Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium

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2019-09-11 Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium

Beh, Daniel Wei Ming

Beh, D. W. M. (2019). Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium (Unpublished doctoral thesis). University of Calgary, Calgary, AB. http://hdl.handle.net/1880/110922 doctoral thesis

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Synthesis and Reactivity of Organometallic Complexes of Scandium and Titanium

Daniel Wei Ming Beh

A THESIS

SUBMITTED TO THE FACULTY OF GRADUATE STUDIES

IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF DOCTOR OF PHILOSOPHY

GRADUATE PROGRAM IN CHEMISTRY

CALGARY, ALBERTA

SEPTEMBER, 2019

Abstract

Rare-earth metal alkyl and hydrides are highly reactive species that can act as a metal precatalyst or reactive intermediate in a variety of homogeneous catalytic reactions such as hydroelementation and polymerization of olefins. Rare-earth metal hydrides play an important role for researchers to study sigma bond metathesis reactions and develop better homogeneous catalysts. However, these complexes tend to oligomerize in order to stabilize the highly polarizable, soft hydride at the Lewis acidic metal center.

This thesis presents the synthesis various scandium complexes supported by the tetrapodal pentadentate B2Pz4Py ligand with the aim to stabilize these reactive intermediates and to isolate their monomeric species. The single site of reactivity at the apical position allows for controlled reactivity and a better understanding of these transformations. The scandium alkyl complexes were shown to be highly robust and do not undergo sigma bond metathesis to afford the hydride complex. Alternatively, the scandium hydride was synthesized from the reaction between a ligated scandium chloro complex and

NaHBEt3 and was found to exist as a dimeric species. These complexes reacted with small molecules like H2O, N2O and CO2, and the mechanism for CO2 insertion was studied by

DFT calculations. Cationic scandium complexes were also generated by alkyl or hydride abstraction with B(C6F5)3 and employed as a catalyst for hydrosilylation of CO2. Efforts to synthesize an anionic scandium terminal oxo complex showed that the complex is highly basic and immediately scavenges any acidic protons to form the corresponding scandium hydroxo complex which condenses to form the more thermodynamically stable scandium

µ-O dimer.

The analogous B2Pz4Py titanium complexes were also synthesized which showed some differences in the coordination chemistry. The dimeric titanium hydride complex is further stabilized by π stacking interactions which made it unreactive to N2O and CO2.

Although the titanium µ-O dimer is once again a common by-product in the presence of adventitious moisture, it can be further oxidized by O2 to form other peroxo and oxo products.

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Acknowledgements

First and foremost, I would like to thank my supervisor, Professor Warren Piers, for this invaluable experience and opportunity to work in such an amazing research group.

His continuous guidance and support helped me to develop my personal and professional character, and I learned so much in the process.

Many thanks to Prof. Roland Roesler and Prof. Gregory Welch for being part of my supervisory committee and providing helpful feedback over the years. I would also like to acknowledge Prof. Thomas Back and Prof. George Shimizu for participating in my

Qualifying examination, and Prof. Stephen Larter for reading this thesis and taking part in my defense. Thank you to Prof. Ian Tonks for taking the time to review this thesis and serving as my external examiner.

I would also like to extend my gratitude to the instrumentation staff at the

University of Calgary who assisted in collecting various spectroscopic data: Michelle

Forgeron, Michelle Thibault, Wade White and Johnson Li. Also thank you to our collaborators Drs. Laurent Maron and Iker del Rosal (Université de Toulouse) who helped with DFT calculations. A big thank you to the X-ray crystallographers Drs. Chris Gendy

Denis Spasyuk, Benjamin Gelfand, Jian-Bin Li and David Bi who helped with obtaining crystal structures and putting up with the countless times I gave them crystals of µ-O dimers or free ligand.

It was a great pleasure to work with many past and present members of the Piers group. They participated in helpful discussion and were always there to support one another along. They are a fun group of people to hang out with both in and out of the lab.

My friends and family have continuously supported me throughout this journey ever since I started undergraduate studies abroad, and I will be forever grateful for all the opportunities that came along the way.

Finally, I would like to thank the University of Calgary and the Department of

Chemistry for financial support.

Table of Contents

Abstract ...... ii Acknowledgements ...... iv Table of Contents ...... vi List of Tables ...... viii List of Figures ...... x List of Schemes ...... xiv List of Symbols, Abbreviations and Nomenclature ...... xvi List of Numbered Compounds ...... xviii Chapter 1: Introduction ...... 1 1.1 Transition Metal Alkyl Complexes ...... 1 1.2 Transition Metal Hydride ...... 6 1.3 Rare-earth M=E Complexes ...... 9 1.4 Pentadentate Ligand Systems ...... 16 1.4.1 Dianionic B2Pz4Py Ligand ...... 20 1.5 Thesis Goals ...... 22

Chapter 2: B2Pz4Py Supported Scandium Alkyl and Hydride Complexes ...... 24 2.1 Introduction ...... 24 2.2 Synthesis of B2Pz4Py Scandium Chloro Complex ...... 24 2.3 Synthesis of B2Pz4Py Scandium Alkyl Complexes ...... 28 2.4 Synthesis of B2Pz4Py Scandium Hydride Complex ...... 36 2.5 Reactivity Studies with Small Molecules ...... 41 2.5.1 DFT Studies on CO2 Insertion...... 45 2.6 Generation of Cationic Scandium Complexes ...... 48 2.6.1 Contact Ion Pair Complex ...... 48 2.6.2 Scandium Formatoborate Complex ...... 51 2.6.3 Preliminary Studies on Hydrosilylation of CO2 ...... 59 2.7 Summary and Conclusions ...... 66 Chapter 3: Scandium Oxo and Hydroxo Complexes ...... 67 3.1 Introduction ...... 67 3.2 Towards Scandium Anionic Terminal Oxo Complex ...... 70 3.3 Scandium Hydroxo Complexes ...... 74 3.4 Summary and Conclusions ...... 87

Chapter 4: Comparative Studies with Analogous B2Pz4Py Titanium Complexes .....88 4.1 Introduction ...... 88 4.2 Synthesis of B2Pz4Py Titanium Chloro Complex ...... 90 4.3 Synthesis of B2Pz4Py Titanium Alkyl and Hydride Complexes ...... 94 4.4 Generation of Cationic Titanium Complexes ...... 105 4.4.1 Titanium Formatoborate Complex ...... 105 4.4.2 Titanium Triflimide ...... 111 vi

4.5 Titanium Peroxo and Terminal Oxo Complexes ...... 115 4.6 Summary and Conclusions ...... 120 Chapter 5: Thesis Summary and Future Directions ...... 121 5.1 Thesis Summary ...... 121 5.2 Future Directions ...... 123 5.2.1 Mixed Metal Oxides ...... 123 5.2.2 Anionic Terminal Imido Scandium Complexes ...... 126 5.2.3 Small Molecule Activation and Functionalization ...... 130 Chapter 6: Experimental Details ...... 138 6.1 General Considerations ...... 138 6.2 Experimental Procedures ...... 140 Appendix A: Crystallographic Data...... 178 Appendix B: Copyright Permissions ...... 189 References ...... 191

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List of Tables

Table 2-1 Selected bond distances (Å) and angle (°) for 1Sc-Ph and 1Sc...... 26 Table 2-2 Synthetic yields, chemicals shifts of α-H and C resonances in the 1H and 13C{1H} 1 NMR spectra, and JCH coupling constants of 2Sc-R...... 29

Table 2-3 Selected metrical parameters for 2Sc-Me (top) and 2Sc-CH2SiMe3 (bottom). . 30

Table 2-4 Selected metrical parameters for 3Sc...... 34

Table 2-5 DOSY results for 2Sc-H, 2Sc-Me and 3Sc...... 38

Table 2-6 Selected metrical parameters for 4Sc-H and 4Sc-Me...... 44

Table 2-7 Selected metrical parameters for 5Sc-THF...... 50

Table 8 Selected metrical parameters for 6Sc...... 52

Table 2-9 Selected metrical parameters for 7Sc...... 55

Table 2-10 Selected metrical parameters for 8Sc-H...... 59

Table 2-11 Selected metrical parameters for 9Sc-Et...... 66

Table 3-1 Selected metrical parameters for [2Sc-OH]2...... 78

Table 3-2 Selected metrical parameters for 10Sc...... 82

Table 3-3 Selected metrical parameters for 12Sc...... 86

Table 4-1 Selected bond distances (Å) and angle (°) for 1M-Ph and 1M (M = Sc or Ti). .. 92

Table 4-2 Selected metrical parameters for 2Ti-CH2SiMe3...... 96

Table 4-3 Selected metrical parameters for 3Ti...... 97

Table 4-4 Selected metrical parameters for [2Ti-H]2...... 101

Table 4-5 Selected metrical parameters for 11Ti...... 114

Table 4-6 Selected metrical parameters for 13Ti...... 117

Table 4-7 Selected metrical parameters for 14Ti...... 119

Table 5-1 Selected metrical parameters for 15Sc (top) and 16Sc (bottom)...... 129

Table 5-2 Selected metrical parameters for 17Sc...... 133 Table A-1 Crystallographer assignment...... 178

Table A-2 Crystal and structure refinement details for 1Sc-Ph, 1Ti-Ph and 1Sc...... 179

Table A-3 Crystal and structure refinement details for 1Ti, 2Sc-CH2SiMe3 and 2Sc-Me 180

Table A-4 Crystal and structure refinement details for 2Ti-CH2SiMe3, 2Sc-Ph-OH and 2Sc- OH...... 181

Table A-5 Crystal and structure refinement details for 2Ti-H, 3Sc-Ph and 3Ti-Ph...... 182

Table A-6 Crystal and structure refinement details for 3Sc, 3Ti and 4Sc-H...... 183

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Table A-7 Crystal and structure refinement details for 4Sc-Me, 5Sc-THF and 6Sc...... 184

Table A-8 Crystal and structure refinement details for 7Sc, 8Sc-H and 9Sc-Et...... 185

Table A-9 Crystal and structure refinement details for 10Sc, 11Ti and 12Sc...... 186

Table A-10 Crystal and structure refinement details for 13Ti, 14Ti and 15Sc...... 187

Table A-11 Crystal and structure refinement details for 16Sc and 17Sc...... 188

List of Figures

Figure 1-1 Structure of Ti(dmpe)EtCl3 β-agostic metal alkyl complex (left) with bond lengths and angles for the agnostic ethyl group (right)...... 4 Figure 1-2 Coordination modes of H ligand to transition metal(s)...... 7

Figure 1-3 Relative energies of M(dπ) and C(2pz) in Schrock-type carbenes...... 10 Figure 1-4 Transition metal bis-iminophosphorane (left) or bis-sulfidophosphorane (right) complexes...... 11 Figure 1-5 Selected examples of transition metal phosphinoalkylidene complexes.75-77 12 Figure 1-6 Examples of bridging and Lewis acid-supported imido scandium complexes.62,78,79 ...... 13 Figure 1-7 Examples of other terminal imido rare-earth metal complexes.81-83 ...... 15 Figure 1-8 Examples of neutral tetrapodal pentadentate ligand...... 18 Figure 1-9 Statistical representation of tetrapodal pentadentate ligands grouped by their κ5 coordination environment (left) and net formal charge of their coordinating atoms (right). (Adapted with permission from Ref. 96. Copyright 2016 American Chemical Society.) 19 Figure 1-10 Selected examples of dianionic pentadentate ligands.97,101,102 ...... 20

Figure 2-1 Molecular structures of 1Sc-Ph (left) and 1Sc (right). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 26

Figure 2-2 Side view of the molecular structure of 1Sc with an arrow to indicate dihedral angle between the mean plane of the axial pyridyl ring and that defined by the equatorial nitrogen atoms...... 27 1 Figure 2-3 H NMR spectra of [Ph-B2Pz4LiPyH]2 (top) and 1Sc-Ph (bottom) in THF-d8 highlighting chemical shift changes upon metalation...... 28

Figure 2-4 Molecular structures of 2Sc-Me (left) and 2Sc-CH2SiMe3 (right). Except for those on C32 of 2Sc-Me, hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 30

Figure 2-5 Top-down view of the molecular structure of 2Sc-Me with arrows indicating narrow and wide grooves around scandium...... 31

Figure 2-6 Superimposed FT-IR spectra of 2Sc-Me and 2Sc-CH2SiMe3...... 32

Figure 2-7 Molecular structure of 3Sc. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 34 1 i Figure 2-8 H NMR spectra in C6D6 showing 2Sc- Bu (bottom) and after prolonged heating (top) with inset expansion region of isobutene...... 35 1 Figure 2-9 H NMR spectrum of 2Sc-H with inset expansion region of the Sc–H signal.37

Figure 2-10 Superimposed FT-IR spectra of 2Sc-H and d1-2Sc-H (left) with expansion of the Sc–H region showing the effect of isotopic substitution (right)...... 37

Figure 2-11 Signer method for the molecular weight determination of 2Sc-H...... 39

Figure 2-12 Computed optimized structure of [2Sc-H]2 by DFT with B3PW91 functional...... 40 1 Figure 2-13 H NMR of the reaction progression of 2Sc-H and N2O in C6D6 with inset expansion of the H2 region. Ligand resonances for 2Sc-H change over time to 3Sc when the reaction mixture is heated...... 42

Figure 2-14 Molecular structures of 4Sc-H (left) and 4Sc-Me (right). Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 44

Figure 2-15 Computed reaction profile (DFT, B3PW91) for the insertion of CO2 into the Sc–H and Sc–C bonds of 2Sc-H (green line) and 2Sc-Me (black line). ΔG values given in parentheses...... 45

Figure 2-16 Computed transition state structures (DFT, B3PW91) for the insertion of CO2 into the Sc–H and Sc–C bonds of 2Sc-H (left) and 2Sc-Me (right)...... 46 1 Figure 2-17 H NMR spectra of 2Sc-Me (top) and after addition of B(C6F5)3, 5Sc (bottom) in C6D6. Change in Sc–CH3 peak is indicated with dots. Solvent residual signal indicated with an asterisk...... 49

Figure 2-18 Molecular structure of 5Sc-THF. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 50

Figure 2-19 Molecular structure of 6Sc. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 52

Figure 2-20 Molecular structure of 7Sc. Hydrogen atoms on the cation have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 54 1 Figure 2-21 H NMR spectra in C6D6 of 7Sc (top) and after addition of 1 atm of CO2 (bottom). Coloured dots indicate assignment of products to 1Sc and 8Sc-H. Solvent residual signal indicated with an asterisk...... 56

Figure 2-22 Molecular structure of 8Sc-H. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 58

Figure 2-23 Before (top) and after (bottom) heating the reaction mixture of 5 mol% 8Sc-Ph- Me, Et3SiH in 1 atm CO2 at 50 °C for 18 h in C6D5Br...... 61 1 Figure 2-24 Stacked H NMR spectra in C6D6 showing the initial reaction mixture of 8Sc- H, Et3SiH and B(C6F5)3 (bottom), and after heating at 80 °C for 2.5 h (middle). Complete formation of the unknown complex with further addition of Et3SiH and heating overnight (top)...... 64

Figure 2-25 Molecular structure of 9Sc-Et. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 65

Figure 3-1 GC (left) and GC-MS (right) traces of the headspace from the reaction of 4Sc- H or 4Sc-H’ and KHMDS with gas identification labels for the observed peaks...... 71 1 Figure 3-2 H NMR spectrum of the resulting solid in C6D6 after exposing 2Sc-CH2SiMe3 briefly to air, with red dots corresponding to 3Sc. Only certain non-overlapping ligand xi resonances for [2Sc-OH]2 have been integrated. Solvent residual signal indicated with an asterisk...... 75 1 Figure 3-3 H NOESY spectrum of the mixture of [2Sc-OH]2 and 3Sc in C6D6. Through space correlation between pyrazolyl protons (green dots) and hydroxyl proton in [2Sc-OH]2 is indicated by a dash line...... 76

Figure 3-4 Molecular structure of [2Sc-OH]2. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 77

Figure 3-5 DOSY spectrum of the mixture of 2Sc-OH and 3Sc in C6D6. Some cross peaks corresponding to 2Sc-OH are indicated with arrows and red dots...... 79

Figure 3-6 Top-down view of the molecular structure of 10Sc. Most of the B2Pz4Py ligand displayed as wireframe style and most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 81

Figure 3-7 Side view of the molecular structure of 10Sc. Most of the B2Pz4Py ligand displayed as wireframe style and most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 81

Figure 3-8 Asymmetric unit of the molecular structure of 10Sc. Hydroxyl groups from the adjacent unit (labelled with a prime symbol) are added to show hydrogen bonding interaction. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 82 1 Figure 3-9 H NMR spectra in C6D6 of the addition of 1 (middle) and 2 (bottom) equivalents of HNTf2 to 3Sc. Complete formation of 11Sc with excess HNTf2. Solvent residual signal indicated with an asterisk...... 83

Figure 3-10 Coordination modes of bis(trifluoromethanesulfonyl)imide anion (NTf2) to a metal center...... 84

Figure 3-11 Connectivity map of Tol-B2Pz4PySc-NTf2 (11Sc)...... 85

Figure 3-12 Side (left) and front (right) view of the molecular structure of 12Sc. Triflimide anions on the right side and most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 86

Figure 3-13 FT-IR spectrum of 12Sc showing prominent O–H and N–H bond stretches at 3590 and 3329 cm-1 respectively...... 87

