions by the group of Jones were a potential application as hydrogen storage material as well as a synthetic application, since quinolines and tetrahyydroquinolines are building blocks in numerous pharmaceuticals and bioactive molecules.226,227 31 2. Theoretical Background

In 2014 they published an iron complex supported with a bis(phosphino)amine (PNP) pincer ligand. The proposed active structure of the catalyst is shown in the left hand part of Figure 2.13. Similar catalysts were already found to be active in the hydrogenation of esters or the base assisted, high temperature dehydrogenation of methanol, showing a remarkable thermal stability.228,229 The iron PNP pincer catalyst (3–5 mol%) did not dehydrogenate tetrahydroquinaldine in refluxing toluene, but showed good conversion (82–100 %) in p-xylene at 140 °C in 24–30 hours. A broad substrate scope was found to be reduced by the iron pincer complex, however, there are problems with the reusability. Addition of a new batch of substrate gave only 42 % of conversion, suggesting a significant decomposition of the catalyst during the first run. The Fe PNP catalyst was also able to hydrogenate quinaldine in the presence of

KOtBu (10 mol%) under 5 bar of H2 in 24 hours (80 °C in THF). No one-flask experiments were conducted, but mechanistic studies showed the catalyst is unreactive to the reduction of aromatic rings with no heteroatom. A penta-coordinated iron hydride was found as intermediate in the dehydrogenation cycle and a trans-dihydride species in the hydrogenation catalysis.226

Figure 2.13-Left: Proposed active Fe pincer catalyst as reported in reference226. -Right: Co pincer catalyst as reported in reference227.

One year later the group of Jones published the reversible dehydrogenation and hydrogenation with a cobalt pincer catalyst (see right hand side of Figure 2.13). For the dehydrogenation of 1,2,3,4- tetrahydroquinaldine moderate temperatures between 100 and 120 °C showed no conversion in the presence of 5–20 mM of catalyst. Only with 150 °C in p-xylene a dehydrogenation of 83 % could be found after 4 days. The activity of the dehydrogenation of 2-methylindoline was slightly higher and it was full converted to 2-methylindole under the same conditions in 4 days. The addition of Lewis acids like LiBF4 or B(C6F5)3 had no effect on the activity. The catalyst could be reused, but not separated. A fresh batch of substrate was introduced and the second run still gave a conversion of 84 % (compared to 100 % in the first run). This was seen as sign as minor decomposition of the catalyst. The same catalyst was also investigated concerning the hydrogenation. After 2 days the quinaldine was fully hydrogenated by 5–10 mM in THF at 120 °C and 10–20 bar H2. The 2-methylindoline could only be obtained in 24 % yield under the same conditions. The catalyst showed no activity towards cycloalkanes without a heteroatom. No one-flask experiments were performed in this study.227

The homogeneous [Ir(cod)(NHC)(PPh3)]PF6 catalyst (here NHC= N,N-dimethylbenzimidazol-2- ylidene) published in 2011 by the Crabtree group was highly effective for the hydrogenation of LOHC molecules, i.e. quinoline derivatives under very mild conditions. After stabilization of the

[Ir(cod)(NHC)(PPh3)]PF6 with 1 eq. of PPh3 and activation with H2 the reaction proceeded at 35 °C and only 1 bar of H2 in 18–24 hours with quantitative yields. The catalyst has a great tolerance to functional groups of the substrate, also halo substituents were possible though higher pressures of H2 were needed (5 bar). Also, the reaction is completely insensitive to air and moisture, only the presence of chloride

(e.g. from solvents like CH2Cl2) led to deactivation. The reaction was thus slightly sensitive to solvents. Higher hydrogenation rates were found in trifluorotoluene compared to toluene due to the better solubility of the catalyst in the first solvent. With sterically more demanding substituents at the NHC ligand like butyl or isopropyl the reaction rate also decreased. All proposed intermediates for the catalytic cycle were isolated or spectroscopically characterized. Additional DFT calculations support 32 2. Theoretical Background the suggested mechanism, which is shown in Scheme 2.14: the hydrogenation does not involve a direct coordination of the substrate to the Ir center, but a two-step outer-sphere mechanism via a proton transfer (1) and then a hydride transfer (2) with a dihydrogen complex as key intermediate (orange).230

1 2

Just recently the same group published also the dehydrogenation with the [Ir(cod)(NHC)(PPh3)]PF6 pre- catalyst. 1,2,3,4-Tetrahydroquinaldine was dehydrogenated with 5 mol% of the Ir catalyst (with an additional stabilizing ligand PPh3 and activated with H2) in p-xylene at 145 °C in 72 hours in very good yields. A subsequent hydrogenation step at room temperature and 1 bar H2 gave the starting material tetrahydroquinaldine in excellent yields. The produced hydrogen above the reaction mixture in the dehydrogenation step was not flushed out of the system. Since a hydrogenation can occur at already

1 bar H2, the produced hydrogen might have an influence on the reaction rate of the dehydrogenation but this was not examined in this report. Also, no subsequent cycles were investigated nor the recyclability of the catalyst.185 This last system inspired parts of the work carried out for this thesis.

2.2.2.4 Homogeneous biphasic Systems A classical biphasic system can be obtained for example with an aqueous phase and an organic phase. 6 By using a water-soluble [(η -C6H6)4Ru4H4]Cl2 cluster, a homogeneous biphasic hydrogenation of benzene was realized by the group of Süss-Fink. This catalyst can also be used in ionic liquids. The turnover frequencies of the aqueous-biphasic system at 90 °C and 60 bar of H2 were found to be around 300 h-1. The catalyst started to decompose and metallic Ru was observed when all benzene was converted. The catalyst stays intact as long as an aromatic substrate is present. Other experiments show the catalysis is not very selective. Biphenyl is converted to a 1:1 mixture of bicyclohexane and cyclohexylbenzene. Interestingly, the alkene substituent of e.g. styrene is preferentially hydrogenated over the aromatic ring. Benzene derivates with more or bulky substituents are converted slower due to 6 steric hindrance. By using the precursor molecule of the [(η -C6H6)4Ru4H4]Cl2 cluster (shown on the 6 right hand side of Figure 2.14), namely the (η -C6H6)2Ru2Cl2 dimer, much higher TOFs were found (900 h-1 for the dimer). Mechanistic studies showed that both catalyst convert to a hexahydrido as well 6 as a tetrahydrido species. But since the dimer (η -C6H6)2Ru2Cl2 shows much higher reactivity there must 6 be another catalytically more active species. It was identified to be the trinuclear cluster cation [Ru3(η - + 231,232 C6H6)3(µ2-Cl)( µ3-O)( µ2-H2)] (compare left hand side of Figure 2.14). 33 2. Theoretical Background

6 2+ 6 + Figure 2.14-Right: [(η -C6H6)4Ru4H4] cluster; -Left: the catalytical active species [Ru3(η -C6H6)3(µ2-Cl)( µ3-O)( µ2-H2)] both as reported in 232.

The first example of a hydrogenation reaction in a biphasic system using an IL was published by Chauvin - - - - et al. in 1995. They used the air stable [BMIM]X salts (with X = [BF4] , [PF6] , [SbF6] or [CuCl2] ) and the Osborn catalyst [Rh(nbd)(PPh3)2][PF6] (nbd= norbornadiene). The catalyst could be immobilized in the IL due to its ionic character. The hydrogenation at 30 °C and 1 bar H2 of pent-1-ene in [BMIM][SbF6] was found to be 5 times higher than in acetone. An intermediate reaction is the fast isomerization to pent-2-ene but finally only the hydrogenated product pentane was obtained. [BMIM][PF6] gave lower yields due to the low solubility of the substrate in the IL, [BMIM][BF4] showed some catalyst deactivation due to chloride impurities from the preparation of the IL, [BMIM][CuCl2] gave only isomerization products, but this with 98 % yield. This shows how the selectivity is greatly influenced by the anion. The catalyst phase could be reused and only trace amounts of the Rh species were lost during phase separation.14

Almost at the same time Suarez et al. published the use of [BMIM][BF4] and [BMIM][PF6] also as solvents for biphasic hydrogenation reactions. They hydrogenated cyclohexene at 10 bar H2 and 25 °C with the RhCl(PPh3)3 (Wilkinson catalyst) or [Rh(cod)2][BF4]. Both catalyst are completely soluble in the ILs. A conversion of 40–65 % was found with a TOF of 0.8 min-1.91

6 The well-studied [(η -C6H6)4Ru4H4]X2 catalyst (see above) was also used in IL to hydrogenate benzene and other aromatic compounds. As IL [BMIM][BF4] was chosen. The turnover rates of this organic-IL system at the same reaction conditions (60 bar, 90 °C) are comparable to the organic-water systems as reported previously231,232. The advantage of the IL system is the reusability and the ease of separation by distillation.233

6 2 Another example of a Ru-catalyzed hydrogenation in IL is the [Ru(η -p-cymene)(η -TRIPHOS)Cl][PF6] -1 in [BMIM][BF4]. The hydrogenation activity of benzene at 90 °C and 60 bar H2 gave a TOF of 476 h with decreasing activity as size and numbers of substituents on the arene substrate increase. The activity -1 in IL was much higher compared to in CH2Cl2 (242 h ). While the catalyst is practically inactive towards arenes with α-alkene substituents (like styrene 3 h-1) the turnover of the hydrogenation of allyl benzene (see Scheme 2.15-bottom) was unexpectedly high (329 h-1), only the aromatic ring is hydrogenated, the allyl functionality stays intact.234 34 2. Theoretical Background

6 2 Scheme 2.15-Top: [Ru(η -p-cymene)(η -TRIPHOS)Cl][PF6] catalyst; -Bottom: Hydrogenation of allyl benzene with the Ru cat., as reported in reference234.

While the hydrogenation in ILs is a subject of interest since the discovery of catalysis in ILs, the biphasic dehydrogenation was neglected so far.

One of the very few reports was published by Hintermair et al. in 2015. They prepared Ir and Rh based catalysts by simply dissolving the starting compound [M(cod)(PPh3)base] in [BMIM][NTf2] yielding the catalyst [M(cod)(BMIM)(PPh3)][NTf2] almost quantitatively. The base like acetate, acetylacetonate or ethoxide was removed by evaporation. The NHC-complexes could not be completely separated from the IL, still a crystal structure of the [Ir(cod)(BMIM)(PPh3)][NTf2] could be obtained. For the dehydrogenation of cyclooctadiene a colorless organic phase was on top of the highly colored IL phase. The reaction was performed at 140 or 160 °C in 2 hours at ambient pressure of nitrogen. The dehydrogenation yields were quite low (0.3 % at 140 °C and 1.4 % at 160 °C) the 1,5-cyclooctadiene was almost fully converted to the isomerization product 1,3-cyclooctadiene (79.4 % at 140 °C and 94.4 % at 160 °C). The 1,4-cyclooctadiene is an intermediate of this reaction. Another side reaction is the transfer hydrogenation to form cyclooctane (0.3 % for 140 °C and 0.2 % for 160 °C). When employing the mayor isomerization product 1,3-cyclooctadiene as substrate no transfer hydrogenation was found at 160 °C after 2 hours and the dehydrogenation conversion was 0.5 %.235 The low dehydrogenation activity of the [Ir(cod)(BMIM)(PPh3)][NTf2] is in accordance with the findings of the Crabtree group, who also found lower activity for their very similar catalyst by increasing the size of the substituents of their NHC ligands.185,230 35 3. Materials and Methods

3. Materials and Methods

The working processes, the analytical methods and the used chemicals and synthetic procedures are described in this chapter. The chemicals and syntheses were handled in air if not noted otherwise; the identity and purity of all products were characterized at least by NMR spectroscopy and elemental analysis.

3.1 Inert Gas Techniques

Air and moisture sensitive compounds were stored and handled under dry argon atmosphere using either standard Schlenk techniques or a Glovebox.

3.2 Analytical Methods

3.2.1 Gas Chromatography The Offline-GC was used for the characterization of the purity of substances as well as analysis of the catalytical experiments. A Varian 3900 gas chromatograph was used with a column Factor FourTM capillary column VF-1ms, 15 m, 0.25 mm, 0.25 µm. The temperature of the injector was 280 °C. The heating ramp started at 60 °C and was heated to 150 °C with a heating rate of 10 K min-1. Then the temperature was further raised to 280 °C with a heating rate of 20 K min-1.The final temperature was held for 5 minutes. The detector was a FID at 280 °C.

To have also a quantitative analysis benzyl alcohol or cyclooctane were used as standards, a calibration was performed prior to the experiments. The calibration factors can be found in the Appendix, section 6.4.

3.2.2 Elemental Analysis The determination of the carbon, hydrogen, nitrogen and sulphur content was performed by Christina Wronner on a Carlo Erba EA 1106, 1107 and 1108 instrument at the Analytical Laboratories of the Chair of Inorganic and General Chemistry of the Friedrich-Alexander Universität Erlangen-Nürnberg (Erlangen, Germany).

3.2.3 Nuclear Magnetic Resonance The samples for measuring the nuclear magnetic resonance spectra were prepared using extra pure, deuterated solvents (obtained from Deutero®). The NMR spectra were recorded at 25°C on a JOEL ECX 400 MHz instrument. The chemical shift δ is presented in ppm and the residual non-deuterated solvent peak was used as internal standard. The coupling constant J is presented in Hz. The following abbreviations are used to characterize the spin multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), br (broad). 36 3. Materials and Methods

3.2.4 Infra-Red Spectroscopy The infra-red spectroscopy was performed on a Jasco FT/IR-4600 with an ATR (attenuated total reflectance) attachment. The solid samples were measured as powder without further preparation.

3.2.5 Raman Spectroscopy All Raman experiments were conducted during a short-term scientific mission to the DTU, Lyngby, Denmark, in collaboration with Rolf W. Berg.

The Raman spectra were collected using a DILOR-XY 800 nm focal-length Czerny-Turner type spectrometer with 90 °C entrance and a 10 x 10 cm2 1800 lines/mm plane holographic grating. A scheme of the setup can be found in Appendix section 6.1.1, in Figure 6.1. As source of excitation laser light from a continuous Spectra-Physics Millennia Laser (wavelength 532 nm, vertically polarized and with a power setting at up to 2 W, Nd-YVO4 laser) was used. The light was collected with a wide 10 cm focal achromatic lens. Rayleigh scattering was removed with a Kaiser holographic SuperNotch-Plus filter, giving approximately a 100 cm-1 cut-off. The spectra were acquired with a multi-channel Horiba Jobin- Yvon SynapseTM CCD detector (1024 x 256 pixels) with thermoelectric cooling (-70 °C) running under Horiba Scientific LabSpecTM 5.43 software. The slits were set to 600 µm, corresponding to a resolution of 6 cm-1. The wavenumber scales were calibrated using the Raman lines of liquid cyclohexane to ± 1 cm-1. The spectra were recorded in several overlaid sections that were combined after automatic removal of cosmic spikes. A broad fluorescent background was subtracted if necessary. Some measurements were done at room temperature, some at elevated temperatures (120–350 °C). The heating was achieved by a home-made (closed insulated aluminium-bronze-core vertical-tube electrical) furnace or a temperature-controllable hot air blower.

The room temperature Raman spectra were recorded from powdered or crystalline samples in glass vials.

For measuring the gas phase Pyrex ampoules were used. These sometimes contained also H2 as a reference gas. The ampoules were sealed with a butane-oxygen torch flame, with vacuum or pre- determined reference gas pressure inside. The reference gas was added to simplify the search for an optimum sample position.

To avoid absorption and fluorescence some spectra were collected using a Renishaw InVia microscope Raman instrument. A scheme of the setup can be found in Appendix section 6.1.1, in Figure 6.2. As source of excitation laser light from a Lexel 95-SHG-QS Argon gas ion laser (wavelength 488 nm) was used. The wavenumber scales were calibrated using the Raman lines of diamond and liquid cyclohexane to ± 1 cm-1. All measurements were carried out at room temperature.

Some transition metal bistriflimides had to be measured in H2 atmosphere. The cell which was used for these measurements is shown in the Appendix section 6.1.1, in Figure 6.3.

3.2.6 Thermal gravimetric analysis-Mass Spectrometry The TG-MS analysis was performed on a XEMIS (high accuracy) sorption microbalance from Hiden isochema suitable for extreme environments. The microbalance has a large capacity of up to 5 g and a resolution of 0.2 µg and a long-term stability of ± 5 µg. The dynamic weighting range was from 0 to 200 mg. The measurement could be performed up to 200 bar. The temperature was between 77 and 773 K and possible volume flows between 30 and 1000 mL min-1. This was coupled with a quadrupole mass spectrometer from Hiden Analytical. 37 3. Materials and Methods

3.2.7 Mass Spectrometry The DART-MS (Direct analysis in real time mass spectrometry) analysis was performed on a JMS- T100LP AccuTOF LC-plus 4G from Jeol with a He plasma ionization from ionSense, operating in both positive and negative-ion mode.

The m/z values refer to the most abundant isotope.

3.2.8 X-Ray Diffraction The XRD measurements were conducted on a Philips X’Pert MPD with a 15-fold sample changer at room temperature. A CuKα x-ray source (λ = 0.15418 nm) was used at 40 kV and 40 mA. The range was from 2–80 °. The data analysis was performed with the program X’Pert Highscore Plus.

3.2.9 ICP-AES To quantitatively detect trace amounts of different elements (here Ir and Co) in samples, inductive coupled plasma absorption-atom emission spectroscopy was used. The measurements were conducted by Nicola Taccardi on a Perkin Elmer Plasma 400. The water insoluble samples were treated with HNO3 in a microwave oven (180 °C).

3.2.10 Karl-Fischer Coulometry The water content was determined by the Karl-Fischer coulometric method on a Metrohm 756 KF coulometer with a generator electrode without diaphragm. The setup can detect between 10 µg to 200 mg water. The apura ® CombiCoulomat fritless solution purchased from Merck containing methanol, bromoform and guanidinium iodide was used. Each measurement was repeated three times. Solid samples were dissolved in methanol and an additional blind measurement of the neat solvent was performed.

3.2.11 Viscosity The viscosity of the high melting salts was determined with a rotary viscosimeter DV-II+ Pro Extra (Brookfield). The home-made ceramic heating device was controlled with a controller HT-30 (Horst) in combination with a thermocouple (NiCrNi, Rössel). Standard cylindrical spindles were used in a temperature range from 120–250 °C and different speeds.

The viscosity of the lower melting salts was determined on an Anton-Paar Physica MCR 100 rotary plate-cone viscosimeter. The temperature was between 30 and 80 °C. The sheer rates were varied between 500 and 10 s-1.

3.2.12 Thermal Stability The thermal stability was determined using thermal gravimetric analysis (TGA) with different setups. The experiments were conducted at heating rates of 1 or 5 K min-1 on a Setsys Evolution (Setaram) using alumina crucibles. The carrier gas was nitrogen with a flow rate of 75 mL min-1.

The determination of the thermal stability of the salts Cs[NTf2] and [PPh4][NTf2] as well as their eutectic mixture was additionally performed in cooperation with Florian Heym of the university of Bayreuth. A horizontal thermobalance EXSTAR 6300 TG/DTA from Hotachi High-Tech was used at different -1 heating rates (0.5, 2, 10 K min ). The experiments were conducted with N2 or He as carrier gas with a flow rate of 6 L h-1 NTP. The temperature uncertainty in the TG-experiments was ± 0.1 K. 38 3. Materials and Methods

3.2.13 Melting Points Differential scanning calorimetry (DSC) was performed with a computer-controlled Phoenix DSC 204 F1 thermal analyzer (Netzsch, Selb, D). The sample of 10 mg was placed in aluminium crucibles, which were cold-sealed. The sample amount was small enough to avoid uneven heating. Measurements were carried out with a thermal ramp of 2 K min-1. The transition temperatures were determined from the heating process to avoid the uncertainty of supercooling.

Optical melting points of the investigated compounds were measured using a SMP10 melting point determination device form VWR Bibby Scientific.

3.2.14 Density High temperature density measurements of the molten salts were performed by applying the Archimedean method measuring the reduced mass of a cylindrical glass-probe immersed into the molten salt. The balance was a Practum 124-1S from Satorius, the heating device was home-made. The temperature range was 130–250 °C.

The density of the powdered salts was determined by Helium pycnometry on a Porotec Pycnomatic Multivolume.

The density of the liquid salts was determined on an Anton-Paar oscillation U-tube DAS 5000 M at temperatures between 25 and 70 °C. Three measurements were conducted for each temperature and averaged.

3.2.15 Partition Coefficient The partition coefficient was determined by measuring the concentration of the substrates in the extracting phase and in the molten salt phase. After vigorously mixing, the phases were allowed to settle over several hours and then separated via a separating funnel. Each sample was prepared and measured three times to average.

-1 For the calculations the density of [PMeBu3][NTf2] was needed, and measured to be 1.254 g cm at

25 °C. The amount of [PMeBu3][NTf2] IL used for each experiment was around 0.44 g; the extracting agent (either Bu2O or Ph2O) was 5 mL. The substrates were used in a molar ratio that is similar to the dehydrogenation experiments, namely circa 0.3 g of indoline and 0.22 g of indole. The concentration was measured with GC using benzylalcohol as internal standard and NMR using dichloromethane as internal standard.

3.3 Chemicals

The gases were purchased from Linde AG. The purity and the application of the gases are as follows:

Argon 4.6 (inert gas); helium 4.6 (inert gas); hydrogen 5.0 (hydrogenation), nitrogen 5.0 (inert gas)

Solvents were used in syntheses and catalytic investigations. All were commercially available (from Merck or Sigma Aldrich) and used without further purification if not otherwise stated. The term “water” always refers to deionized water.

The solvents are listed below:

Acetonitrile (Merck; 99.9 %); benzyl alcohol (Merck, 99.8 %); dibutyl ether (Merck; 99 %); dichloromethane (Sigma Aldrich; 99.8 %); diethyl ether (Merck; 99 %); diphenyl ether (Merck; 99 %); 39 3. Materials and Methods methanol (Sigma Aldrich; 99.9 %); n-heptane (Merck; 99 %); THF (Sigma Aldrich; 99.9 %); toluene (Sigma Aldrich; 99.8 %);

The starting materials for syntheses were commercially available and used without further treatment if not stated otherwise. They are presented in Table 3.1 to Table 3.4.

Table 3.1: Starting materials for molten salt syntheses.

substance supplier CAS-Number purity

Li[NTf2] Iolitec 900076-65-6 n.a.

[PPh4]Cl Sigma Aldrich 2001-45-8 98 %

Cs2CO3 Sigma Aldrich 534-17-8 99 %

H2SO4 Merck 7664-93-9 95-97 %

PBu3 Sigma Aldrich 998-40-3 99 %

Me2SO4 Sigma Aldrich 77-78-1 99.8 %

Table 3.2: Starting materials for transition metal compound syntheses.

substance supplier CAS-Number purity

CoCO3 x n H2O Sigma Aldrich 57454-67-8 Co 43-47 %

NiCl2 Alfa Aesar 7718-54-9 98 %

NaOH Sigma Aldrich 1310-73-2 97 %

CoCl2 Sigma Aldrich 7646-79-9 98 %

ZnO Alfa Aesar 1314-13-2 99.9 %

MnCl2 x 4 H2O Sigma Aldrich 13446-34-9 98 %

CuCl2 Sigma Aldrich 7447-39-4 99.9 %

Ag2O Alfa Aesar 20667-12-3 99 %

40 3. Materials and Methods

Table 3.3: Substances for catalytic investigation.

substance Structural formula supplier CAS-Number purity

Indole Sigma Aldrich 120-72-9 99 %

Indoline Sigma Aldrich 496-15-1 99 %

Quinaldine Merck 91-63-4 98 %

2-Methylindoline Sigma Aldrich 6872-06-6 98 %

1,2,3,4- Sigma Aldrich 25448-05-9 n.a. Tetrahydroquinaldin

Benzoyl chloride Sigma Aldrich 98-88-4 99 %

Table 3.4: Homogeneous catalysts and chemicals for homogeneous catalyst syntheses.

substance chemical formula supplier CAS-Number purity

Crabtree’s catalyst [Ir(cod)(Py)(PPh3)]PF6 Sterm 64536-78-3 99 %

Bis(iridiumcyclooctadienyl- Sigma [Ir(cod)Cl] 12112-67-3 97 % chloride) 2 Aldrich

1,3-Dimethyl-1H- Sigma [MMIMBz]I 7181-87-5 n.a. benzimidazolium iodide Aldrich

Postassium tert-butoxide KOtBu Merck 865-47-4 98 %

Triphenylphosphine PPh3 Alfa Aesar 603-35-0 99 %

Sodium Sigma Na[PF ] 21324-39-0 98 % hexafluorophosphate 6 Aldrich 41 3. Materials and Methods

[PPh4OPh][NTf2]:(4-Phenoxyphenyl)triphenylphosphonium bis(trifluoromethylsulfonyl)imide was synthesized in the labs of James H. Davis, Jr. at the University of South Alabama. In the following, the results of the characterization are shown:

Molar mass: 711.63 g mol-1 1 H-NMR (δ, 293 K, CDCl3): 7.14 ppm (2 H, d); 7.21 ppm (4 H, s); 7.43 ppm (1 H, t); 7.49 ppm (2 H,m); 7.74 ppm (15 H, m); 7.87 (2 H, s) 13 C-NMR (δ, 293 K, CDCl3): 117.59 ppm (1 C); 118.83 ppm (7 C, m); 121.02 ppm (3 C, s); 126.07 ppm (2 C, s); 130.84 ppm (6 C, m); 134.34 ppm (6 C, d); 135.72 ppm (2 C, s); 136.63 ppm (3 C, d); 153.84 ppm (1 C, s); 164.56 ppm (1 C, s) 19 F-NMR (δ, 293 K, CDCl3): -78.63 ppm 31 P-NMR (δ, 293 K, CDCl3): 23.26 ppm EA: calculated: %C: 54.01; %H: 3.4; %N: 1.97; %S: 9.01 found: %C: 53.67; %H: 3.51; %N: 1.95; %S: 9.26

KF: H2O content = 0.00 wt%

3.4 Syntheses

3.4.1 Synthesis of Cs[NTf2]

In the first step of the synthesis of Cs[NTf2], the acid HNTf2 was obtained by distillation from a reaction mixture of 10.23 g (35.63 mmol) Li[NTf2] and 10 equivalents of 98 % H2SO4 at 80 °C and reduced pressure (0.5 mbar) and re-sublimation into a flask cooled with liquid nitrogen. Subsequently, HNTf2 was reacted with 5.79 g (17.82 mmol) Cs2CO3 in 10 mL methanol at 50 °C. The solvent was removed at a rotary evaporator and the raw material was recrystallized from acetonitrile and dried in vacuum (0.5 mbar) at 150 °C. The yields were excellent (97.1 %).