Figure 4-1 Molecular structures of 1Ti-Ph (left) and 1Ti (right). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 92 1 Figure 4-2 H NMR spectrum of 1Ti in C6D6. Solvent residual signal indicated with an asterisk...... 94

Figure 4-3 Connectivity map of 2Ti-Me (left) and molecular structure of 2Ti-CH2SiMe3 (right). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 96

Figure 4-4 Molecular structure of 3Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 97

xii

1 Figure 4-5 H NMR spectrum of [2Ti-H]2 in C6D6 with inset expansion region of the Ti-H signal. Ligand resonances are assigned with numbers. Solvent residual signal indicated with an asterisk...... 99

Figure 4-6 Signer method for the molecular weight determination of [2Ti-H]2...... 100

Figure 4-7 Molecular structure of [2Ti-H]2. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 100

Figure 4-8 Geometry optimized structures of the dissociation of [2Ti-H]2 into the monomers proceeding through a loosely associated dimer (middle)...... 103

Figure 4-9 DFT calculated reaction profile for the insertion of CO2 into the Ti–H bond of 2Ti-H. ΔG values given in parentheses...... 104

Figure 4-10 Superimposed FT-IR spectra of 4Ti-H and 4Sc-H (left) with expansion of the C–O region showing the effect of denticity on bond stretches (right)...... 105 1 Figure 4-11 H NMR spectra of 7Ti (top) and 8Ti-H (bottom) in C6D6. Solvent residual signal indicated with an asterisk...... 107 1 Figure 4-12 H NMR spectrum of 6Ti in C6D6 with coloured dots to indicate assignment of ligand resonances. Solvent residual signal indicated with an asterisk...... 110 1 19 1 Figure 4-13 H (left) and F{ H} (right) NMR spectra of 11Ti in C6D6. Solvent residual signal indicated with an asterisk...... 112 19 1 Figure 4-14 F{ H} NMR spectrum of the triflimide region of 11Ti in Tol-d8 at variable temperatures...... 113

Figure 4-15 Molecular structure of 11Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 114 1 Figure 4-16 H NMR spectrum of the reaction mixture of 3Ti and 1 atm O2 in CDCl3. Peak assignment indicated by coloured dots while solvent residual signal indicated with an asterisk...... 116

Figure 4-17 Molecular structure of 13Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 117

Figure 4-18 Molecular structure of 14Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 119

Figure 5-1 DFT calculated HOMO-2 and HOMO-4 orbitals of the ground state of 3ScCo showing π bonding interaction between scandium and oxygen...... 125

Figure 5-2 Molecular structures of 15Sc (left) and 16Sc (right). Isopropyl groups on 16Sc and most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 129

Figure 5-3 Molecular structure of 17Sc. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level...... 133

xiii

List of Schemes

Scheme 1-1 General methods for synthesizing transition metal methyl complexes...... 2 Scheme 1-2 Representation of β-C–H bond agostic interaction (left) and β-elimination in a transition metal ethyl complex. Vacant coordination site is indicated by □...... 3 Scheme 1-3 Selected examples of alkyl abstraction of a scandium dialkyl complex for their use in olefin polymerization.36,37...... 5 Scheme 1-4 Scandium-catalyzed enantioselective intermolecular hydrosilylation of unsaturated C–C bonds with secondary silanes...... 6 Scheme 1-5 Preparation of dimeric benzamidinato-supported hydrido complex by hydrogenolysis of a dialkyl complex. Dipp = 2,6-diisopropylphenyl...... 7 Scheme 1-6 Hydride migration or formation of stable bridging hydrido complex upon hydrogenolysis of scandium and yttrium alkyl complexes...... 8 Scheme 1-7 Synthesis of monomeric Sc–H complexes by sigma bond metathesis (top) or activation of H2 by scandium terminal imido complex (bottom)...... 9 Scheme 1-8 Synthesis of terminal imido scandium complex supported by DMAP.80 ..... 14 Scheme 1-9 Synthesis of a terminal oxo scandium complex via methylidene transfer to benzophenone...... 16 Scheme 1-10 Synthesis of a terminal oxo cerium complex stabilized by hydrogen bonding to acetamide...... 16 103,104 Scheme 1-11 Synthesis of [Ar-B2Pz4LiPyH]2 proligand...... 21

Scheme 2-1 Synthesis of 1Sc-Ph and 1Sc by salt metathesis...... 25

Scheme 2-2 Synthesis of 2Sc-R by salt metathesis...... 29

Scheme 2-3 Reactions of 2Sc-R with water...... 33

Scheme 2-4 Synthesis of scandium hydride complex 2Sc-H...... 36

Scheme 2-5 Synthesis of 4Sc-R either through CO2 insertion with 2Sc-R or salt metathesis with 1Sc...... 43

Scheme 2-6 Synthesis of 5Sc and 5Sc-THF via alkyl abstraction followed by addition of THF...... 48

Scheme 2-7 Synthesis of hydridoborate complexes 6Sc and 7Sc...... 53

Scheme 2-8 Formation of 8Sc-H with the elimination of 1Sc when 7Sc is exposed to CO2...... 56

Scheme 2-9 Alternate synthesis of 8Sc-R and contributing resonance structures...... 57 134 Scheme 2-10 Reduction of CO2 to CH4 with decamethylscandocenium hydridoborate...... 60

Scheme 2-11 Hydrosilylation of formate moiety starting from 8Sc-H to form 6Sc which can activate CO2 once again...... 63

xiv

Scheme 2-12 Suggested deactivation pathway in the hydrosilylation of CO2...... 64 Scheme 3-1 Synthesis titanium(IV) terminal oxo anion complexes.163,164...... 68

Scheme 3-2 Deprotonation of 4Sc-H resulting in the immediate formation of 3Sc...... 70

Scheme 3-3 Trapping reactive intermediate [K][Sc=O] and proposed pathways to 1Sc with excess Me3SiCl...... 73

Scheme 3-4 Formation of 10Sc from successive hydrolysis of a scandium alkyl complex 2Sc-R...... 80

Scheme 3-5 Protonation of 3Sc with HNTf2 to form 2Sc-OH and 11Sc...... 83

Scheme 4-1 Synthesis of 1Ti-Ph and 1Ti by salt metathesis...... 91

Scheme 4-2 Synthesis of 2Ti-R by salt metathesis...... 95

Scheme 4-3 Synthesis of titanium hydride complex [2Ti-H]2...... 98

Scheme 4-4 Synthesis of 8Ti-H with B(C6F5)3-activated hydrosilane in the presence of CO2...... 106

Scheme 4-5 Hydrosilylation of formate moiety starting from 8Ti-H, and the suggested deactivation pathway to form [2Ti-H]2...... 108

Scheme 4-6 Synthesis of 11Ti by halide abstraction...... 111

Scheme 4-7 Oxidation of 3Ti with O2...... 115

Scheme 4-8 Synthesis of 13Ti by oxidation with H2O2...... 116 II Scheme 4-9 Synthesis of 14Ti by oxidation from a Ti species or by oxygen atom abstraction from 13Ti...... 118 Scheme 5-1 General synthesis of mixed metal oxides through dealkylation or deamination pathways...... 123

Scheme 5-2 Synthesis of oxo-bridged heterobimetallic 3ScCo and its contributing resonance structures via dealkylation of 2Sc-CH2SiMe3...... 124

Scheme 5-3 Synthesis of aryl (15Sc) or borylamido (16Sc) scandium complexes...... 127 Scheme 5-4 Scandium-mediated synthesis cycle for the formation of hydrazine derivatives with (E = carbon-based electrophile).248 ...... 131 II Scheme 5-5 Synthesis of side-on dinitrogen complex 17Sc from a reduced Sc species...... 132

Scheme 5-6 Synthesis of an oxorhenium Lewis acid-base adduct with B(C6F5)3 and its equilibrium to form a reactive FLP. Mes = 1,3,5-trimethylphenyl.265 ...... 135

Scheme 5-7 Synthesis of Lewis acid-base adduct between 14Ti and B(C6F5)3 and subsequent activation of H2...... 136

List of Symbols, Abbreviations and Nomenclature

Symbol Definition Å angstrom APCI atmospheric pressure chemical ionization Ar aryl group asym asymmetric atm atmospheres (unit of pressure) B3PW91 Becke three parameter hybrid functional with non-local correlation provided by Perdew/Wang in 1991 µB Bohr magneton (unit) br broad nBu butyl (normal) tBu tertiary-butyl ºC degrees Celsius ca. circa (approximately) calcd. calculated CASSCF complete active space self consistent field cm-1 wavenumbers Cp cyclopentadienyl Cp* 1,2,3,4,5-pentamethylcyclopentadienyl Cy cyclohexyl 19 Δδm,p difference in F NMR chemical shifts of meta and para fluorine peaks in species incorporating B(C6F5)3 d doublet dn deuterated, n = number of deuterium atoms dn n number of d-electrons D deuterium or diffusion constant DFT density functional theory Dipp 2,6-diisopropylphenyl DMAP dimethylaminopyridine DME 1,2-dimethoxyethane DOSY diffusion ordered spectroscopy µeff effective magnetic moment e.g. exempli gratia, “for example” Et ethyl equiv. equivalents FLP frustrated Lewis pair g gram GC-MS gas chromatography-mass spectrometry HOMO highest occupied molecular orbital hr hours HRMS high resolution mass spectrometry Hz hertz IR infra-red

xvi n JX-Y n (e.g. 1, 2, 3,…) bond coupling between atoms X and Y K Kelvin (unit) kB Boltzmann’s constant kcal kilocalories L generic ligand Ln lanthanide LUMO lowest unoccupied molecular orbital m multiplet m meta M metal or molar (mol L-1) Me methyl Mes 1,3,5-trimethylphenyl (mesityl) min minutes mg milligram MHz megahertz mL milliliter mmol millimole MS mass spectrometry nacnac β-diketiminate NOESY nuclear Overhauser effect spectroscopy NMR nuclear magnetic resonance NTf2 triflimide o ortho p para Ph phenyl ppm parts per million iPr iso-propyl nPr propyl (normal) Py pyridyl Pz pyrazolyl q quartet r hydrodynamic radius R hydrocarbon group s seconds or singlet sept septet sym symmetric t triplet T temperature THF tetrahydrofuran δ chemical shift in ppm κ denticity η hapticity or viscosity coefficient ν frequency μ bridging μL microliter {1H} proton decoupled

xvii

List of Numbered Compounds

xviii

xix xx

Chapter 1: Introduction

1.1 Transition Metal Alkyl Complexes

Transition metal complexes containing hydrocarbyl ligands M–R (where R = alkyl, aryl, vinyl or alkynyl) have played an immense role in catalyzing various organic transformations like hydrogenation, cross coupling and olefin polymerization.1 When viewed as an ionic model, metal alkyls result from the combination of an alkyl anion with a metal cation. Hence the alkyl anion is stabilized to a different extent depending on the electronegativity of the metal.2 While transition metal–alkyl bonds are largely covalent, these bonds are more polar when electropositive early transition metals are involved. As a result, alkyls of early transition metals in groups 3 and 4 are more susceptible to protonolysis and are highly air and water sensitive. Efforts to prepare transition metal alkyls date back to 1907 when Pope and Peachey reported the isolation of Me3PtI generated

3 from the reaction of PtCl4 and MeMgI. Development of transition metal alkyl complexes continued slowly over the next several decades with a growing focus on the synthesis of

4 homoleptic alkyl complexes MRn. The first hexamethyl transition metal complex, WMe6, containing the most common and simplest alkyl ligand was reported by Wilkinson and coworkers in 1973.5 This soon led to subsequent reports of other hexamethyl transition metal complexes.6-11 The preparation of these transition metal methyl complexes have been established to typically involve one of the following reactions: (1) metathesis of metal halides with a nucleophilic main group organometallic reagents, such as LiMe, MeMgX,

δ+ ZnMe2, etc.; (2) electrophilic attack on a metal complex using a Me –X reagent; (3) oxidative addition of Me–X reagent (X = halide, Scheme 1-1).2

Scheme 1-1 General methods for synthesizing transition metal methyl complexes.

Earlier work on the synthesis of stable d-block metal alkyl complexes with longer alkyl chain lengths or substituted alkyls were often unsuccessful. While it was originally thought to be a thermodynamic factor, strong transition metal–carbon bonds with typical bond strengths of 30-65 kcal mol-1 indicate that kinetic decomposition pathways were likely to be the cause for these synthetic failures.2 One major decomposition pathway is β- hydride elimination, in which a metal alkyl is converted into a hydridometal alkene complex (Scheme 1-2). However this process requires that (1) a hydrogen substituent on the β-carbon of the alkyl, (2) an open coordination site adjacent to the migrating hydride ligand, and (3) coplanar M–C–C–H unit for favourable alignment of the β-hydrogen atom with the metal orbital.12,13 Furthermore, the elimination is promoted for non-d0 metal centers as the formation of the σ bonded C–H intermediate is accompanied by backdonation into the C–H σ* orbital, hence reducing the C–H bond order.

Scheme 1-2 Representation of β-C–H bond agostic interaction (left) and β-elimination in a transition metal ethyl complex. Vacant coordination site is indicated by □.

Agostic interactions, coined by Brookhart and Green,14 consist of σ donation of the electron pair of a C–H bond from one of the pre-coordinated ligands to an empty d metal orbital.15 This 3-center-2-electron interaction can be detected by X-ray diffraction; the first structurally characterized β-agostic metal alkyl complex was reported by Green and coworkers in 1982.16 The compound, 1,2-bis(dimethylphosphino)ethane (dmpe) titanium ethyl trichloride (Ti(dmpe)EtCl3), features an acute angle of 85.9(6)° at the α-carbon whereas a typical angle close to 109° would be expected for a sp3 hybridized carbon atom

(Figure 1-1). The short H···Ti distance of 2.29 Å is also considerably shorter than the sum of the van der Waals’ radii, hence indicating an interaction between the C–H bond and titanium center. The agostic interaction is also accompanied by a 1H NMR peak that is shifted upfield from that of an uncoordinated aryl of alkyl C–H bond. The reduced C–H bond order in the agostic system also translates to a reduced νCH in the IR spectrum and

1 3 lowering of the JCH to around 50-100 Hz, compared to that of ≈125 Hz for a normal sp hybridized carbon atom.15,17 Although non-d0 metal centers and agostic C–H→M interactions are factors to consider when stabilizing metal alkyl complexes, the β-hydride elimination decomposition pathway can be avoided simply by utilizing alkyls that lack a

β-hydrogen atom. Introduction of bulky ligands is also an additional way to kinetically stabilize organometallic complexes. They aid in slowing down associative decomposition

3 pathways like reaction with solvent molecules or another molecule of the complex. Hence bulky ligands such neopentyl (CH2CMe3) or trimethylsilylmethyl (CH2SiMe3) are particularly useful in the preparation of homoleptic alkyl complexes or unique organometallic compounds.18

Figure 1-1 Structure of Ti(dmpe)EtCl3 β-agostic metal alkyl complex (left) with bond lengths and angles for the agnostic ethyl group (right).

The design and synthesis of new σ-bonded rare-earth (Group 3 and lanthanides) metal alkyl complexes have seen significant progress over the past several decades.19-22

Continuing interest in these complexes stems from their high reactivity and application in the catalytic transformation of unsaturated substrates such as hydrogenation,23,24 hydrosilylation25 and polymerization.26-31 Two different synthetic approaches are often used for the preparation of these rare-earth alkyl complexes. The first is based on metathesis reactions of mono or dihalo complexes LLnXn with an appropriate alkyl derivate of alkaline or alkaline-earth metals (vide supra). The second route is through an alkane elimination reaction of a rare-earth tris(alkyl) species LnR3 with the protio form of the ligand LH. Although mono(alkyl) rare-earth complexes have predominantly been the focus of attention, there is a continuing interest in bis(alkyl) species for the synthesis of cationic mono(alkyl) rare-earth complexes.19,32,33 This was prompted by the emergence of

4 cationic alkyl complexes of group 4 metals as the active species in metallocene-based

Ziegler-Natta olefin polymerization catalysts.34 The larger ionic radii of the rare-earth metals, in contrast group 4 metals, stabilizes the cationic portion better and the use of appropriate donor ligands L have enabled the isolation of these ionic organometallic complexes.35 These cationic species are typically generated by alkyl activation with an equimolar amount of an appropriate Brønsted acid such as [HNMe2Ph][B(C6F5)4] or strong

26 Lewis acid like B(C6F5)3 or [Ph3C][B(C6F5)4] (Scheme 1-3). These selected examples in the following scheme of cationic half-sandwich scandium complexes serve as excellent catalyst for the copolymerization of 1-hexene and ethylene (A) or 1-hexene and dicyclopentadiene (B), and report very high catalyst activity for a rare-earth metal catalyst.

Scheme 1-3 Selected examples of alkyl abstraction of a scandium dialkyl complex for their use in olefin polymerization.36,37

1.2 Transition Metal Hydrides

Molecular transition metal hydride complexes continue to attract much interest due to their structural diversity and its role in synthetic and catalytic organometallic chemistry.38-40 Early transition metal hydrides have been employed for a wide variety of organic transformations such as olefin hydrogenation,41 polymerization42 and CO reduction.43-45 Rare-earth metal hydrides in particular are extremely reactive compounds38,46,47 that are key intermediates in several hydroelementation reactions48,49 and

C–H bond activation processes; both of which typically proceed via a sigma bond metathesis mechanism.50-54 For example, Hou and co-workers reported enantioselective intermolecular hydrosilylation of alkene by a chiral half-sandwich scandium catalyst while employing reaction pathways that are well established for d0 early transition metal complexes (Scheme 1-4).55 The reactive scandium hydride is generated which inserts an alkene to give an alkyl species. It subsequently undergoes sigma bond metathesis with a silane to yield the silylation product and regenerating the hydride species. These reactive metal hydrides are usually generated in situ; when isolated, they tend to form dimers or higher oligomers due to the high Lewis acidity and coordinative unsaturation of this family of compounds and the facility with which the hydride ligand can act to bridge metal centers

(Figure 1-2).56,57

Scheme 1-4 Scandium-catalyzed enantioselective intermolecular hydrosilylation of unsaturated C–C bonds with secondary silanes.

Figure 1-2 Coordination modes of H ligand to transition metal(s).

The reactivity of hydride complexes is greatly dependent on the electronic and geometrical properties of the metal center. As such, the design of auxiliary ligand systems is an attractive avenue to synthesize and isolate hydride complexes with unique properties.

However, rare-earth metal hydride complexes are mostly supported by the η5- cyclopentadienyl ligand and its substituted derivatives.21,58 Much work has since then progressed into designing post-metallocene type ligand scaffolds to impart electronic and steric control around the metal center. Hou and co-workers reported the hydrogenolysis of amidinate-ligated rare-earth dialkyl complexes to afford the corresponding binuclear tetrahydride complex in which three hydrides adopt a µ2-bonding mode to bridge the two metal atoms, while one is a terminal hydride ligand (Scheme 1-5).59 However the dimeric species slowly reacts with THF to form an nbutoxide/hydride complex in which the terminal hydride ligand ring opens a THF molecule.

Scheme 1-5 Preparation of dimeric benzamidinato-supported hydrido complex by hydrogenolysis of a dialkyl complex. Dipp = 2,6-diisopropylphenyl.

The Piers group also investigated the reactivity of base free organoscandium and organoyttrium complexes supported by the bulky salicylaldiminate ligand with H2 (Scheme

1-6).60 Hydrogenolysis of the yttrium alkyl complex resulted in the corresponding hydride complex which is dimeric in both solution and solid state. In the case of scandium, the hydride species which is unstable as a monomer was not observed, and the product was proposed to form through the nucleophilic attack of the hydride onto the aldimine carbon.

This reactivity was attributed to the bulky ancillary ligand and scandium’s smaller ionic radius which caused it to be too sterically encumbered to dimerize.

Scheme 1-6 Hydride migration or formation of stable bridging hydrido complex upon hydrogenolysis of scandium and yttrium alkyl complexes.

Fine tuning the steric properties of the supporting ancillary ligands is therefore crucial for the stabilization of the highly polarizable, soft hydride at the Lewis acidic rare- earth metal center. As a result, examples of monomeric group 3/lanthanide metal hydrides remain rare and they typically involve highly sterically crowded or multidentate ligand environments.61 This is even true for the smallest of this family of elements, scandium.35,62-

65 To date, very few monomeric Sc–H have been reported and only one has been structurally characterized. Bis(pentamethylcyclopentadienyl)scandium hydride is of unknown nuclearity, but its tetrahydrofuran (THF) adduct is proposed to be monomeric as determined by solution molecular weight measurements (Scheme 1-7).54 Chen and co- workers recently reported the β-diketiminato (nacnac) supported monomeric scandium complex that features a terminal Sc–H moiety generated by the addition of dihydrogen (H2) across the Sc=N bond of a scandium imido precursor (Scheme 1-7).66 Evidently, the pendant amine donor is able to prevent transfer of the hydride to the imine functional groups of the nacnac ligand, as has been observed in complexes with more conventional nacnac or salicylaldiminato ligands.60

Scheme 1-7 Synthesis of monomeric Sc–H complexes by sigma bond metathesis (top) or activation of H2 by scandium terminal imido complex (bottom).

1.3 Rare-Earth M=E Complexes

The field of transition metal–ligand multiple bonds (M=E/M≡E; M = transition metal, E = main-group element) continues to be of great interest in organometallic

9 chemistry not just from a fundamental and chemical bonding approach, but also from a practical perspective in which they exhibit versatile stoichiometric and catalytic activities.

Terminal alkylidene, imido and phosphinidene complexes (M=E, where E = C, N and P respectively) involve favourable bonding interactions between the valence shell d orbitals of the metal ion and p orbitals of the ligand atom if they have similar orbital energies.

However, examples of multiple bonds to rare-earth metals remain scarce due to relative mismatch of metal and ligand orbital energies. The more electropositive metals have higher energy, or less stable, d orbitals which result in a greater difference in the HOMO/LUMO orbital energies, thus creating a highly polarized Mδ+=Eδ- bond (Figure 1-3).67

Consequently various strategies to stabilize these complexes include the use of labile pendant chelating ligand, or an external Lewis acid or base to coordinate to the element or metal center respectively.

Figure 1-3 Relative energies of M(dπ) and C(2pz) in Schrock-type carbenes.

Early transition metal alkylidene, commonly viewed as a Schrock carbene, are nucleophilic in nature and stabilize high oxidation state metal centers by the generation of a covalent double bond.68 The bonding becomes increasingly ionic on moving further left

10 in the periodic table and these alkylidenes react with ketones and aldehydes to give the corresponding metal oxide and alkene. Tebbe’s reagent, Cp2Ti(μ-CH2)(μ-Cl)Al(CH3)2, is a well-known isolable group 4 alkylidene that upon addition of a Lewis base such as pyridine, DMAP or THF, generates the transient methylidene Cp2Ti=CH2 which readily undergoes [2+2] cycloaddition with olefins or alkynes to afford metallacyclobutane or metallacyclobutane products respectively.69 Another strategy to stabilize early transition metal carbenes is the incorporation of phosphorus substituents on the carbenes as it was calculated to have a carbanion stabilization energy of –89.1 kJ mol-1, without which would typically result in polymetallic rare earth clusters with bridging carbenes (Figure 1-4).70

Employing these methandiide dianions with P(V) substituents thus led to the isolation of mononuclear rare-earth metal complexes.71-74 Although these complexes participate in metallo-Wittig chemistry with carbonyl compounds, [2+2] cycloadditions and C–H bond activation chemistry across the M=C double bond, reactivity of such complexes were quite sluggish. This was attributed to the strong electronic withdrawing P(V) substituents which reduced the overall polarity of the M=C double bond.

Figure 1-4 Transition metal bis-iminophosphorane (left) or bis-sulfidophosphorane (right) complexes.

A more uncommon approach to stabilize the alkylidene moiety is to have one phosphorus substituent in the +3 oxidation state on the carbene in which dative bonding of

11 the lone pair of electrons on phosphorus to the metal center occurs. The installation of the phosphinoalkylidene can occur through a variety of methods; early work by Gibson and co-workers showed that the reduction of Cp*TaCl4 with sodium and neat PMe3 resulted in

75 double C–H activation of a bound PMe3 (left, Figure 1-5). Alternatively the addition of

LiCH2PPh2 to a PNP-pincer supported titanium alkylidene triflate complex

(PNP)Ti=CHtBu(OTf) resulted in the formation of the phosphinoalkylidene complex. It

t was proposed that elimination of LiOTf and H3C Bu generated a reactive Ti≡CPPh2 moiety which subsequently activates benzene via a 1,2-C–H bond addition to afford the said complex (middle, Figure 1-5).76 More recently, the Chen group showed that thermolysis of a nacnac supported scandium methyl phosphinoalkyl complex

(nacnac)Sc(CH3)(CHSiMe3PPh2) at 50 °C in THF eliminated methane to give the phosphinoalkylidene complex (right, Figure 1-5).77

Figure 1-5 Selected examples of transition metal phosphinoalkylidene complexes.75-77

Advances in the synthesis and reactivity of terminal imido complexes of rare earth metals have only flourished since 2010. Prior to this, these complexes were generally isolated as dimeric species or higher nuclearity clusters which contained bridging imido

12 ligands. In 2003, Hessen and co-workers reported the unexpected dimeric µ2-bridging imido scandium complex formed from a reaction between scandium but-2-ene-1,4-diyl and benzonitrile (left, Figure 1-6).78 Formation of the bridging imide complex proceeds via insertion of the C≡N bond into an Sc–C bond and subsequent cyclization and dimerization of the three-coordinate imido complex. Mindiola and co-workers also showed that a PNP- pincer scandium methyl amide complex can eliminate methane to form an AlMe3-masked scandium imide complex at room temperature in the presence of AlMe3 (middle, Figure

79 1-6). Although displacement of AlMe3 with pyridine is possible, this resulted in ortho-

2 C–H activation of pyridine to afford the pyridyl complex (PNP)Sc(NHDipp)(η -NC5H4).

A scandium imido trimer was reported by the Piers group which was obtained from the reaction between a β-diketiminato supported scandium dichloride complex and LiHBEt3

(right, Figure 1-6).62 The reaction proceeds through a transient scandium hydride chloride species, which undergoes hydride transfer to the imine carbon of the β-diketiminato ligand.

This induces fragmentation of the ancillary ligand to give an organic imine and the scandium imido cluster in the presence of LiHBEt3.

Figure 1-6 Examples of bridging and Lewis acid-supported imido scandium complexes.62,78,79

It was in 2010 that Chen and co-workers successfully isolated the first terminal imido rare-earth metal complex using 4-dimethylaminopyridine (DMAP) as a strong external Lewis base.80 Heating a nacnac supported scandium methyl amido complex in the presence of DMAP gave the terminal imido complex with the elimination of methane

(Scheme 1-8). Following the preparation of (nacnac)Sc(NDipp)(DMAP), other research groups employed a similar strategy to prepare other terminal imido scandium complexes.

The Piers group also noted the slow formation of the scandium imide when a simplified nacnac supported, without the pendant amine donor group, scandium alkyl amido complex was heated at 50 °C over 5 days (left, Figure 1-7).81 The Chen group further modified their nacnac ligand to incorporate a second pendant amine donor which only chelates to scandium upon formation of the imide, hence stabilizing metal center without the need for

DMAP (right, Figure 1-7).82 More recently Anwander and co-workers prepared the first terminal lanthanide-imido complex supported by a tris(pyrazolyl)borate ligand via the same approach of methane elimination in the presence of DMAP (right, Figure 1-7).83

Scheme 1-8 Synthesis of terminal imido scandium complex supported by DMAP.80

Figure 1-7 Examples of other terminal imido rare-earth metal complexes.81-83

Although arylimido functionality is an increasingly common feature in rare-earth– ligand multiple bonded complexes, terminal oxo complexes still remain elusive. To date only one example of a terminal oxo scandium complex has been reported. Mindiola and co-workers reported a PNP-pincer scandium methylidene complex supported by two

AlMe3 ligands being able to transfer the methylidene ligand to a ketone to yield the scandium oxo complex and the corresponding olefin (Scheme 1-9).84 This was supported

1 by the formation of the terminal olefin, H2C=CPh2, in the H NMR spectrum as well as the solid state structure. The scandium oxo complex features a short Sc–O bond length of

2.008(2) Å, while longer than a scandium alkoxide distance (~1.9 Å) due to strong interaction with AlMe3 that lowers π interaction between scandium and oxygen, is shorter than a dative bond Sc←OR2 (~2.2 Å) typical of a coordinating ether.

Scheme 1-9 Synthesis of a terminal oxo scandium complex via methylidene transfer to benzophenone.

It was only later in 2014 that Williams, Lin, Leung and co-workers reported tetravalent cerium terminal oxo complex, the first of the lanthanide series.85 Treatment of a bulky cerium dichloride complex with silver oxide led to the formation of a cerium oxo aquo complex stabilized by acetamide which was formed in the oxidative hydrolysis of acetonitrile. The strong nucleophilicity of the Ce=O moiety causes it to react readily with

IV III CO2 and CO to give Ce and Ce bridging carbonate complexes respectively.

Scheme 1-10 Synthesis of a terminal oxo cerium complex stabilized by hydrogen bonding to acetamide.

1.4 Pentadentate Ligand Systems

The high electropositivity of early transition metals has created synthetic challenges in terms of isolating reactive functionalities at the metal center, such as metal hydrides or

16 metal–element multiple bonds. Due to their high polarity, these complexes often dimerize or require some sort of external Lewis acid or base to stabilize the complex which typically comes at the expense of lowered reactivity towards further functionalization. Therefore ligand design is fundamental when targeting and isolating these reactive mononuclear species. A single open coordination site on the metal center of a molecular species would be ideal as it would allow for systematic studies on stepwise transformations which could potentially lead to stabilization of reactive intermediates. The ease of synthesis of supporting ligands should be of importance as it would allow for facile tuning of electronic and steric properties around the metal center, thereby influencing its stability and activity.

Pentadentate pentaamine ligands are a widely used set of ligands which have five nitrogen donor atoms binding to a single metal ion, and provides greater stability compared to monodentate or bidentate ligands.86 These ligands can be comprised entirely of 5 aliphatic N-donor atoms, however this can lead to multiple stereoisomers based on connectivity which could complicate characterization of the metal complex.87

Alternatively, incorporating more aromatic N-donor atoms would aid in rigidifying the ligand scaffold and impose a square pyramidal coordination mode to the metal center.

These tetrapodal pentadentate ligands form discrete mononuclear octahedral complexes and restrict reactivity of substrates to a single coordination site on the metal center. A well known example of the pentadentate ligand is the neutral 2,6-bis[methoxybis(2- pyridyl)methyl]pyridine (PY5) ligand which was reported separately by Stack88 and

Feringa89 in 1997 (Figure 1-8). Its coordination properties were further studied on first row transition metals of groups 7–12 and it was shown that the axial pyridyl ring tilts away from orthogonality from the plane defined by the four equatorial nitrogens.90 This

17 distortion affects σ- and π-interactions between the axial pyridyl ring and the metal’s d orbitals, thereby affecting reactivity of these complexes at the trans position. Although the

PY5 ligand features methoxy substituents on the quaternary carbons of the ligand backbone, other substituents can be employed to vary the ligand’s overall donor properties.

For example, the methyl derivative of the PY5 ligand (PY5Me2) has been extensively studied in its application in water reduction catalysis.91-94 Modifications to the PY5 ligand by substituting the equatorial pyridyl rings for pyrazolyl donors led to the generation

95 of a related ligand framework, pz4lut (Figure 1-8). The smaller pyrazolyl rings result in a larger steric accessibility in comparison to the PY5 ligand and complexes supported by pz4lut were also shown to have nearly orthogonal dihedral angles in contrast the analogous

PY5 supported complexes. Hence chemical reactivity, steric and electronic properties of resulting complexes can be tuned through simple ligand modifications.

Figure 1-8 Examples of neutral tetrapodal pentadentate ligand.

Although most of these pentadentate ligands contain neutral aliphatic or aromatic nitrogen-donating groups, incorporation of carbon-, phosphorus-, oxygen-, or sulfur-based donors in the ligand scaffold still remain uncommon. A comprehensive survey reported by

Chang and Long on tetrapodal ligands forming mononuclear complexes and exhibiting κ5 coordination mode up to 2016 reveals that about 77% of ligands are purely nitrogenous

(left, Figure 1-9).96 Interestingly there are no reported examples of early transition metal complexes supported by tetrapodal ligands and these ligands have only been employed on

Mn, Fe, Co, Ni, Cu and Zn.

Figure 1-9 Statistical representation of tetrapodal pentadentate ligands grouped by their κ5 coordination environment (left) and net formal charge of their coordinating atoms (right). (Adapted with permission from Ref. 96. Copyright 2016 American Chemical Society.)

Given that the majority of these pentadentate ligands are neutral, cationic complexes supported by counteranions result upon complexation. Since it has been shown that anionic donors are able to support high-valent metal complexes,97-100 employing a suitable dianionic tetrapodal ligand would be advantageous in generating neutral MIII octahedral complexes with an X-type ligand at the apical position where reactivity can occur. However only a few examples of dianionic pentadentate ligands have been reported thus far (right, Figure 1-9). Bominaar, Münck and Que reported a mononuclear FeIV oxo complex supported by dicarboxylate groups that mimicked the active site of non-heme iron

19 enzymes (left, Figure 1-10),97 while Aime and co-workers studied the stability of a diamino dicarboxylate copper complex (middle, Figure 1-10).101 More recently, Fout and co- workers incorporated the dianionic charge through equatorial pyrrolyl groups and studied the interconversion between iron aquo and hydroxo complexes (right, Figure 1-10).102 This was facilitated by taking advantage of the tautomerizable pyrrole-imine motif which converts it into a datively bonded azafulvene-amine.

Figure 1-10 Selected examples of dianionic pentadentate ligands.97,101,102

1.4.1 Dianionic B2Pz4Py Ligand

In the past few years, the Piers group has been focusing on a new class of dianionic pentadentate ligand in which the charges are brought about through the incorporation of borate functionality in the ligand framework. Given that the apical M–X bond is sterically more accessible when equatorial pyrazolyl groups are present, a synthetic methodology was then developed to introduce borate functionality at the quarternary carbon position of

103 pz4lut (Scheme 1-11).

103,104 Scheme 1-11 Synthesis of [Ar-B2Pz4LiPyH]2 proligand.

The bromo(dimethylamino)arylborane and 2,6-bis(trimethylstannyl)pyridine building blocks can by synthesized in excellent yields from commercially available starting materials and the proligand was obtained over five steps in moderate yield. The proligand was found to exist as a dimeric species by X-ray crystallography and the dianionic charge from the borate functionality is balanced by a lithium cation, which coordinates to pyrazolyl rings from opposing units in a tetrahedral environment, and a pyridinium proton.