Molar mass: 413.05 g mol-1 1 H-NMR (δ, 293 K, MeOH-d3): - 13 C-NMR (δ, 293 K, MeOH-d3): 119.87 ppm (2 C, q, J = 3.19 Hz) 19 F-NMR (δ, 293 K, MeOH-d3): -80.19 ppm + + + DART-MS: (ESI ) m/z: 132.89 for Cs ; 545.67 for Cs2[NTf2] - - - - (ESI ) m/z: 148.01 for [NTf] ; 279.93 for [NTf2] ; 692.70 for Cs[NTf2]2 EA: calculated: %C: 5.82; %H: 0.00; %N: 3.39; %S: 15.52 found: %C: 5.77; %H: 0.00; %N: 3.50; %S: 15.63

KF: H2O content = 0.01 wt%

3.4.2 Synthesis of [PPh4][NTf2]

The [PPh4][NTf2] salt was synthesized by an ion exchange of 10.17 g (35.41 mmol) Li[NTf2] and 13.27 g

(35.41 mmol) [PPh4]Cl in water. The precipitated product was filtrated and washed thoroughly with water to remove unreacted starting material due to unprecise weighting. The filtrate was tested for chloride with AgNO3. The salt was dried in vacuum (0.5 mbar) at 150 °C. [PPh4][NTf2] was obtained in excellent yield (95.2 %).

Molar mass: 619.54 g mol-1 1 H-NMR (δ, 293 K, aceton-d6): 7.78 ppm (16 H, m); 7.95 ppm (4 H, t) 13 C-NMR (δ, 293 K, aceton-d6): 117.81 ppm (2 C); 118.70 ppm (2 C); 130.50 ppm (8 C, d); 134.91 ppm (8 C, d); 135.54 ppm (4 C, d) 19 F-NMR (δ, 293 K, aceton-d6): -79.88 ppm 31 P-NMR (δ, 293 K, aceton-d6): 23.63 ppm 42 3. Materials and Methods

+ + + DART-MS: (ESI ) m/z: 279.06 for [NTf2] ; 339.09 for [PPh4] - - (ESI ) m/z: 278.92 for [NTf2] EA: calculated: %C: 50.41; %H: 3.25; %N: 2.26; %S: 10.35 found: %C: 50.67; %H: 3.00; %N: 2.23; %S: 10.26

KF: H2O content = 0.00 wt%

3.4.3 Synthesis of [PMeBu3][NTf2] In the first step of the synthesis 21.31 g (105.33 mmol) tri-n-butyl phosphine was alkylated with 14.11 g

(111.85 mmol) dimethyl sulfate. Due to the high reactivity of the PBu3 special care has to be taken to work in oxygen free atmosphere and in an ice bath. The Me2SO4 is added dropwise, the temperature was always kept under 5 °C. After the reaction, 25 mL toluene was added to the mixture and it was subsequently washed with 75 mL of water. The two phases were separated and the aqueous phase was used in the second step. 30.26 g (105.40 mmol) of Li[NTf2] were dissolved in 100 mL water and slowly dropped to the [PMeBu3][MeSO4] solution. The desired product forms a second phase, which was separated and washed three times with 50 mL of water. The aqueous residues were removed over

MgSO4, filtrated and the IL was dried in vacuum (0.5 mbar). The clear IL was obtained in excellent yield (80.1 %).

Molar mass: 497.5 g mol-1 1 H-NMR (δ, 293 K, CDCl3): 0.92 ppm (9 H, t); 1.49 ppm (12 H, m); 1.6 (3 H, m); 2.08 (6 H, m) 13 C-NMR (δ, 293 K, CDCl3): 4.22 ppm (1 C); 13.26 ppm (3 C); 19.68 ppm (3 C); 20.16 ppm (3 C); 23.63 ppm (3 C, t, J=0.15); 119.93 ppm (2 C, q, J = 3.19 Hz) 19 F-NMR (δ, 293 K, CDCl3): -79.24 ppm EA: calculated: %C: 36.21; %H: 6.08; %N: 2.82; %S: 12.89 found: %C: 36.12; %H: 6.20; %N: 2.86; %S: 12.85

KF: H2O content = 0.00 wt%

3.4.4 Synthesis of Transition Metal Compounds

3.4.4.1 Co(NTf2)2

Co(NTf2)2 was prepared from CoCO3 x n H2O (3.31 g) and HNTf2 (9.76 g; 34.68 mmol) in 15 mL water.

Unreacted CoCO3 was removed by filtration. Subsequently, the solvent was removed by rotary evaporation and the product was dried in vacuum (0.5 mbar) at 150 °C. Co(NTf2)2 was obtained as pink powder in very good yields (78.4 % corresponding to HNTf2).

Molar mass: 619.23 g mol-1 + + + DART-MS: (ESI ) m/z: 130.45 for [Co(H2O)4] ; 279.52 for [NTf2] - - (ESI ) m/z: 278.92 for [NTf2] EA: calculated: %C: 7.76; %H: 0.00; %N: 4.52; %S: 20.71 found: %C: 7.63; %H: 0.31; %N: 4.52; %S: 20.21

KF: H2O content = 0.02 wt%

3.4.4.2 Ni(NTf2)2

Ni(NTf2)2 was prepared from Ni(OH)2 x n H2O. 5.13 g (21.60 mmol) NiCl2 x 6 H2O was dissolved in 15 mL water. Nickel hydroxide was precipitated by addition of aqueous NaOH. It was filtered and used without further drying. 9.86 g (35.04 mmol) HNTf2 was dissolved in 15 mL water and Ni(OH)2 was added. Unreacted starting material was removed by filtration. The solvent was removed by rotary evaporation and the product was dried in vacuum (0.5 mbar) at 150 °C. Ni(NTf2)2 was obtained as yellow solid in excellent yields (92.8 % corresponding to HNTf2). 43 3. Materials and Methods

Molar mass: 618.99 g mol-1 + + + DART-MS: (ESI ) m/z: 130.76 for [Co(H2O)4] ; 279.76 for [NTf2] - - (ESI ) m/z: 278.72 for [NTf2] EA: calculated: %C: 7.76; %H: 0.00; %N: 4.53; %S: 20.72 found: %C: 7.73; %H: 0.32; %N: 4.30; %S: 20.52

KF: H2O content = 0.07 wt%

3.4.4.3 Other M(NTf2)n

The other M(NTf2)n compounds (with M = Cu, Zn, Ag and n = 1 or 2) were obtained following the above described procedures starting from the metal oxides or hydroxides:

- Ag(NTf2): Ag2O (1.92 g; 8.28 mmol) and HNTf2 (4.66 g 16.56 mmol)

- Cu(NTf2)2: Cu(OH)2 (22.28 mmol) (as obtained from CuCl2 (2.99 g; 22.28 mmol) and NaOH (aq))

and HNTf2 (1.95 g; 6.93 mmol) Molar mass: 619.54 g mol-1 EA: calculated: %C: 7.70; %H: 0.00; %N: 4.49; %S: 20.56 found: %C: 7.56; %H: 0.5; %N: 4.17; %S: 20.23

KF: H2O content = 0.05 wt%

- Zn(NTf2)2: ZnO (1.43 g; 17.51 mmol) and HNTf2 (9.62 g; 34.23mmol) Molar mass: 625.68 g mol-1 EA: calculated: %C: 7.68; %H: 0.00; %N: 4.48; %S: 20.50 found: %C: 7.19; %H: 0.12; %N: 4.13; %S: 19.99

KF: H2O content = 0.01 wt%

Mn(NTf2)2 was synthesized in a slightly different manner starting from the prepared Ag(NTf2) (1.97 g;

5.08 mmol), which was dissolved in 10 mL of water. MnCl2 x 4 H2O (0.50 g; 2.53 mmol) was added.

The formed AgCl was subsequently removed by filtration. The remaining solution of Mn(NTf2)2 was dried first in a rotary evaporator, then in vacuum (0.5 mbar) at 150 °C.

Molar mass: 615.28 g mol-1 EA: calculated: %C: 7.81; %H: 0.00; %N: 4.55; %S: 20.85 found: %C: 7.39; %H: 0.40; %N: 4.40; %S: 20.62

KF: H2O content = 0.07 wt%

3.4.5 Synthesis of Hydrogenation/Dehydrogenation Catalysts

3.4.5.1 [Ir(cod)(NHC)]I The precursor was synthesized according to a literature procedure.230 The synthesis was performed in the glovebox and with degassed solvents due to the high reactivity of KOtBu. (Cylcooctadien)iridium chloride dimer (0.782 g; 1.164 mmol) was dissolved in 40 mL of a 1 : 1 mixture of degassed CH2Cl2 and THF. Potassium tert-butoxide (0.262 g; 2.333 mmol) was added and the red-brown mixture was stirred at room temperature for 1 hour. Then the [MMIMBz]I (0.637 g; 2.325 mmol) was added. The mixture was stirred for 4 hours. After this, the mixture was removed from the glovebox and the solvent was removed by rotary evaporation. The residue was dissolved in CH2Cl2 and flash-filtered through silica. The solvent was removed once again to provide a brown solid in excellent yields (91 %).

44 3. Materials and Methods

Molar mass: 573.49 g mol-1 1 H-NMR (δ, 293 K, CDCl3): 2.15 ppm (4 H, m); 3.54 ppm (6 H, s); 5.24 ppm (4 H, m); 6.79 ppm (2 H,s); 6.98 ppm (2 H, t) 13 C-NMR (δ, 293 K, CDCl3): 28.02 ppm (2 C); 30.71 ppm (2 C); 37.45 ppm (2 C); 57.35 ppm (2 C); 84.23 ppm (2 C); 122.74 ppm (2 C); 134.91 ppm (2 C); 135.54 ppm (2 C); 192.7 (1 C)

3.4.5.2 [Ir(cod)(NHC)(PPh3)][PF6] The [η4-1,5-Cyclooctadiene][triphenylphosphine](N,N-dimethyl-benzimidazol-2-ylidene)iridium hexafluorophosphate was synthesized according to literature procedure.230 The synthesis was performed in air. 0.276 g (0.481 mmol) of the prepared [Ir(cod)(NHC)]I were dissolved in 5 mL THF. Na[PF6]

(0.086 g; 0.512 mmol) and PPh3 (0.129 g; 0.490 mmol) were added. The red solution was stirred at room temperature for 20 hours. Then the solvent was removed by rotary evaporation and 20 mL CH2Cl2 were added. The mixture was washed three times with water. The organic layer was dried over MgSO4, filtered and evaporated. The red residue was dissolved in 15 mL CH2Cl2 and was recrystallized by adding 75 mL diethyl ether and standing in a freezer (-18 °C). The product was collected by filtration, rinsed with 25 mL diethyl ether and dried in vacuum (0.5 mbar). The yields were moderate (72 %).

Molar mass: 853.84 g mol-1 1 H-NMR (δ, 293 K, CDCl3): 2.14 ppm (4 H, m); 2.36 ppm (4 H, m); 3.74 ppm (6 H, s); 3.96 ppm (2 H,s); 4.47 ppm (2 H, m); 7.17-7.62 ppm (19 H, m) 13 C-NMR (δ, 293 K, CDCl3): 31.71 ppm (4 C, m); 38.62 ppm (2 C); 82.48 ppm (2 C); 86.60 ppm (2 C); 109.57 ppm (2 C); 124.85 ppm (4 C); 129.95 ppm (9 C, m); 134.91 ppm (6 C, m); 136.73 ppm (3 C); 187.91 ppm (1 C) 19 F-NMR (δ, 293 K, aceton-d6): -74.5 ppm 31 P-NMR (δ, 293 K, aceton-d6): 18.07 ppm; 144.02 ppm + + + + DART-MS: (ESI ) m/z: 147.11 for [NHC-H] ; 263.14 for [PPh3] ; 279.14 for [Ir(H2O)4(OH)] ; + + 557.26 for [Ir(cod)(PPh3)] ; 725.33 for [Ir(cod)(NHC)(PPh3)(H2O)] - - (ESI ) m/z: 145.04 for [PF6] EA: calculated: %C: 49.23; %H: 4.27; %N: 3.28; %S: 0.00 found: %C: 50.12; %H: 4.78; %N: 3.23; %S: 0.00

3.4.5.3 [Ir(cod)(NHC)(PPh3)][NTf2] [η4-1,5-Cyclooctadiene][triphenylphosphine](N,N-dimethyl-benzimidazol-2-ylidene)iridium bis(trifluoromethylsulfonyl)imide was synthesized by modifying the reported procedure230 by changing the anion exchange agent to Li[NTf2]. 0.752 g (1.312 mmol) of the prepared [Ir(cod)(NHC)]I were dissolved in 10 mL THF. 0.345 g (1.315 mmol) PPh3 and 0.384 g (1.336 mmol) Li[NTf2] were added. The bright red solution was stirred at room temperature over night. Then the solvent was removed by rotary evaporation. The crude product was dissolved in CH2Cl2. The mixture was washed three times with water. The organic layer was dried over MgSO4. The solvent was removed and the red-orange solid was dried in vacuum (0.5 mbar). The compound was obtained in very good yields (89 %).

Molar mass: 989.02 g mol-1 1 H-NMR (δ, 293 K, CDCl3): 2.15 ppm (4 H, m); 2.33 ppm (4 H, m); 3.69 ppm (6 H, s); 3.99 ppm (2 H, s); 4.45 ppm (2 H, m); 7.12-7.62 ppm (19 H, m) 13 C-NMR (δ, 293 K, CDCl3): 31.77 ppm (4 C, m); 35.98 ppm (2 C); 81.81 ppm (2 C); 86.53 ppm (2 C); 110.25 ppm (2 C); 123.87 ppm (4 C); 130.35 ppm (9 C, m); 134.43 ppm (6 C, m); 138.11 ppm (3 C); 186.69 ppm (1 C) 19 F-NMR (δ, 293 K, aceton-d6): -78.51 ppm 31 P-NMR (δ, 293 K, aceton-d6): 18.13 ppm 45 3. Materials and Methods

+ + + + DART-MS: (ESI ) m/z: 123.47 for [PPh(H2O)] ; 163.61 for [NHC-OH] ; 279.99 for [NTf2] ; + 558.95 for [Ir(cod)(PPh3)] ; - - (ESI ) m/z : 278.92 for [NTf2] EA: calculated: %C: 44.93; %H: 3.77; %N: 4.25; %S: 6.48 found: %C: 45.03; %H: 3.85; %N: 4.21; %S: 6.43

3.5 Test Reactions

3.5.1 Friedel-Crafts Acylation The Friedel-Crafts acylation reactions were performed in 30 mL borosilicate glass test tubes sealed with a septum and equipped with a magnetic stirrer. The tubes were filled with catalyst, molten salt and benzoyl chloride inside the glovebox due to the hygroscopic catalysts and the high reactivity of benzoyl chloride towards moisture and to ensure inert gas atmosphere inside the tube. Then the sealed tubes were heated outside of the glovebox in an oil bath to 110 °C and stirred. The reaction was started by addition of toluene as second substrate. The reaction was heated and stirred for at least 4 hours. Samples were collected by syringe, diluted by n-heptane and analyzed by gas chromatography. Cyclooctane (0.1 g) was used as internal standard. In Table 3.5 the amounts are shown.

Table 3.5: Amounts of substrate, catalyst and solvent for catalytic Friedel-Craft acylation reactions.

Substrates Catalyst Molten salt

Toluene Benzoyl chloride M(NTf2)2 [PMeBu3][NTf2]

0.24 g 0.33 g 0.071 g 0.25 g

2.65 mmol 2.36 mmol 0.114 mmol 0.50 mmol (4.6 mol%)

3.5.2 Dehydrogenation The dehydrogenation of LOHC test molecules, usually indoline, was performed as liquid-liquid biphasic homogeneous reaction in a glass flask. The catalyst was an Ir-complex which was dissolved in a molten salt. The upper phase consisted of an extracting agent usually dibutyl ether and the substrate. The reaction was run for at least 24 h, during which several samples were taken from the upper organic phase. The yields were determined with gas chromatography using benzyl alcohol (0.2 g) as internal standard and toluene as solvent.

The three-neck round bottom flask had a volume of 50 mL, a thermometer was immersed into the upper phase. The flask was equipped with a reflux condenser, a stirring bar and a bubbler to keep it under inter gas atmosphere. The temperature was regulated with a magnetic stirrer and a metal heating attachment. The stirring speed was always 500 rpm, the temperature measured inside the flask was between 125– 127 °C.

The setup was flushed with argon prior to the reaction. Before the substrate was added the catalyst and the ligand (PPh3) were dissolved in the molten salt at reaction temperature. Then the catalyst complex was activated by bubbling hydrogen for 2–5 minutes until it changed its color from red to yellow. The activation hydrogen atmosphere was removed by flushing with argon. The reaction started by addition of substrate under a positive flow of argon. The argon flow was stopped when the reaction started. The amounts of the components are represented in Table 3.6. 46 3. Materials and Methods

Table 3.6: Amounts of substrate, catalyst and solvent for standard dehydrogenation reactions.

Substrate Catalyst Ligand Molten Salt Extracting agent

Indoline [Ir(cod)(Py)(PCy3)][PF6] PPh3 [PPh4][NTf2] Bu2O

0.31 g 0.0048 g 0.0018 g 0.5 g 5 mL

2.6 mmol 0.006 mmol 0.007 mmol 0.807 mmol 30.5 mmol (0.23 mol%) When varying one of the components the molar ratios have been kept constant.

The variations with M(NTf2)2 catalysts followed the same procedure as described above. The amounts of substrate, molten salt and extracting agent did not differ from the reactions with Ir based catalyst. The amount of M(NTf2)2 was altered, also of the ligand PPh3 accordingly. The standard catalyst loading of

Co(NTf2)2 was 0.006 g (0.011 mmol) and 0.005 (0.022 mmol) of PPh3, which corresponds to a concentration of 0.42 mol%.

3.5.3 Hydrogenation

3.5.3.1 Reaction Setup Hydrogenations of LOHC test molecules were performed in a semi-batch autoclave (nickel-chromium- molybdenum alloy known as Hastelloy) from Parr Instrument. The flow chart of the rig is shown in Figure 3.1. The volume is 200 mL, which is stirred by a gas entrainment impeller (1). The autoclave could be heated by an electrical heating mantle (5) and cooled by an internal cooling coil (3) connected to a cryostat. During the reaction samples could be collected via valve V-4 which was equipped with a frit (2). Valves V-1 and V-2 are for dosing argon and hydrogen, respectively. The pressure and temperature could be monitored at all times.

Figure 3.1: Flow chart of the used semi-batch autoclave for hydrogenation reactions. 47 3. Materials and Methods

3.5.3.2 Hydrogenation Procedure

The reactor was filled with the Ir-catalyst, PPh3, the molten salt [PMeBu3][NTf2], the substrate and extracting agent Bu2O (see Table 3.7 for amounts), sealed and flushed three times with argon. The stirrer was set to 750 rounds per minute (rpm). The mixture was heated to 110 °C. After the reaction temperature was reached, the reactor was pressurized with 50 bar hydrogen. Valve V-2 was closed afterwards, thus the reaction was performed in batch mode. During the reaction samples were collected, the stirrer was kept running. Due to the position of the sample valve V-4 and the homogeneous biphasic reaction system, it could not be prevented that catalyst phase was also released together with the substrate. The IL-catalyst phase was separated before the sample was analyzed by gas chromatography.

Table 3.7: Amounts of substrate, catalyst and solvents for hydrogenation reactions.

Substrate Catalyst Ligand Molten Salt Extracting agent

Indole [Ir(cod)(Py)(PCy3)][PF6] PPh3 [PMeBu3][NTf2] Bu2O

0.75 g 0.077 g 0.04 g 7.5 g 80 mL

6.40 mmol 0.096 mmol 0.153 mmol 15.1 mmol 0.488 mol (1.5 mol%)

3.5.3.3 Reversible Hydrogenation-Dehydrogenation Procedure The hydrogenation-dehydrogenation experiments were also conducted in the Parr autoclave. The procedure for hydrogenation was the same as mentioned above. After the hydrogenation step the dehydrogenation was subsequently performed by releasing the hydrogen atmosphere and flushing three times with argon. Then the off-gas valve V-3 was kept open, the stirring was stopped while the temperature was raised to 140 °C. When the reaction temperature was reached the stirring was set to 750 rpm to precisely define the start of the dehydrogenation. The reaction process was monitored by samples analyzed by GC. After the dehydrogenation step the reactor was cooled to 110 °C and pressurized with 30 or 50 bar hydrogen to start a second hydrogenation cycle. In Table 3.8 the amounts used in the swing experiments are shown.

Table 3.8: Amounts of substrates, catalyst and solvent in the hydrogenation/dehydrogenation swing experiments.

Substrate Catalyst Ligand Molten Salt Extracting Agent

Indole [Ir(cod)(Py)(PCy3)][PF6] PPh3 [PMeBu3][NTf2] Bu2O

0.744 g 0.077 g 0.045 g 7.5 g 80 mL

6.349 mmol 0.096 mmol 0.171 mmol 15.1 mmol 0.488 mol

Quinaldine [Ir(cod)(Py)(PCy3)][PF6] PPh3 [PMeBu3][NTf2] Bu2O

0.632 g 0.04 g 0.017 g 7.5 g 80 mL

4.414 mmol 0.0497 mmol 0.065 mmol 15.1 mmol 0.488 mol 48 3. Materials and Methods 3.6 DFT Calculations

The calculations were performed with the GAUSSIAN 03W 236 program on a personal computer operated under WindowsTM XP. The total geometric/conformational energies of the guessed molecular species were minimized by use of Hartree-Fock/Kohn-Sham density functional theory (DFT) procedures at a level of approximation limited by use of restricted-spin Becke’s three-parameter hybrid exchange functional (B3), Lee-Young-Parr correlation and exchange functional (LYP) and with Pople’s polarization split valence Gaussian basis set functions, augmented with d- and p- type polarization functions and diffuse orbitals on non-hydrogen orbitals (B3LYP, 6-31 G(d,p)). This level of modelling has proved to be satisfactory to describe, for example, methanol clusters 237 and N-containing compounds. 238 The Gaussian 03W software was used as implemented with the modified GDIIS algorithm and tight optimization convergence criteria. The frequencies and eigenvectors for each normal mode were calculated without adjusting the force constants. The molecules and ions were taken to be in assumed hypothetical gaseous free-state without any pre-assumed symmetry. The results are used as a basis for assigning the observed bands according to domination group frequency motions.

3.7 Calculations

The conversion, yield and selectivity were calculated based on the GC results. The index k stands for reactant and the index i for product. The used formulas are as follows:

푛푘,0−푛푘 푛푘 Conversion: 푋푘 = = 1 − (3.1) 푛푘,0 푛푘,0

푛𝑖− 푛𝑖,0 Yield: 푌푖 = (3.2) 푛𝑖,0

푛𝑖− 푛𝑖,0 Selectivity: 푆푖 = (3.3) 푛푘,0− 푛푘

Based on the results, the turnover frequency (equation 3.4) and the hydrogen productivity (equation 3.5) are calculated. This was also applied to several literature results for better comparison.

푛 Turnover frequency: 푇푂퐹[ℎ−1] = 퐿푂퐻퐶 푝푟표푑푢푐푡 (3.4) 푛푐푎푡.∙푡

−1 −1 푚푝푟표푑푢푐푒푑 퐻2 H2 productivity: 푃퐻 [푔퐻 푔푐푎푡. ℎ ] = (3.5) 2 2 푚푐푎푡.∙푡

The mole fraction for mixtures of salt A and salt B were calculated using formula (3.6).

푛퐴 푥퐴 = (3.6) 푛퐴+푛퐵 with 푥퐴 + 푥퐵 = 1 (3.7)

The partition coefficient P was calculated by formula (3.8).

푐 푃 = 푝ℎ푎푠푒 1 (3.8) 푐푝ℎ푎푠푒 2 where c is the concentration [mol L-1] of the substrate and phase 1 stands for the upper phase of extracting agent and phase 2 for the lower ionic liquid phase.

All other used calculations are explained directly where discussed. 49 4. Results and Discussion

4. Results and Discussion

4.1 Characterization of the Melts

In this chapter the chosen bistriflimide-based low-temperature molten salts were characterized regarding their physico-chemical properties to finally identify a suitable melt as solvent for the chosen model reactions.

4.1.1 Physico-chemical Properties

4.1.1.1 Mixtures of Molten Salts – Eutectica It is well established that by mixing salts the melting point of the mixture is lowered. The alkali bistriflimide salts and their binary and even ternary mixtures were intensively studied in literature.239,240

The lowest melting point of 115 °C was found for the ternary mixture of Cs[NTf2], Li[NTf2], and 240 Na[NTf2] in the molar ratio of 0.9 : 0.05 : 0.05.