Initial research on the work presented herein focused primarily on the phenyl-substituted ligand [Ph-B2Pz4LiPyH]2 however hydrolyzed by-products were often obtained, and their solid-state structures will only be mentioned very briefly. It was soon realized that the resulting phenyl-substituted complexes were sparingly soluble in common organic solvents. Thus the more soluble tolyl-substituted [Tol-B2Pz4LiPyH]2 proligand was employed which was synthesized through the same procedure as outline in Scheme 1-11 21 except starting from trimethyl(4-methylphenyl)silane. Furthermore, installation of the solubilizing methyl groups also served as a diagnostic signal when analyzing complexes by 1H NMR spectroscopy.

1.5 Thesis Goals

In view of the ongoing interest in the chemistry of rare-earth metal hydrides and metal–element multiple bonds, the need for a sterically crowded or polydentate ligand environment is necessary for stabilizing these rare-earth metal complexes. This will be accomplished by using the B2Pz4Py ligand framework to form strong Sc–N bonds between the acidic and hard Sc3+ ion, and the nitrogen donor atoms. It is hoped that tetrapodal pentadentate ligand would be sterically bulky to discourage dimerization of complexes containing reactive M–H or M=E bonds. Furthermore, the B2Pz4Py ligand can be used to stabilize cationic complexes formed by hydride or alkyl abstraction with a strong Lewis acid, and examining the activity of these complexes as CO2 hydrosilylation catalysts. This would aid in the isolation of unique monomeric octahedral complexes with a single site of reactivity at the apical position. Hence allowing for controlled reactivity studies with small molecules and better understanding of these transformations. Chapter 2 describes the synthesis of a scandium chloro complex supported by the B2Pz4Py ligand, which creates avenues for further functionalization through a salt elimination route. This allows for the isolation of the targeted scandium alkyl and hydride complexes, and their reactivity with small molecules will be discussed. Cationic scandium complexes will be generated with the use of B(C6F5)3 and their potential as a catalyst for the hydrosilylation of carbon dioxide

22 will be studied. Chapter 3 investigates the formation and reactivity of the elusive scandium hydroxo and anionic terminal oxo complexes based on the compounds covered in the preceding chapter. Finally, Chapter 4 presents the synthesis of B2Pz4Py titanium complexes, and comparisons of its coordination chemistry and reactivity to the analogous scandium system.

Chapter 2: B2Pz4Py Supported Scandium Alkyl and Hydride Complexes

2.1 Introduction

This chapter focuses on the synthesis and characterization of the various scandium complexes supported by the pentadentate B2Pz4Py ligand. The scandium chloro precursor complex is functionalized through salt metathesis reactions to afford a variety of stable scandium alkyl complexes. Their reactivity through sigma bond metathesis and β-hydride elimination to form the scandium hydride complex is also explored. These alkyl and hydride complexes react with small molecules like H2O, N2O and CO2, and the reaction profile for CO2 insertion was analyzed by DFT calculations. The synthesis of cationic scandium complexes via alkyl or hydride abstraction with strong Lewis acid B(C6F5)3 will also be discussed. In particular, the cationic scandium hydridotris(pentafluorophenyl)borate complex is able to activate CO2 to a formatoborate moiety, which is subsequently transferred to a B(C6F5)3-activated Et3SiH to form triethylsilylformate. The resultant B(C6F5)3-catalyzed reaction can further hydrosilylate triethylsilylformate to ultimately yield CH4.

2.2 Synthesis of B2Pz4Py Scandium Chloro Complex

Installation of ligands by salt metathesis is a commonly used strategy to synthesize organoscandium alkyl complexes.47,54,105,106 Although three steps are involved (lithiation of pro-ligand, metalation with metal halides, alkylation by salt metathesis) instead of the

24 more direct alkane elimination route by reacting a protio-ligand with a homoleptic metal alkyl complex, metathesis to afford the corresponding chloride complex is appealing as they can be easily functionalized to other derivatives.107

Previously established procedures for the synthesis of late metal B2Pz4Py complexes utilized LiOiPr as a base to deprotonate the proligand, however it had to replaced with LiHMDS to avoid potentially forming the corresponding scandium isopropoxide complex from any unreacted base due to the high oxophilicity of early transition metals.103,104,108 Deprotonation of either the phenyl or p-tolyl substituted ligand precursor [Ar-B2Pz4LiPyH]2 with LiHMDS in THF resulted in a translucent solution within a few minutes and subsequent addition of solid ScCl3(THF)3 to the in situ generated dilithio salt and stirring overnight led to the formation of the colourless Sc–Cl starting material 1Sc-Ph or 1Sc in excellent yield (Scheme 2-1). Colourless crystals of these moisture sensitive compounds were obtained from slow vapour diffusion of pentane into a saturated benzene solution, and their structures confirmed via X-ray crystallography (Figure 2-1).

Scheme 2-1 Synthesis of 1Sc-Ph and 1Sc by salt metathesis.

Figure 2-1 Molecular structures of 1Sc-Ph (left) and 1Sc (right). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 2-1 Selected bond distances (Å) and angle (°) for 1Sc-Ph and 1Sc.

1Sc-Ph 1Sc Sc–Cl 2.419(1) 2.396(1) Sc–NPz(avg) 2.205(6) 2.204(7) Sc–NPy 2.272(3) 2.306(3) Sc–NPz Plane 0.250(2) 0.257(2) Pytilt 73.7(1) 75.3(2)

As seen in Table 2-1, both 1Sc-Ph and 1Sc have similar metrical data. The Sc atom protrudes by about 0.256(2) Å from the plane defined by the four pyrazolyl nitrogens in

1Sc; this is in contrast to related structures for Fe(III) and Co(III) compounds, where the metal is 0.146(1) and 0.0148(11) Å out of the plane respectively, and is likely due to the

35 larger size of the Sc(III) ion. The average Sc–NPz distance of 1Sc is 2.204(7) Å and is identical to that of 1Sc-Ph and the dihedral angle (Pytilt) between the planes defined by the six pyridyl atoms and four datively bonded pyrazolyl nitrogen atoms is not significantly different in both complexes (Figure 2-2).

Figure 2-2 Side view of the molecular structure of 1Sc with an arrow to indicate dihedral angle between the mean plane of the axial pyridyl ring and that defined by the equatorial nitrogen atoms.

Both phenyl-substituted free ligand and 1Sc-Ph are sparingly soluble in THF at room temperature, but complete dissolution was achieved upon heating thus allowing for multinuclear NMR spectroscopic analysis. The chemical shifts of all the signals in the 1H

NMR spectrum are shifted upon binding to scandium (Figure 2-3). Most of the ligand resonances shift downfield upon complexation due to induction of electron density towards the Lewis acidic scandium center. This results in lower electron density around the protons and therefore become deshielded. Although the para-pyridyl peak is also expected to shift downfield, further aided by resonance structures that places a positive charge on the para position and a negative charge on nitrogen, it instead shifts upfield from 7.68 ppm to 7.30 ppm. Since orientation of the pyridyl ring changes drastically from proligand to complex, this could potentially affect the shielding of protons brought about by the induced magnetic field from aromatic ring currents of the flanking phenyl/tolyl rings. The disappearance of the peak at 15.32 ppm corresponding to the protonated pyridine in [Ph-B2Pz4LiPyH]2 is

27 also observed which indicates that no free ligand present (purple dot). The structurally similar tolyl-substituted complex 1Sc was also synthesized, and it was found to be slightly soluble in benzene and toluene due to installation of methyl groups on the ligand backbone.

It also served to provide a diagnostic signal at ≈2.32 ppm in the 1H NMR spectrum when studying reactivity of these complexes.

1 Figure 2-3 H NMR spectra of [Ph-B2Pz4LiPyH]2 (top) and 1Sc-Ph (bottom) in THF-d8 highlighting chemical shift changes upon metalation.

2.3 Synthesis of B2Pz4Py Scandium Alkyl Complexes

The reaction between 1Sc with a variety of alkyllithium reagents in toluene is seen to proceed quickly as observed by the complete dissolution of sparingly soluble 1Sc, to result in a translucent solution caused by insoluble LiCl within a few hours. The reactions were left to stir overnight at room temperature to ensure complete conversion to the alkyl complexes 2Sc-R, particularly for 2Sc-Me due to the poor solubility or MeLi in toluene.

These complexes are readily synthesized in moderate to excellent yield; the lower yield of

2Sc-CH2SiMe3 is attributable to its slight solubility in pentane when washing the product

(Table 2-2).

Scheme 2-2 Synthesis of 2Sc-R by salt metathesis.

Table 2-2 Synthetic yields, chemicals shifts of α-H and C resonances in the 1H and 13C{1H} 1 NMR spectra, and JCH coupling constants of 2Sc-R.

1 R Compound Yield (%) δH (ppm) δC (ppm) JCH (Hz) CH3 2Sc-Me 93 0.58 26.4 106 CH2SiMe3 2Sc-CH2SiMe3 55 0.71 42.3 100 CH2SiMe2Ph 2Sc-CH2SiMe2Ph 77 0.87 38.3 101 n CH2CH2CH3 2Sc- Pr 81 1.15 55.0 105 i CH2CH(CH3)2 2Sc- Bu 61 1.17 64.6 104

The pattern of ligand resonances for all of the compounds 2Sc-R is indicative of C2v symmetry, implying free rotation about the Sc–C bonds. The solid state structures of 2Sc-

Me and 2Sc-CH2SiMe3 were determined (Figure 2-4) and that of the latter shows that, as

103,108 in several related Co and Fe structures, the SiMe3 group aligns with the “narrow groove” defined by the smaller of the NPz–Sc–NPz (N2–Sc1–N4 or N6–Sc1–N8, in this instance, Figure 2-5). Although these angles are smaller, this vector is the most sterically open in the coordination sphere sculpted by this ligand as there could be some steric interactions with pyrazolyl protons on the 3 position. However the barrier to rotation by

≈90° to place the R group in the “wide groove” must be small, and so, in solution, an averaged structure is always manifested.

Table 2-3 Selected metrical parameters for 2Sc-Me (top) and 2Sc-CH2SiMe3 (bottom).

Compound Bond Distances (Å) Bond Angles (°) 2Sc-Me Sc1–N1 2.342(1) N2–Sc1–N8 99.38(7) Sc1–N2 2.211(2) N2–Sc1–N4 82.33(7) Sc1–N4 2.195(2) N4–Sc1–N6 91.56(7) Sc1–N6 2.185(2) N6–Sc1–N8 82.88(7) Sc1–N8 2.222(2) Sc1–C32 2.224(2) 2Sc-CH2SiMe3 Sc1–N1 2.333(1) N2–Sc1–N8 96.78(8) Sc1–N2 2.244(2) N2–Sc1–N4 82.91(8) Sc1–N4 2.204(3) N4–Sc1–N6 91.21(8) Sc1–N6 2.192(2) N6–Sc1–N8 85.19(8) Sc1–N8 2.228(3) Sc1–C32–Si1 131.1(4) Sc1–C32 2.185(6)

Figure 2-5 Top-down view of the molecular structure of 2Sc-Me with arrows indicating narrow and wide grooves around scandium.

The resonances for the α carbon atoms exhibited some broadening due to the quadrupolar 45Sc nucleus (I = 7/2, 100% natural abundance) but were easily detectable in the 13C{1H} NMR spectra, and the corresponding α hydrogen resonances in the 1H NMR spectra were sharp (Table 2-2). Similarly, the α carbons to the boron atoms in the ligand backbone are broadened due to the quadrupolar 11B nucleus and can be determined by in

1 13 1 the H- C HMBC spectra. The JCH coupling constant for the α C–H bonds are somewhat lower than those of free alkanes,17 likely a reflection of the electropositive nature of the

(B2Pz4Py)Sc substituent rather than any α agostic interactions, with the former being common in metal centers that have a ≤ 16 electron configuration.15 The latter typically

1 features unusually low JCH values ranging from 50 to 100 Hz and furthermore no low energy C–H stretching bands were observed in the IR spectra of compounds 2Sc-Me and

2Sc-CH2SiMe3 (Figure 2-6). This is consistent with normal Sc–C bond distances of

2.224(2) and 2.185(6) Å in 2Sc-Me and 2Sc-CH2SiMe3 respectively. Furthermore, the Sc1–

C32–Si1 bond angle of 131.1(4)° in 2Sc-CH2SiMe3 is in close agreement with other reported examples.‡

Figure 2-6 Superimposed FT-IR spectra of 2Sc-Me and 2Sc-CH2SiMe3.

The alkyl complexes 2Sc-R are extremely sensitive to protic reagents, especially adventitious water; treatment of any of the compounds with H2O leads to conversion to the

Sc–O–Sc dimer 3Sc (Scheme 2-3). Hence great care was taken to exclude traces of moisture from glassware, solvents and the glovebox atmosphere during the syntheses. However in all but the driest of media, small amounts of 3Sc can be detected, and it can be made preparatively in 93% yield with intentional addition of water.

‡ From the CCDC accessed on 19 March 2019, the average Sc–C bond distance in 39 hits for Sc–CH3 is 2.260 Å. The bond lengths range from 2.162 to 2.499 Å. For Sc-CH2SiMe3 moieties, the average Sc–C bond distance is 2.233 Å from 299 hits and the range is 2.115 to 2.350 Å. The average Sc–C–Si angle is 130.5° ranging from 115.5 to 155.4°. 32

Scheme 2-3 Reactions of 2Sc-R with water.

In fact during multiple attempts of crystallizing 2Sc-R, it was noted that solvents used that were dried over sodium/benzophenone ketyl tend to cause the formation of 3Sc within a few days as seen by the characteristic colourless thin needles, while solvents that were stored under 3 Å molecular sieves in the glovebox led to colourless blocks of 2Sc-R.

Storage of these crystals in the crystallization solution for a prolonged period of time ultimately forms 3Sc as well. Solvents used that were stored over 3 Å molecular sieves delayed the formation of the µ-O dimer seemingly due to its better solvent drying

109 capabilities than sodium/benzophenone. The molecular structure of 3Sc is depicted in

Figure 2-7, and despite short essentially equal Sc–O bonds of 1.8827(5) Å, the Sc1–O1–

Sc1’ is slightly bent (166.62(14)°) from linearity, probably due to packing forces. These data compare to distances of 1.910(5) and 1.925(5) Å and an angle of 174.1° for the related

110 complex [(C5Me4H)2Sc]2(μ-O).

Figure 2-7 Molecular structure of 3Sc. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 2-4 Selected metrical parameters for 3Sc.

Bond Distances (Å) Bond Angles (°) Sc1–N1 2.378(2) N2–Sc1–N8 102.31(7) Sc1–N2 2.275(2) N2–Sc1–N4 81.64(7) Sc1–N4 2.208(2) N4–Sc1–N6 91.74(7) Sc1–N6 2.226(2) N6–Sc1–N8 80.27(7) Sc1–N8 2.229(2) Sc1–O1–Sc1’ 166.62(14) Sc1–O1 1.8838(6)

While the signature reaction of organoscandium complexes supported by soft cyclopentadienyl ligands is sigma bond metathesis,54,111 this reactivity is largely absent in harder ligand environments where the more positively charged scandium center’s orbitals are less available to interact with sigma bonds.33,107,112,113 The coordinative saturation of compounds 2Sc-R exacerbates the low tendency to engage in sigma bond metathesis and, indeed, these alkyls are completely unreactive towards hydrogen, even with heating, and

34 so the hydride 2Sc-H was not accessible via this route. Compounds 2Sc-Me, 2Sc-CH2SiMe3 and 2Sc-CH2SiMe2Ph were initially targeted due to their lack of β hydrogens, thus avoiding the possibility of β-hydride elimination from occurring and aiding in the isolation of these scandium alkyl complexes.12 The n-propyl and i-butyl derivatives were subsequently prepared to see if β elimination to 2Sc-H was possible; all compounds 2Sc-R are thermally

i robust, with 2Sc- Bu being the only compound that thermally converts to another species upon prolonged heating albeit only ~40% conversion after 4 days at 80 °C (Figure 2-8).

4 Isobutene was detected as the by-product with a JHH coupling constant of 1.2 Hz and the other species was identified to be hydride 2Sc-H. No insertion product was observed when independently prepared 2Sc-H was heated at 80 °C with 1 atm of isobutene, thus ruling out

i the possibility of the reformation of 2Sc- Bu that could explain the slow conversion to product.

1 i Figure 2-8 H NMR spectra in C6D6 showing 2Sc- Bu (bottom) and after prolonged heating at 80 °C (top) with inset expansion region of isobutene.

2.4 Synthesis of B2Pz4Py Scandium Hydride Complex

Scheme 2-4 Synthesis of scandium hydride complex 2Sc-H.

i Attempts to promote β-hydride elimination in 2Sc- Bu by heating it to elevated temperatures of 110 °C and above, heating while under reduced pressure or heating while open to a positive flow of argon were unsuccessful and often led decomposition products.

A more efficient route to synthesizing 2Sc-H in 80% yield was instead achieved by treating

1Sc with NaHBEt3 (Scheme 2-4). Similarly, the deuteride d1-2Sc-H could be also be prepared using LiDBEt3. In addition to the familiar patterns for the ligand protons, a broad signal at 9.17 ppm in the 1H NMR spectrum was observed and could be assigned as that due to Sc–H based on the observation of a sharper signal at this chemical shift in the

2 1 H{ H} NMR spectrum (Figure 2-9). A comparison of the IR spectra for 2Sc-H and d1-2Sc-

H suggested that a band at 676 cm-1 (that shifted to 499 cm-1) could be assigned to the Sc–

H moiety (Figure 2-10).

1 Figure 2-9 H NMR spectrum of 2Sc-H in C6D6 with inset expansion region of the Sc–H signal.

676 cm-1

499 cm-1

Figure 2-10 Superimposed FT-IR spectra of 2Sc-H and d1-2Sc-H (left) with expansion of the Sc–H region showing the effect of isotopic substitution (right).

However none of these spectroscopic data points definitively at a monomeric structure and all attempts to grow X-ray quality crystals of 2Sc-H were unsuccessful. In order to determine the nuclearity of 2Sc-H, diffusion ordered spectroscopy (DOSY) was employed.114 Comparison of relative diffusion constant measurements are often used for the determination of monomer and dimer species in solution state in organometallic

114-117 synthesis. Due to the unequivocal solid-state structural data, monomeric 2Sc-Me and homo-bimetallic 3Sc served as excellent molecular volume references to compare with 2Sc-

H. Solutions of 2Sc-H, 2Sc-Me and 3Sc of similar concentrations in C6D6 were prepared individually and the DOSY experiment was performed at room temperature. The gradient amplitude was varied from 2% to 95% with an optimized δ (gradient pulse length) of 2600

µs and a Δ (diffusion time) of 0.075 s. In order to make a more quantitative comparison of the diffusion constant obtained from the resultant 2D DOSY representation, one can utilize the Stokes equation (Equation 2-1) where D is the measured diffusion constant, kB is the

Boltzmann constant, T is the temperature, η is the viscosity coefficient of C6D6 and r is the hydrodynamic radius. The quantitative results of the DOSY experiments are displayed in

Table 2-5.

푘 푇 퐷 = 퐵 6휋휂푟

Equation 2-1

Table 2-5 DOSY results for 2Sc-H, 2Sc-Me and 3Sc.

Compound D (×10-10 m2/s) r (Å) 2Sc-H 4.8±0.5 7.1±0.7 2Sc-Me 6.9±0.5 5.0±0.4 3Sc 4.8±0.5 7.1±0.7

118 By using the viscosity coefficient for C6D6 at 298 K (0.6392 mPa·s), the hydrodynamic radius of 2Sc-H was determined to be 7.1±0.7 Å, while 2Sc-Me and 3Sc had calculated radii 5.0±0.3 and 7.1±0.7 Å respectively. Its estimated standard error was obtained from the width of a Gaussian lineshape in the diffusion domain.119 Although these values assume a perfectly spherical molecule, these approximations strongly suggest that

2Sc-H and 3Sc are similar in size while 2Sc-Me is much smaller. Thus, it can be concluded that 2Sc-H is indeed dimeric in solution. The Signer method of molecular weight 38 determination was also performed to provide additional evidence of 2Sc-H’s dimeric structure.120 This involves the principle of isothermal distillation in which two solutions of the same solvent are enclosed in an evacuated apparatus with only their vapours in contact.

This causes the solvent from the less concentrated solution to distill into the more concentrated solution until the apparent concentrations of the solutions are equal. At equilibrium, the molecular weight of the compound of interest can be calculated from the following equation (Equation 2-2) where MW, M and V refer to the apparent molecular weight, mass in mg, and volume in mL respectively, while subscripts s and u refer to the standard and unknown (or 2Sc-H in this case). Azobenzene was chosen as the standard due to its high solubility in organic solvents, and the colour of its solution distinguishes it from the unknown. Both compounds were dissolved in benzene and the Signer method resulted in a value of 1190 g mol-1, consistent with the calculated molecular weight of 1105.22 g mol-1 for a dimeric structure (Figure 2-11).

푀푢 · 푀푊푠 · 푉푠 푀푊푢 = 푀푠 · 푉푢

Equation 2-2

Figure 2-11 Signer method for the molecular weight determination of 2Sc-H. 39

Figure 2-12 Computed optimized structure of [2Sc-H]2 by DFT with B3PW91 functional.

Furthermore, density functional theory (DFT) computations using the B3PW91 functional indicated that the dimer was favoured by 28.3 kcal mol-1 over the monomer in a monomer/dimer equilibrium. Thus, despite the pentadentate nature of the B2Pz4Py ligand,

2Sc-H is favoured in its dimeric form [2Sc-H]2 as shown in Figure 2-12. The DFT optimized structure of the dimer features a Sc···Sc distance of 3.329 Å while calculated Sc–H and

Sc–H’ distances are 1.968 and 1.974 Å respectively, both of which are consistent with other dimeric scandium hydride complexes that have been structurally characterized by X-ray crystallography.‡

‡ From the CCDC accessed on 18 July 2019, the average Sc–H bond distance in 12 hits for bis-µ-H scandium dimers is 1.997 Å and they range from 1.859 to 2.137 Å. The average Sc···Sc bond distance is 3.220 Å and they range from 2.848 to 3.422 Å. 40

2.5 Reactivity Studies with Small Molecules

Despite the fact that the dimeric form of 2Sc-H is favoured, its reactivity with D2,

N2O and CO2 was explored. Assuming that reactivity proceeds through the monomer,

-1 barriers of at least 28 kcal mol would be expected and, indeed, reaction of 2Sc-H did not proceed at room temperature with any of these reagents. Heating benzene or toluene solutions of 2Sc-H to 80 °C under 1 atm of D2 did not result in any production of d1-2Sc-H, suggesting that even the monomer was immune to this most facile of sigma bond

54 metathesis processes. When treated with 1 atm of N2O at 80 °C, however, clean conversion of 2Sc-H to the oxo-bridged dimer 3Sc was observed. Such reactions with other, monomeric electropositive metal hydrides (Hf–H121 and Mg–H122) are quite facile at room temperature, resulting in M–OH complexes123 that may or may not undergo further reactions to M–O–M complexes. Heating is required to generate the monomer from [2Sc-

H]2, which undergoes reaction with N2O to produce the unobserved presumed intermediate

2Sc-OH which reacts further with 2Sc-H to give 3Sc and H2 in quantitative yield after 6 hours

(Figure 2-13). The dihydrogen byproduct was identified by the signal at 4.47 ppm in the

1H NMR spectrum.124

18 h, 80 °C

5 h, 80 °C

2 h, 80 °C

1 h, 80 °C

2Sc-H

1 Figure 2-13 H NMR of the reaction progression of 2Sc-H and N2O in C6D6 with inset expansion of the H2 region. Ligand resonances for 2Sc-H change over time to 3Sc when the reaction mixture is heated.

The accessibility of monomeric 2Sc-H through heating prompted further reactivity studies of [2Sc-H]2 and genuinely monomeric 2Sc-Me with carbon dioxide (Scheme 2-5).

Both reactions required heating to 80 °C for 3-4 hours in benzene in order to proceed to completion and the products were identified as the acetate and formate complexes 4Sc-H and 4Sc-Me respectively. While these insertion reactions were clean on a small scale (as

1 determined by H NMR spectroscopy), an alternate salt elimination route starting from 1Sc and sodium acetate or formate was employed for preparative scale experiments. This separate synthesis confirmed the identity of the CO2 insertion products 4Sc-R, which were

42 structurally characterized by X-ray crystallography (Figure 2-14). The structures are very similar in terms of the metrical data associated with the molecular core, and there are no atypical distances or angles present. The κ2 formate and acetate ligands lie along the axis that bisects the two wide N–Sc–N angles (wide groove); since the steric bulk of the ligands is minimal, this must be an electronic preference. The κ2(O,O) binding mode is also unique for scandium as the high oxophilicity of the metal center renders monomeric scandium formate and acetate highly susceptible to dimerization.125-127 The 1H and 13C{1H} NMR spectra of 4Sc-H showed resonances at 8.60 and 178.1 ppm, respectively characteristic of the formate moiety, while the acetate carbonyl carbon of 4Sc-Me showed up at 189.1 ppm

13 1 in the C{ H} NMR spectrum. The FT-IR spectra of 4Sc-H and 4Sc-Me feature strong intensity bands at 1566 cm-1 and 1535 cm-1 respectively which correspond to the C=O stretch of a κ2(O,O) CO(O)R moiety, in contrast to a κ1(O) formate C=O stretch of ≈1640 cm-1.128

Scheme 2-5 Synthesis of 4Sc-R either through CO2 insertion with 2Sc-R or salt metathesis with 1Sc.

Figure 2-14 Molecular structures of 4Sc-H (left) and 4Sc-Me (right). Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 2-6 Selected metrical parameters for 4Sc-H and 4Sc-Me.

Compound Bond Distances (Å) Bond Angles (°) 4Sc-H Sc1–N1 2.311(1) N2–Sc1–N8 102.48(8) Sc1–N2 2.266(2) N2–Sc1–N4 80.55(8) Sc1–N4 2.208(2) N4–Sc1–N6 92.37(8) Sc1–N6 2.216(2) N6–Sc1–N8 80.16(8) Sc1–N8 2.265(2) O1–Sc1–O2 58.19(7) Sc1–O1 2.230(1) O1–C32–O2 121.2(2) Sc1–O2 2.256(2) C32–O1 1.243(3) C32–O2 1.255(3) 4Sc-Me Sc1–N1 2.358(6) N2–Sc1–N8 102.4(2) Sc1–N2 2.288(6) N2–Sc1–N4 79.0(2) Sc1–N4 2.213(7) N4–Sc1–N6 91.6(2) Sc1–N6 2.195(7) N6–Sc1–N8 81.5(2) Sc1–N8 2.231(7) O1–Sc1–O2 59.1(2) Sc1–O1 2.198(6) O1–C32–O2 118.5(7) Sc1–O2 2.216(7) C32–O1 1.250(12) C32–O2 1.281(12)

2.5.1 DFT Studies on CO2 Insertion

The reaction profiles for these insertion reactions were explored via DFT calculations at the B3PW91 level and were found to proceed through quite different transition states, despite having qualitatively similar rates. The relative energies of the reactants, intermediates, transition states and products (relative to 2Sc-H and 2Sc-Me at 0 kcal mol-1) for each insertion are depicted in Figure 2-15, while the transition structures for the insertion step in each process are given in Figure 2-16.

Figure 2-15 Computed reaction profile (DFT, B3PW91) for the insertion of CO2 into the Sc–H and Sc–C bonds of 2Sc-H (green line) and 2Sc-Me (black line). ΔG values given in parentheses.

For both reactions, a weak, entropically disfavoured van der Waals complex with

CO2 was found, which leads to the insertion transition states. For the 2Sc-Me reaction, TS2-

-1 Me is enthalpically 17 kcal mol above the ground state of 2Sc-Me, and features a weak Sc–

O interaction with one of the CO2 oxygen atoms (Sc–O = 2.52 Å) and a bent O–C–O angle of 161°. The nascent C–C bond is not yet fully formed (C–C = 2.39 Å), indicating an early transition state, and is likely due to the highly directional nature of the sp3 hybridized orbital

1 on the methyl group. This TS2-Me connects to a  acetate complex in a highly exothermic path (–46.9 kcal mol-1 downhill) which converts to the even more stable 2 observed product via a nearly barrierless Sc–O bond rotation.

Figure 2-16 Computed transition state structures (DFT, B3PW91) for the insertion of CO2 into the Sc–H and Sc–C bonds of 2Sc-H (left) and 2Sc-Me (right).

In contrast, the barrier to insertion into the Sc–H bond of 2Sc-H is much smaller, almost negligible relative to the energy of monomeric Sc–H; the barrier is only 0.8 kcal

-1 mol starting from the van der Waals complex between 2Sc-H and CO2. Therefore, the

46 experimentally observed sluggishness of this reaction must be due to the need to dissociate

-1 [2Sc-H]2 into the highly reactive monomer, creating an effective barrier of > 28 kcal mol and accounting for the fact that production of 4Sc-H proceeds at similar rates to those observed for 4Sc-Me. From monomeric 2Sc-H, the transition state for CO2 insertion can be characterized as a direct nucleophilic attack of the Sc–H moiety on unactivated CO2; the

O–C–O angle is only slightly bent (168°) and the closest Sc–O distance is 3.06 Å.129,130

This is reflective of the high basicity of the hydride ligand in 2Sc-H. Charge density analysis indicates that the charge at H is –0.38 and the Wiberg bond index of the Sc–H is 0.8, indicating a significant degree of ionicity in the bond (1.0 would be a pure covalent bond).

Natural bond analysis indicates that the Sc–H bond is indeed strongly polarized toward H

(30% Sc and 70% H). Thus, while the forming C–H bond in TS2-H is still quite long (2.15

Å), the high hydricity of the scandium hydride makes it basic enough to react directly with

CO2 without Lewis acid activation of a carbon dioxide oxygen atom as seen in TS2-Me.

2.6 Generation of Cationic Scandium Complexes

2.6.1 Contact Ion Pair Complex

The first route investigated for the generation of cationic scandium complexes involved the abstraction of the alkyl group with the strong Lewis acid B(C6F5)3 (Scheme

131 2-6). Treatment of 2Sc-Me with one equivalent of B(C6F5)3 forms the contact ion pair 5Sc cleanly, and although a colourless oil remains upon evaporation of the reaction mixture, multiple triturations of the residue with pentane results in a white solid in 92% yield.

Scheme 2-6 Synthesis of 5Sc and 5Sc-THF via alkyl abstraction followed by addition of THF.

Apart from the usual minor shifts in the B2Pz4Py ligand peaks when the apical R group has been functionalized, the most prominent change in the 1H NMR spectrum is the disappearance of the sharp singlet at 0.58 ppm that corresponds to Sc-CH3 and the formation of a broad singlet at 2.36 ppm (Figure 2-17). The borate anion also features a sharp singlet at –14.2 ppm in the 11B NMR spectrum and a difference in chemical shifts

19 1 between meta and para fluorine atoms Δδm,p = 4.6 ppm in the F{ H} NMR spectrum.

Generally, the latter is sensitive to the environment at the boron center and therefore is a used as a diagnostic handle; a small Δδm,p of ≈3 ppm would indicate a fairly weak interaction between the cation and anion of the ion pair.132,133 Since attempts to grow X-

48 ray quality crystals of 5Sc were unsuccessful, it cannot be established with certainty if the association of the ionic components is through a µ-methyl contact between the boron and

1 2 scandium centers, or κ (F) or κ (F)-C6F5 bonds between the counteranion and scandium solely based on solution NMR spectroscopy at room temperature.112,113,134-137

1 Figure 2-17 H NMR spectra of 2Sc-Me (top) and after addition of B(C6F5)3, 5Sc (bottom) in C6D6. Change in Sc–CH3 peak is indicated with dots. Solvent residual signal indicated with an asterisk.

Reversible intramolecular methyl ligand transfer from boron to scandium in 5Sc was not observed as a new product (5Sc-THF) is produced rapidly in the presence of a coordinating solvent like THF. The interaction between the two ions are now disrupted and the scandium centre is capped with a molecule of THF (Scheme 2-6). A complete shift in the ligand resonances peaks are observed once again in the 1H NMR spectrum along with a sharp singlet at 1.36 ppm for the methyl borate anion. The molecular structure of 5Sc-

THF is depicted in Figure 2-18 and it features a Sc–O bond length of 2.145(4) Å which is

II slightly shorter but comparable to other (B2Pz4Py)M -THF (M = Fe or Co) complexes due to the more positively charged Sc(III) center.103,104,108 Minor change of the boron chemical

19 1 shift to –13.6 ppm and a Δδm,p = 2.6 ppm in the F{ H} NMR spectrum still indicate the presence of a tetracoordinate boron center.

Figure 2-18 Molecular structure of 5Sc-THF. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 2-7 Selected metrical parameters for 5Sc-THF.

Compound Bond Distances (Å) Bond Angles (°) 5Sc-THF Sc1–N1 2.232(3) N2–Sc1–N8 97.38(16) Sc1–N2 2.175(4) N2–Sc1–N4 84.34(15) Sc1–N4 2.152(4) N4–Sc1–N6 93.82(15) Sc1–N6 2.150(4) N6–Sc1–N8 83.12(15) Sc1–N8 2.204(4) Sc1–O1 2.230(1)

2.6.2 Scandium Formatoborate Complex

Hydride abstraction of 2Sc-H was likewise investigated through a similar approach with 5Sc to form the corresponding hydridoborate 6Sc. No reactivity was observed when

2Sc-H was treated with one equivalent B(C6F5)3 at room temperature which could be attributed to the need for prior dissociation of [2Sc-H]2 to access the monomer and that Sc–

-1 -1 138 H bonds are stronger than Sc–C (49 kcal mol vs. 28 kcal mol for Sc–CH3). Heating the reaction to 70 ºC for 1 hour in toluene affords 6Sc relatively cleanly and the complex can be purified by extracting it with pentane to give an overall 28% yield (Scheme 2-7).