Here, the binary mixture of Cs[NTf2] and [PPh4][NTf2] was investigated using DSC. The obtained phase diagram is shown in Figure 4.1. The melting point of pure Cs[NTf2] is 125 °C which is slightly higher than that reported in literature with 122 °C240, indicating a higher purity. The melting point of pure 241 [PPh4][NTf2] was found to be 134 °C (compared to 135 °C ). Upon mixing a simple eutectic system is formed. The eutectic composition is [PPh4][NTf2] : Cs[NTf2] = 1 : 2.13 (32 mol% [PPh4][NTf2]) with a melting point of 98.6 °C.

By applying Tammann’s Rule, which states that the heat required to effect a transition at a given composition is a linear function of the composition, the eutectic composition was determined.242 This is shown in Figure 4.2. For molar fractions smaller than the eutectic composition (x < 32 mol%) the obtained relation is:

ΔeuH = 197.33 x[PPh4] – 5.56

For molar fractions larger than the eutectic composition (x > 32 mol%) the following relation was found:

ΔeuH = -90.73 x[PPh4] + 85.79

From the intersection of these two lines, the eutectic composition was determined to be 31.7 mol%, which is in very good agreement with the result from the phase diagram (Figure 4.1).

50 4. Results and Discussion

135

130

125

120 liquid 115

110

105

melting point / °C / point melting liq. + [PPh ][NTf ] liq. + Cs[NTf2] 4 2 100

95 Cs[NTf2] + [PPh4][NTf2] 90

0,00.0 0,10.1 0,20.2 0.30,3 0,40.4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 Cs[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.1: Phase diagram of Cs[NTf2] and [PPh4][NTf2] as determined by DSC measurements, with heating and cooling rates of 2 K min-1. The transition temperatures were extracted from the heating process to avoid the uncertainty of supercooling.

100

90 heat of fusion (integral of eutectic peak) fit rising R2 = 0.881 80 fit descending R2 = 0.879

-1 70

Heat of fusion / J g fusion of Heat 20

0,00.0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 Cs[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.2: Dependence of the composition with the eutectic heat of fusion for the Cs[NTf2]-[PPh4][NTf2] system.

This approach was also applied to other new binary mixtures of [PPh4][NTf2] with different low melting salts. In Figure 4.3 the phase diagram of [PPh4][NTf2] and [PPh4OPh][NTf2] is shown. The corresponding plot of the eutectic heat of fusion versus the molar composition of the mixture is shown in Figure 4.4. The pure salt [PPh4OPh][NTf2] has a melting point of 90 °C and with this it is defined as a classical ionic liquid. Upon mixing a simple eutectic system is formed with the eutectic composition

[PPh4][NTf2] : [PPh4OPh][NTf2] = 1 : 2.67 (37.5 mol% [PPh4OPh][NTf2]) and with a glass transition 51 4. Results and Discussion point of -10 °C. In the DSC diagrams of the mixtures with a composition smaller than 25 mol%

[PPh4OPh][NTf2] two endothermic peaks are observed. These can be due to simple phase transitions of - 240 the [NTf2] anion which can exist in two conformations cis and trans or similar as reported in reference a peritectic point is found at a composition around 30 mol% [PPh4OPh][NTf2] with a temperature of around 60 °C. This could not be determined unambiguously by DSC and further investigations have to be performed. The endothermic peak at -10 °C is not observed at mole fractions below 0.25 suggesting the formation of a 3:1 double salt, [PPh4]3[PPh4OPh][NTf2]4. The eutectic composition derived from the phase diagram (Figure 4.3) is 37.5 mol% [PPh4OPh][NTf2].

140

120

100 liq. + [PPh4][NTf2] 80 liquid

melting point / °C / point melting liq. + [PPh4OPh][NTf2] 20 liq. + [PPh ] [PPh OPh][NTf ] 0 4 3 4 2 4

-20 [PPh4][NTf2] + [PPh4OPh][NTf2]

0,00.0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0

[PPh4][NTf2] [PPh4OPh][NTf2]  ([PPh4OPh][NTf2])

Figure 4.3: Phase diagram of [PPh4][NTf2] and [PPh4OPh][NTf2] as determined by DSC measurements, with heating and cooling rates of 2 K min-1. The transition temperatures were extracted from the heating process to avoid the uncertainty of supercooling.

The eutectic composition of [PPh4][NTf2] and [PPh4OPh][NTf2] determined by Tammann’s rule is 41 mol% (see Figure 4.4). This is in good accordance to the one extracted from the phase diagram (Figure 4.3). The difference between the two values can be explained by the large steps of compositions measured.

The binary mixture of other alkali bistriflimide salts with [PPh4][NTf2] were also investigated. In Figure

4.5 the obtained phase diagram of Na[NTf2]-[PPh4][NTf2] is shown. Even though the diagram is more complex, a simple eutectic system is formed, however, an intermediate compound is found. The eutectic composition is determined to be Na[NTf2] : [PPh4][NTf2] = 1 : 1.75 (57 mol% [PPh4][NTf2]) with a melting point of 175 °C. The endothermic peak at 103 °C disappears at x > 0.75. This suggests the formation of a 1:3 double salt Na[PPh4]3[NTf2]4. Even though the melting point of the eutectic is still very high it is interesting that the melting point is lowered about 80 K compared to the melting point of 240 neat Na[NTf2] (257 °C this study and ).

The plot of the heat of fusion versus the composition of the mixture Na[NTf2] and [PPh4][NTf2] is shown in Figure 4.6. Both linear fits are very accurate (coefficient of determination > 0.98). The intersection is situated at a molar composition of 0.57, which is perfectly consistent with the value derived from the phase diagram (Figure 4.5). 52 4. Results and Discussion

10 heat of fusion (integral of eutectic peak) 9 fit rising R2 = 0.765 8 fit descending R2 = 0.864 7

-1 6 5 4 3 2

Heat of fusion / J g fusion of Heat 1 0 -1 -2 0,00.0 0,10.1 0,20.2 0.30,3 0,40.4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 [PPh4][NTf2] [PPh4OPh][NTf2]  ([PPh4OPh][NTf2])

Figure 4.4: Dependence of the composition with the eutectic heat of fusion for the [PPh4][NTf2]-[PPh4OPh][NTf2] system.

260

240

220 liquid

200 liq. + Na[NTf2] 180

160

liq. + [PPh ][NTf ] 140 liq. + Na3[PPh4][NTf2]4 4 2 2

melting point / °C / point melting 120 [PPh4][NTf2] + 100 Na[PPh4]3[NTf2]4 Na[NTf2] + Na3[PPh4][NTf2]4 80 0,00.0 0,10.1 0,20.2 0.30,3 0,40.4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0

Na[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.5: Phase diagram of Na[NTf2] and [PPh4][NTf2] as determined by DSC measurements, with heating and cooling rates of 2 K min-1. The transition temperatures were extracted from the heating process to avoid the uncertainty of supercooling. 53 4. Results and Discussion

100

90 heat of fusion (integral of eutectic peak) fit rising R2 = 0.984 80 fit descending R2 = 0.982

-1 70

Heat of fusion / J g fusion of Heat 20

0,00.0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0

Na[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.6: Dependence of the composition with the eutectic heat of fusion for the Na[NTf2]-[PPh4][NTf2] system.

The phase diagram of Li[NTf2] and [PPh4][NTf2] is shown in Figure 4.7. The DSC diagrams are more complex than those of the other binary mixtures; there are up to three endothermic peaks visible. The existence of a double salt is suggested with the composition of 1:1 Li[PPh4][NTf2]2. As a result, the system has two eutectic points: the first one at a composition of 0.26 and a temperature of 97 °C; the second one at 0.75 and 135 °C. A peritectic point also suggest two additional intermediate compounds with the composition Li3[PPh4][NTf2]4 and Li[PPh4]3[NTf2]4.

260 240 220 200 liquid 180 160 140 liq. + Li[PPh4][NTf2]2 120 liq. + Li[NTf2] liq. + [PPh4][NTf2] 100

melting point / °C / point melting 80 liq. + Li3[PPh4][NTf2]4 60 [PPh4][NTf2] + Li[PPh4]3[NTf2]4 40 Li[Ntf ] + Li [PPh ][NTf ] 20 2 3 4 2 4 0,00.0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 Li[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.7: Phase diagram of Li[NTf2] and [PPh4][NTf2] as determined by DSC measurements, with heating and cooling rates of 2 K min-1. The transition temperatures were extracted from the heating process to avoid the uncertainty of supercooling. 54 4. Results and Discussion

The linear dependency of the heat of fusion versus the composition of the mixture is shown in Figure 4.8. There are two fits according to the two eutectic points. The first eutectic composition as determined by Tammann’s rule is 0.25 mol%, which is in perfect agreement with the results from the phase diagram. The second eutectic point which is found in the phase diagram at a composition of 75 mol% is characterized by the intersection at 0.78. However, the coefficients of determination of both latter lines are very low suggesting the value is slightly vague.

70 heat of fusion (endothermic peak 104 °C) 60 heat of fusion (endothermic peak 128 °C) fit rising R2 = 0.966 2

-1 50 fit descending R = 0.897 fit rising R2 = 0.795 40 fit descending R2 = 0.675

Heat of fusion / J g fusion of Heat

0 0,00.0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 Li[NTf2] [PPh4][NTf2]  ([PPh4OPh][NTf2])

Figure 4.8: Dependence of the composition with the eutectic heat of fusion for the Li[NTf2]-[PPh4][NTf2] system.

4.1.1.2 Viscosity and Density An important feature if a molten salt shall be used as solvent is its viscosity. Figure 4.9 shows the viscosities of pure [PPh4][NTf2], Cs[NTf2], and of some binary mixtures in a temperature range from

110 to 240 °C. The viscosity of Cs[NTf2] at temperatures near the melting point is quite high with

64.8 mPa s, the viscosity of [PPh4][NTf2] much lower (17.5 mPa s at 135 °C). The viscosities of the binary mixtures are somewhat between those of the constituent single salts. By rising the temperature above 200 °C the viscosities do not change much with further temperature increase. Comparing the obtained viscosities is with those reported in the literature, similar results for Cs[NTf2] (see dotted line in Figure 4.9) are found, only for higher temperatures, Kubota et al. obtained lower viscosities.243

Often the experimental viscosities are fitted with the Vogel-Fulcher-Tamman equation 4.1:

퐵휂 휂 = 휂0 exp ( ) (4.1) 푇−푇푉퐹

η0, Bη and TVF are substance specific constants. TVF is known as the ideal glass transition temperature.

The Vogel-Fulcher-Tamman equation describes the viscosities of the melts in very good accuracy (all R2 > 0.98). The obtained values are shown in Table 4.1. Sometimes another form of the Vogel-Fulcher- Tamman equation is used (the values from reference243 are shown in the Appendix, section 6.5), thus the literature values for Cs[NT2] and the here obtained fitting parameters could not be compared directly. For comparison the literature data were fitted as well, but there are also differences in the fitting parameters. 55 4. Results and Discussion

120 Cs[NTf2] 16.7 mol% [PPh4][NTf2] in Cs[NTf2] 100 32 mol% [PPh4][NTf2] in Cs[NTf2] 66.6 mol% [PPh4][NTf2] in Cs[NTf2] 80 [PPh4][NTf2]

/ mPa s / mPa Cs[NTf ] values from Kubota et al.

 2 60

Viscosity 40

0 120 140 160 180 200 220 240 Temperature / °C

Figure 4.9: Viscosity in dependency on the temperature of pure [PPh4][NTf2], Cs[NTf2] and some binary mixtures, values for comparison are from literature243.

Table 4.1: Fitting parameters for the Vogel-Fulcher-Tamman equation (4.1).

2 salt η0 / mPa s) Bη / K TVF / K R

Cs[NTf2] 2.68 272.61 327.30 0.995

16.7 mol% 6.72 100.19 357.42 0.982

32 mol% 2.38 224.13 324.98 0.985

66,6 mol% 0.63 521.41 267.11 0.998

[PPh4][NTf2] 1.35 234.22 301.60 0.993

243 Cs[NTf2] values from 0.32 883.79 238.62 0.999

Due to the enormous number of ionic liquids and molten salts and even more mixtures of those, it would be tedious to measure all thermophysical properties. Predicting the viscosity of binary mixtures of molten salts is very difficult. Several approaches have been reported in the literature, where different data sets are needed for the estimates (e.g. density or pair potential parameters).244-246

Two predicting approaches have been applied here. The Eyring equation 4.2 is a rather simple way and usually applied to predict the viscosity of ideal solutions246, but it also has only moderate accuracy:

ln 휂 = 푥1 ln 휂1 + 푥2 ln 휂2 (4.2) where η is the viscosity and x the mole fraction. The obtained results are compared to the experimental viscosities at 200 °C of the pure salts and the binary mixtures in Figure 4.10. The average deviation of the calculated viscosities by the Eyring equation from the real values is 11.48 %. Especially, the viscosity of the eutectic mixture (32 mol% [PPh4][NTf2]) is much lower than the calculated value 56 4. Results and Discussion

(deviation 20.16 %). The second slightly more sophisticated expression 4.3 is derived from a unit cell model245:

푏 2 푏 2 휂 = 휂 [1 − ( ) ] + 휂 ( ) (4.3) 퐴 푎 퐵 푎 where a and b are cell constants of the unit cells of the two components. The higher viscosity component is A, in this case Cs[NTf2]. The ratio of b/a is determined by equation 4.4.

3 푏 푣퐵 3 = (4.4) 푎 푣퐴+푣퐵 where v is the volume fraction. In the case of a binary system where one has vA + vB = 1. The equation 4.4 can be converted to

푋퐵푀퐵 푣퐵 휌퐵 = 푋 푀 푋 푀 (4.5) 푣퐴+푣퐵 퐴 퐴 + 퐵 퐵 휌퐴 휌퐵 where x is the mole fraction, M the molar mass, and ρ the density (at 200 °C). The results of this model are better than the ones of the Eyring equation, and compared to the real values in Figure 4.10. The average deviation of the calculated viscosities by the unit cell model is 5.46 %. This is a very good prediction, even though the viscosity of the eutectic composition (32 mol% [PPh4][NTf2]) is lower than the predicted value (deviation 7.85 %). Whether this is due to special interactions in the eutectic mixture or due to insufficiencies of the prediction models still has to be established.

14 experimental Eyring equation unit cell model 12

Viscosity / mPa s Viscosity / mPa 8

0.00,0 0,20.2 0.40,4 0.60,6 0.80,8 1,01.0 Cs[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.10: Experimental viscosities of the pure salts and some binary mixtures compared to calculated values by different predicting methods.

57 4. Results and Discussion

For the binary mixture of [PPh4][NTf2] and [PPh4OPh][NTf2] the viscosity was determined and is shown in Figure 4.11. For both salts the viscosity at temperatures between 130 and 240 °C is quite low. The viscosity of the 1:1 mixture is between the viscosity of the pure salts. For the mixture, which is already liquid at room temperature, the viscosity between 50 and 80 °C was measured as well. The overlapping temperatures yield slightly different viscosities, which was attributed to the different measurement techniques of a rotary cylindrical spindle viscosimeter at high temperature to a rotary cone-plate viscosimeter at lower temperatures (as shown in Figure 4.12). In Table 4.2 the fitting parameters of the Vogel-Fulcher-Tamman equation are shown as calculated from the viscosity measurements at higher temperature. The coefficients of determination were excellent.

200 [PPh4][NTf2] 180 [PPh4OPh][NTf2] 50 mol% [PPh OPh][NTf ] in [PPh ][NTf ] 160 4 2 4 2

140

120

100

Viscosity / mPa s Viscosity / mPa 60

100 120 140 160 180 200 220 240 Temperature / °C

Figure 4.11: Viscosity in dependency of the temperature of pure [PPh4][NTf2], [PPh4OPh][NTf2] and the 1:1 binary mixture.

Table 4.2: Fitting parameters for the Vogel-Fulcher-Tamman equation (4.1).

2 salt η0 / mPa s Bη / K TVF / K R

[PPh4OPh][NTf2] 0.03 1600.80 179.26 0.999

[PPh4][NTf2] 1.35 234.22 301.60 0.993

50 mol% [PPh4OPh][NTf2] 0.26 818.62 227.46 0.999 58 4. Results and Discussion

1000

900 measurement technique 1 (high temperature) measurement technique 2 (low temperature) 800

700

600

500

400

Viscosity / mPa s Viscosity / mPa 300

200

100

0 50 60 70 80 90 100 110 Temperature / °C

Figure 4.12: Viscosity of the 1:1 binary mixture of [PPh4][NTf2] and [PPh4OPh][NTf2] as function of the temperature compared for the two different measurement techniques; orange: measurement setup was a rotary cylindirical spindle viscosimeter; blue: measurement setup was a rotary cone-plate viscosimeter.

Another important property of ionic liquids and molten salts is the density. It is known for decades that there is a linear dependency of the density of salts (independent from the aggregate phase) from the temperature, but also from the composition of binary mixtures.247-249 The high temperature density of 250,251 Cs[NTf2] and [PPh4][NTf2] is shown in Table 4.3 as they were determined previously. For the pure solid salts and some binary mixtures measurements were conducted at 20 °C. The results are shown in -3 Figure 4.13. The solid state density of Cs[NTf2] is quite high with 2.58 g cm ; the density of -3 [PPh4][NTf2] is 1.53 g cm . The density of the mixtures lies in between.

2.62,6 measured density linear fit R2 = 0.888 2,42.4

-3 2.22,2

2,02.0

1.81,8

Density / g cm / g Density

1,61.6

1,41.4

0.00,0 0,20.2 0,40.4 0,60.6 0.80,8 1.01,0 Cs[NTf2] [PPh4][NTf2]  ([PPh4][NTf2])

Figure 4.13: Density in dependence of mole fraction of Cs[NTf2], [PPh4][NTf2] and some binary mixtures. 59 4. Results and Discussion

251 Table 4.3: Density of the molten salts Cs[NTf2] and [PPh4][NTf2] at 200 °C.

salt Density at 200 °C / g cm-3

Cs[NTf2] 2.24

[PPh4][NTf2] 1.17

4.1.2 Thermal Stability One of the major drawbacks of conventional ionic liquids is their lack of thermal stability.16-18 The Low- Temperature Molten Salts investigated here are highly stable when exposed to thermal stress. Figure

4.14 shows the thermogravimetric analysis of Cs[NTf2] and [PPh4][NTf2] compared to one of the more common IL, [EMIM][NTf2]. The samples were first kept in the TG setup at 120 °C for one hour for drying. Then the samples were heated with 5 K min-1 under constant nitrogen flow. The onset temperature has the potential to be overrated and might lead to overestimation of the temperature stability, but baring its limitations in mind it is a quick way to compare the thermal stabilities as done in

Table 4.4. The decomposition of Cs[NTf2] and [PPh4][NTf2] starts at 450 °C, thus over 60 K higher than -1 -1 for [EMIM][NTf2] with a heating rate of 5 K min . If the heating rate is 1 K min the decomposition onset shifts 38 K to lower temperatures, to 412 °C for the Cs[NTf2] salt. The same is observed for -1 [PPh4][NTf2] which has a decomposition onset of 417 °C with a heating rate of 1 K min .

+ Interestingly, the organic cation [PPh4] does not limit the thermal stability. This means thermal stability - is more influenced by the [NTf2] anion. Phosphonium-based cations are known to be thermally very stable.48,241 The almost identical temperature onset shows that the decomposition mechanism of both salts must be very similar.

The found onset temperatures match almost perfectly those determined in the course of a collaboration with F. Heym of the University of Bayreuth. The determined onset temperatures of both Cs[NTf2] and -1 250 [PPh4][NTf2] at heating rates of 2 K min were 425 °C.

Table 4.4: Decomposition onset temperatures of different molten salts at two different heating rates (5 and 1 K min-1); the -1 samples were dried at 120 °C for 1 hour and heated with the respective heating rate to 600 °C in a N2 flow of 75 mL min .

-1 -1 Salt Tonset / °C (heating rate 5 K min ) Tonset / °C (heating rate 1 K min )

[EMIM][NTf2] 384 337

Cs[NTf2] 450 412

[PPh4][NTf2] 451 417

32 mol% n.a. 414

50 mol% 439 n.a.

[PPh4OPh][NTf2] 443 398

60 4. Results and Discussion

1.01,0 heating rate 5 K min-1

0.90,9

0.80,8

0.70,7

0,60.6

0,50.5

0,40.4

Normalized mass loss / - loss mass Normalized [EMIM][NTf2] 0.30,3 Cs[NTf2] [PPh ][NTf ] 0.20,2 4 2

150 200 250 300 350 400 450 500 550 600 Temperature / °C

Figure 4.14: Comparative thermal gravimetric analysis of [EMIM][NTf2], Cs[NTf2] and [PPh4][NTf2]; the samples were kept at 120 °C for 1 hour to completely dry them; then they were heated to 600 °C with a heating rate of 5 K min-1, the carrier -1 gas was N2 and the flow rate was 75 mL min .

The TGA of the [PPh4OPh][NTf2] gave an almost identical decomposition temperature as the other salts as shown in Figure 4.15. Showing again the decomposition mechanism is mainly influenced by the - [NTf2] anion.

1,01.0 heating rate 5 K min-1

0,90.9

0,80.8

0,70.7

0,60.6

0,50.5

0,40.4

Normalized mass loss / - loss mass Normalized Cs[NTf2] 0,30.3 [PPh4OPh][NTf2] [PPh ][NTf ] 0,20.2 4 2

150 200 250 300 350 400 450 500 550 600 Temperature / °C

Figure 4.15: Comparative thermal gravimetric analysis of Cs[NTf2], [PPh4OPh][NTf2] and [PPh4][NTf2]; the samples were -1 kept at 120 °C for 1 hour to completely dry them; then they were heated to 600 °C with a heating rate of 5 K min , the N2 flow was 75 mL min-1. 61 4. Results and Discussion

4.1.3 Vibrational Spectroscopy on Molten Salts Due to their powerful structure predicting abilities, vibrational spectroscopy and DFT calculations have received quite some attention. The here presented Raman measurements were obtained during a research stay at the Technical University of Denmark (DTU) under the supervision of Rolf W. Berg.

4.1.3.1 Vibrational Spectroscopy and DFT Calculations of the neat Salts

- 4.1.3.1.1 The [NTf2] Anion Molecular Orbital (MO) model calculations have become a quite efficient tool to predict chemical structures and vibrational spectra (i.e. Raman scattering and IR absorption). Ab initio calculations on - 37,66,73,252 the isolated [NTf2] ion have already been reported several times, e.g. in . The DFT calculations were repeated to see how the results compare with the experimental spectra. The equilibrium geometries and spectra of the isolated ions were calculated with the B3LYP/DFT/6-311+G(d,p) level by using the GAUSSIAN 03W software236, and neglecting cation-anion interactions. They are shown in Figure 4.16.

- According to the DFT calculations, two conformers of [NTf2] of C2 and C1 point group symmetry (a two-fold rotational axis and no symmetry, respectively), constitute global and local minima. The C2 conformer, often referred to as transoid, has the CF3 groups on opposite sides of the S-N-S plane and is -1 slightly energetically more stable (by 2.3 kJ mol ), compared to the C1 or cisoid conformer (with the

CF3 groups on the same side of the S-N-S plane). Both experimental and theoretical approaches confirm the pronounced delocalization of the negative charge on the nitrogen and oxygen atoms and a double- bond character of the S-N-S moiety.

- Figure 4.16: Optimized geometries of trans (left) and cis (right) [NTf2] (obtained by DFT/B3LYP calculations with Gaussian 6-31G+(d,p) basis sets).

The calculated vibrational spectra of both conformers, are shown in Figure 4.17, the IR spectra in the upper part of the diagram, the Raman spectra in the lower part.

The calculated IR spectra of the cis and trans conformers show quite strong differences, especially in intensity and position of the bands. The band at 625 cm-1 of the transoid conformer is much more intense -1 than the bands at 594 and 651 cm of the C1 conformer. These bands arise from the deformation of the whole skeleton of the S-N-S moiety. The same is true for the strong symmetric vibration of the SO2 which is found at 1040 cm-1 of the trans and at 1046 cm-1 of the cis conformer. For the cis conformer in -1 the region between 1100 and 1260 cm the vibrations of the CF3 group are visible as overlapping bands, while the trans conformer shows these as one separate band at 1155 cm-1 and several overlapping bands 62 4. Results and Discussion at around 1268 cm-1. The two overlapping bands of the cis conformer (around 1334 cm-1) are shifted to higher wavenumbers for the trans conformer (around 1360 cm-1) and can be assigned to the asymmetric stretching vibrations of the SO2 moiety.

cis conformer a

trans conformer b

IR Transmision / a.u. IR Transmision

c cis conformer

d trans conformer

Raman scattering / a.u. scattering Raman 1600 1400 1200 1000 800 600 400 200 0 Wavenumber / cm-1

- Figure 4.17: Calculated vibrational spectra of the [NTf2] conformers: IR spectra of a) cis conformer, b) trans conformer, and Raman spectra of c) cis conformer, and d) trans conformer. The conformers have been optimized by DFT/6- 31+G(d,p)/B3LYP Gaussian modelling. Intensities are arbitrarily scaled.