Apart from the usual ligand resonance peaks in the 1H NMR spectrum, the B–H proton was

1 11 observed as a quartet at 4.00 ppm ( JB–H = 75.7 Hz), while the B NMR spectrum showed

1 19 a doublet at –20.0 ppm ( JB–H = 76.2 Hz). The room temperature F NMR showed three

- sharp o-/m-/p-C6F5 resonances with Δδm,p = 3.9 ppm, thus indicative of a [HB(C6F5)3] anion. Although NMR spectroscopic data suggests that the ions are fully separated in solution, the molecular structure of 6Sc shows that the contact between the cation and anion occurs through coordination of an ortho fluorine of one of the perfluoroaryl groups (Figure

2-19). The Sc1–F1 bond distance of 2.280(1) Å is consistent with the related

1 137 [Cp*2Sc(κ (F)-C6H5F)][BPh4] as reported by Hessen. Solid phase π–π stacking between a pyrazolyl ring and perfluoroaryl ring is also observed as the distance between the centroids is 3.54 Å, and is comparable with arene-hexafluorobenzene interactions which have interplanar distances around 3.43 – 3.55 Å.139

Figure 2-19 Molecular structure of 6Sc. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 2-8 Selected metrical parameters for 6Sc.

Compound Bond Distances (Å) Bond Angles (°) 6Sc Sc1–N1 2.236(1) N2–Sc1–N8 92.23(5) Sc1–N2 2.157(1) N2–Sc1–N4 86.48(5) Sc1–N4 2.207(1) N4–Sc1–N6 96.86(5) Sc1–N6 2.186(2) N6–Sc1–N8 83.67(5) Sc1–N8 2.192(1) Sc1–F1 2.280(1) B3–H3A 1.24(2)

Since the synthesis of the hydridoborate complex 6Sc via hydride abstraction is a two step reaction starting from 1Sc, a more efficient preparation of these hydridoborate complexes was pursued which closely follows a method developed for the synthesis of a scandocenium hydridoborate.140 The procedure involves the reaction of the starting chloro complex, 1Sc, with one equivalent of B(C6F5)3 and a slight excess of Et3SiH (Scheme 2-7).

The hydrosilane is activated through interaction with the Lewis acidic borane,132 resulting in a strongly polarized system with an electrophilic Si center and a nucleophilic hydride.

This putative borane-silane adduct has been utilized greatly for the hydrosilylation of various organic functional groups, such as carbonyls,141,142 alcohols,143 imines144 and

145 arenes. Notably, no reaction is observed between 1Sc and silane in the absence of borane.

From this reaction, the Et3SiCl byproduct was easily removed by washing the solid product with pentane and drying it under vacuum. Despite identifying all of the resonance peaks that could suggest the presence of a hydridoborate moiety, the product turned out to be a scandium µ-chloride dimer hydridoborate complex, 7Sc as determined by X-ray crystallography (Figure 2-20).

Scheme 2-7 Synthesis of hydridoborate complexes 6Sc and 7Sc.

This reaction can be seen as only half an equivalent of 1Sc reacting with B(C6F5)3 and Et3SiH to form 6Sc, and the empty coordination site at scandium is immediately capped with the remaining starting complex to give 7Sc in 95% yield. Heating the reaction mixture or addition of a solution of 1Sc slowly into a stirred mixture of B(C6F5)3 and Et3SiH did not

53 result in the desired product 6Sc. The Sc1–Cl1 and Sc2–Cl1 bonds are essentially equal at

2.526(3) and 2.520(2) Å respectively which is longer than that of mononuclear 1Sc at

2.396(1) Å, while the Sc1–Cl1–Sc2 is bent at 129.87(8)°. This makes it unique as a structurally characterized mono-µ-chloride scandium dimer whereas all reported examples

‡ are of bis-µ-chloride dimers. The Sc–NPz distance around both scandium centers are also equal with Sc1–NPz(avg) at 2.186(10) and Sc2–NPz(avg) at 2.181(12) Å, both of which are shorter than the neutral Sc(III) complexes due to the overall electropositive dimeric cation.

Figure 2-20 Molecular structure of 7Sc. Hydrogen atoms on the cation have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

‡ From the CCDC accessed on 19 March 2019, the average Sc–Cl bond distance in 29 hits for bis-µ-Cl scandium dimers is 2.570 Å and they range from 2.466 to 2.684 Å. The average Sc–Cl–Sc angle is 98.4° ranging from 82.2 to 105.5°. 54

Table 2-9 Selected metrical parameters for 7Sc.

Compound Bond Distances (Å) Bond Angles (°) 7Sc Sc1–N1 2.216(6) N2–Sc1–N8 100.4(2) Sc1–N2 2.202(4) N2–Sc1–N4 85.3(2) Sc1–N4 2.170(6) N4–Sc1–N6 91.1(2) Sc1–N6 2.178(4) N6–Sc1–N8 82.1(2) Sc1–N8 2.195(6) Sc1–Cl1 2.526(3) Sc2–N10 2.216(4) N11–Sc2–N17 95.7(2) Sc2–N12 2.184(4) N11–Sc2–N13 85.3(2) Sc2–N14 2.165(7) N13–Sc2–N15 92.5(2) Sc2–N14 2.167(4) N15–Sc2–N17 85.2(2) Sc2–N16 2.207(8) Sc2–Cl2 2.520(2) Sc1–Cl1–Sc2 129.87(8)

While it is conceivable that the ion pair 6Sc is in equilibrium with the neutral

140 scandium hydride, 2Sc-H, and free borane B(C6F5)3, it is more unlikely to occur in 7Sc due to the bridging chloride ligand; dissociation has to occur prior to reversible hydride transfer to scandium. This is important in light of 7Sc’s high reactivity towards CO2 as exposure of 7Sc to 1 atm of CO2 immediately yields two distinct products in a 1:1 ratio as seen in the 1H NMR spectrum (Scheme 2-8 and Figure 2-21). One set of ligand signals is attributed to 1Sc while the other was assigned to a new formatoborate complex 8Sc-H. The formation of this complex is presumably through the coordination of CO2 to the Lewis

- acidic metal center followed by hydride transfer from the [HB(C6F5)3] anion to the

134 electrophilic carbon on CO2. Repeating the synthesis outlined in Scheme 2-7 while under an atmosphere of CO2 affords 8Sc-H cleanly as the sole product in 81% yield.

Scheme 2-8 Formation of 8Sc-H with the elimination of 1Sc when 7Sc is exposed to CO2.

1 Figure 2-21 H NMR spectra in C6D6 of 7Sc (top) and after addition of 1 atm of CO2 (bottom). Coloured dots indicate assignment of products to 1Sc and 8Sc-H. Solvent residual signal indicated with an asterisk.

In relation to the reaction profile of CO2 insertion into scandium hydride and methyl complexes shown in Figure 2-15, the kinetic products of insertion are the κ1 formate and

-1 acetate complexes that are only 5.4 (4Sc-H) or 7.5 (4Sc-Me) kcal mol higher in energy than

2 1 the κ thermodynamic products. Attempts to trap the κ isomers by treating with B(C6F5)3 gave clean conversion of 4Sc-R to the µ-formato and acetato complexes 8Sc-H and 8Sc-Me

56 in 76 and 95% yield respectively (Scheme 2-9). A broad resonance was observed at 0 ppm

11 in the B NMR spectrum for each of the compounds 8Sc-R although it cannot be confidently assigned, as the B2Pz4Py ligand has a boron chemical shift at ≈0 ppm as well.

19 1 Nevertheless a set of three sharp resonances in the F{ H} NMR spectrum with Δδm,p =

6.9 and 6.7 ppm for 8Sc-H and 8Sc-Me, respectively, establishes the presence of the

- 134,146-148 [HCO2B(C6F5)3] ion. These compounds are best described as tending towards a

“zwitterionic” resonance structure as opposed to the alternate “neutral” variant shown in

Scheme 2-9.134

Scheme 2-9 Alternate synthesis of 8Sc-R and contributing resonance structures.

The molecular structure of 8Sc-H, depicted in Figure 2-22, determined by X-ray diffraction reveals metrical parameter that are consistent with the solution structure derived from the NMR data. The Sc–N distances are all somewhat shorter than those observed in other neutral (B2Pz4Py)Sc complexes, consistent with a more positively charged Sc(III) center, while Sc1–O1 (2.056(2) Å) and O2–B3 (1.549(4) Å) distances are more in line with dative and covalent bonds, respectively. Furthermore, the C50–O1 distance is slightly

57 shorter than the C50–O2 distance, as would be expected. The Sc1–O1–C50 angle of

162.9(2)° is at first glance not consistent with a dative carbonyl to metal interaction, but in cases where ketones or aldehydes bind to d0 centers such as this, sp hybridization at the oxygen is not uncommon.149 The parameters associated with the bridging formato unit are

* comparable to those observed in the related Cp 2Sc-OC(H)O-B(C6F5)3 previously reported

134 in the Piers group, where the Sc–O and O–B distances are 2.088(3) and 1.560(5) Å respectively. The smaller Sc–O–C bond angle of 151.2(3)° and longer Sc–O bond distance is likely due to steric repulsion between the methyl groups on Cp* and C6F5 groups on

B(C6F5)3.

Figure 2-22 Molecular structure of 8Sc-H. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 2-10 Selected metrical parameters for 8Sc-H. Compound Bond Distances (Å) Bond Angles (°) 8Sc-H Sc1–N1 2.217(2) N2–Sc1–N8 99.09(11) Sc1–N2 2.219(2) N2–Sc1–N4 82.55(11) Sc1–N4 2.175(3) N4–Sc1–N6 94.10(10) Sc1–N6 2.170(3) N6–Sc1–N8 82.88(10) Sc1–N8 2.206(3) Sc1–O1–C50 162.9(2) Sc1–O1 2.056(2) O1–C50–O2 122.6(3) O1–C50 1.234(4) C50–O2–B3 122.9(3) O2–C50 1.256(4) O2–B3 1.549(4)

2.6.3 Preliminary Studies on Hydrosilylation of CO2

Cationic scandium systems developed in the Piers group have either utilized a decamethylscandocenium framework134 or an anilido bipyridyl ligand system.150 These systems have been shown to be capable of activating CO2 at the scandium center (Scheme

2-10). Considering the former example, the catalytic system involves the initial activation of CO2 to form the corresponding scandocenium formatoborate (step a), which is in equilibrium with the neutral scandocene formate and free B(C6F5)3 (step b). The formate moiety is then transferred to a B(C6F5)3-activated hydrosilane, forming triethylsilyl formate and regenerating the hydridoborate complex (step c). Subsequent hydrosilylation of triethylsilyl formate ultimately yields CH4 (steps d – f). The dissociation step to form neutral scandocene formate and free B(C6F5)3 is small, therefore leading to the observation of incomplete reduction products. Addition of excess B(C6F5)3 facilitates the hydrosilylation of the intermediates, thus favouring the formation of CH4.

134 Scheme 2-10 Reduction of CO2 to CH4 with decamethylscandocenium hydridoborate.

Preliminary work presented herein was performed prior to switching over to the more soluble tolyl-substituted B2Pz4Py ligand, hence compound labels have “Ph” denoting a different ligand. The synthesis of 8Sc-Ph-Me is identical to 8Sc-Me as outlined in Scheme

2-9.

As the role of the scandocene catalyst is to activate CO2 and transfer a formate moiety to a stoichiometric hydrosilane reductant,134 we were interested to see how the

(B2Pz4Py)Sc would compare considering the significantly different steric and electronic properties around the scandium center.

Initial mixing of 8Sc-Ph-Me and one equivalent of Et3SiH under 1 atm of CO2 showed no observable reactivity at room temperature up till 24 hours. Heating the reaction mixture to 55 °C saw the consumption of Et3SiH as observed by the disappearance of the

Si–H peak at 3.87 ppm. The 19F{1H} NMR spectrum also showed a slight shift of less than

1 ppm for the trio of peaks corresponding to the o-/m-/p-C6F5 resonances of 8Sc-Ph-Me,

- presumably to the formatoborate [HCO2B(C6F5)3] ion. This led to the possibility that 8Sc-

Ph-Me could be a viable catalyst for hydrosilylation of CO2. Treatment of excess Et3SiH with CO2 at 50 °C in the presence of catalytic amounts of 8Sc-Ph-Me (5 mol%) resulted in

1 the formation resonances for various organic products of CO2 reduction in the H NMR spectrum: (Et3SiO)2CH2 being the majority product at 5.06, Et3SiOCH3 at 3.32, and CH4 at 0.17 ppm (Figure 2-23).140 However, exact product distribution cannot be accurately quantified due to evolution of CH4 into the headspace of the sealed NMR tube. The absence of observable concentrations of triethylsilyl formate is due to the rapid reduction to

(Et3SiO)2CH2 in the presence of Et3SiH and B(C6F5)3, arising from the dissociation equilibrium between 8Sc-Ph-H and 4Sc-Ph-H (Scheme 2-10).

Et3Si–H

CH4

Figure 2-23 Before (top) and after (bottom) heating the reaction mixture of 5 mol% 8Sc-Ph- Me, Et3SiH in 1 atm CO2 at 50 °C for 18 h in C6D5Br.

These organic products were accompanied by the formation of a new scandium complex as identified by a distinct set of Ph-B2Pz4Py ligand resonances. While the identity of this complex is unknown, its formation could be due to a deactivating pathway that results in catalyst death. Subsequent addition of fresh Et3SiH did not result in further formation of reduced CO2 products. Surprisingly when the reaction was repeated under the same reaction conditions but with 1 mol% of 8Sc-Ph-Me, a negligible amount of CO2 was

1 hydrosilylated as the H NMR spectrum showed resonances for unreacted Et3SiH, a small amount of CH4 and the unknown complex. Monitoring the reaction mixture at a slightly cooler temperature of 40 °C indicated about 70% conversion of 8Sc-Ph-Me to the unknown complex over the course of five hours with a small amount of (Et3SiO)2CH2. Subsequently heating the reaction mixture to 65 °C resulted in the complete formation of the unreactive complex and further hydrosilylation of the small amount of (Et3SiO)2CH2 to CH4. As preliminary reactivity studies with 8Sc-Ph-Me suggested the need for higher catalyst loading and elevated reaction temperature, in-depth kinetic studies were not performed due its inferior performance to other ionic scandium complexes.134,150

In order to gain some insight into the identity of the unknown complex, 8Sc-H was used for subsequent reactions. Not only is 8Sc-H part of the active cycle that hydrosilylates

CO2, it is suggested to be the resting state the catalyst as CO2 addition to 6Sc or addition of

B(C6F5)3 to 4Sc-H to form 8Sc-H is rapid (Scheme 2-10). Furthermore, resonances for 6Sc

1 or 4Sc-H were not observed in the H NMR spectra throughout the reaction. Scandium formatoborate complex 8Sc-H was mixed with an equivalent of Et3SiH with an additional

0.1 eq. of B(C6F5)3 to aid in the activation of Et3SiH (Scheme 2-11).

Scheme 2-11 Hydrosilylation of formate moiety starting from 8Sc-H to form 6Sc which can activate CO2 once again.

Heating the reaction mixture resulted in the complete consumption of Et3SiH within

2.5 h at 80 °C, and the products from the hydrosilylation of the formate moiety in 8Sc-H

1 were identified to be the unknown complex, (Et3SiO)2CH2 and Et3SiOCH3 in the H NMR spectrum (Figure 2-24). Some unreacted 8Sc-H remained likely due to the faster rate of

Et3SiH hydrosilylating (Et3SiO)2CH2 than dissociation of B(C6F5)3 from 8Sc-H and subsequent hydrosilylation of 4Sc-H. Subsequent addition of another equivalent of Et3SiH and heating overnight resulted in the complete formation of the unknown complex together with the fully hydrosilylated formate moiety, CH4, at 0.16 ppm in C6D6. The formation of

19 free borane B(C6F5)3 is also noted in the F NMR spectrum, indicating that there is an intramolecular hydride ligand transfer from boron to scandium in 6Sc to form 2Sc-H.

1 Resonances for 2Sc-H were not observed in the H NMR spectrum which suggested it could have reacted further since monomeric 2Sc-H can be accessed at elevated temperatures.

1 Figure 2-24 Stacked H NMR spectra in C6D6 showing the initial reaction mixture of 8Sc- H, Et3SiH and B(C6F5)3 (bottom), and after heating at 80 °C for 2.5 h (middle). Complete formation of the unknown complex with further addition of Et3SiH and heating overnight (top).

Scheme 2-12 Suggested deactivation pathway in the hydrosilylation of CO2.

It is postulated that reactive monomeric 2Sc-H, could have reacted with triethylsilyl formate through a formal 1,2-addition of the Sc–H bond across the O=C bond to yield the

64 corresponding scandium alkoxide complex (Scheme 2-12). The methylene resonance was not observed in the 1H NMR spectrum and it is plausible for the ScO–C bond to be hydrosilylated by B(C6F5)3-activated Et3SiH, thus forming the corresponding (tol-

B2Pz4Py)ScOSiEt3 9Sc-Et complex and Et3SiOCH3. The identity of the unknown complex was confirmed to be 9Sc-Et through its independent synthesis from 1Sc and KOSiEt3 in 51% yield. The molecular structure of 9Sc-Et as depicted in Figure 2-25 features a Sc1–O1 distance of 1.905(1) Å which is consistent with other scandium siloxide complexes.‡ The

Sc1–O1–Si1 bond angle of 169.41(9)° indicates involvement of both pπ-orbitals in

O(pπ)→Sc(dπ) donation which is typical in electrophilic early transition metals that have a ≤ 16 electron configuration.151 The formation of this thermodynamic product could suggest why hydrosilylation of CO2 does not proceed when it is present in the reaction mixture.

Figure 2-25 Molecular structure of 9Sc-Et. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

‡ From the CCDC accessed on 18 July 2019, the average Si–O bond distance in 4 hits is 1.933 Å, ranging from 1.901 to 1.974 Å. 65

Table 2-11 Selected metrical parameters for 9Sc-Et.