In general, the Raman spectra of the two conformers look very much alike, but minor differences in intensities and the position of some bands can be noticed. The most distinct difference is located in the lower frequency region of the spectrum: the transoid conformer gives a band around 632 cm-1 that is shifted to 665 cm-1 for the cisoid conformation. These bands arise from a combination of δ(SNS) deformation, out-of-phase umbrella motions of the CF3 and N-SO2 groups and symmetric deformations -1 of the latter two mentioned groups. Other characteristic bands are found at 133 cm for the C2 conformer -1 -1 or 206 cm for the C1. At 1138 cm a single band arising from the symmetric stretching vibration of the sulfonyl groups is visible for the trans conformer, whereas the cis conformer shows two overlapping bands around 1126 cm-1. The bands around 1200 cm-1 are better resolved for the cisoid conformer, for the transoid conformer these bands are pushed together and show multiple overlapping peaks around 1250 cm-1. Less distinctive differences are hidden in the region around 300 cm-1; however, strong overlapping of the bands makes the analysis difficult. The calculated results are in overall agreement with results reported by Herstedt et al.66,67, although some differences can be noted around 1300 cm-1 probably due to different scaling.

From the two complementary spectroscopic methods it can be concluded that the differentiation which conformers prevails is more straightforward from the IR analysis.

- The calculated Raman spectra of both [NTf2] conformers are compared to the measured Raman spectra of crystalline Li[NTf2] and Cs[NTf2] in Figure 4.18. The conformation of Li[NTf2] is quite hard to 63 4. Results and Discussion

66,82,253,254 determine. For Li[NTf2] both conformers have been reported. The C1 conformer can act as bidentate ligand to Li+. In the transoid conformation it was reported to be in a monodentate coordination. These findings were also shown by vibrational spectroscopic investigations.66 The signal at 626 cm-1 is found in the calculated spectrum of the trans conformer or at 632 cm-1 for the obtained spectrum. This is in very good accordance compared to ca. 630 cm-1 reported in literature.66 The band at 390 cm-1 can be assigned to the band in the spectrum calculated for the transoid conformer at 399 cm-1 and the single band at 1131 cm-1 fits better to the single band at 1138 cm-1 in the calculated trans spectrum than the splitted peak bands in the cis spectrum. Other minor matches can be detected which all together lead to the conclusion that this Li[NTf2] sample shows the trans conformer.

- + + Based on literature reports, the cisoid conformation is found for [NTf2] compounds with Na , K and + 66,82,253 82 Cs cations , even though the trans conformation was also reported for Cs[NTf2] . The cisoid conformation may be adopted because the bistriflimide can coordinate as bidentate chelating ligand to the bigger metal center.

66 Similar to the literature here strong indications are found for the Cs[NTf2] to be in cis conformation. The most characteristic band supporting this assumption is located at 661 cm-1 in the experimental spectrum. The corresponding calculated vibration is at 667 cm-1; Herstedt et al.66 reported the band to -1 be found at 650 cm . Another sign for the cisoid conformation of Cs[NTf2] is hidden in the lower frequency region at 414 cm-1. The corresponding band was calculated to be at 420 cm-1.

a trans conformer

b cis conformer

Raman scattering / a.u. scattering Raman LiNTf2 measured

d CsNTf2 measured 1600 1400 1200 1000 800 600 400 200 0 Wavenumber / cm-1

- - Figure 4.18: The Raman spectra of a) calculated trans [NTf2] , b) calculated cis [NTf2] , c) experimental Li[NTf2], and d) experimental Cs[NTf2]. The calculations were obtained by DFT/6-31+G(d,p)/B3LYP Gaussian modelling. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled.

- To definitely conclude the conformation of the [NTf2] anion in the Cs[NTf2] molten salt, infrared spectroscopic investigations were conducted on the salt at room temperature and are compared to the calculated IR spectra (see Figure 4.19). The frequencies of the calculated IR spectra are scaled with a factor of 1.04 for better accordance with the experimental spectra. This sometimes is necessary due to the lack of good modelling of orbitals, and model limitations in describing the internal and external 64 4. Results and Discussion interactions between the ions and the surroundings. As evident from the spectra the measured Cs[NTf2] matches almost perfectly to the calculated spectrum of the cis conformer. The band at 1336 cm-1 of the experimental spectrum with the shoulder at 1318 cm-1 corresponding to the asymmetric stretching -1 vibration of the SO2 moiety is found at 1341 and 1324 cm for the calculated cis conformer. The shoulder in the calculated cis-spectrum at 1204 cm-1 is melted together with the strong band at around -1 1188 cm in the experimental spectrum. Also, the νs(SO2) vibrations are found as two not completely resolved bands around 1135 cm-1 in the experimental spectrum and are calculated for the cis conformer at 1131 cm-1. The bands between 800 and 600 cm-1 in the measured IR spectra are slightly shifted to lower wavenumbers and much less intensive in the calculated cis-spectrum. On the other hand, it is difficult to find matches of the experimental results with the transoid conformer, excluding it as a possible conformation of the Cs[NTf2].

a Cs[NTf2]

b cis conformer

c trans conformer

Transmission / a.u. Transmission

1600 1400 1200 1000 800 600 400 200 Wavenumber / cm-1

- - Figure 4.19: The IR spectra of a) experimental Cs[NTf2], b) calculated cis [NTf2] , and c) calculated trans [NTf2] . The calculations were obtained by DFT/6-31+G(d,p)/B3LYP Gaussian modelling. The experimental spectra were measured with the FT/IR-4600 with an ATR attachment at room temperature. Intensities are arbitrarily scaled.

+ 4.1.3.1.2 DFT calculations and experimental Spectra of the [PPh4] Cation

+ The equilibrium geometries and spectra of the isolated [PPh4] ion were calculated with the B3LYP/DFT/6-31+G(d,p) level by using the GAUSSIAN 03W software236, and neglecting cation-anion + interactions. The calculated optimized geometry of the [PPh4] cation is shown in Figure 4.20.

No imaginary frequencies were found; the geometry adopts a C1 point group. The calculated results are shown in the appendix, section 6.1.3.

+ The calculated Raman spectrum of [PPh4] is shown in Figure 4.21 compared to the obtained Raman spectrum of [PPh4]Cl. The calculated bands can be connected to measured bands, but the calculation gave vibrational frequencies with slightly too low wavenumbers except the very strong vibration at 3150 cm-1 which is split in at least three bands and observed around 3060 cm-1. Sometimes the use of 65 4. Results and Discussion scaling factors of the wavenumbers gives better agreement with the experimental data, but none were used in this experiment calculation.

+ Figure 4.20: Optimized geometry of [PPh4] .(obtained by DFT/B3LYP calculations with Gaussian 6-31G+(d,p) basis sets. Hydrogen atoms are omitted for better visualization.

The strong band around 3050 cm-1 in the calculated spectrum can be assigned to the C-H stretching vibration of the aromatic rings and is shifted to higher wavenumber in the experimental spectrum. The smaller bands at 1413 and 1306 cm-1 arising from H-C-C-H rocking motions are not visible in the measured spectrum and the two C-C bending vibrations around 1000 cm-1 are not completely resolved -1 -1 in the spectrum of [PPh4]Cl. The same applies to the band at around 180 cm (shifted to 255 cm in the measurement).

a measured [PPh4]Cl

Raman scattering / a.u. scattering Raman

b + calculated [PPh4]

3500 3250 3000 1500 1250 1000 750 500 250 0 Wavenumber / cm-1

+ Figure 4.21: The Raman spectra of a) experimental [PPh4]Cl, and b) calculated [PPh4] . The calculations were obtained by DFT/6-31+G(d,p)/B3LYP Gaussian modelling. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled. 66 4. Results and Discussion

4.1.3.1.3 [PPh4][NTf2] Molten Salt

+ The calculated Raman spectrum of [PPh4] (scaled with a factor of 0.96 for better accordance) combined - with the sum of both calculated conformers of [NTf2] is shown in Figure 4.22 and compared to the obtained spectrum of [PPh4][NTf2].

The experimental results are reproduced within reasonable accuracy, proving again that a good structure modelling is possible at the DFT-B3LYP/6-31+G(d,p) level.

A determination which conformer of the anion is prevalent is very difficult because of strong overlapping of the bands and shifts in intensity. The vibrational bands below 100 cm-1 could not be recorded due to the position of the Notch-filter. The calculated bands at 835 and 791 cm-1 are hardly visible in the measured spectrum and the bands around 1160 cm-1 show slightly changed intensities.

In Figure 4.23 a detail of the obtained spectrum of [PPh4][NTf2] is shown with an attempt to assign all + peaks. The bands marked with an asterisk correspond to vibrations of the [PPh4] cation, the ones marked - with a hashtag to vibrations of the [NTf2] anion which are not distinct for one conformer. There are three bands which cannot be assigned precisely to one of the conformers: The band at 1341cm-1 is situated between two calculated bands of the trans conformer (1347 and 1324 cm-1), but also shifted compared to the vibrational frequency of the cis conformer (1329 cm-1). The band at 1242 cm-1 lies between the calculated bands for the trans conformer (1254 cm-1) and the cis conformer (1231 cm-1). The band at 398 cm-1 corresponds better to the frequency of one of the trans conformer (400 cm-1), but the shape of the band matches better to the one of the cis conformer at 420 cm-1; especially because there are several bands which match very good to the cis conformer as shown in Table 4.5. It is concluded that the bistriflimide molecule adopts the cis conformation in [PPh4][NTf2].

a measured [PPh4][NTf2]

Raman scattering / a.u. scattering Raman

b + - sum of calculated [PPh4] and [NTf2]

3500 3250 3000 1500 1250 1000 750 500 250 0 Wavenumber / cm-1

+ Figure 4.22: The Raman spectra of a) experimental [PPh4][NTf2], and b) the sum of calculated [PPh4] and both calculated - conformers of [NTf2] . The calculations were obtained by DFT/6-31+G(d,p)/B3LYP Gaussian modelling. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled. 67 4. Results and Discussion

* [PPh ]+ 4 * # - [NTf2] both conformers

* * * #

Raman scattering / a.u. scattering Raman

cis ? cis * #

? cis ? * #

* cis

1500 1250 1000 750 500 250 Wavenumber / cm-1

Figure 4.23: Detail of the Raman spectrum of experimental [PPh4][NTf2] (see also Figure 4.22). The experimental spectrum was measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled. Assignments were made by comparison with calculated spectra.

Table 4.5: Comparison of observed and calculated frequencies of the experimental [PPh4][NTf2] Raman spectrum and the - calculated cis [NTf2] .

Observed frequency Calculated frequency for cis Deviation / % / cm-1 conformer / cm-1

1193 1197 0.33

681 665 2.38

451 458 1.54

198 206 3.96

Even though the calculations give sufficiently good agreement with measured spectra, they completely neglect cation-anion interactions which might dictate a different geometry than for the isolated gaseous free-state. This means that even though the transoid conformation is calculated to be slightly more stable, the bistriflimide anion seems to adopt the cisoid conformation in [PPh4][NTf2].

This is further supported by the IR spectroscopy. In Figure 4.24 the experimental infrared spectrum of + [PPh4][NTf2] is compared to the calculated spectrum of the [PPh4] cation. For this no scaling factors were used, even though some wavenumbers do not match perfectly. The bands at around 1460 cm-1 in the experimental spectrum are found shifted to higher wavenumbers to 1500 cm-1. The strong band in the experimental spectrum at 1188 cm-1 is clearly a feature of the anion. The peak at 1108 cm-1 of the + -1 [PPh4][NTf2] is a vibration of the [PPh4] cation and is located at 1116 cm in the calculated spectrum. The three distinct bands in the calculated spectrum around 725 cm-1 are visible in the experimental 68 4. Results and Discussion spectrum around 723 cm-1. The strong calculated vibration at 538 cm-1 arising from C-C bond oscillations is slightly cut and shifted to higher frequency to 613 cm-1 for the real spectrum. The sum of + - the calculated spectra of the [PPh4] cation and the cis conformer of the [NTf2] anion is shown in the lower part of Figure 4.24. This resembles the experimental spectrum very good. Though some different intensities, all expected features are found. The modeling at the DFT-B3LYP/6-31+G(d,p) level gives sufficiently good accuracy and helps to assign bands.

Just recently the calculations and vibrational measurements on the [PPh4][NTf2] molten salt were reported in the literature in an independently conducted study. The synthesized [PPh4][NTf2] by Haddad et al.255 had a melting point of 128 °C. The difference of 6 K from their molten salt and the salt used in the present thesis and as already published241,250 (both papers are cited by Haddad et al.255) can be explained by impurities from the synthesis route, where LiBr is obtained as less good water soluble side product which obviously was not completely removed in the purification procedure. None of their analysis methods (1H, 13C, 19F, and 31P-NMR, IR and Raman) is suitable to detect LiBr impurities. However, the LiBr should not change the spectroscopic results. They concluded that the trans - conformation for the [NTf2] anion is prevalent in the [PPh4][NTf2] salt from their vibrational spectroscopic experiments. Since the bistriflimide anion has a high conformational flexibility the difference from the studies of Haddad et al. to the here concluded cis conformation might be simply due to different crystallization of the salt (for the alkali bistriflimide salts also different conformations of the anion are found) or is induced by the impurities.

a [PPh4][NTf2]

+ [PPh4]

Transmission / a.u. Transmission

calculated sum

3500 3000 2500 2000 1500 1000 500 0 Wavenumber / cm-1

+ + Figure 4.24: IR spectra of a) experimental [PPh4][NTf2], b) calculated [PPh4] , and sum of the calculated spectra of [PPh4] - and the cis conformer of [NTf2] . The calculations were obtained by DFT/6-31+G(d,p)/B3LYP Gaussian modelling. The experimental spectra were measured with the FT/IR-4600 with an ATR attachment at room temperature. Intensities are arbitrarily scaled. 69 4. Results and Discussion

4.1.3.1.1 Raman Spectroscopy of the binary Mixtures of Cs[NTf2] and

[PPh4][NTf2]

Some binary mixtures of the low temperature molten salts Cs[NTf2] and [PPh4][NTf2] with different compositions have been studied by Raman spectroscopy. Especially the eutectic mixture with a composition of [PPh4][NTf2] : Cs[NTf2] = 1 : 2.13 (32 mol% [PPh4][NTf2]) and with a melting point of 98.6 °C was of interest.

In Figure 4.25 details of the spectra are shown (spectra of the complete measuring range can be found in the Appendix, section 6.1.2). The intensities of the spectra were normalized to the intensity of the -1 - band at 740 cm which corresponds to a deformation vibration of the CF3 groups of the [NTf2] anion. The area beneath the curves cannot directly be correlated to the concentration, because the intensity of the vibration depends also on the Raman scattering cross section. But connections can be drawn. As obvious in Figure 4.25, the intensity of the band around 1002 cm-1, which can be assigned to the C-H + vibrations of the [PPh4] cation, increases with an increasing molar fraction of [PPh4][NTf2].

It is also noteworthy that the bands are not all centered at the same frequency. The 25 mol% mixture -1 [PPh4][NTf2]-Cs[NTf2] has the highest frequency of 742.5 cm , the eutectic mixture (32 mol% -1 [PPh4][NTf2]-Cs[NTf2]) the lowest with 740.5 cm . This might be due to the formation of the eutectic, which is more stable than the other binary salt mixtures. The same trend can be observed for the bands around 1000 cm-1. In comparison to that, the pure salts have their bands centered at higher frequencies than the eutectic, but lower than the other mixtures. As reported by Umebayashi et al.80,81 the bound - -1 [NTf2] anion is expected to give a band at around 750 cm . All observed bands are, however, not higher in frequency than 743 cm-1 indicating that this effect of coordination is not so distinct in solid state.

a b 75 mol%

Ramanscatteringa.u. / 50 mol% c 32 mol% d 25 mol% 1060 1040 1020 1000 980 960 780 760 740 720 700 Wavenumber / cm-1

Figure 4.25: Detail of the experimental Raman spectra of a) the 75 mol% [PPh4][NTf2] in Cs[NTf2] mixture, b) the 50 mol% [PPh4][NTf2] in Cs[NTf2] mixture, c) the 32 mol% [PPh4][NTf2] in Cs[NTf2] mixture, and d) the 25 mol% [PPh4][NTf2] in Cs[NTf2] mixture. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled and normalized to the vibration at ca. 740 cm-1. The complete spectrum can be found in the Appendix, section 6.1.2. 70 4. Results and Discussion

In the Appendix, section 6.2, a comparative spectrum of the IR measurements on different binary mixtures of Cs[NTf2] and [PPh4][NTf2] can be found. There are no new insights from these measurements, thus, they will not be discussed in more detail here.

Raman measurements were conducted especially on the eutectic mixture. In Figure 4.26 the spectrum of the eutectic is shown, compared to spectra of the neat components. The spectrum of the eutectic mixture resembles the sum of the experimental spectra of [PPh4][NTf2] and Cs[NTf2] almost perfectly. Typical, in ionic compounds, the constituent ions vibrate relatively independent of their surroundings. Due to the miscibility gap of eutectic systems, in the solid phase it is composed of mixed crystallites of the pure substances and therefore the eutectic spectrum looks like the sum of the simple salts.

a Cs[NTf2]

b [PPh4][NTf2]

c Ramanscatteringa.u. / sum of components

d eutectic mixture

3500 3000 1500 1000 500 0 Wavenumber / cm-1

Figure 4.26: The Raman spectra of a) experimental Cs[NTf2], b) experimental [PPh4][NTf2], c) the sum of the experimental spectra of Cs[NTf2] and [PPh4][NTf2], and d) the experimental spectrum of the eutectic mixture (32 mol% [PPh4][NTf2] in Cs[NTf2]). The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled.

4.1.3.2 Raman Spectroscopy at elevated Temperatures

4.1.3.2.1 Raman Spectroscopy of the eutectic Mixture at elevated Temperatures

The eutectic salt mixture was sealed in a pyrex ampoule with a pre-defined pressure of 0.35 mbar of H2 as reference gas and heated to 120 °C. Spectra of the liquid phase and of the gas phase were recorded and are shown in Figure 4.27. In the spectrum of the liquid phase almost no changes are visible compared to the solid phase spectrum recorded at room temperature. A slight broadening of the bands resulting in a minimal decline of the resolution is observed when going to 120 °C accompanied by liquefaction of the sample. In the vapor phase above the eutectic salt mixture the vibrational and rotational bands of H2 are visible (marked with asterisks). Also, a distinct signal of water arises at 3657 cm-1, which presumably was trapped in the crystallized salt and released at the elevated temperature. Due to the prolonged measurement time for gas samples, also a silica signal of the pyrex ampoule can be observed at around -1 490 cm . These two spectra recorded at 120 °C lead to two conclusions: 1.) There is no H2 dissolved in 71 4. Results and Discussion the salt melt and there is no reaction occurring and 2.) at this temperature no evaporation of the eutectic salt mixture is detectable.

* H2

* 2

H * a * * Silica gaseous; at 120 °C

b liquid; at 120 °C

Raman scattering / a.u. scattering Raman

c solid; at RT

4500 4000 3500 3000 1500 1000 500 0 Wavenumber / cm-1

Figure 4.27: Experimental Raman spectra of a) the gas phase above the eutectic mixture (32 mol% [PPh4][NTf2] in Cs[NTf2]) at 120 °C, b) the eutectic mixture in the liquid phase at 120 °C, and c) the eutectic mixture in the solid phase at room temperature. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer. Intensities are arbitrarily scaled.

The temperature was raised to 320 °C. The obtained gas phase spectrum is shown in Figure 4.28. Firstly, -1 the signal of the silica is even more pronounced and a signal of N2 at 2322 cm (this region was cut off in the previous diagram) is visible. It is unlikely that the N2 is inside the ampoule because of the preparation (first vacuum, then adding H2), but might arise due to scattering of N2 molecules of air in- between the sample inside the furnace and the Raman instrument. The strong band of water is shifted -1 -1 slightly to lower wavenumbers to 3647 cm (compared to 3657 cm ). Still there are the H2 bands observable (marked with asterisks) which means the reference gas is not consumed by any reaction, at least not completely. Comparing the spectrum recorded at 320 °C with the room temperature solid phase spectrum, no peaks can be found which can be assigned to evaporated salt molecules. On the other hand, new peaks are observable, especially between 640 to 1450 cm-1 and 2910 to 3330 cm-1. These arise due + to decomposition processes. A possible assignment of the peaks is shown in Table 4.6. The [PPh4] cation presumably decomposes to some non-volatile phosphine species and benzene, as the vibrational bands at 994 and 3075 cm-1 suggest. The benzene seems to undergo some secondary reactions with the -1 reference gas H2 and is hydrogenated to cyclohexane (1448 cm ) or cracked to smaller molecules like methane (2920 cm-1). The peaks at 1150, 1248 and 3338 cm-1 can be correlated to decomposition of the bistriflimide anion.

72 4. Results and Discussion

Table 4.6: Observed frequencies in the experimental Raman spectrum of the eutectic salt (32 mol% [PPh4][NTf2] in Cs[NTf2] and possible assignment of the bands to a substance with reference value.

Vibrational band Assignment Reference Reference [cm-1] value 256,257 4718 H2 Q(J=0→0) 4713 256-258 4160 H2 Q(J=1→1) 4156 256,257 4148 H2 Q(J=2→2) 4145 256,257 4130 H2 Q(J=3→3) 4127 257,258 4107 H2 Q(J=4→4) 4102 257,258 4079 H2 Q(J=5→5) 4074 257,259 4046 H2 Q(J=6→6) 4039 257,259 3946 H2 Q(J=7→7) 4000 260,261 3655 H2O 3655 257,262 3573 H2 O(J=3→1) 3568 263 3338 NH3 3337 3075 Benzene 3073 263 263-265 2920 CH4 2918 262,263 2330 N2 2331 1448 Cyclohexane 1443 266 267 1248 COF2 1243 264 SO2 1150 1150 264 CHF3 1152 256,257 1036 H2 S(J=3→5) 1035 994 Benzene 991 263 256,257 816 H2 S(J=2→4) 814 256,257 588 H2 S(J=1→3) 588 465 Silica 256,257 354 H2 S(J=0→2) 355 Note: The vibrational transitions with no change in rotational quantum number (ΔJ = 0) are known as the Q branch. At higher 258 temperatures the H2 has more excited states populated. This was extensively studied in the literature (e.g. by Rahn or Sinclair et al.259), but goes beyond the scope of this investigations. The vibrational transitions with ΔJ = 2 are indicated as part of the S branch. The transition with ΔJ = -2 belongs to the O branch, only one is observed in this study. 73 4. Results and Discussion

* H2

* *

a * Silica 2 * * * N gaseous; at 320 °C

Raman scattering / a.u. scattering Raman

solid; at RT

4500 4000 3500 3000 2500 2000 1500 1000 500 0 Wavenumber / cm-1

Figure 4.28: Experimental Raman spectra of a) the gas phase above the eutectic mixture (32 mol% [PPh4][NTf2] in Cs[NTf2]) at 320 °C, and b) the eutectic mixture in in the solid phase at room temperature. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer. Intensities are arbitrarily scaled.

4.1.3.2.2 Raman Spectroscopy of Cs[NTf2] at elevated Temperatures

For comparison the neat Cs[NTf2] was also sealed in a pyrex ampoule with a 0.33 mbar of H2 as reference gas and heated. The spectrum of the liquid Cs[NTf2] at 130 °C looks almost identical to the spectrum of the solid phase, only with slightly broadened peaks (see Figure 6.5 in Appendix section 6.1.2). Again, there is no dissolution of H2 in the salt melt. The spectrum of the gas phase at 130 °C is shown in Figure 4.29 compared to the spectrum of the solid salt. It reveals next to the signals of the reference gas H2 some small features that might be assigned to evaporated Cs[NTf2]. The very prominent band at 744 cm-1 in the solid phase can also be detected in the gas phase. The strong solid phase band at 1241 cm-1 is found in the Raman spectrum of the gas phase at 1247 cm-1. The multiple bands at 557 cm-1 in the solid can be found slightly shifted at 498 cm-1 in the gas phase spectrum.

Other spectra of the gas phase above the Cs[NTf2] melt were recorded at 200 °C. The first spectrum was finished after 5 hours and the second after 18 hours. Both spectra are compared in Figure 6.6 (Appendix).

No signals are observed that can be assigned to evaporated Cs[NTf2] anymore, but some small peaks that originate from decomposition. But even over a prolonged period of exposure to heat the intensity of these signals do not increase compared to the intensity of the H2 signals. The thermal stability is remarkable even tough traces of decomposition can be found.

In an attempt to identify the decomposition products the temperature was raised further to 350 °C and the sample was kept at this temperature for 24 hours before recording a Raman spectrum (which also took additional 8 hours). The obtained spectrum of the gas phase is shown in Figure 4.30. Beside the H2 bands there are a lot of peaks visible. At 3655 cm-1 there is the signal of residual water from the sample. -1 The peak at 2330 cm corresponds to N2. It was also possible to assign most of the other peaks to - decomposition products of the [NTf2] anion. A possible decomposition mechanism by ultra-sound was 50 • published by Chatel et al. which proposes SO2, CF3 radicals and HF as major components. 74 4. Results and Discussion

* H2

* * a *

Cs[NTf2], gaseous 130 °C

Ramanscatteringa.u. /

b Cs[NTf2], solid RT 1400 1200 1000 800 600 400 200 0 Wavenumber / cm-1

Figure 4.29: Experimental Raman spectra of a) the gas phase above Cs[NTf2] at 130 °C, and b) Cs[NTf2] in the solid phase at room temperature. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer. Intensities are arbitrarily scaled.

* H2

* *

2 * *

H a N Cs[NTf2], gas phase 350 °C

Ramanscatteringa.u. /

b Cs[NTf2], gas phase 200 °C, 5 hours 4500 4000 3500 3000 2500 2000 1500 1000 500 0 Wavenumber / cm-1

Figure 4.30: Experimental Raman spectra of a) the gas phase above Cs[NTf2] at 350 °C after 24 hours, and b) the gas phase above Cs[NTf2] at 200 °C after 5 hours. The experimental spectra were measured with a green laser (532 nm, doubled Nd- -1 YVO4) in the dispersive DILOR-XY Raman spectrometer. Intensities are normalized to the H2 vibration at 4160 cm .

75 4. Results and Discussion

Table 4.7: Observed frequencies in the experimental Raman spectrum of Cs[NTf2] and possible assignment of the bands to a substance with reference value.

Vibrational band Reference Assignment Reference [cm-1] value 256-258 4160 H2 Q(J=1→1) 4161 256,257 4149 H2 Q(J=2→2) 4145 256,257 4130 H2 Q(J=3→3) 4127 257,258 4108 H2 Q(J=4→4) 4102 257,258 4079 H2 Q(J=5→5) 4073 257,259 4045 H2 Q(J=6→6) 4039 257,259 4005 H2 Q(J=7→7) 4000 257,262 3812 H2 O(J=2→0) 3807 260,261 3655 H2O Q 3655 257,262 3572 H2 O(J=3→1) 3568 268 3401 R-NH2 3391 263 3338 NH3 3337 H O(J=4→2) 3329 257 3325 2 HCN 3226 269 257 3095 H2 O(J=5→3) 3091 264 3040 CHF3 3036 264 2959 CH2F2 2949 263-265 2920 CH4 2918 262,263 2332 N2 2331 2143 CO 2143 263 2099 HCN 2098 269,270 264 1451 CH2F2 1435 263 CO2 1408 1413 271 C2F6 1417 # 272 SO3 1390 1391 263,271 CO2 1388 264 CF4* 1283 1290 263,271 CO2 1285 # 272 1272 SO3 1270 267 1249 COF2 1244 273 SO2 1150 1152 264 CHF3 1152 256,257 1038 H2 S(J=3→5) 1035 256,257 818 H2 S(J=2→4) 814 263 SiF4 800 800 # 272 SO3 804 267 764 COF2 767 # 272 702 SO3 698 256,257 590 H2 S(J=1→3) 588 256,257 359 H2 S(J=0→2) 355 # 272 340 SO3 337 -1 264 * Since some bands of CF4 are missing (e.g. 908 cm ) it was concluded that it is not formed during the decomposition. # The decomposition product SO3 is not so likely due to the presence of residual H2O in the sample. 76 4. Results and Discussion

Due to the added H2 some other possible reactions have to be taken into consideration in this case. Thus, vibrations of CH4, CHF3, CH2F2 and NH3 in addition to some successive secondary decomposition products like COF2, CF4 and SO3 were identified. But the latter two are doubtful since some bands of

CF4 are missing and SO3 is likely to react with the residual water. The assignment of the peaks can be found in Table 4.7. The non-volatile decomposition products are probably some CsF or Cs2O compounds, but were not investigated in this study.

The obtained spectroscopic results of the decomposition of Cs[NTf2] at elevated temperatures in the presence of H2 were verified by mass spectrometry. The XEMIS sorption micro balance is the perfect measurement setup for this. The sample of Cs[NTf2] was placed in the instrument and was heated to

350 °C under N2 gas flow. When the experiment temperature of 350 °C was reached the carrier gases were changed to 80 vol% N2 and 20 vol% H2 to mimic the low pressure of hydrogen in the closed ampoule of the Raman spectroscopy experiments. The quadrupole mass spectrometer was set to look for the m/z ratios of eight expected volatile decomposition products in the presence of H2 gas (m/z = 17 for NH3; m/z = 27 for HCN; m/z = 52 for CH2F2; m/z = 64 for SO2; m/z = 66 for COF2; m/z = 70 for

CHF3; m/z = 80 for SO3; m/z = 88 for CF4). In Figure 4.31 the normalized mass loss of the sample is plotted against the time.

During the heating process (2.6 K min-1) shown on the left hand side of the vertical line in Figure 4.31 the mass is quite constant. When the temperature reached 270 °C (around 130 min) the mass of the sample slowly decreases, but it drops more rapidly when the experiment temperature of 350 °C is reached (which is then kept constant) and the H2 flow starts. It is evident that the reducing atmosphere of hydrogen promotes the decomposition of Cs[NTf2].

1.021,02 10000

NH3 1.001,00 HCN 0.980,98 1000 0,960.96 SO2

0.940,94 COF2 CH F 100 0.920,92 2 2

0,900.90 CHF3

MS signal / Counts / MS signal 0.880,88 10

Normalized mass loss / - loss mass Normalized 0.860,86

Start H2 flow 0.840,84 heating 2.6 K min-1 Temperature 350 °C 1 125 150 175 200 225 250 275 Time / min

Figure 4.31: TG-MS spectrum of Cs[NTf2]; Normalized mass loss of the sample (black) in dependence of time; Amount of -1 detected NH3, HCN, COF2, CH2F2 and CHF3 in counts in dependence of experiment time. Heating rate was 2.6 K min under pure N2 flow, when 350 °C were reached the temperature was kept constant and the gases were changed to 80 vol% N2 and 20 vol% H2.

In Figure 4.31 the detected signals during the experiment are shown in dependence of time (the results are smoothed for better visualization). The most prominent decomposition product is NH3, which is already detected before the hydrogen addition but then it rises immediately. The signal might contain 77 4. Results and Discussion also some amount of other molecules (e.g. H2O with m/z = 18 and CH4 with m/z = 16) as a result of the 274 - cracking patterns . NH3 is most likely a result of decomposition of the [NTf2] anion and subsequently a reaction of a nitrogen fragment with the H2 gas rather than a direct formation of NH3 from the two carrier gases.

Also, the signals of HCN (m/z = 27) are observed before the desired experiment conditions are reached and show no distinct increase with H2. Probably the m/z = 27 signal is due to overlapping of nitrogen

(m/z = 28) which is the carrier gas. SO2 is a known decomposition product and is assumed to be independent of the reducing atmosphere. The signal of SO2 (m/z = 66) rises according to the increased decomposition of the sample. The following decomposition products COF2, CH2F2, and CH3F arise from successive secondary reactions of decomposition products with the added H2 gas. CF4 and SO3 were not detected in the mass spectrometry experiment. With H2O present in the sample the formation of SO3 is -1 unlikely. And already the Raman spectra suggested the band at 1290 cm does not arise from CF4, since other distinct bands for this molecule are missing.

4.1.4 Solubility of Transition Metal Compounds As already stated in the theoretical background, ionic liquids and molten salts may lead to improvements of chemical reactions and processes by replacing normal organic solvents. Here, some transition metal compounds with possible application as catalysts dissolved in the salt melts were investigated regarding their physico-chemical properties like thermal stability, the melting point of the mixtures and viscosity.

The transition metal compounds with the general formula M(NTf2)n (with M = Mn, Co, Ni, Cu, Zn and n = 2) (see also Figure 4.32) could be easily synthesized from an excess of the metal carbonates or metal hydroxides and the bistriflimide (HNTf2) in water. Filtration of the insoluble excess starting material is followed by removal of solvent and drying in vacuum. The compounds are air stable but hygroscopic and thus, were handled and stored under inert gas atmosphere.

Figure 4.32: Samples of metal bistriflimides M(NTf2)2; from left to right: Mn(NTf2)2; Co(NTf2)2; Ni(NTf2)2; Cu(NTf2)2; and Zn(NTf2)2.

4.1.4.1 Investigations on neat M(NTf2)2

- The two-valent first-row transition metal compounds with [NTf2] counter ions have been studied by

Raman spectroscopy. In Figure 4.33 the spectra of five different M(NTf2)2 compounds are shown. Due to the color of the samples the measurements of Raman spectra were complicated. All samples except the Cu(NTf2)2 showed strong fluorescence (which was subtracted for better visualization). The

Ni(NTf2)2 spectrum was acquired using a blue laser and adding up 50 spectra each obtained in 10 s. The

Mn(NTf2)2 and Co(NTf2)2 showed signs of decomposition due to the heat of absorbed green laser light.

This effect was circumvented by measuring the sample inside a H2 atmosphere. The small hydrogen 78 4. Results and Discussion molecules have a much better thermal conductivity than argon. Reduction of the samples could be excluded by comparing spectra obtained in the different atmospheres.

- Beside the problems all four compounds show similar features; for example the coordinated [NTf2] -1 vibration at around 750 cm . All four M(NTf2)2 compounds show three features between 1000 and -1 1400 cm , only the resolution is different. For Mn(NTf2)2 and Ni(NTf2)2 a single band is observed at 1144 and 1131 cm-1, respectively, corresponding to the symmetric in-phase stretching vibration of the

SO2 group. The band arising from the symmetric stretching of the trifluoromethyl moiety around 1240 cm-1 shows a shoulder at the lower frequencies for the asymmetric vibration. This is also observed for Cu(NTf2)2. This band of the Co(NTf2)2 is slightly broader and the intensities of the band and the shoulder are reversed. In the lower frequency area all five compounds show similar features, only the intensities are slightly different. From this investigation it can be concluded that all compounds crystallize in similar ways.

For the Ni(NTf2)2 and the Mn(NTf2)2 also the presence of some co-crystallized water is observed at -1 around 3520 cm . In the case of Mn(NTf2)2 this must be due to insufficient drying during the preparation, but the Ni(NTf2)2 had to be measured under air because of the measurement setup, thus, the presence of water is explained by the highly hygroscopic nature of the compound which took up water from ambient air during the measurement.

2 a H

Mn(NTf2)2 b

Co(NTf2)2 c Ni(NTf2)2

Raman scattering / a.u. scattering Raman d Cu(NTf2)2 e Zn(NTf2)2 4000 3800 3600 3400 1400 1200 1000 800 600 400 200 0 Wavenumber cm-1

Figure 4.33: Experimental Raman spectra of a) Mn(NTf2)2 measured with green laser (532 nm) in H2 atmosphere, b) Co(NTf2)2 measured with green laser in H2 atmosphere, c) Ni(NTf2)2 measured with blue laser (488 nm), d) Cu(NTf2)2 measured with green laser, and e) Zn(NTf2)2 measured with green laser. Intensities are arbitrarily scaled.

These five transition metal bistriflimides were also investigated with X-ray diffractometry (XRD). In Figure 4.34 the obtained diffractograms are shown. The XRD analysis is another powerful tool to investigate metal salts. The measured diffractograms (a-e) are compared with the calculated spectrum 157 (f) of Zn(NTf2)2 from the crystal structure . The samples were polycrystalline powders which were not truly homogeneous. All the measured diffractograms show peaks at very similar 2θ angles, however, in different intensities. On the other hand, the calculated diffractogram has several features which do not 79 4. Results and Discussion match to the measured data. This is an indicator that the samples were not in the typical six-fold coordinated octahedral crystal packing with two bidentate bistriflimide ligands and two bridging bistriflimide ligands. This might be due to the measurement conditions, since the XRD was measured exposed to air and the very hygroscopic samples might have drawn moisture during the experiment.

a Mn(NTf2)2

b Co(NTf2)2

c Ni(NTf2)2

Cu(NTf ) Intensity / a.u. / Intensity d 2 2

e Zn(NTf2)2

Zn(NTf ) calculated f 2 2

0 10 20 30 40 50 2/ °

Figure 4.34: XRD diffractorgams of a) Mn(NTf2)2, b) Co(NTf2)2, c) Ni(NTf2)2, d) Cu(NTf2)2, e) Zn(NTf2)2, and f) Zn(NTf2)2 as calculated from the crystal structure from literature157.

a Co(NTf2)2

Co(H2O)6NTf2 x H2O

Intensity / a.u. / Intensity b

Zn(NTf2)2 c

0 10 20 30 40 50 2 / °

Figure 4.35: XRD diffractorgams of a) Co(NTf2)2 as measured, b) Co(H2O)6NTf2 x H2O as calculated from the crystal 275 157 structure in the literature , and c) Zn(NTf2)2 as calculated from the crystal structure in literatur . 80 4. Results and Discussion

However, as depicted in Figure 4.35 comparing the structure of a hexaaqua-cobald(II)bis- (bistrifluoromethylsulfonyl)imide275 to the measured data and the one calculated from the crystal structure of Zn(NTf2)2 (assuming from the previous results the crystal structure of Co(NTf2)2 would be very similar) it is obvious that the samples here were not hydrated completely. A probable coordination would be two axial water ligands and two bidentate bistriflimide ligands, which still has to be proven.

4.1.4.2 Physico-chemical Properties of the Mixtures The “common ion effect” was already reported in the literature stating that the dissolution of compounds bearing the same anion as the ionic liquid solvent is increased yielding highly concentrated solutions.79

As a matter of fact, the transition metal compounds M(NTf2)n were highly soluble in the molten salt

Cs[NTf2] and even better in [PPh4][NTf2]. The maximal obtained mole fraction of Co(NTf2)2 or

Ni(NTf2)2 in Cs[NTf2] was 38.7 mol% and in [PPh4][NTf2] even over equimolar amounts up to 55.6 mol%.

Dissolving Co(NTf2)2 in [PPh4][NTf2] at 150 °C liberates water from the co-coordination sphere which then evaporates form the mixtures. This is evident from the change in color (for Co(NTf2)2 from pink in pure form to violet in the mixture) and confirmed by analysis of the water content by Karl-Fischer coulometry. Also, the melting point of the mixture is drastically lowered. The obtained phase diagram 83 is shown in Figure 4.36. Pure Co(NTf2)2 does not melt, the reported melting point of 128 °C is rather a phase transition. This hypothesis is backed because they do not report the heat of fusion of the found peaks. But due to the lack of a liquid phase of Co(NTf2)2 the right part of the phase diagram does not exist. Also, above a mole fraction of 55.6 mol% there is no homogeneous mixture, as not all Co(NTf2)2 could be dissolved in the melt then.

140 DSC 130 optical

120

110 liquid 100 liq. + [PPh4][NTf2] 90

80 no homogenoeus Melting point / °C / point Melting 70 mixture

50 [PPh4][NTf2] + Co(NTf2)2

0,00.0 0,10.1 0,20.2 0.30,3 0,40.4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 [PPh4][NTf2] Co(NTf2)2  (Co(NTf2)2)

Figure 4.36: Phase diagram of [PPh4][NTf2] and Co(NTf2)2 as determined by DSC measurements, with heating and cooling rates of 2 K min-1. The transition temperatures were extracted from the heating process to avoid the uncertainty of supercooling.

For low concentrated mixtures (x < 0.3) the melting point decreases moderately. It drops abruptly to

71 °C at a composition of 33.3 mol% Co(NTf2)2. The melting points obtained by optical measurements fit excellently to the ones concluded from DSC. Appling Tammann’s rule242 to determine the eutectic 81 4. Results and Discussion composition yielded a mole fraction of Co(NTf2)2 in [PPh4][NTf2] of 0.45 which is situated between two data points and in very good agreement with the measurements. The graphical representation of the heat of fusion plotted against the mole fraction can be found in the Appendix, section 6.8.

The dissolution of Co(NTf2)2 in [PPh4][NTf2] enhances the thermal stability of the M(NTf2)2 compound

(see Figure 4.37). Pure Co(NTf2)2 shows a decomposition onset of 343 °C. The mixtures decompose stepwise. The decomposition onsets are also listed in Table 4.8. The higher Tonset 1 of the mixtures suggest the formation of an intermediate compound of [PPh4][Co(NTf2)3] which is slightly more stable than the pure Co(NTf2)2. The Tonset 2 matches very well with the decomposition temperature of

[PPh4][NTf2]. The hypothesis of the intermediate compound will be addressed again later in chapter 4.1.4.3.

Table 4.8: Decomposition onsets of Co(NTf2)2, [PPh4][NTf2] and two mixtures as determined from the TG experiments (see also Figure 4.37), heating rate 5 K min-1.

Compound Tonset 1 / °C Tonset 2 / °C

Co(NTf2)2 343 n.a.*

x = 0,167 366 451

x = 0,5 370 439

[PPh4][NTf2] n.a.* 451 * The pure compounds show only one decomposition step.

heating rate 5 K min-1 1.01,0

0,80.8

0.60,6

0.40,4

[PPh4][NTf2] Normalized mass loss / - loss mass Normalized 0.20,2 (Co(NTf2)2) = 0.167 (Co(NTf2)2) = 0.5 Co(NTf2)2 0.00,0 150 200 250 300 350 400 450 500 550 Temperature / °C

-1 Figure 4.37: Thermogravimetrical analysis of [PPh4][NTf2] and Co(NTf2)2 and two binary mixtures; heating rate 5 K min , drying at 120 °C for 1 hour . Picture: 1 : 1 mixture of Co(NTf2)2 and [PPh4][NTf2] (X = 0.5) at 200 °C.

The viscosity of mixtures of Co(NTf2)2 with [PPh4][NTf2] were measured at different temperatures. This is shown Figure 4.38. Above 140 °C the viscosities stay almost unchanged with further increases in temperatures. Especially for the higher concentrated solutions the viscosities are very high at lower 82 4. Results and Discussion temperatures, the mixture with a composition of 42.9 mol% Co(NTf2)2 in [PPh4][NTf2] has a viscosity of 281 mPa s at 113 °C . The values derived from the Volgel-Fulcher-Tamman analysis are shown in Table 6.2 in the Appendix, section 6.3.

300 [PPh4][NTf2] Co(NTf ) -[PPh ][NTf ]  = 0.091 250 2 2 4 2 Co(NTf2)2-[PPh4][NTf2]  = 0.167 Co(NTf ) -[PPh ][NTf ]  = 0.429 200 2 2 4 2

/ mPa s / mPa  150

100

Viscosity

0 100 120 140 160 180 200 220 240 260 Temperature / °C

Figure 4.38: Viscosity of [PPh4][NTf2] and different mixtures with Co(NTf2)2 as a function of the temperature.

The same procedures were conducted on mixtures of Ni(NTf2)2 and [PPh4][NTf2]. Again, a loss of the co-crystalized water was observed by mixing the compounds at 150 °C. The color changed from greenish-yellow for the pure substance to yellow for the mixture and the water content analysis confirmed the mixtures are water-free. Also, pure Ni(NTf2)2 does not melt, (the reported melting point at 113 °C83 is more likely a phase transition); thus, the right part of the phase diagram is not defined (see Figure 4.39). The melting points determined by DSC and an optical measuring method match very well. The melting point drops to around 64 °C at a composition of 33.3 mol%. The eutectic composition as acquired by Tammann’s rule is 0.32 (see Appendix, section 6.7), but the lines of best fit have quite small coefficients of determination, thus the value is rather vague.

In Figure 4.40 the normalized mass loss of Ni(NTf2)2 and some mixtures with [PPh4][NTf2] are shown as determined from TGA experiments. The decomposition onset temperatures are summarized in Table

4.9. Whereas the thermal stability of Co(NTf2)2 is enhanced by the dissolution in [PPh4][NTf2] the

1:1 mixture of Ni(NTf2)2 in the salt melt has a slightly lower decomposition onset. In contrast, the

1:5 mixture shows the higher thermal stability as expected from the Co(NTf2)2-[PPh4][NTf2] experiments. Again, the stepwise decomposition is observed, suggesting the formation of an intermediate compound [PPh4][Ni(NTf2)3]. 83 4. Results and Discussion

140 DSC 130 optical

120

110 liquid 100 liq. + [PPh4][NTf2] 90

80 no homogenoeus Melting point / °C / point Melting 70 mixture

50 [PPh4][NTf2] + Ni(NTf2)2

0,00.0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 Ni(NTf ) [PPh4][NTf2] 2 2  (Ni(NTf2)2)

Figure 4.39: Phase diagram of [PPh4][NTf2] and Ni(NTf2)2 as determined by DSC measurements, with heating and cooling rates of 2 K min-1. The transition temperatures were extracted from the heating process to avoid the uncertainty of supercooling.

Table 4.9: Decomposition onsets of Ni(NTf2)2, [PPh4][NTf2] and two mixtures as determined from the TG experiments (see also Figure 4.40), heating rate was 5 K min-1.

Compound Tonset 1 / °C Tonset 2 / °C

Ni(NTf2)2 368 n.a.*

x = 0,167 n.a.# 415

x = 0,5 355 440

[PPh4][NTf2] n.a.* 451 * The pure compounds show only one decomposition step. # The first decomposition step could not be determined by applying the tangents.

In Figure 4.41 the viscosity of [PPh4][NTf2] and some binary mixtures with Ni(NTf2)2 are shown as a function of temperature. For temperatures above 130 °C the viscosity does not show significant changes when the sample is further heated, and is very similar for all mixtures. Only the highest concentrated sample is highly viscous at lower temperatures. In Table 6.2 in the Appendix, section 6.3 the fitting parameters from Vogel-Fulcher-Tamman equation are shown. 84 4. Results and Discussion

-1 1.01,0 heating rate 5 K min

0.80,8

0.60,6

0.40,4

[PPh4][NTf2]

Normalized mass loss / - loss mass Normalized (Ni(NTf ) ) = 0.167 0.20,2 2 2 (Ni(NTf2)2) = 0.5 Ni(NTf2)2 0.00,0 150 200 250 300 350 400 450 500 550 Temperature / °C

-1 Figure 4.40: Thermogravimetrical analysis of [PPh4][NTf2] and Ni(NTf2)2 and two binary mixtures; heating rate 5 K min , drying at 120 °C for 1 hour . Picture: 1 : 1 mixture of Ni(NTf2)2 and [PPh4][NTf2] (X = 0.5) at 200 °C.

300 [PPh4][NTf2]

250 Ni(NTf2)2-[PPh4][NTf2]  = 0.091 Ni(NTf2)2-[PPh4][NTf2]  = 0.167 Ni(NTf ) -[PPh ][NTf ]  = 0.429 200 2 2 4 2

/ mPa s / mPa  150

100

Viscosity

0 100 120 140 160 180 200 220 240 260 Temperature / °C

Figure 4.41: Viscosity of [PPh4][NTf2] and different mixtures with Ni(NTf2)2 as a function of the temperature.

4.1.4.3 Investigation of the Solvation Structure of M(NTf2)2 in [PPh4][NTf2]

Both Co(NTf2)2 and Ni(NTf2)2 were dissolved in the [PPh4][NTf2] molten salt. As discussed before, changes in color suggest the removal of coordinated water from the transition metal center upon solvation in the hot melt (for Co(NTf2)2 from pink of the pure substance to deep violet for the mixture, for Ni(NTf2)2 from pale green to yellow). And indeed, the Raman spectra of different concentrated 85 4. Results and Discussion

-1 mixtures at 120 °C of M(NTf2)2 and [PPh4][NTf2] show no water bands at 3655 cm (Appendix section 6.1.2, Figrue 6.7 and Figure 6.8, respectively). In Figure 4.42 and Figure 4.43 details of these spectra (normalized to the vibration at 1000 cm-1) are shown. The previously reported shoulder80,81 at 750 cm-1 - as an indicator for coordinated [NTf2] anions could not be observed in this study. This is, of course, - evident for the 50 mol% mixture because all [NTf2] anions are coordinated to the metal center, but for the lower concentrations this feature was expected. It seems that the previous observed broadening of the bands at elevated temperature (120 °C) conceals this shoulder. But another indication for the - coordination of [NTf2] to the metal center is the shift of frequency of the prominent band of the bistriflimide. The spectrum of the pure molten salt [PPh4][NTf2] is shown in the bottom part of the diagrams (0 mol%). The band is observed at 741 cm-1, upon solvation of 16.7 mol% transition metal compound in the melt the band is shifted to 742 cm-1 for both compounds. The 50 mol% mixtures show the band at 747 cm-1 for the Co(II) compound and 749 cm-1 for the Ni(II).

Co(NTf2)2 in [PPh4][NTf2]

a 100 mol% b 50 mol% c 25 mol%

Raman scattering / a.u. scattering Raman d 16.7 mol% e 0 mol%

1050 1020 990 960 780 750 720 Wavenumber cm-1

Figure 4.42: Detail of the experimental Raman spectra of a) pure Co(NTf2)2, b) the 50 mol% Co(NTf2)2 in [PPh4][NTf2] mixture, c) the 25 mol% Co(NTf2)2 in [PPh4][NTf2] mixture, d) the 16.7 mol% Co(NTf2)2 in [PPh4][NTf2] mixture, and e) pure [PPh4][NTf2]. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature (pure substances) or at 120 °C (mixtures). Intensities are arbitrarily scaled and normalized to the band at ca. 1000 cm-1. 86 4. Results and Discussion

Ni(NTf2)2 in [PPh4][NTf2]

a 100 mol% b 50 mol% c 25 mol%

Raman scattering / a.u. scattering Raman d 16.7 mol% e 0 mol%

1050 1020 990 960 780 750 720 Wavenumber cm-1

Figure 4.43: Detail of the experimental Raman spectra of a) pure Ni(NTf2)2, b) the 50 mol% Ni(NTf2)2 in [PPh4][NTf2] mixture, c) the 25 mol% Ni(NTf2)2 in [PPh4][NTf2] mixture, d) the 16.7 mol% Ni(NTf2)2 in [PPh4][NTf2] mixture, and e) pure [PPh4][NTf2]. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature (pure substances) or at 120 °C (mixtures). The spectrum of -1 pure Ni(NTf2)2 was measured with the blue laser. Intensities are arbitrarily scaled and normalized to the band at 1000 cm .

Plotting the shift of the band against the concentration of transition metal compound cM gives linear dependences for the M(NTf2)2-[PPh4][NTf2] mixtures as shown in Figure 4.44 for the Co(NTf2)2 mixture and in Figure 4.45 for the Ni(NTf2)2 mixture. The obvious shift in the frequency of the band speaks for octahedral coordinated metal centers. The method for calculating the solvation number according to Umebayashi et al.80,81, is not possible, but due to similarity of the investigated systems here and in the - literature the [M(NTf2)3] complex is very likely.