Compound Bond Distances (Å) Bond Angles (°) 9Sc-Et Sc1–N1 2.346(1) N2–Sc1–N8 94.79(6) Sc1–N2 2.244(2) N2–Sc1–N4 82.67(6) Sc1–N4 2.211(2) N4–Sc1–N6 96.48(6) Sc1–N6 2.195(2) N6–Sc1–N8 81.61(6) Sc1–N8 2.250(2) Sc1–O1–Si1 169.41(9) Sc1–O1 1.905(1) O1–Si1 1.622(1)

2.7 Summary and Conclusions

This chapter described the synthesis and characterization of remarkably stable, monomeric organoscandium compounds. The hydrido derivative 2Sc-H is very reactive in its monomeric form which can be accessed through dissociation when heated. The Sc–H bond is highly ionic and therefore the hydrido ligand is significantly basic. This is evidenced by the facile dimerization of this species despite the six coordinate nature of the monomer; this dimerization may also be favoured by London dispersion forces152 but the lack of dimerization in the chloro derivative 1Sc and the alkyl complexes 2Sc-R point to the importance of the basicity of the hydride. The alkyl and hydride ligand can be abstracted with B(C6F5)3 to form ion pair complexes. These ionic complexes were targeted for their

Lewis acidic scandium center to activate CO2. Preliminary work shows that hydrosilylation of CO2 is possible with scandium formatoborate complex 8Sc-H albeit poorer reactivity compared to previous examples in literature, likely due to the formation of stable scandium siloxide complex.

Chapter 3: Scandium Oxo and Hydroxo Complexes

3.1 Introduction

The utilization of the B2Pz4Py ligand framework has led to the synthesis of a variety of robust scandium complexes which allowed for controlled reactivity studies. Another motivation to use this ligand is to aid in the stabilization and potentially isolation of unique transition metal complexes containing highly polarized metal–ligand multiple bonds.

These complexes are of practical interest due to their applications in small molecule activation, metathesis and atom-transfer reactions.68,153-155 Although scandium terminal imidos156 and alkylidenes157,158 have been documented, there is only one example of an

84 2- 2- isolated scandium terminal oxo. Unlike the imido ([NR] ) or alkylidene ligand ([CR2]

), the oxo (O2-) ligand does not allow for any necessary steric protection and electronic tunability due to the absence of the R substituents. To complicate things further, the supporting dianionic pentadentate ligand would render the scandium(III) oxo complex anionic thereby increasing its basicity and reactivity.

Although there are no examples of group 3 oxo anions in the literature, there are few well-defined and structurally characterized complexes of group 4. Zirconium oxo anions often dimerize with the associated cation,159-161 although a monomeric terminal anionic Zr=O species was obtained from the deprotonation of a heterobimetallic

i 162 zirconium/cobalt hydroxide complex Co( Pr2PNMes)3ZrOH with LiHMDS. Cummins and co-workers previously reported that the deprotonation of a tris(anilido) titanium(IV) formate complex by a non-nucleophilic base like NaHMDS or KHMDS resulted in facile

67 decarbonylation of the CO2 anion to afford the terminal titanium oxo anion (Scheme 3-1, top).163 More recently, Okuda and co-workers also employed the same strategy of deprotonating a titanium(IV) formate complex supported by a trisamido-amine to afford a polymeric anionic oxo complex with bridging potassium cations (Scheme 3-1, bottom).164

Reactivity studies have shown that the nucleophilic oxo anion readily binds CO2 to form a carbonate complex which can then react with electrophiles resulting in simple

165,166 functionalization of CO2.

Scheme 3-1 Synthesis titanium(IV) terminal oxo anion complexes.163,164

In contrast, the analogous lanthanide metal-oxo complexes are much less developed with only a few examples reported on cerium. Schelter and co-workers reported a monomeric cerium imido complex that was stabilized by a tripodal, anionic hydroxylaminato ligand framework, TriNOx3-, and a rubidium cation.167 This complex underwent an aza-Wittig reaction with benzophenone to form an aggregation of

[(TriNOx)Ce=O]- moieties with rubidium cations to form an tetrameric cluster. Hayton and

68 co-workers reported the synthesis of a anionic Ce(IV)=O complex supported by a

168 tetradentate trisamido-amine ligand (NN’3) in the presence of 12-crown-4. The reaction was proposed to proceed through a [Li(12-crown-4)][(NN’3)Ce(III)(NO3)] precursor that undergoes inner sphere nitrate reduction in diethylether to form the oxo complex with the elimination of NO2 (Scheme 3-2).

Scheme 3-2 Formation of Ce(IV) oxo complex by inner sphere nitrate reduction.168

This chapter describes the attempts to prepare a terminal oxo scandium anion via deprotonation and decarbonylation of the scandium formate complex 4Sc-H. The oxo anion is highly reactive and rapidly reacts with proton sources to afford a scandium hydroxo intermediate 2Sc-OH that subsequently condenses to give the thermodynamic μ-O dimer product. Hence different approaches to accessing 2Sc-OH will be discussed, which in the process formed unprecedented neutral dihydroxo and cationic hydroxo scandium complexes by partial hydrolysis of the B2Pz4Py ligand.

3.2 Towards Scandium Anionic Terminal Oxo Complex

Cummins and Okuda have previously reported that the deprotonation of a titanium(IV) terminal formate complex by a non-nucleophilic base like NaHMDS or

KHMDS results in decarbonylation of the formate moiety to afford an anionic oxo titanium complex supported by the corresponding alkali metal. We envisioned that employing this protocol to the scandium formate complex 4Sc-H (outlined in Section 2.5) would allow for the synthesis of a terminal anionic oxo scandium complex supported by the B2Pz4Py ligand framework. Treatment of 4Sc-H with 1 equivalent of KHMDS at room temperature resulted in the clean formation of a new set of ligand resonances in the 1H NMR spectrum within minutes of initial mixing of reagents. Those new signals were assigned to be the scandium

µ-O dimer 3Sc (Scheme 3-3).

Scheme 3-3 Deprotonation of 4Sc-H resulting in the immediate formation of 3Sc.

Headspace analysis of the reaction mixture by gas chromatography (GC) showed the presence of argon and CO, together with O2 and N2 that arose from air during the injection process (Figure 3-1). The formation of CO was further corroborated by

13 performing this reaction with a C–labelled scandium formate complex 4Sc-H’. The

13 reaction between 1Sc and C–labelled sodium formate afforded 4Sc-H’ which has identical

70 resonances in the 1H NMR spectrum to the unlabelled compound except for the formate

13 1 proton, which shows up as a doublet due to coupling to the C labelled formate ( JCH =

208.7 Hz). Deprotonation of 4Sc-H’ was observed through the disappearance of the formate

13 1 peak at 178.1 ppm in the C{ H} NMR spectrum in C6D6 along with the appearance of a new peak at 184.5 ppm attributed to CO. Similarly, 13CO was detected when the headspace of the reaction was analyzed by gas chromatography-mass spectrometry (GC-MS, Figure

3-1). No other distinctly 13C–labelled products were observed in the 13C{1H} NMR spectrum which suggests that the deprotonation of 4Sc-H only forms the highly reactive scandium oxo anion [K][Sc=O] and CO.

Figure 3-1 GC (left) and GC-MS (right) traces of the headspace from the reaction of 4Sc- H or 4Sc-H’ and KHMDS with gas identification labels for the observed peaks.

However [K][Sc=O] was not observed in the 1H NMR spectrum. It was hypothesized that the highly basic terminal oxo moiety scavenges any acidic protons to form the corresponding scandium hydroxo complex 2Sc-OH, which subsequently condenses to form the thermodynamic bridged oxo product 3Sc. The source of protons could either be the formate proton in 4Sc-H or the free amine HMDS that origins from the initial deprotonation step. Complete deprotonation of 4Sc-H could not be achieved with 71 catalytic amounts of base since water that is produced in the condensation step would react with [K][Sc=O] and eliminate KOH, instead of [K][Sc=O] deprotonating HMDS to regenerate KHMDS. Formation of an insoluble white precipitate is observed in the reaction mixture when more base (up to one equivalent) is added in small portions. When

LiCH2SiMe3 was used as a base instead of KHMDS in which the conjugate acid, SiMe4, is weaker than (SiMe3)2NH, immediate decomposition of the reaction mixture occurred leading to a mixture of unidentifiable products. Attempts to stabilize [K][Sc=O] with cryptands or crown ethers and performing the reaction at lower temperatures were unsuccessful as 3Sc was always obtained. This inherent instability was not too surprising as even Lewis acidic AlMe3 had be to employed to stabilized the highly polarized Sc=O ligand in a neutral scandium oxo complex.84

The presence of [K][Sc=O] is also supported by a trapping experiment in which

Me3SiCl was added to the reaction mixture prior to deprotonation (Scheme 3-4). No reactivity was observed when 4Sc-H was initially mixed with one equivalent of Me3SiCl; however, subsequent deprotonation with KHMDS at room temperature resulted in

1 scandium µ-O dimer 3Sc and a new set of ligand resonances in about 79:21 ratio in the H

NMR spectrum after an hour of mixing. The new product was assigned to be the scandium trimethylsiloxide complex 9Sc, and its identity was confirmed by synthesizing it independently from the reaction of the scandium chloro complex 1Sc and KOSiMe3 in 58% yield. The lower yield was attributed to 9Sc’s high solubility in pentane. Familiar ligand

1 resonances for the tol-B2Pz4Py ligand were observed in the H NMR spectrum with a boron chemical shift of 0.2 ppm, while the –SiMe3 group showed up at 0.26 ppm.

Scheme 3-4 Trapping reactive intermediate [K][Sc=O] and proposed pathways to 1Sc with excess Me3SiCl.

Repeating the reaction with an excess of Me3SiCl (42 equivalents) at 0 °C and leaving it to stir overnight at room temperature resulted in the formation of both 9Sc and 1Sc

1 as observed in the H NMR spectrum. The formation of 1Sc is likely due to the further reaction of 9Sc and Me3SiCl, with the formation of the Si–O bond as driving force of the reaction. Alternatively, scandium µ-O dimer 3Sc, which was produced rapidly upon initial deprotonation of 4Sc-H and subsequent condensation, could have reacted with Me3SiCl as

1 well since 3Sc was not observed in the H NMR spectrum. In order to prove these two pathways to 1Sc, individual samples of 3Sc and 9Sc were mixed with Me3SiCl to probe its reactivity. When 3Sc was mixed with an equivalent of Me3SiCl in C6D6, the emergence of two distinct sets of ligand resonances was observed in the 1H NMR spectrum slowly over

2 hours. Although the reaction is slow, leaving the reaction mixture to stir overnight results in a complete reaction to form 1Sc and 9Sc in a 1:1 ratio. No reaction was observed when

9Sc was stirred with one equivalent of Me3SiCl at room temperature for 18 hours; however,

73 heating the reaction mixture overnight at 80 °C resulted in the clean formation of 1Sc along with HMDSO which was identified by the signal at 0.12 ppm in the 1H NMR spectrum.124

Therefore while both pathways outlined in Scheme 3-4 are plausible, the formation of 1Sc at room temperature suggests that protonation of [K][Sc=O] and subsequent condensation to form 3Sc is quicker than direct silylation of [K][Sc=O].

3.3 Scandium Hydroxo Complexes

Since the introduction of scandium alkyl complexes 2Sc-R in Section 2.3, the scandium hydroxo complex 2Sc-OH has been proposed numerous times to be an intermediate species that could quickly react with 2Sc-R, or condense with another equivalent of 2Sc-OH to form the scandium µ-O dimer 3Sc as the thermodynamic product.

However, multiple attempts to purposefully synthesize 2Sc-OH were unsuccessful. These include addition of degassed water at lower temperature, stirring a solution of scandium alkyl in air, or addition of wet toluene (degassed toluene that did not undergo the normal drying procedure). Instead, when a vial containing solid 2Sc-CH2SiMe3 was taken out of the glove box, exposed to air for a few minutes and subsequently brought back into the glove box while uncapped, the 1H NMR spectrum of the resulting solid showed that it had hydrolyzed (Figure 3-2). Two distinct sets of ligand resonances were observed with one of them identified to be 3Sc. Although some peaks corresponding to the pyrazolyl, pyridyl and tolyl handle were overlapping at 7.53, 7.22 and 2.32 ppm, the remaining ligand resonances for the other product integrated to what was expected for [2Sc-OH]2. Furthermore, a singlet was observed at 5.47 ppm which was assigned as the hydroxyl proton of a B2Pz4Py

74 scandium hydroxo complex [2Sc-OH]2. The assignment was further supported by a Nuclear

Overhauser effect spectroscopy (NOESY) experiment in which through space correlation was observed between the hydroxyl group and the pyrazolyl proton on the 3-position at

7.69 ppm (Figure 3-3). The latter proton resonance is easily identified as the most

3 deshielded doublet with a JHH coupling constant of 2.2 Hz, which is much smaller than

3 169 the aromatic pyrazolyl and tolyl protons ( JHH = 7.7 Hz). An O–H stretch was also

-1 170 observed at 3716 cm in the IR spectrum which is in line with a [Sc(μ-OH)]2 moiety.

1 Figure 3-2 H NMR spectrum of the resulting solid in C6D6 after exposing 2Sc-CH2SiMe3 briefly to air, with red dots corresponding to 3Sc. Only certain non-overlapping ligand resonances for [2Sc-OH]2 have been integrated. Solvent residual signal indicated with an asterisk.

1 Figure 3-3 H NOESY spectrum of the mixture of [2Sc-OH]2 and 3Sc in C6D6. Through space correlation between pyrazolyl protons (green dots) and hydroxyl proton in [2Sc-OH]2 is indicated by a dash line.

Although there are examples of salen170 and porphyrin171 scandium hydroxo complexes that were synthesized through the addition of excess water to a scandium amide complex, most of the reported bridging hydroxo scandium complexes are typically characterized by X-ray crystallography as they are often obtained as a by-product in low quantities from the presence of adventitious water during prolonged crystallization

170,172-175 attempts. Following a similar approach, when a crystallization vial of 2Sc-

CH2SiMe3 in toluene was left out of the glovebox, colourless crystals of [2Sc-OH]2 were obtained through selective crystal picking amidst 3Sc. The molecular structure of [2Sc-OH]2

76 depicted in Figure 3-4 feature Sc–O bonds of 2.118(3) and 2.196(3) Å, and O1–Sc1–O1’ bond angle of 64.9(1)° which are comparable with other scandium μ-OH dimers.‡ The longer Sc–O bond length correlates with a smaller O1–Sc1–O1’ bond angle, although these differences are probably due to packing forces. In comparison to the electronically similar phenyl substituted Ph-B2Pz4Py scandium hydroxo dimer [2Sc-Ph-OH]2, metrical parameters indicate Sc–O bond distances of 2.090(1) and 2.107(1) Å with a separation between both metal centres (Sc1···Sc1’) of 3.4726(8) Å. The O1–Sc1–O1’ bond angle is also larger at

67.08(5)°.

Figure 3-4 Molecular structure of [2Sc-OH]2. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

‡ From the CCDC accessed on 23 April 2019, the average Sc–O bond distance in 40 hits for Sc μ-OH dimer is 2.072 Å. The bond lengths range from 2.033 to 2.117 Å. Average O–Sc–O bond angle is 73.6° ranging from 69.4 to 78.4°, while the average Sc–O–Sc bond angle is 107.5° ranging from 101.6 to 111.3°. 77

Table 3-1 Selected metrical parameters for [2Sc-OH]2.

Bond Distances (Å) Bond Angles (°) Sc1–N1 2.404(2) N2–Sc1–N8 101.96(6) Sc1–N2 2.312(2) N2–Sc1–N4 81.34(7) Sc1–N4 2.206(2) N4–Sc1–N6 90.61(7) Sc1–N6 2.252(2) N6–Sc1–N8 80.61(6) Sc1–N8 2.231(2) O1–Sc1– O1’ 64.9(1) Sc1–O1 2.118(3) Sc1–O1–Sc1’ 113.8(1) Sc1–O1’ 2.196(3) Sc1···Sc1’ 3.6157(5)

In order to determine the nuclearity of the 2Sc-OH in solution, a DOSY experiment in C6D6 at room temperature was performed. The previously isolated sample mixture of

2Sc-OH and 3Sc was used as it enabled direct comparison of diffusion constants between the two compounds. The resultant 2D DOSY spectrum illustrated in Figure 3-5 shows that

-10 2 2Sc-OH has a measured diffusion constant of 4.8±0.3×10 m /s, which indicates strong similarity to that of 3Sc since each component of the mixture has a unique set of horizontal cross peaks. By employing the Stokes equation (Equation 2-1), it was calculated that 2Sc-

OH has a hydrodynamic radius of 7.1±0.5 Å, thus supporting the fact that 2Sc-OH is dimeric in solution as well as in the solid state. Although dimerization of 2Sc-OH stabilizes it long enough for various NMR spectroscopy experiments to be performed, leaving it in solution for a few days or heating it causes 2Sc-OH to convert to 3Sc likely through dissociation into the reactive monomer.

Figure 3-5 DOSY spectrum of the mixture of 2Sc-OH and 3Sc in C6D6. Some cross peaks corresponding to 2Sc-OH are indicated with arrows and red dots.