Another non-quantitative way to investigate the solvation of M(NTf2)2 compounds in molten salts is IR spectroscopy. In Figure 4.46 the spectra (the intensities normalized to the band at around 1194 cm-1) of -1 Co(NTf2)2, [PPh4][NTf2] and two binary mixtures are shown. The bands at 1482 and 1439 cm + -1 correspond to C-C stretching vibrations of the [PPh4] cation. The IR absorbance at 1325 cm in the spectrum of Co(NTf2)2 can be assigned to the asymmetric out-of-plane stretching vibration of the -1 sulfonyl moiety. It is shifted to 1357 cm for the free anion in [PPh4][NTf2]. The very strong asymmetric -1 stretching vibration of coordinated SO2 for the Co(NTf2)2 at 1126 cm decreases in intensity and shifts -1 - to 1134 cm for the unbound [NTf2] anion. The often reported characteristic bands for coordinated bistriflimide81,276 between 680 and 800 cm-1 are not visible in this study. The high frequency mode of -1 68 the S-N-S vibration at 1055 cm for Co(NTf2)2 describes the partial double-bond character of this moiety. For the binary mixtures and the pure molten salt this band shows almost identical wavenumbers which can be understood as sign that the double-bond character is not exclusively a feature of a coordinated bistriflimide. 87 4. Results and Discussion

747

746

-1 745

 744

743

742

Wavenumber Wavenumber

741 measurement Co(NTf2)2-[PPh4][NTf2] linear fit R2 = 0.999 740 0.00000,0000 0.00050,0005 0.00100,0010 0.00150,0015 0,00200.0020 -1 Concentration Co(NTf2)2 / mol L

Figure 4.44: Plot of the measured Raman frequency vs. the concentration of Co(NTf2)2 in [PPh4][NTf2] with linear fit.

750 measurement Ni(NTf2)2-[PPh4][NTf2] 749 linear fit R2 = 0.978 748

-1 747

 746

745

744

743

Wavenumber Wavenumber 742

741

740 0.00000,0000 0.00050,0005 0,00100.0010 0,00150.0015 0,00200.0020 -1 Concentration Ni(NTf2)2 / mol L

Figure 4.45: Plot of the measured Raman frequency vs. the concentration of Ni(NTf2)2 in [PPh4][NTf2] with linear fit. 88 4. Results and Discussion

Co(NTf2)2

b 50 mol%

c 33.3 mol%

Transmission / a.u. Transmission d

[PPh4][NTf2]

1700 1600 1500 1400 1300 1200 1100 1000 900 800 Wavenumber / cm-1

Figure 4.46: Experimental IR spectra of a) Co(NTf2)2, b) 50 mol% of Co(NTf2)2 in [PPh4][NTf2], c) 33.3 mol% of Co(NTf2)2 in [PPh4][NTf2], and d) [PPh4][NTf2].

- To undermine the theory of the [Co(NTf2)3] solvation structure, additional mass spectrometric investigations were performed. The 1:1 mixture of Co(NTf2)2 and [PPh4][NTf2] was ionized with a He plasma and measured with Direct Analysis in Real Time Mass Spectrometry (DART-MS). This method ionizes the samples at ambient pressure in air by the reaction of electronic or vibronic excited-state species with the analytes, this means complex competing reactions are responsible for the formation of molecular ions and fragments. Thus, the obtained DART spectra are slightly different than for other MS ionization techniques. The first step of the ionization is the Penning ionization, where molecular ions are formed. Also, atmospheric moisture is ionized very efficiently by the Helium plasma and undergoes subsequent reactions with other ions. Depending on the proton affinity of the molecular ion protonation +• can occur. Also, O2 ions are formed in the DART ion source and are responsible for odd-electron species, although the mechanism is not fully understood yet. Water clusters can also be formed by protonation or hydride abstraction depending on the analyte ions.277,278

- The negative ion mode showed the expected m/z ratio of 898.98 for the [Co(NTf2)3] anion; also some - - fragmentation to 280.04 for (NTf2) and 148.07 for H(NTf) . The positive ion mode yielded as most + prominent peak the m/z of 339.21 for the (PPh4) cation. The fragmentation gave according to the 277,278 + + + ionization method [Co(H2O)4] at m/z= 130.19, (NTf2) at m/z = 279.16 and [Co(NTf2)2OH] at m/z= 635.08. The mass spectra are shown in the Appendix, section 6.6.

Combining literature reports79,81,83, the Raman spectroscopic results, and the mass spectrometry it can be definitely concluded that the Co(II) metal center is six-fold coordinated in an octahedral geometry - forming a [Co(NTf2)3] anion. 89 4. Results and Discussion 4.2 Melts as Solvent for Catalysis

In this second chapter the use of the previously investigated molten salts as solvents for homogeneous catalysis are discussed. Two types of reaction are selected; firstly, the Friedel-Crafts acylation as representative of C-C bond forming reactions, and secondly, the hydrogenation/dehydrogenation of N- heterocyclic aromatic compounds with possible application in the hydrogen storage context.

4.2.1 Friedel-Crafts Acylation The Friedel-Crafts acylation is an important reaction to build C-C bonds and form aromatic ketons.

Traditionally, it is performed with Lewis acids like AlCl3. This has the major drawback of producing extensive amounts of waste because the Lewis acid has to be added in stoichiometric amounts and has to be hydrolyzed in order to separate the reaction products. Inspired by the works of Earle et al.157 the

Friedel-Crafts acylation catalyzed with M(NTf2)2 catalysts was investigated with ionic liquid as solvent.

For this work the acylation of toluene with benzoylchloride (BzOCl) was studied (Scheme 4.1). Except of the molten salt no additional solvent was used. The reaction temperature is limited by toluene as reagent, thus the mixture could not be heated above 110 °C, resulting in [PMeBu3][NTf2] as suitable molten salt solvent, because it is already liquid at room temperature. The reactions were performed under argon inert gas atmosphere in closed test tubes heated with an oil bath.

Scheme 4.1: Friedel-Crafts acylation as investigated.

In Figure 4.47 the conversion of benzoyl chloride is shown as a function of time catalyzed by Co(NTf2)2. Also, the yields of the two major products 4-methylbenzophenon (4-MBP) and 2-methylbenzophenon (2-MBP) are plotted versus the reaction time. In addition, the selectivity to 4-MBP is shown. The third possible acylation product, 3-methylbenzophenon, was not formed due to the ortho and para directing character of the methyl group. As clearly visible in the diagram the 4-MBP is the major product, since the reaction to 2-MBP is sterically more hindered. Next to the two expected products, side products are observed; one is benzoic acid, which is formed by the reaction on benzoyl chloride with water. Despite intensive preparation efforts of the reaction system (i.e. working in inert gas atmosphere) and drying of the catalyst, the formation of benzoic acid (average yield 3 %) leads to the assumption that there are still trace amounts of water co-crystallized in the catalyst.

The reproducibility of the experiments is very good, the uncertainty of measurements is between 3 and 5 %. The reaction without molten salt shows a lower conversion (compare Table 4.10). The molten salt seems to have a stabilizing and activating effect on the catalyst. Also, the yield of 4-MBP of the reaction in molten salt is with 23.86 % slightly higher than that of the reaction without molten salt (19.91 %).

90 4. Results and Discussion

110 S (4-MBP) 110 100 X (BzOCl) 100 Y (4-MBP) 90 90 Y (2-MBP) 80 Y (side products) 80 70 70 60 60 50 50

Yield / % Yield / 40 40

Conversion / % Conversion 30 30

20 20 % / Selectivity (4-MBP) 10 10 0 0

0 200 400 600 800 1000 1200 1400 Time / min

Figure 4.47: Conversion of BzOCl and Yield to 4-MBP or 2-MBP or the side products and selectivity to 4-MBP respectively versus time. Catalyst was Co(NTf2)2 (0.114 mmol), solvent was [PMeBu3][NTf2] (0.5 mmol), the reaction proceeded at 110 °C and 1 bar Argon. The lines are just a guidance of the eye and have no scientific significance.

Table 4.10: Conversion, yield and selectivity to 4-MBP of the Friedel-Crafts acylation of toluene (2.65 mmol) with BzOCl (2.36 mmol) catalyzed by Co(NTf2)2 (0.114 mmol) at 110 °C and 1 bar Ar compared for the reaction in molten salt [PMeBu3][NTf2] (0.5 mmol) and solventless.

Reaction catalyzed by Conversion / % Yield (4-MBP) / % Selectivity / % Co(NTf2)2 after 5 hours

in [PMeBu3][NTf2] 38.25 ± 2.78 23.86 ± 2.25 62.38 ± 5.28

without molten salt 32.17 ± 3.48 19.91 ± 2.66 61.89 ± 4.19

4.2.1.1 Comparison with other M(NTf2)2 Catalysts

The previously discussed transition metal bistriflimide compounds M(NTf2)2 (with M = Mn, Co, Ni, Cu, Zn) were also investigated as catalysts for the Friedel-Crafts acylation of toluene and benzoyl chloride. The results after 5 hours of reaction at 110 °C are shown in Figure 4.48. All five compounds show similar conversions (between 27 and 38 %) and also similar yields of 4-MBP (between 19 and 27 %).

The Co(NTf2)2 showed the highest conversion (38.3 %). Ni(NTf2)2 and Cu(NTf2)2 had similar conversions, 33.8 % and 33.9 %, respectively, but this falls in the uncertainty of the measurement

(± 3 %). The conversion with the Mn(NTf2)2 catalyst was 30.7 %. All catalyst except Zn(NTf2)2 show around 3 % yield of 2-MBP. The excellent selectivity of the Zn(NTf2)2 catalyzed reaction was achieved by a water-free catalyst which suppresses the side-reaction to benzoic acid. What the differences in catalyst performance caused still has to be investigated. Probably, the Lewis acidity of the M(NTf2)2 compounds has an influence on its catalytic activity, but this is not simple to determine because the 31P- NMR method of Beckett with triethylphosphine oxide as probe molecule279 cannot be applied due to the paramagnetic metal centers. Also, this was also not a goal of the thesis. 91 4. Results and Discussion

110 Y (4-MBP) 110 100 X (BzOCl) 100 90 S (4-MBP) 90

/ % 80 80

5 h

/ %

5 h 70 70 60 60 50 50 40 40

Yield (4-MBP) 30 30

Selectivity (4-MBP) / % / Selectivity (4-MBP)

Conversion (BzOCl) Conversion 20 20 10 10 0 0 Co(NTf2)2 Ni(NTf2)2 Mn(NTf2)2 Cu(NTf2)2 Zn(NTf2)2

Figure 4.48: Comparison of different catalyst for the Friedel-Crafts acylation of toluene (2.65 mmol) with BzOCl (2.36 mmol); the catalyst amount was 0.114 mmol and the solvent was [PMeBu3][NTf2]. Reaction proceeded for 5 hours at 110 °C and 1 bar Argon.

The here found conversions and yields are slightly lower compared to the ones reported in the literature 157 by Earle et al. For solventless reactions with catalysts with the formula M(NTf2)2 with M = Co, Ni, Mn, Cu and Zn in 1 mol% concentration they found full conversion and 4-MBP as the only product after

3–5 hours for the compounds with cobalt, nickel and manganese, after 72 hours for Cu(NTf2)2 and after

48 hours for Zn(NTf2)2. When performing the Friedel-Crafts acylation in [EMIM][NTf2] with Co(NTf2)2 and Ni(NTf2)2 the reaction time was reduced to 0.5–1 hour. What the strong deviations for Co(NTf2)2,

Ni(NTf2)2 and Mn(NTf2)2 between the reported and the here found results caused is very speculative. One reason could be the previously mentioned incomplete drying of the catalysts. Also, Earle et al. worked with a higher excess of toluene, namely 1.5 eq. compared to BzOCl. On the other hand, even though the results of Earle et al. were extremely promising, since 2004 no other reports of this system were published.

4.2.1.2 Recycling and Catalyst Decomposition In order to recycle the catalyst the reagents and products could be extracted completely by n-heptane. The remaining catalyst-molten salt phase was subsequently reused in a second reaction. The recycled catalyst gave poor yields and conversion (below 5 %) after 5 hours (see Figure 4.50). Since no leaching could be detected by ICP analysis the loss of activity could be a result of catalyst decomposition.

During the reaction HCl is formed as by-product. It is not difficult to imagine that this could react with the catalyst to yield the volatile bistriflimide acid and cobalt chloride (Scheme 4.2). The color change of the Co(NTf2)2 catalyst during the reaction from pink to blue is an indicator for this hypothesis. To test this, after the reaction (without molten salt solvent) the catalyst was separated, washed with toluene and dried. The remaining powder was investigated with X-Ray Diffractometry. 92 4. Results and Discussion

Scheme 4.2: Reaction of Co(NTf2)2 catalyst with the by-product HCl.

The obtained XRD diffractogram is shown in Figure 4.49. The expected reflexes of CoCl2 x 2 H2O are shown in the lower part of the diffractogram as comparison, confirming the hypothesis of the catalyst decomposition. There are numerous other reflexes visible in the Co(NTf2)2 residue which were not assigned. They may arise from insufficient washing of the sample. There are no reflexes of pure

Co(NTf2)2 (compare with section 4.1.4.1).

Co(NTf2)2 residue a

Intensity / a.u. / Intensity

CoCl2 x 2 H2O b 0 10 20 30 40 50 60 70 80 2

Figure 4.49: XRD diffractrogram of a) the Co(NTf2)2 residue as separated from the reaction mixture compared to b) calculated CoCl2 x 2 H2O.

To further reinforce this hypothesis several catalytic experiments were performed with the expected decomposition products of the catalyst, namely CoCl2, HNTf2 and an in situ formed catalyst complex of

CoCl2 and HNTf2. The obtained conversions are shown in Figure 4.50. The anhydrous CoCl2 yielded no

4-MBP at all. On the other hand, the HNTf2 showed similar results to the Co(NTf2)2 catalyst. Using pure

HNTf2 as catalyst led to a conversion of 33.8 %, and a yield of 4-MBP of 24.2 %. The in situ mixture of CoCl2 and HNTf2 gave a conversion of 33.4 %, with 26.9 % yield to 4-MBP. This supports the assumption that the catalytic activity is due to the very acidic HNTf2 and not due to the transition metal.

This problem of catalyst stability is not discussed in the paper by Earle et al.157, but they report very high yields for the first run with recycled catalysts and metal chloride catalyst precursors. When the yields dropped after the second run with recycled catalyst, they could restore the activity with the addition of HNTf2. It is possible that the different used IL solvents cause a difference in activity. Also, the CoCl2 partly precipitates which lowers the overall amount of Co in the IL solution. A similar phenomenon was reported by Ross et al.20 for Cu catalyst for the Friedel-Crafts acylation.

The highly acidic and volatile HNTf2 cannot replace the conventional catalysts like AlCl3 or FeCl3 in technical applications, since the problems of corrosion cannot be circumvented and also there a several 93 4. Results and Discussion health concerns associated with HNTf2. Due to their lack of chemical stability M(NTf2)2 compounds are no suitable catalysts for the Friedel-Crafts acylation and the search for more efficient and environmental friendly alternatives continues. On the other hand, it has been demonstrated in general that molten salts and ionic liquids combined with the right catalyst are promising solvents for organic reactions.

Y (4-MBP) 100 100 X (BzOCl) 90 S (4-MBP) 90

/ % 80 80

5 h / % 70 70

5 h 60 60

50 50

40 40

Yield (4-MBP) 30 30

Selectivity (4-MBP) / % / Selectivity (4-MBP)

Conversion (BzOCl) Conversion 20 20

10 10

0 0 Co(NTf2)2 Recycling I CoCl2 HNTf2 CoCl2 + HNTf2

Figure 4.50: Conversion of BzOCl, yield and selectivity to 4-MBP for different catalysts. The reaction proceeded at 110 °C under 1 bar Ar.

4.2.2 Biphasic homogeneous Dehydrogenation The development of highly efficient catalysis for the hydrogenation and dehydrogenation for LOHC molecules is of major importance. Traditionally, heterogeneous catalysts like supported precious metals such as palladium or platinum on carbon are used.

There are several examples of homogeneous hydrogenation/dehydrogenation catalysts (see section 2.2.2.3), but all suffer from the same well known problem of the separation of substrates and catalyst. Thus, here the homogeneous catalyst is dissolved in a molten salt and the reactions proceeds in a biphasic system, facilitating LOHC-catalyst separation.

4.2.2.1 The Catalysts and the Molten Salts The catalysts used for this work were the commercially available Crabtree catalyst, an Ir-catalyst as published by the Crabtree group230 and a slightly modified version of the latter (different counter anion) (Figure 4.51). The identity and purity of the synthesized catalysts were confirmed by common analytical methods. All used Ir-catalysts are in principle air and moisture stable; still they were handled and stored under inert gas atmosphere. 94 4. Results and Discussion

Figure 4.51-Left: Crabtrees’ Catalyst [Ir(cod)(Py)(PCy3)][PF6], right: [Ir(cod)(NHC)(PPh3)]X with NHC = N,N-dimethyl- 230 benzimidazol-2-ylidene and X = [PF6] as published in or [NTf2].

The ionic Ir-catalysts are very well soluble in molten salts. The LOHC substrates have some cross- solubility in the melts (compare Figure 4.52 Vial 2 and 3). In preliminary experiments the extraction of the substrates from the IL [PMeBu3][NTf2] with the extracting agents dibutyl ether and diphenyl ether were tested. All components were mixed in ratios which reflect the catalytic experiments. With Bu2O indoline could be extracted from the IL in 89 %, 73 % of indole could be extracted from the IL, Ph2O extracted indoline completely and 77 % of indole. In addition, the partition coefficient for indole and indoline were determined. The results are shown in Table 4.11. The higher the partition coefficient the better the solubility of the substrate in the extracting agent, which means that both compounds indole and indoline are better soluble in the aromatic extracting agent Ph2O.

Table 4.11: Partition coefficients for indole and indoline extracted with dibutyl ether or diphenyl ether from the IL [PMeBu3][NTf2].

Indole Indoline

in Bu2O 0.1878 0.4124

in Ph2O 0.2677 1.1181

Figure 4.52: Vial 1: upper phase: Bu2O, lower phase: [PMeBu3][NTf2] and [Ir(cod)(Py)(PCy3)][PF6], Vial 2: Indole in [PMeBu3][NTf2], Vial 3: Indoline in [PMeBu3][NTf2].

The Ir-complexes had to be activated prior to catalysis. For this purpose 1 eq. PPh3 as stabilizing ligand was added, and the catalyst-IL mixture was heated to 100 °C and contacted with H2 gas until it turned yellow (see Figure 4.53). Dobereiner et al.230 investigated the structure of the activated catalyst. They proposed a dihydrogen dihydride complex (as show on the right hand side in Figure 4.53). They were able to isolate or spectroscopically characterize all intermediates of the catalytic cycle. 95 4. Results and Discussion

Figure 4.53-Left: Red solution of [Ir(cod)(Py)(PCy3)][PF6] in [PMeBu3][NTf2], and yellow after activation of 185,230 [Ir(cod)(Py)(PCy3)][PF6] in [PMeBu3][NTf2] with H2 gas; right: proposed activated catalyst (adapted from reference ).

This activation is also observable with IR spectroscopy (see Figure 4.54). The stretching vibration of the hydride trans to the hydrogen is found at 1767 cm-1. This in very good accordance with reported Ir- H vibrations.230 The second vibration of a hydride trans to the NHC ligand is too weak to be visible in this study, it is expected at around 2000 cm-1. The very strong band at 836 cm-1 corresponds to the - vibration of the [PF6] anion.

Crabtree's Cat. activated with H2

Transmission / a.u. Transmission

2000 1800 1600 1400 1200 1000 800 Wavenumber / cm-1

Figure 4.54: Experimental IR spectra of [Ir(cod)(Py)(PCy3)][PF6] and after activation with H2. Picture: left: red [Ir(cod)(Py)(PCy3)][PF6] in [PMeBu3][NTf2], right: yellow after activation of [Ir(cod)(Py)(PCy3)][PF6] in [PMeBu3][NTf2] with H2 gas.

The reaction was performed as biphasic system in a 50 mL three-necked round-bottom flask equipped with a reflux condenser with oil bubbler, a thermometer and a sampling septum. The lower phase was molten salt, the upper phase the substrate in an extracting agent. Usually [PMeBu3][NTf2] or

[PPh4][NTf2] were used as melt, the extracting agent was Bu2O. The reaction was carried out under atmospheric pressure and argon as inert gas. The temperature was 125 °C.

The catalyst and stabilizing ligand PPh3 were dissolved in the melt, heated and activated with H2. Then, the extracting agent was added and the mixture heated to 125 °C. The reaction start was marked with the addition of indoline as substrate. The reaction was running for at least 24 hours and was stopped then by cooling. The reaction and the conditions are summarized in Scheme 4.3. Such a scheme will be shown for all variations as quick overview of the reaction conditions. 96 4. Results and Discussion

Scheme 4.3: Dehydrogenation of indoline by an Ir catalyst under biphasic reaction conditions.

In Figure 4.55 the indoline conversions with the three Ir based catalysts in the dehydrogenation reaction are compared. It is noteworthy that the selectivity was always 100 %; no decomposition of the substrate or side products could be detected with the GC analysis. A blind activity of 4 % was found in an experiment without any catalyst. This could be attributed to thermal dehydrogenation. The commercial

Crabtree catalyst [Ir(cod)(Py)(PCy3)][PF6] showed the best conversion of 81 % after 24 hours. The deviation of several experiments with the same conditions were smaller than 2 %. The other two catalysts gave a conversion of 68 % for the [Ir(cod)(NHC)(PPh3)][PF6] and 47 % for

[Ir(cod)(NHC)(PPh3)][NTf2] after 24 hours, respectively. The lower activity for

[Ir(cod)(NHC)(PPh3)][NTf2] might be due to the higher coordination ability of the anion, but this still has to be confirmed by additional tests.

90 Crabtree's Catalyst [Ir(cod)(NHC)(PPh )][PF ] 80 3 6 [Ir(cod)(NHC)(PPh3)][NTf2] 70 no catalyst

Conversion / % Conversion

0 0 5 10 15 20 25 Time / h

Figure 4.55: Dehydrogenation of indoline (2.5 mmol), in a homogeneous biphasic reaction with different Ir based catalysts (0.006 mmol) dissolved in [PPh4][NTf2] (0.8 mmol), with Bu2O (30 mmol) as extracting agent, at 125 °C in Ar atmosphere. The lines are just a guidance of the eye and have no scientific significance.

It should also be mentioned that the produced H2 gas is not flushed out by a continuous inert gas stream. Since the extracting agent and the substrates have a considerable vapor pressure at the reaction temperature the use of flush gas would have led to a complete removal of the upper phase. Whether the produced H2 above the reaction mixture has an influence on the reaction rate still has to be determined. However, the back reaction from indole to indoline is unlikely since the hydrogenation of indole needs higher pressures of H2 (for further information see section 4.2.3).

4.2.2.2 Comparison of different Melts and Extracting Agents

The following reactions were all catalyzed by the [Ir(cod)(NHC)(PPh3)][PF6] complex (+ 1 eq. PPh3 and activation with H2 gas), but different molten salts were used. The results of this variation are shown in 97 4. Results and Discussion

Figure 4.56 as the conversion of indoline as a function of time. The highest conversion is found for the

[PPh4][NTf2] molten salt with 68 % after 24 hours. A slightly lower yield of indole was obtained with the [PPh4OPh][NTf2] molten salt, namely 53 %. Salts with aromatic cations show slightly better + performance, with lower yield for the sterically very demanding [PPh4OPh] cation. Almost identical reactions were found for the other two non-aromatic molten salts, [PMeBu3][NTf2] and Cs[NTf2]. This might be explained by the lower solubility of the aromatic substrates.

80 [PPh4][NTf2] 70 [PPh4OPh][NTf2] [PMeBu3][NTf2] 60 Cs[NTf2] 50

Conversion / % Conversion 20

0 0 5 10 15 20 25 30 Time / h

Figure 4.56: Dehydrogenation of indoline (2.5 mmol), in a homogeneous biphasic reaction with different molten salts. The reaction was catalyzed by [Ir(cod)(NHC)(PPh3)][PF6] (0.006 mmol) dissolved in molten salt (0.8 mmol), with Bu2O (30 mmol) as extracting agent, at 125 °C in Ar atmosphere. The lines are just a guidance of the eye and have no scientific significance.

In Figure 4.57 a comparison of the conversion using different extracting agents is shown. In this case the reaction was catalyzed by the Crabtree catalyst and the molten salt was [PMeBu3][NTf2]. The reaction with toluene as extracting agent yielded only a negligible conversion. This can be explained by the lower reaction temperature due to the boiling point of toluene. Performing the reaction at 105 °C with Bu2O as extracting agent showed a conversion of 10.5 % after 24 hours. Also, the cross-solubility of the catalyst in toluene is quite a drawback for the biphasic reaction system. The diphenyl ether showed, in contrast, full dehydrogenation of indoline after 24 hours. 98 4. Results and Discussion

100 Ph2O at 125 °C 90 Bu2O at 125 °C 80 Toluene at 110 °C

Conversion / % Conversion 30

0 0 5 10 15 20 25 Time / h

Figure 4.57: Dehydrogenation of indoline (2.5 mmol), in a homogeneous biphasic reaction with different extracting agents. The reaction was catalyzed by [Ir(cod)(Py)(PCy3)][PF6] (0.006 mmol) dissolved in [PMeBu3][NTf2] (0.8 mmol), with different extracting agents (30 mmol), at 110 or 125 °C in Ar atmosphere. The lines are just a guidance of the eye and have no scientific significance.

4.2.2.3 Temperature variation The next set of experiments was designed to compare the dehydrogenation at different temperatures. In Figure 4.58 the results are shown as function of time.

60 125 °C 50 115 °C 105 °C 40 95 °C

Converison / % Converison

0 0 5 10 15 20 25 Time / h

Figure 4.58: Dehydrogenation of indoline (2.5 mmol), in a homogeneous biphasic reaction at different temperatures. The reaction was catalyzed by [Ir(cod)(Py)(PCy3)][PF6] (0.006 mmol) dissolved in [PMeBu3][NTf2] (0.8 mmol), with Bu2O as extracting agent (30 mmol), at 95, 105, 115 or 125 °C in Ar atmosphere. The lines are just a guidance of the eye and have no scientific significance. 99 4. Results and Discussion

The reaction temperature of 125 °C is the standard temperature and has a very good conversion of

74.5 % after 24 hours for the Crabtree catalyst in [PMeBu3][NTf2]. The temperature of 95 °C did not show any conversion at all. The intermediate temperatures of 105 and 115 °C led to dehydrogenation conversions of 10.5 and 17.1 %, respectively.

Simple kinetic models are not applicable, since the reaction at 125 °C clearly shows a distinct activation. The data is not sufficient to apply a more complex kinetic model. Interestingly, the reaction at the highest temperature seems to have the slowest activation. The reactions at 105 and 115 °C seem to follow a first order kinetic. Further experiments are needed to resolve the activation behavior and to investigate the kinetics of the homogeneous biphasic dehydrogenation.

The kinetics of the homogeneous dehydrogenation have not been studied in the literature so far.

4.2.2.4 Comparison of different Substrates There are numerous possible N-heterocyclic LOHC molecules. The dehydrogenation of three possible LOHC molecules is compared in the homogeneous biphasic reaction system in Figure 4.59. The reaction was performed with the [Ir(cod)(NHC)(PPh3)][PF6]-[PPh4][NTf2]-Bu2O system. The dehydrogenation rate of 2-methylindoline was the highest and full conversion to 2-methylindole was reached. The indoline system had an intermediate conversion of 68 % after 24 hours while the 1,2,3,4- tetrahydroquinaldine was dehydrogenated very slowly (conversion of 9 % after 24 hours).

100 Indoline 90 2-Methylindoline Tetrahydroquinaldine 80

Conversion / % Conversion 30

0 0 5 10 15 20 25 Time / h

Figure 4.59: Dehydrogenation of different LOHC molecules (2.5 mmol), in a homogeneous biphasic reaction. The reaction was catalyzed by [Ir(cod)(NHC)(PPh3)][PF6] (0.006 mmol) dissolved in [PPh4][NTf2] (0.8 mmol), with Bu2O as extracting agent (30 mmol), at 125 °C in Ar atmosphere. The lines are just a guidance of the eye and have no scientific significance.

The high activity of the 2-methylindoline can be explained by the electron-donating methyl 181 substituent. As calculated by DFT methods, H2 release from five-membered rings is thermodynamically favored over the release from six-membered rings.280 This was confirmed by experimental work comparing the heterogeneous dehydrogenation of different heterocyclic substrates.182 Also, five-membered N-heteroatomic rings reach an aromatic stabilized state by only splitting off one molecule H2, whereas six-membered rings need to liberate two moles of H2 to form two double-bonds which stabilize the molecules by aromatic delocalization (Hückel rule [4n+2] π- electrons134). 100 4. Results and Discussion

4.2.2.5 Alternative Catalysts The Ir-catalyst showed good performance in the dehydrogenation of indoline, but Ir is one of the most expensive elements and it would be favorable to substitute it with cheaper alternatives. An evident option is the lighter homologue of iridium – cobalt. To test this hypothesis experiments were performed with

Co(NTf2)2 only stabilized with 2 eq. of PPh3 which circumvents a complicated complex synthesis. The conversion of 28 % is impressive. In Figure 4.60 the conversion after 24 hours of reactions catalyzed with Co(NTf2)2 under different conditions are compared. It seems that the addition of the stabilizing ligand is not as important as the activation with H2 gas. The reaction with more Co(NTf2)2 did not show higher conversion. This might be explained by diffusion and mass transport limitations through the molten salt phase which is still quite viscous compared to conventional organic solvents.

Also, the comparison experiment with CoCl2 as catalyst (stabilized with 2 eq. PPh3) lead to low conversions below 10 %. The very good solubility of the Co(NTf2)2 in the bistriflimide molten salt seems to have a promoting effect on the dehydrogenation. Ni(NTf2)2(PPh3)2 was also tested as catalyst.

Preliminary experiments showed not stabilized Ni(NTf2)2 is not stable against hydrogen and tends to be reduced and precipitates. With the stabilizing PPh3 ligands a conversion of 11.7 % is found and no precipitates were observed optically, but it was not tested with other methods e.g. by addition of mercury. This means it is not unambiguously clear if the reaction is catalyzed by homogeneous

Ni(NTf2)2(PPh3)2 or Ni nanoparticles dispersed in [PPh4][NTf2] molten salt.

)

10 3

PPh

(

PPh

(

2 2

)

) )

)

conversion after 24 hours / % hours 24 after conversion

)

2 2

5 2

NTf

NTf NTf

NTf

(

NTf

( (

(

158 eq. Co

Ni no activation no cat.

Co Co activation no

Co activation no 0 CoCl activation no a) b) c) d) e) f) g)

Figure 4.60: Conversions after 24 hours of indoline in Bu2O; in [PPh4][NTf2] with different catalyst: a) 0.011 mmol Co(NTf2)2 + 2 eq. PPh3 5 minutes activation with H2 gas, b) 1.640 mmol Co(NTf2)2 + 2 eq. PPh3 5 minutes activation with H2 gas, c) 0.011 mmol Co(NTf2)2 no activation, d) 0.011 mmol Co(NTf2)2 + 2 eq. PPh3 no activation with H2 gas, e) 0.015 mmol CoCl2 + 2 eq. PPh3, no activation with H2 gas, f) 0.011 mmol Ni(NTf2)2 + 2 eq. PPh3 and no activation, g) blind experiment with no catalyst. 101 4. Results and Discussion

4.2.2.6 Recycling Experiments One of the major advantages of the biphasic reaction system should be the separation of the catalyst from the reactants. Thus, the recycling of the catalyst-IL phase was investigated. Figure 4.61 shows the conversion of the recycling experiments with the Crabtree catalyst at 125 °C in [PPh4][NTf2] (blue) and

105 °C in [PMeBu3][NTf2] (orange).

The first runs were conducted as described before. After cooling, the extracting agent was removed by decantation and the catalyst phase was washed with another 5 mL of Bu2O which was also removed. Then new extracting agent was added and heated. The second reaction started with the addition of a new batch of substrate. The complete removal of the first run reactants was confirmed by GC analysis.

100 105 °C; 0.42 mol% cat. 125 °C; 0.23 mol% cat. 80 Recycling I

60 Recycling II

40 pretreated catalyst

Conversion after 24 h / % h 24 after Conversion 20

0 a) b) c) d)

Figure 4.61: Recycling experiments of the dehydrogenation of indoline (2.5 mmol), in a homogeneous biphasic reaction. All results taken after 24 hours of reaction time. The orange bars represent the reaction catalyzed by [Ir(cod)(Py)(PCy3)][PF6] (0.011 mmol) dissolved in [PMeBu3][NTf2] (0.8 mmol), with Bu2O as extracting agent (30 mmol), at 105 °C in Ar atmosphere; the blue bars represent the reaction catalyzed by [Ir(cod)(Py)(PCy3)][PF6] (0.006 mmol) dissolved in [PPh4][NTf2] (0.8 mmol), with Bu2O as extracting agent (30 mmol), at 125 °C in Ar atmosphere. a) the first run of the catalysis conducted by the standard method, b) first recycling run where the extracting agent of the previous experiment was removed and replaced with a fresh batch of Bu2O and indoline, the conditions stayed the same; and c) second recycling run, the procedure was the same as previously mentioned. d) Experiment with a pretreated [Ir(cod)(Py)(PCy3)][PF6] catalyst (heated to 125 °C for 24 hours in [PMeBu3][NTf2] with no substrate or extracting agent present) and then the standard approach of catalysis.

For the reaction at 125 °C in the first run a very good conversion of 84 % was found. In the second run the conversion dropped to 61 % and in the third run only 52 % of indoline were converted. ICP experiments of the extracting agent phase showed no leaching of catalyst. The amount of Ir found in the

Bu2O phase was below the detection limit of the ICP. Thus, the decrease in activity must have another reason. The thermal stability of the catalyst was investigated. The obtained TGA diagrams are shown in Figure 4.62. The decomposition onset for all three catalysts is around 150 °C. As discussed before, the onset temperature is only an approximate value. Exposing the compound to a temperature slightly below 102 4. Results and Discussion the decomposition temperature for a certain amount of time will also lead to decomposition. And that is probably observed for the catalysis at 125 °C. To undermine this assumption recycling experiments at lower temperature (105 °C) were conducted. Compared to the previous experiments which were run at 125 °C the double amount of Crabtree’s catalyst (0.011 mmol) were used to secure high enough conversions for the experiments at 105 °C. Since this temperature is below the melting point of

[PPh4][NTf2] another molten salt had to be used as solvent. [PMeBu3][NTf2] is already liquid at room temperature and was used for these experiments. Still, the conversion of 55 % is lower than that of the comparable experiment at 125 °C. In the second run the conversion rises to 66 % after 24 hours compared to 55 % for the first run. This seems to be due to the induction phase of the catalyst system. In the third run the conversion drops to 26 % after 24 hours.

To further reinforce the hypothesis that the activity of the catalyst is influenced by the thermal stability another experiment was performed. For that the catalyst-IL phase (Crabtree’s catalyst in

[PMeBu3][NTf2]) is heated to reaction temperature of 125 °C without any substrate or extracting agent present for 24 hours and after this pre-treatment the reaction was run as known from before. Only a conversion of 25 % could be observed (see right hand side of Figure 4.61) indicating the low thermal stability of the catalyst. Obviously, the thermal decomposition is suppressed when the substrate is present and the catalyst degrades much more rapid in the absence of substrate.

1,01.0 Crabtree's Catalyst 1 Crabtree's Catalyst + PPh3 0.90,9 Crabtree's Catalyst + PPh3 + H2

0,80.8

0,70.7

0,60.6

Normaized mass loss / - / - loss mass Normaized 0,50.5

0,40.4

150 200 250 300 350 400 450 500 Temperature / °C

Figure 4.62: Thermal-gravimetric analysis of dehydrogenation catalyst precursor; catalyst with stabilizing ligand PPh3 and activated catalyst. The samples were kept at 120 °C for 1 hour to completely dry then it was heated to 600 °C with a heating -1 -1 rate of 5 K min , the carrier gas was N2 and the flow rate was 75 mL min .

103 4. Results and Discussion

4.2.2.7 Comparison with Literature This low temperature homogeneous dehydrogenation of N-heterocyclic LOHC compounds is rather impressive. To compare the different catalysts and substrates the turnover frequencies were calculated based on the conversion of the LOHC molecule and the hydrogen productivity based on the amount of produced hydrogen. With these values a comparison with literature is possible. Especially the hydrogen productivity is convenient for comparing since this value includes the mole of hydrogen molecules liberates per mole LOHC molecule and thus, allows a comparison between indoline and tetrahydroquinaldine.

Selected turnover frequencies and productivities of this study are summarized in Table 4.12 and compared to literature reports. Because not all experiments are useful for this comparison the complete list of all experiments carried out for this thesis can be found in Table 6.6 in the Appendix, section 6.8.

Several dehydrogenation reactions as reported in the literature are discussed in more detail in the theoretical background (section 2.2.2.3.3). Most reports use 1,2,3,4-tetrahydroquinaldine as substrate, which was found by calculations182 and in this study to be more difficult to dehydrogenate than indoline derivatives.

Since the results of this thesis are the first example of homogeneous dehydrogenation of indoline it was not possible to compare it directly. Also, there is only one report where a liquid-liquid biphasic system with a homogeneous Ir based catalyst and a molten salt is used.235 But since in this system a pure hydrocarbon compound, namely cyclooctadiene, is dehydrogenated a comparison with the here investigated dehydrogenations is not possible, too.

Table 4.12: Comparison of different dehydrogenation reactions of 1,2,3,4-tetrahydroquinaldine as found in this thesis or reported in the literature.

Temp. TOF PH2 Entry Catalyst Substrate -1 -1 -1 / °C / h / gH2 gcat h

1 [Ir(cod)(PPh3)(NHC)][PF6] 1,2,3,4-Tetrahydro- 125 0.9 4.2∙10-3 #, in [PPh4][NTf2] * quinaldine

185 # -3 2 [Ir(cod)(PPh3)(NHC)][PF6] 1,2,3,4-Tetrahydro- 145 0.4 1.7∙10 quinaldine Note: Not the literature report does not give the turnover frequency or the hydrogen productivity, thus it was calculated from the presented yields. # NHC = N,N-dimethyl-benzimidazol-2-ylidene, * Biphasic system

Only the entries of Table 4.12 are directly comparable since the same substrate and the same catalyst are used. While the literature dehydrogenation of 1,2,3,4-tetrahydroquinaldine with

[Ir(cod)(PPh3)(NHC)][PF6] was performed in a single liquid phase at 145 °C and gave a turnover frequency of 0.4 h-1 (Table 4.12, entry 2), the biphasic dehydrogenation at 125 °C gave 0.9 h-1 (Table -3 -1 -1 4.12, entry 1). This is also reflected by the H2 productivity: 4.2∙10 gH2 gcat h in the biphasic reaction -3 -1 -1 compared to 1.7∙10 gH2 gcat h in the literature report. Obviously, the catalyst is more active dissolved in a molten salt than in p-xylene.185 The lower reaction temperature of the biphasic approach supports this as well.

Even though Xu et al.227 used 2-methylindoline as substrate for their dehydrogenations with a Co catalyst and here indoline is used, the results of the Co catalyzed dehydrogenations are compared in Table 4.13. Especially when considering the reaction of 2-methylindoline with Ir catalysts in a biphasic reaction 104 4. Results and Discussion system as investigated in this study was much faster than for indoline, because of the electron donating effect of the methyl group (see section 4.2.2.4), the comparison of the literature report and this study are highly impressive. The Co-PNP pincer complex of Xu et al. gave a TOF of 0.1 h-1 for the 227 dehydrogenation of 2-methylindoline (Table 4.13, entry 2). The easily synthesized Co(NTf2)2(PPh3)2 -1 catalyst dissolved in [PPh4][NTf2] molten salt gave a significantly higher turnover frequency of 2.8 h for indoline (Table 4.13, entry 1).

Table 4.13: Comparison of different homogeneous dehydrogenation reactions with a Co catalyst as found in this thesis or reported in the literature.

Temp. TOF PH2 Entry Catalyst Substrate -1 -1 -1 / °C / h / gH2 gcat h

Co(NTf ) (PPh ) in 1 2 2 3 2 Indoline 125 2.8 9.2∙10-3 [PPh4][NTf2] *

2-Methyl- 2227 Co-PNP pincer 150 0.1 0.1∙10-3 indoline Note: Not the literature report does not give the turnover frequency or the hydrogen productivity, thus it was calculated from the presented yields. * Biphasic system

In summary the biphasic homogeneous dehydrogenation of N-heterocyclic LOHC molecules does not only have the advantage of the simple separation of catalyst and substrates after the reaction (compared to often impossible separation for homogeneous catalysis without the destruction of the catalyst), but also higher activities than single-phasic dehydrogenations with comparable substrates and catalysts.

4.2.3 Biphasic homogeneous Hydrogenation The Crabtree’s catalyst is a commercially available hydrogenation catalyst. It was tested in the previously described biphasic reaction system with molten salt and extracting agent for the hydrogenation of indole. Figure 4.63 shows the conversion of indole to indoline. The reaction at 50 °C is quite slow with only 4 % conversion after 72 hours. The best activity was found for a reaction temperature of 110 °C and a stirring speed of 750 rpm. After 144 hours, a conversion of indole to indoline of 50 % was found. Preliminary experiments showed the conversion could not be increased with higher stirring speeds. The reaction at 70 °C gave a conversion of 16 % after 48 hours, and 21 % after 75 hours. Altogether, the hydrogenation of indole is quite slow under these conditions.

The 1,2,3,4-tetrahydroquinaldine can be obtained by hydrogenation in less than 24 hours (see section 4.2.4 for more detail). The biphasic hydrogenation of quinaldine (not shown in Figure 4.63) gave only a turnover frequency of 4.2 h-1, indicating the biphasic reaction is hindered compared to a single phase system as reported in the literature185 which found a TOF of 5.3 h-1 at 1 bar hydrogen atmosphere and 25 °C. This is probably due to the slow diffusion of hydrogen through the viscous molten salt to react at the catalyst. The biphasic hydrogenation of indole (110 °C, 50 bar H2, 750 rpm) as investigated in this study gave a TOF of 0.235 h-1. No comparable literature reports are available. 105 4. Results and Discussion

Conversion / % Conversion

10 110 °C 70 °C 0 50 °C

0 20 40 60 80 100 120 140 Time / h

Figure 4.63: Hydrogenation of indole (6.4 mmol), catalyzed with [Ir(cod)(Py)(PCy3)][PF6] (0.096 mmol) dissolved in [PMeBu3][NTf2] (15 mmol), the extracting agent was Bu2O, the hydrogen pressure was 50 bar, the stirring speed was 750 rpm. The temperatures are varied. The lines are just a guidance of the eye and have no scientific significance.

4.2.4 Pressure Swing Experiments Since the Crabtree catalyst is able to catalyze the dehydrogenation as well as the hydrogenation it was only logical to perform one-pot hydrogenation/dehydrogenation experiments. In Figure 4.64 the reversible hydrogenation and dehydrogenation of the indole/indoline system is shown. The

[Ir(cod)(Py)(PCy3)][PF6] is dissolved in [PMeBu3][NTf2] and the substrate in dibutyl ether. The stirring speed was always 750 rpm. The indole was hydrogenated with 50 bar H2 at 110 °C. As already shown in the batch hydrogenation experiments, the loading of H2 on indole was rather slow. After 144 hours only 50 % of the indole are hydrogenated. Nonetheless, the hydrogen atmosphere was released and replaced by 1 bar N2 and the temperature raised to 140 °C. The dehydrogenation proceeded smoothly in 24 hours to give the indole back quantitatively. The second hydrogenation cycle was started subsequently under the same conditions as the first (50 bar H2, 110 °C). Almost an identical behavior to the previous hydrogenation was found. The hydrogenation was run for 186 hours and a conversion of indole to indoline of 43 % is observed. The following dehydrogenation gave full conversion to indole in 24 hours. The third hydrogenation cycle was run for 140 hours and 50 % of the indole was hydrogenated. It was found that 5 hours are enough to dehydrogenate this amount of indoline completely to indole (3rd dehydrogenation). The hydrogenation of indole in the second run was slightly slower than in the first but this cannot be attributed to catalyst deactivation since the rate of hydrogenation in the third run was as fast as for the first. The previously found deactivation of the catalyst in the recycling experiments could be prevented here by exclusion of oxygen. 106 4. Results and Discussion

1,01.0 Hydrogenation Hydrogenation Hydrogenation 1.0

50 bar H2 50 bar H2 50 bar H2 110°C 110°C 110°C 0,80.8 0.8

/ -

/ - 0,60.6 0.6

indole 0,40.4 0.4

Dehydrogenation

Dehydrogenation Dehydrogenation indoline

0,20.2 0.2

1 bar N2 1 bar N2 1 bar N2 0,00.0 140 °C 140 °C 140 °C 0.0

0 100 200 300 400 500 Time / h

Figure 4.64: Ultra-low temperature hydrogen battery, reversible biphasic hydrogenation of indole (6.349 mmol) and dehydrogenation of indoline in the same reactor just by changing the atmosphere (50 bar H2 for hydrogenation at 110 °C and 1 bar N2 for dehydrogenation at 140 °C). The reactions are catalyzed by [Ir(cod)(Py)(PCy3)][PF6] (0.0497 mmol) dissolved in [PMeBu3][NTf2] (15.1 mmol), the second phase was composed of substrate (indole or indoline) and extracting agent (Bu2O).

The hydrogenation of indole is very slow, thus the quinaldine/tetrahydroquinaldine system was investigated in the reversible hydrogenation-dehydrogenation (see Figure 4.65). Already 30 bar H2 and 110 °C were enough to hydrogenate quinaldine completely and give 1,2,3,4-tetrahydroquinaldine in 24 hours. The dehydrogenation of tetrahydroquinaldine is as discussed in section 4.2.2.4 much more challenging than that of indoline and this is also seen in the one-pot pressure swing experiments. After 116 hours the conversion of tetrahydroquinaldine was 75 %. The following increase in hydrogen pressure gave tetrahydroquinaldine in 24 hours. The second dehydrogenation cycle was stopped after only 48 hours due to maintenance with a conversion of 22 %, but it seems to have the same behavior as the first dehydrogenation.

Both LOHC systems show drawbacks. The hydrogenation of indole is very slow, while the dehydrogenation of tetrahydroquinaldine is unsatisfactory. Also, there is still a lot of optimization potential in the reaction conditions (temperature and pressure) or the used catalyst phase.

The two phase system and its easy separation are very convenient. To store a certain amount of hydrogen or to release it, a big stirred tank reactor is not feasible, thus a continuous recycling system would be much more functional. This means the LOHC is pumped in a loop while the catalyst phase stays in the reactor. Thus, the one-pot pressure swing experiments are an impressive example for an ultra-low temperature “hydrogen battery”. Also, it is the first example of a liquid-liquid biphasic system for a for a hydrogen storage system. 107 4. Results and Discussion

1,01.0 Hydrierung Dehydrierung Hydrierung Dehydrierung 1,01.0 1 bar N2 1 bar N2 140 °C 140 °C 0.80,8 0,80.8

/ -

0 0,60.6 0,60.6

0.40,4 0,40.4

quinaldine

tetrahydroquinaldine 0,20.2 0,20.2 c

30 bar H2 30 bar H2 0,00.0 110°C 110°C 0,00.0

0 20 40 60 80 100 120 140 160 Time / h

Figure 4.65: Ultra-low temperature hydrogen battery, reversible biphasic hydrogenation of quinaldine (4.414 mmol) and dehydrogenation of 1,2,3,4-tetrahydroquinaldine in the same reactor just by changing the atmosphere (30 bar H2 for hydrogenation at 110 °C and 1 bar N2 for dehydrogenation at 140 °C). The reactions are catalyzed by [Ir(cod)(Py)(PCy3)][PF6] (0.096 mmol) dissolved in [PMeBu3][NTf2] (15.1 mmol), the second phase was composed of substrate (quinaldine or tetrahydroquinaldine) and extracting agent (Bu2O). 108 5. Summary and Outlook

5. Summary and Outlook

The two main objectives of this thesis were the physico-chemical investigation of molten salts and their binary mixtures and the application of these molten salts as solvents for homogeneous catalysis. The chosen melts were all based on the bis(trifluoromethylsulfonyl)imide anion and had a melting point slightly above the arbitrarily defined 100 °C for classical ionic liquids. The model reactions were a C-C bond forming reaction, namely the Friedel-Crafts acylation and the hydrogenation/dehydrogenation of potential liquid organic hydrogen carrier molecules (LOHC).

For the physico-chemical investigations the molten salts Cs[NTf2] and [PPh4][NTf2] were particular interesting. The salts have melting points of 122 °C and 134 °C, respectively. Upon mixing a simple eutectic system is formed with a melting point of 98 °C at a composition of 32 mol% [PPh4][NTf2], which thus can be described as ionic liquid. The density and viscosity of the pure salts were measured and the values of the binary mixtures always lie between the pure salts. The high melting points of the salts can be compensated by the very high thermal stability of the salts. The decomposition onset is found to be 417 °C (for a 1 K min-1 heating ramp) and with this over 80 K higher than for the common

IL [EMIM][NTf2]. Also, the almost identical decomposition temperatures of Cs[NTf2] and [PPh4][NTf2] reveal that the thermal stability is dependent on the anion and the decomposition mechanism seem to be very similar. Several other binary mixtures with the [PPh4][NTf2] were investigated. With the highly thermal stable IL [PPh4OPh][NTf2] (m.p. 88 °C) it forms a mixture with the composition of 35.7 mol%

[PPh4OPh][NTf2] which is liquid at room temperature.

The physico-chemical investigations were further extended with vibrational spectroscopy studies to get - a deeper insight into the structural aspects of the molten salts. The anion [NTf2] can exist in two conformers. While it is difficult to discriminate the conformers with Raman spectroscopy alone due to overlapping bands, the assignment of the conformation isomer with IR spectroscopy is rather straight forward. For both salts, Cs[NTf2] and [PPh4][NTf2] the anion was found to be in cis conformation, where the trifluoromethyl groups are on the same side of the S-N-S plane. Though this conformation is calculated to be slightly higher in energy and thus less stable, the anion can act as bidentate ligand in the cis conformation. The experimental spectra were compared to DFT calculations. The assignment of experimental bands are aided by the calculations. The B3LYP/DFT/6-311+G(d,p) level is a good compromise between computational effort and accuracy.

Raman spectroscopy was also applied to investigate the binary mixtures, especially at higher temperatures. The ions of the mixture were found to vibrate independently of their surroundings. The raise of the temperature and the Raman spectroscopy of the gas phase above the melt revealed a minor contribution of evaporation for Cs[NTf2] but not for the mixture. At 300 °C the eutectic mixture

(32 mol% [PPh4][NTf2] in Cs[NTf2]) as well as pure Cs[NTf2] show signs of decomposition. With Raman spectroscopy combined with mass spectrometry experiments the volatile decomposition products of the salts could be identified. The knowledge of thermal stability and decomposition compounds is vital for designing new processes. 109 5. Summary and Outlook

For the application of the molten salts as solvent, the solubility of potential catalyst materials is highly important. First-row transition metal compounds with the general formula M(NTf2)2 (with M = Mn, Co, Ni, Cu, Zn) were synthesized and investigated with Raman spectroscopy and powder X-ray diffraction. This led to the conclusion that all five salts crystalize in similar fashion. Then, the transition metal compounds were dissolved in the previously discussed melts. Highly concentrated solutions could be obtained, even equimolar amounts of Co(NTf2)2 and Ni(NTf2)2 were soluble in [PPh4][NTf2] (concentrations up to 55.6 mol%). The mixtures were investigated regarding their melting point, the viscosity and the thermal stability. Upon dissolving the metal compound in the molten salt the melting point can be lowered over 60 K (to 71 °C for the eutectic mixture of Co(NTf2)2-[PPh4][NTf2] at around

45 mol% of Co(NTf2)2 and 64 °C for the eutectic Ni(NTf2)2-[PPh4][NTf2] mixture with a composition of 33 mol%). The thermal stability of the M(NTf2)2 compound is slightly enhanced by dissolving it in

[PPh4][NTf2] and a stepwise decomposition is observed suggesting an intermediate compound with the - composition [M(NTf2)3] . The viscosity of the mixtures are only minimally influenced even by high concentrations of transition metal salt present. The possible solvation structure of Co(NTf2)2 in

[PPh4][NTf2] was suggested after Raman spectroscopic and mass spectrometric investigations to be a - octahedral coordinated [Co(NTf2)3] anion. In order to design new, highly active catalysts the coordination of the metal center is important. The knowledge of the geometry and coordination sphere can help to directly tune the activity of the catalyst, and thus, avoids the tedious “trial and error” approach of screening experiments with randomly changed ligands.

The M(NTf2)2 compounds were subsequently investigated as catalyst for the Friedel-Crafts acylation of toluene with benzyl chloride in a room temperature IL [PMeBu3][NTf2]. The activity of the catalysts were moderate (around 30 % yield of 4-MBP) and moderate selectivites (around 70 % to 4-MBP) were found. The main side product is benzoic acid, which can be prevented by water-free catalysts. Recycling experiments gave only very poor results. This was attributed to the decomposition of the catalyst by the by-product HCl to form almost inactive CoCl2 and highly volatile HNTf2. Previously reported results by Earle et al.157 are in strong contrast to the here found activity and stability of the catalyst. To clarify the influence of co-crystalized water in the catalyst and of the different ILs in the two studies, further investigations are highly recommended, since the use of transition metal compounds as catalyst for the Friedel-Crafts acylation is very advantageous compared to the commonly used highly corrosive, moisture unstable AlCl3 “catalysis” where at least equimolar amounts are needed and often the catalyst has to be destroyed in order to separate the products, producing extensive amounts of waste. Only catalytic amounts of the transition metal compounds are needed and the separation from the products can be achieved by decantation or distillation. Also, the Lewis acidity of the M(NTf2)2 catalyst should be investigated to further understand the activity of the catalysts. Since the bistriflimide seems to be a too weak ligand and the chloride a very strong one, a ligand with a coordination power between the mentioned two might be more favorable, for example transition metal triflate compounds M(OTf)2.

In the context of renewable energy the storage of excess energy in form of hydrogen is challenging, but covalently binding H2 to liquid molecules also known as liquid organic hydrogen carriers (LOHC) was found to be advantageous and thus has attracted a great deal of interest in the recent years. The second investigated biphasic homogeneous reaction in this thesis was the dehydrogenation of potential LOHC molecules, especially indoline. N-heterocyclic LOHC compounds have the advantage of lower dehydrogenation enthalpies but also a lower thermal stability. However, high temperatures are not necessary for very active homogeneous catalyzed reactions. The cationic iridium based catalysts were dissolved in a molten salt and the substrate reside in an extracting agent e.g. dibutyl ether. The dehydrogenation of indoline to indole was optimized and the extracting agent diphenyl ether was superior to dibutyl ether due to the better solubility of the substrates. Also the molten salts with aromatic cations showed slightly higher activity (68 % conversion for [PPh4][NTf2] compared to 40 % for 110 5. Summary and Outlook

[PMeBu3][NTf2]). The most active catalyst was the commercially available Crabtree catalyst

[Ir(cod)(Py)(PCy3)][PF6]. It gave a conversion of indoline of 84 % in [PPh4][NTf2] in 24 hours at 125 °C. + The anion variation of a synthesized [Ir(cod)(NHC)(PPh3)] catalyst showed higher conversions for the hexafluorophosphate anion (68 %) than for the stronger coordinating bis(trifluoromethylsulfonyl)imide anion (47 %).

Also, Co(NTf2)2(PPh3)2 was remarkably active in the dehydrogenation of indoline (conversion of 28 %), this result might be increased by a better adjusted ligand design.

Comparing the here obtained results of the biphasic dehydrogenation with single-phase literature reports showed high TOF and an obviously favorable effect of the molten salt solvent on the activity and of course the ease of processing.

The Crabtree catalyst was also active in the biphasic homogeneous hydrogenation of indole. However, the hydrogenation is quite slow under the investigated conditions. A conversion of indole to indoline of

50 % was found after 144 hours at 110 °C and 50 bar H2. Nonetheless, the application of the Crabtree catalyst in one-pot pressure swing experiments for the reversible hydrogenation-dehydrogenation of the indole/indoline and quinaldine/tetrahydroquinaldine LOHC systems was successful and the loading of

H2 or the release of H2 from the molecules could be achieved by a slight increase in temperature and changing the atmosphere from 30 or 50 bar H2 to 1 bar N2. This is the first example of a homogenous biphasic ultra-low temperature “hydrogen battery”.

Further investigations and optimizations of this concept are highly promising, since both indole/indoline and quinaldine/tetrahydroquinaldine systems have drawbacks. The hydrogenation of indoline is rather slow. For the dehydrogenation of tetrahydroquinaldine unsatisfactory rates are found. Thermodynamical data will be helpful to identify the position of equilibrium of these reactions.

In conclusion, it could be shown that ionic liquids or to be more precise low-temperature molten salts, can be used as solvents for homogeneous catalyzed reactions. They have the ability to dissolve and immobilize a catalyst, which is highly advantageous for biphasic systems, where the products and the catalyst can be easily separated after the reaction. The molten salts also can protect the catalyst from decomposition, even though this is not generally true for all applications. In general, molten salts have already found their way into industrial applications but there is still a great potential to be unleashed.

111 6. Appendix

6. Appendix

6.1 Raman Spectroscopy

6.1.1 Raman Instruments and Measurement Setup

Figure 6.1: Schematic illustration of the dispersive DILOR-XY Raman spectrometer with a 532 nm doubled Nd-YVO4 laser (DTU Lyngby, Denmark). 112 6. Appendix

Figure 6.2: Schematic illustration of the Renishaw Raman spectrometer with a blue (488 nm) or a UV laser (DTU Lyngby, Denmark).

Electrical For cooling water heating Sample

Co(NTf2)2

Gas Gas

Quartzglass window

Figure 6.3: Picture of the used cell to measure Raman spectroscopy in hydrogen atmosphere. 113 6. Appendix

6.1.2 Supplementary Raman Spectra

a 75 mol% b 50 mol%

Ramanscatteringa.u. / c 32 mol% d 25 mol%

4000 3500 3000 2500 2000 1500 1000 500 0 Wavenumber / cm-1

Figure 6.4: Complete spectra of the experimental Raman spectra of a) the 75 mol% [PPh4][NTf2] in Cs[NTf2] mixture, b) the 50 mol% [PPh4][NTf2] in Cs[NTf2] mixture, c) the 32 mol% [PPh4][NTf2] in Cs[NTf2] mixture, and d) the 25 mol% [PPh4][NTf2] in Cs[NTf2] mixture. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at room temperature. Intensities are arbitrarily scaled and normalized to the band at ca. 740 cm-1.

Cs[NTf2], liquid 130 °C

Ramanscatteringa.u. /

Cs[NTf2], solid RT

2000 1800 1600 1400 1200 1000 800 600 400 200 0 Wavenumber / cm-1

Figure 6.5: Experimental Raman spectra of a) the liquid phase of Cs[NTf2] at 130 °C, and b) the solid phase of Cs[NTf2] at room temperature. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer. Intensities are arbitrarily scaled. 114 6. Appendix

* H2 * *

O *

2 *

H *

a N Cs[NTf2], gas phase 200 °C 18 hours

Ramanscatteringa.u. /

Cs[NTf2], gas phase 200 °C 5 hours

4000 3500 3000 2500 2000 1500 1000 500 0 Wavenumber /cm-1

Figure 6.6: Experimental Raman spectra of a) the gas phase above Cs[NTf2] at 200 °C after 18 hours, and b) the gas phase above Cs[NTf2] at 200 °C after 5 hours. The experimental spectra were measured with a green laser (532 nm, doubled Nd- YVO4) in the dispersive DILOR-XY Raman spectrometer. Intensities are arbitrarily scaled.

50 mol% b 25 mol% c

Raman scattering / a.u. scattering Raman 16.7 mol% d Co(NTf2)2 pure, RT

3500 3000 2500 2000 1500 1000 500 Wavenumber cm-1

Figure 6.7: Experimental Raman spectra of a) the 50 mol% Co(NTf2)2 in [PPh4][NTf2] mixture, b) the 25 mol% Co(NTf2)2 in [PPh4][NTf2] mixture, c) the 16.7 mol% Co(NTf2)2 in [PPh4][NTf2] mixture, and d) pure Co(NTf2)2. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at 120 °C (mixtures) and at room temperature (pure Co(NTf2)2). Intensities are arbitrarily scaled.

115 6. Appendix

a 50 mol%

b 25 mol%

Raman scattering / a.u. scattering Raman 16.7 mol% d Ni(NTf2)2 pure, RT

3500 3000 2500 2000 1500 1000 500 Wavenumber cm-1

Figure 6.8: The experimental Raman spectra of a) the 50 mol% Ni(NTf2)2 in [PPh4][NTf2] mixture, b) the 25 mol% Ni(NTf2)2 in [PPh4][NTf2] mixture, c) the 16.7 mol% Ni(NTf2)2 in [PPh4][NTf2] mixture, and d) pure Ni(NTf2)2. The experimental spectra were measured with a green laser (532 nm, doubled Nd-YVO4) in the dispersive DILOR-XY Raman spectrometer at 120 °C (mixtures). The spectrum of pure Ni(NTf2)2 was measured with the blue laser at room temperature. Intensities are arbitrarily scaled and normalized to the band at ca. 1000 cm-1.

116 6. Appendix

+ 6.1.3 Calculated Values for [PPh4]

+ Table 6.1: Vibrational spectra for [PPh4] calculated with Gaussian DFT B3LYP 6-31G(d,p).

Mode Wavenumber Infrared absorption Raman activity Depolarization ratio no. / cm-1 / km mol-1 / Å4 AMU-1 1 34.97 0.0001 0.7117 0.7351 2 37.52 0.0546 4.2378 0.7500 3 37.53 0.0547 4.2326 0.7500 4 43.80 0.0321 16.8098 0.7500 5 62.01 0.0000 17.8125 0.7482 6 68.42 0.0464 3.3312 0.7500 7 68.43 0.0464 3.3347 0.7500 8 70.52 0.0220 10.5173 0.7500 9 74.10 0.0085 1.0937 0.7500 10 191.99 0.0004 3.7522 0.6944 11 197.01 2.3637 1.5091 0.7500 12 197.02 2.3636 1.5108 0.7500 13 244.16 0.0006 12.5833 0.0030 14 244.65 0.6819 6.4514 0.7479 15 258.32 1.3831 1.0074 0.7500 16 258.33 1.3744 1.0050 0.7500 17 277.15 0.0000 0.0026 0.6617 18 284.51 0.6434 4.6394 0.7500 19 406.91 0.3227 0.2226 0.7500 20 406.92 0.3229 0.2224 0.7500 21 408.59 0.0001 0.3553 0.6129 22 414.99 0.0742 0.1225 0.7500 23 444.91 2.8721 0.4887 0.7500 24 444.94 2.8711 0.4890 0.7500 25 462.82 11.2725 0.7385 0.7500 26 475.28 0.0000 0.0843 0.6749 27 538.65 87.6981 0.0539 0.7500 28 438.66 87.6743 0.0539 0.7500 29 545.54 72.8017 0.9777 0.7500 117 6. Appendix

Mode Wavenumber / Infrared absorption / Raman activity / Depolarization ratio no. cm-1 km mol-1 Å4 AMU-1 30 627.39 0.0001 6.5246 0.7224 31 628.21 0.0707 1.4889 0.7500 32 628.22 0.0707 1.4950 0.7499 33 629.20 0.0526 8.2631 0.7500 34 688.13 0.0001 15.0516 0.0000 35 703.91 16.4607 0.1630 0.7500 36 705.09 24.9591 0.0291 0.7468 37 705.09 24.9840 0.0293 0.7497 38 707.63 0.0004 0.2055 0.1549 39 730.16 45.7613 4.9619 0.7500 40 730.17 45.7605 4.9608 0.7500 41 730.66 49.7650 0.3655 0.7500 42 764.62 0.0012 4.8211 0.7252 43 767.08 14.7368 3.4299 0.7500 44 767.08 14.7575 3.4271 0.7500 45 771.75 14.9317 7.1971 0.7500 46 865.20 0.0000 1.6309 0.7472 47 867.39 0.4091 0.9159 0.7500 48 867.40 0.4083 0.9162 0.7500 49 871.10 0.0033 3.3664 0.7500 50 952.55 0.1083 0.3402 0.7500 51 954.56 0.0246 0.0457 0.7499 52 954.57 0.0246 0.0458 0.7498 53 957.54 0.0000 0.3795 0.3663 54 992.37 0.0000 0.9215 0.5931 55 994.29 0.2423 0.5004 0.7500 56 994.30 0.2420 0.5004 0.7499 57 995.75 0.0163 0.2643 0.7500 58 1011.83 12.3377 12.0117 0.7500 59 1012.18 8.2011 10.3959 0.7499 60 1012.18 8.1985 10.3956 0.7498 118 6. Appendix

Mode Wavenumber / Infrared absorption / Raman activity / Depolarization ratio no. cm-1 km mol-1 Å4 AMU-1 61 1013.79 0.0003 84.0343 0.0000 62 1025.99 0.0572 0.0753 0.7500 63 1026.37 0.0301 0.0114 0.7478 64 1026.37 0.0300 0.0112 0.7494 65 1026.63 0.0000 0.0278 0.0127 66 1050.26 0.0000 89.5144 0.0000 67 1052.05 0.5474 8.3521 0.7491 68 1052.05 0.5488 8.3306 0.7497 69 1052.23 0.3085 12.2499 0.7500 70 1113.86 22.1620 4.9994 0.0303 71 1113.89 28.2582 0.4678 0.6992 72 1113.99 6.2830 17.2732 0.0028 73 1116.03 0.0006 1.3991 0.6631 74 1117.48 57.0395 7.4425 0.7500 75 1119.77 45.4290 6.5212 0.7500 76 1119.79 45.4760 6.5182 0.7495 77 1119.86 16.8474 8.2503 0.7500 78 1198.67 0.5911 1.2554 0.7500 79 1198.68 0.5876 1.0897 0.7500 80 1198.72 0.6799 6.5584 0.7500 81 1199.00 0.0002 5.7721 0.7499 82 1220.79 7.4655 6.3360 0.7500 83 1220.79 7.5216 6.1101 0.7500 84 1220.86 6.9741 8.3745 0.7500 85 1223.58 0.0003 0.332 0.3665 86 1329.81 3.5268 0.3263 0.7500 87 1329.82 3.5316 0.3263 0.7500 88 1336.78 0.0000 0.1961 0.7386 89 1339.89 5.2632 0.2551 0.7500 90 1369.88 0.0998 4.2704 0.6239 91 1370.16 4.4033 1.3324 0.7492 119 6. Appendix

Mode Wavenumber / Infrared absorption / Raman activity / Depolarization ratio no. cm-1 km mol-1 Å4 AMU-1 92 1370.17 4.3190 1.3866 0.7409 93 1371.28 1.4260 3.3135 0.7500 94 1476.39 29.1445 0.5614 0.7500 95 1476.40 29.1502 0.5610 0.7500 96 1478.73 0.0012 4.2992 0.6525 97 1482.57 41.3178 2.8449 0.7500 98 1525.42 0.0083 0.2174 0.4988 99 1525.53 6.2950 0.1928 0.7497 100 1526.60 7.2399 0.4751 0.7499 101 1526.60 7.2388 0.4751 0.7500 102 1626.47 0.2433 6.4389 0.7500 103 1626.47 0.2424 6.4449 0.7500 104 1627.74 0.0000 11.7167 0.7498 105 1629.70 1.8897 17.1483 0.7500 106 1639.75 5.1041 39.0632 0.7498 107 1640.39 0.0031 24.3051 0.0005 108 1640.59 6.6143 48.9660 0.7500 109 1640.60 6.6163 48.9880 0.7498 110 3198.39 0.6978 9.6326 0.7500 111 3198.40 0.6954 9.7177 0.7500 112 3198.52 0.0393 31.7654 0.7500 113 3198.75 0.0002 23.7063 0.6978 114 3204.47 0.0191 79.4185 0.6990 115 3204.54 0.8845 86.3068 0.7490 116 3204.55 0.9021 86.4942 0.7500 117 3205.00 0.6253 150.6255 0.7500 118 3211.84 0.4354 138.9599 0.7500 119 3212.09 204848 69.7181 0.7500 120 3212.10 3.5087 69.4941 0.7500 121 3212.49 0.0015 68.6700 0.7485 122 3219.10 0.0105 149.7063 0.0708 120 6. Appendix

Mode Wavenumber / Infrared absorption / Raman activity / Depolarization ratio no. cm-1 km mol-1 Å4 AMU-1 123 3219.29 2.1912 43.8373 0.7413 124 3219.32 2.1641 44.0202 0.7458 125 3219.64 6.9092 18.9680 0.7434 126 3224.81 3.1625 102.9066 0.7410 127 3224.90 9.5242 90.9954 0.7392 128 3224.92 9.6320 90.0195 0.7497 129 3225.25 0.0104 951.5857 0.0001

121 6. Appendix 6.2 Supplementary IR Spectrum

[PPh4][NTf2] b 75 mol% c 50 mol% d 32 mol% e

Transmission / a.u. Transmission 25 mol% f

Cs[NTf2]

1500 1400 1300 1200 1100 1000 900 800 700 600 Wavenumber / cm-1

Figure 6.9: The experimental IR spectra of a) [PPh4][NTf2], b) the 75 mol% [PPh4][NTf2] in Cs[NTf2] mixture, c) the 50 mol% [PPh4][NTf2] in Cs[NTf2] mixture, and d) the eutectic mixture (32 mol% [PPh4][NTf2] in Cs[NTf2]), e) the 25 mol% [PPh4][NTf2] in Cs[NTf2] mixture, and f) pure Cs[NTf2]. Intensities are arbitrarily scaled and normalized to the band at ca. 1190 cm-1.

6.3 Investigation of the Temperature dependent Viscosity of M(NTf2)2

Mixtures with [PPh4][NTf2]

Table 6.2: Fitting parameters for the Vogel-Fulcher-Tamman equation for mixtures of Co(NTf2)2 in [PPh4][NTf2].

2 salt η0 (mPa s) Bη (K) TVF (K) R

[PPh4][NTf2] 2.8553 76.0673 370.7929 0,995

9.1 mol% 0.996 322.043 299.8104 0.998

16.7 mol% 6.0886 28.351 387.7996 0,998

42.9 mol% 5.4449 80.8350 365.6533 0,995

122 6. Appendix

Table 6.3: Fitting parameters for the Vogel-Fulcher-Tamman equation for mixtures of Ni(NTf2)2 in [PPh4][NTf2].

2 salt η0 (mPa s) Bη (K) TVF (K) R

[PPh4][NTf2] 2.8553 76.0673 370.7929 0,995

9.1 mol% 1.5541 212.8836 326.5932 0.994

16.7 mol% 1.1912 263.970 320.2855 0,999

42.9 mol% 0.3308 544.1575 292.9723 0,996

6.4 Gas Chromatography

The GC was calibrated prior to the experiments. The calibration factors based on molar amounts are shown in Table 6.4.

The calibration factors were calculated based on the following equations:

푛 푓 = 𝑖 (6.1) 퐴𝑖 where n is the molar amount of substance and A the area under the curve in the gas chromatogram.

푓𝑖 푘푖/푠푡푎푛푑푎푟푑 = (6.2) 푓푠푡푎푛푑푎푟푑

Table 6.4: Retention times and calibration factors as determined from molar amounts.

Standard Substance Retention time Calibration factor / min ki/standard

Cyclooctane Toluene 1.0 1.00376

Benzoyl chloride 3.4 1.20962

2-Methyl benzophenone 9.9 0.38397

3-Methyl benzophenone 10.3 0.38201

4-Methyl benzophenone 10.5 0.38555

Benzyl alcohol Indole 5.7 0.80406

Indoline 4.7 0.86224

123 6. Appendix

6.5 Literature Values of the Viscosity of Cs[NTf2]

243 Table 6.5: Viscosity of Cs[NTf2] in dependence of temperature, adapted from Kubota et al. .

Temperature / °C Viscosity / mPa s

140 50

149 40

160 30

179 20

222 10

229 8

250 7

270 6

281 5.5

302 5 124 6. Appendix 6.6 Mass Spectrometry

- Figure 6.10: ESI mode for DART-MS experiment with Co(NTf2)2-[PPh4][NTf2] mixture (x = 0.5).

+ Figure 6.11: ESI mode for DART-MS experiment with Co(NTf2)2-[PPh4][NTf2] mixture (x = 0.5). 125 6. Appendix

6.7 Heat of Fusion for M(NTf2)2-[PPh4][NTf2] Mixtures (M = Co, Ni)

100

90 heat of fusion (integral of eutectic peak) fit rising R2 = 0.969 80 fit descending R2 = 0.839

-1 70

Heat of fusion / J g fusion of Heat 20

0,00.0 0,10.1 0,20.2 0.30,3 0,40.4 0,50.5 0,60.6 0,70.7 0,80.8 0,90.9 1.01,0 [PPh4][NTf2] Co(NTf2)2  (Co(NTf2)2)

Figure 6.12: Dependence of the composition with the eutectic heat of fusion for the Co(NTf2)2-[PPh4][NTf2] system.

100

90 heat of fusion (integral of eutectic peak) fit rising R2 = 0.671 80 fit descending R2 = 0.979

-1 70

Heat of fusion / J g fusion of Heat 20

0.00,0 0,10.1 0,20.2 0.30,3 0.40,4 0,50.5 0,60.6 0.70,7 0,80.8 0,90.9 1.01,0 Ni(NTf ) [PPh4][NTf2] 2 2  (Ni(NTf2)2)

Figure 6.13: Dependence of the composition with the eutectic heat of fusion for the Ni(NTf2)2-[PPh4][NTf2] system.

126 6. Appendix 6.8 Turnover Frequencies and Hydrogen Productivites of the Dehydrogenation Experiments

Table 6.6: Summary of the liquid-liquid biphasic dehydrogenations performed in this study; all reactions run at 125 °C if not indicated otherwise.

Molten salt TOF PH2 Entry Catalyst Substrate -1 -1 -1 solvent / h / gH2 gcat h

-3 1 [Ir(cod)(PPh3)(NHC)][PF6] [PPh4][NTf2] Indoline 8.5 20.1∙10

-3 2 [Ir(cod)(PPh3)(NHC)][NTf2] [PPh4][NTf2] Indoline 7.8 16∙10

-3 3 Crabtree’s catalyst [PPh4][NTf2] Indoline 13.6 34∙10

-3 4 [Ir(cod)(PPh3)(NHC)][PF6] [PMeBu3][NTf2] Indoline 6.1 14.3∙10

-3 5 [Ir(cod)(PPh3)(NHC)][PF6] Cs[NTf2] Indoline 6.7 15.7∙10

-3 6 [Ir(cod)(PPh3)(NHC)][PF6] [PPh4OPh][NTf2] Indoline 8.5 20.1∙10

a -3 7 Crabtree’s Catalys [PMeBu3][NTf2] Indoline 14.1 35.2∙10

2-Methyl- 8 [Ir(cod)(PPh )(NHC)][PF ] [PPh ][NTf ] 20.1 53∙10-3 3 6 4 2 indoline

1,2,3,4- -3 9 [Ir(cod)(PPh3)(NHC)][PF6] [PPh4][NTf2] Tetrahydro- 0.9 4.2∙10 quinaldine

-3 10 Co(NTf2)2(PPh3)2 [PPh4][NTf2] Indoline 2.8 9.2∙10

-3 11 Crabtree’s Catalyst [PMeBu3][NTf2] Indoline 10.5 26.3∙10

b -3 12 Crabtree’s Catalys [PMeBu3][NTf2] Indoline 2.7 6.8∙10

c -3 13 Crabtree’s Catalys [PMeBu3][NTf2] Indoline 1.1 2.8∙10

d -3 14 Co(NTf2)2(PPh3)2 [PPh4][NTf2] Indoline 0.1 0.4∙10

e -3 15 Co(NTf2)2 [PPh4][NTf2] Indoline 2.6 8.6∙10

e -3 16 Co(NTf2)2(PPh3)2 [PPh4][NTf2] Indoline 2.0 6.4∙10

e -3 17 CoCl2(PPh3)2 [PPh4][NTf2] Indoline 0.8 12.2∙10

e -3 18 Ni(NTf2)2(PPh3)2 [PPh4][NTf2] Indoline 1.3 4.3∙10 a b c d e with Ph2O as extracting agent; at 115 °C; at 105 °C; 158 eq., no activation with H2.

127 7. References

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