During multiple attempts to obtain X-ray quality crystals and the structure of [2Sc-

OH]2, an unexpected structure was obtained through crystal picking. The compound identified is a scandium dihydroxy complex with a partially ligating B2Pz4Py ligand 10Sc, likely formed during successive hydrolysis of a scandium alkyl complex 2Sc-R (Scheme

3-5). The pentadenticity of the B2Pz4Py ligand is lost upon protonation of the nitrogen on the pyridyl ring, resulting in coordination of only two of pyrazolyl groups to scandium.

This coordination motif is typically seen when the pyridyl ring is protonated, for example

103,104 in the [Ar-B2Pz4LiPyH]2 proligand. Hydrolysis of the supporting polydentate ligand framework is a common reaction pathway where the metal center is electronically or

79 sterically saturated.176 This has been shown to occur on cyclometalated complexes in which the metal–carbon bond is cleaved to form a metal hydroxo complex, and is typically regarded as a driving force particularly in the case with early transition metals.177,178 The molecular structure of 10Sc depictured in Figure 3-6 to 3-8 shows a distorted octahedral geometry around scandium with coordination of the hydroxo groups to adjacent scandium metal centres to form an overall hexameric structure. Although hydrolysis of the B2Pz4Py ligand would be kinetically challenging due partial demetallation of the B2Pz4Py ligand, the formation of 10Sc would likely be thermodynamically favourable due to formation of

Sc–O bonds. The structure is further stabilized through hydrogen bonding interactions between either the hydroxyl or pyridinium proton and the pyrazolyl nitrogen (Figure 3-8); representative of the short contact distances of 2.028(2) and 2.002(3) Å between H1’···N7 and H1A···N9 respectively.

Scheme 3-5 Formation of 10Sc from successive hydrolysis of a scandium alkyl complex 2Sc-R.

Table 3-2 Selected metrical parameters for 10Sc.

Bond Distances (Å) Bond Angles (°) Sc1–N3 2.292(2) O1–Sc1–O2 72.86(7) Sc1–N5 2.286(2) Sc1–O1–Sc1’ 107.81(7) Sc1–O1 2.070(3) Sc1–O2–Sc1’ 106.19(7) Sc1–O2 2.089(2) H1’···N7 2.028(2) H1A···N9 2.002(3)

Since the synthesis of 2Sc-OH by exposure of a scandium alkyl complex to air was inconsistent and typically formed 3Sc as the majority product, an alternative approach to accessing the scandium hydroxo complex was taken via protonation of the oxo-bridged dimer. When one equivalent of strong bis(trifluoromethanesulfonyl)imide acid (HNTf2) was added to 3Sc at room temperature in C6D6, 2Sc-OH and an ionic scandium triflimide

11Sc complexes are formed (Scheme 3-6). However, the in situ generated 2Sc-OH quickly

1 condenses to reform 3Sc as observed in the H NMR spectrum (Figure 3-9, middle).

Addition of an excess of HNTf2 (2 equivalents total) resulted in the complete conversion of 3Sc to 11Sc.

Scheme 3-6 Protonation of 3Sc with HNTf2 to form 2Sc-OH and 11Sc.

* 3Sc

* 3Sc and 1 eq. HNTf2

* 3Sc and 2 eq. HNTf2

1 Figure 3-9 H NMR spectra in C6D6 of the addition of 1 (middle) and 2 (bottom) equivalents of HNTf2 to 3Sc. Complete formation of 11Sc with excess HNTf2. Solvent residual signal indicated with an asterisk.

The weakly coordinating bis(trifluoromethanesulfonyl)imide anion is known to coordinate to transition metals through four distinct binding modes: monodentate nitrogen or oxygen, bidentate oxygen-oxygen or nitrogen-oxygen (Figure 3-10).179 In the case of

19 1 11Sc, the F{ H} NMR spectrum features a sharp singlet at –79.2 ppm which indicates that the CF3 groups are equivalent. Colourless crystals of 11Sc were grown through slow vapour diffusion of pentane onto a solution of 11Sc in toluene at –35 °C. However rapid evaporation of the cocrystallized toluene and pentane molecules occurred once the crystals were removed from the mother liquor, thus resulting in loss in crystallinity of the single crystal. This preluded full data collection but nevertheless a connectivity map for 11Sc was obtained, which confirmed that the NTf2 counteranion coordinates to the Sc(III) center through two of the sulfonyl oxygen atoms in a κ2(O,O) fashion (Figure 3-11).

Figure 3-10 Coordination modes of bis(trifluoromethanesulfonyl)imide anion (NTf2) to a metal center.

Unlike the analogous B2Pz4PyCo(III) triflimide complex which was shown to coordinate through κ1(O),104 scandium’s larger ionic radius and oxophilicity allows for the

2 κ (O,O) coordination mode to the metal center. 11Sc was also independently synthesized from the scandium methyl complex 2Sc-Me. Elimination of methane in the form of bubbling was observed upon immediate addition of HNTf2 to 2Sc-Me in toluene, and subsequent evaporation of solvent in vacuo gave 11Sc in 96% yield. 84

Figure 3-11 Connectivity map of Tol-B2Pz4PySc-NTf2 (11Sc).

1 Although water was not detected in the H NMR spectrum for the reaction of 3Sc with HNTf2 likely due to immiscibility in C6D6, its presence once again resulted in hydrolysis of the B2Pz4Py ligand framework. Colourless crystals formed in the sealed NMR tube when the reaction mixture was left to stand for several days at room temperature. The molecular structure determined by X-ray crystallography reveals a cationic dimeric scandium hydroxo complex that is supported by NTf2 counteranions 12Sc (Figure 3-12).

The scandium atoms adopt a slightly distorted octahedral geometry in which the pyrazolyl nitrogens from opposing units coordinate to it. The Sc–O bond lengths of 2.052(3) and

2.064(3) Å are consistent with other scandium μ-OH dimers. 12Sc also has a H1···O1A’ distance of 2.045(5) Å between the hydroxo and NTf2 anion which is characteristic of hydrogen bonding. Furthermore a broad O–H stretch and sharp N–H stretch for the pyridinium proton were observed at 3590 and 3329 cm-1 respectively in the IR spectrum

1 (Figure 3-13). None of the ligand resonances attributed to 12Sc were observed in the H

NMR spectrum, particularly diagnostic pyridinium and hydroxyl protons, likely due to its 85 poor solubility in non-polar C6D6. The reaction mixture only showed a decrease in peak intensities of 11Sc relative to the residual solvent peak of C6D6 due to precipitation of 12Sc over several days. Unfortunately, characterization by multinuclear NMR spectroscopy could not be obtained since a very small quantity of 12Sc was isolated.

Table 3-3 Selected metrical parameters for 12Sc.

Bond Distances (Å) Bond Angles (°) Sc1–N3 2.215(5) O1–Sc1–O2 75.85(15) Sc1–N5 2.231(4) Sc1–O1–Sc1’ 104.15(15) Sc1–N7’ 2.248(4) Sc1–N9’ 2.234(4) Sc1–O1 2.052(3) Sc1–O1’ 2.064(3) H1···O1A’ 2.045(5)

Figure 3-13 FT-IR spectrum of 12Sc showing prominent O–H and N–H bond stretches at 3590 and 3329 cm-1 respectively.

3.4 Summary and Conclusions

Scandium B2Pz4Py complexes were utilized with the aim of stabilizing an anionic terminal oxo compound [K][Sc=O]. Although there is evidence supporting its formation from the deprotonation of a scandium formate complex through trapping experiments with

Me3SiCl, the highly basic oxo moiety scavenges for any source of protons to form the corresponding scandium hydroxo complex [2Sc-OH]2. Slow formation of kinetically trapped μ-OH dimer aids in stabilizing the compound, which would otherwise quickly condense to afford the μ-O dimer 3Sc in solution. Further partial hydrolysis of the B2Pz4Py ligand framework have shown that accessing these unique neutral dihydroxo or cationic hydroxo scandium complexes are possible.

Chapter 4: Comparative Studies with Analogous B2Pz4Py Titanium Complexes

4.1 Introduction

This chapter details the synthesis and characterization of various B2Pz4Py titanium complexes that are analogous to the scandium systems previously covered in Chapters 2 and 3. It was postulated that titanium’s smaller ionic radius would cause the B2Pz4Py ligand framework to present greater steric hindrance around the metal center. The lower oxophilicity and presence of a d electron in titanium could impart differences in the coordination of ligands and reactivity in comparison to the scandium systems. Titanium alkyl and hydride complexes were easily synthesized from previously established procedure outlined in Chapter 2, however the titanium hydride complex was seen to be unreactive towards CO2. This is attributed to its inability to dissociate into reactive monomeric titanium hydrides which was supported by DFT calculations.

A drawback of working with hard early transition metals causes them to be highly reactive to hard bases that contain nitrogen or oxygen atoms. Their high electropositivity causes them to interact with oxygen donating sources such as H2O, O2, and N2O, and pyridine N-oxide, thus requiring strict air and moisture free handling techniques when working with these compounds. This is particularly evident in group (IV) transition metals whereby oxygen can be incorporated into the metal complexes as bridging and terminal oxo or side-on and bridging peroxo ligands (Figure 4-1).180

Figure 4-1 Common coordination modes of oxo and peroxo ligands on transition metals.

Although bridging oxo complexes (M–O–M) are very common and are typically thermodynamic sinks in the B2Pz4Py scandium and titanium systems, there is still a continued interest in synthesizing these oxo-bridged species.181 This involves the dealkylation or deamination reaction between a well-defined metal-hydroxyl compound and an organometallic or metal-amide reagent. Using two different metal centers allows for tuning of the electronic properties in these heterometallic species, which might exhibit different chemical reactivity compared to the homometallic analogues for example in olefin and ring-opening polymerization reactions.182 Alternatively, peroxo complexes of titanium are generally synthesized through the addition of dioxygen to a low valent metal complex

183,184 or by ligand exchange with H2O2. These discrete molecular systems are sought after as models to the proposed reactive intermediate in heterogeneous titanium-silicalite-1 (TS-

1), which have been shown to be a highly efficient catalyst for H2O2-based oxidations of

185 organic molecules.

Terminal oxo complexes in contrast are less prolific due to their propensity to oligomerize which can be avoided by employing sufficiently bulky ligands.186 Generation of these terminal titanium(IV) oxo complexes typically involves the oxidation of

89 titanium(II) precursors187-190 or by cross-metathesis from a titanium(IV) imido191,192 or alkylidene.193 Hence the titanium peroxo and terminal oxo compounds supported by the

B2Pz4Py ligand are targeted and characterized by various spectroscopic methods.

4.2 Synthesis of B2Pz4Py Titanium Chloro Complex

The synthesis of titanium chloro complex 1Ti follows closely the method developed for 1Sc. As titanium is most stable in the +4 oxidation state, there are a wide variety of both homoleptic and heteroleptic Ti(IV) starting compounds used in organometallic chemistry.194 These compounds are typically composed of halides, amides and alkoxides; tetraalkyls are sometimes used but in comparison to scandium, are rather uncommon.

Despite efforts to use bulkier alkyls and without any β hydrogens, these homoleptic alkyl titanium complexes are still thermally unstable and often decompose through bond homolysis or α hydrogen abstraction.195 This is inherent in Ti(III) as well since typical reaction products from the reaction of TiCl3 and alkyllithiums undergo disproportionation

II IV 196 to form Ti R2 and Ti R4. Therefore THF adducts of the titanium chloride compound

197 TiCl3(THF)3, which is synthesized by THF extraction of aluminum from 3TiCl3·AlCl3, is used for the synthesis of the (B2Pz4Py)TiCl precursor.

Scheme 4-1 Synthesis of 1Ti-Ph and 1Ti by salt metathesis.

198 The syntheses of 1Ti-Ph and 1Ti follow closely the method developed for 1Sc. A suspension of [Ar-B2Pz4LiPyH]2 in THF was first deprotonated with LiHMDS to give a translucent solution, to which solid addition of TiCl3(THF)3 to the in situ generated dilithio salt led to the instant formation of a dark brown-orange solution. Stirring the reaction mixture overnight at room temperature led to the precipitation of the pink titanium chloro starting material 1Ti-Ph or 1Ti in moderate yield (Scheme 4-1). Orange crystals of these air and moisture sensitive compounds were obtained by layering cyclohexane onto a saturated

THF solution of 1Ti-Ph, while the more soluble 1Ti crystallizes readily from hot benzene, and their structures were confirmed via X-ray crystallography (Figure 4-2).

Figure 4-2 Molecular structures of 1Ti-Ph (left) and 1Ti (right). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 4-1 Selected bond distances (Å) and angle (°) for 1M-Ph and 1M (M = Sc or Ti).

1Sc-Ph 1Sc 1Ti-Ph 1Ti Ti–Cl 2.419(1) 2.396(1) 2.3096(7) 2.337(1) Ti–NPz(avg) 2.205(6) 2.204(7) 2.143(4) 2.134(6) Ti–NPy 2.272(3) 2.306(3) 2.206(2) 2.229(2) Ti–NPz Plane 0.250(2) 0.257(2) 0.211(1) 0.210(1) Pytilt 73.7(1) 75.3(2) 74.34(6) 75.20(9)

Similar to the analogous scandium complexes, both 1Ti-Ph and 1Ti feature similar metrical data as seen in Table 4-1. The Ti atom protrudes by about 0.210(1) Å from the plane defined by the four pyrazolyl nitrogens in 1Ti; this is in contrast to 1Sc (0.257(2) Å) and is attributed to the smaller ionic radius of Ti(III) (Sc(III) 74.5 pm; Ti(III) 67 pm).35

The average Ti–NPz distance is 2.134(6) Å in 1Ti and is identical to that of 1Ti-Ph. A general recurring trend observed when comparing analogous scandium and titanium complexes is that Ti–R (R = apical ligand, Npz or Npy) bond lengths are ≈0.08 Å shorter than in the scandium systems which is in line with the smaller ionic radius of Ti(III). Although all six bonds to titanium in the octahedral geometry are shorter, the nearly invariant dihedral angle

92 between the pyridyl ring and equatorial plane suggests that any M–NPy π-bonding interactions is not a significant contributor to complex stabilization.90

Metalation of the B2Pz4Py ligand onto titanium results in the formation of 1Ti-Ph

1 and 1Ti which feature paramagnetic NMR spectra due the unpaired electron in the d system of a Ti(III) complex (Figure 4-3). Due to the paramagnetic nature of these complexes, most of the resonances could not be assigned with confidence, apart from the protons on the pyridyl ring and two methyl groups on the tolyl substituents on the B2Pz4Py ligand. The broad singlets at 5.19 (overlapping with another peak that integrates to 4 protons) and 5.27 ppm represent the two meta protons and single para proton in the pyridyl ring, while the sharp singlet at 2.42 ppm is diagnostic of the tolyl ligand methyl groups. The compounds were 13C{1H} NMR spectroscopy silent and no useful coupling information was obtained from the 1H-1H COSY experiment, thus hindering the assignment of the remaining resonances but could correspond to any of the pyrazolyl or tolyl aromatic protons.

Magnetic susceptibility measurements obtained from a Gouy balance199 or an Evans’

200 method by NMR spectroscopy gave a µeff of 1.50 and 1.34 for 1Ti-Ph and 1Ti respectively.

Although slightly lower than the typical spin only value for the observed magnetic moment of 1.73 µB for Ti(III), it can be approximated that these complexes have 1 unpaired electron

(S = ½).201

1 Figure 4-3 H NMR spectrum of 1Ti in C6D6. Solvent residual signal indicated with an asterisk.

4.3 Synthesis of B2Pz4Py Titanium Alkyl and Hydride Complexes

Titanium chloro complex 1Ti acts as an ideal starting material for further derivatization through various salt metathesis reactions. Treatment of 1Ti with different alkyllithium reagents led to the formation of the light orange alkyl complexes 2Ti-R in good yield. Addition of LiCH2SiMe3 to 1Ti in toluene caused the reaction mixture to turn from cloudy pink to translucent orange over a few hours. These reactions were left to stir overnight to ensure complete formation of 2Ti-R. Subsequent filtration of insoluble LiCl and evaporation of solvent in vacuo led to a light orange solid of the alkyl compound. The synthesis of 2Ti-Me with MeLi proceeded much slower in toluene as resonances for 2Ti were still present in the reaction mixture even after stirring for 3 days. However switching the reaction solvent to diethylether aided in solubilizing MeLi which resulted in a complete reaction overnight (Scheme 4-2).

Scheme 4-2 Synthesis of 2Ti-R by salt metathesis.

These paramagnetic Ti(III) alkyl complexes 2Ti-R feature broadened resonances in

1 the H NMR spectrum similar to 2Ti; the terminal –SiMe3 group in 2Ti-CH2SiMe3 shows up as a broad singlet at –0.83 ppm while the α hydrogens in both complexes could not be located likely due to their proximity to the paramagnetic titanium metal center. The effect of the unpaired electron density, predominantly found in the singly occupied molecular orbital (SOMO) on titanium, accompanied by fast electron-nuclear spin relaxation, causes

202 significant line broadening thus these α hydrogens are unobserved. 2Ti-Me and 2Ti-

CH2SiMe3 have µeff values of 1.77 and 2.04 (Evans’ method, S = ½) respectively, and their purity are supported by elemental analyzes. Their identity of 2Ti-Me and 2Ti-CH2SiMe3 were also supported by solid state structures as determined by X-ray crystallography although only a connectivity map was obtained for 2Ti-Me (Figure 4-4).

Figure 4-4 Connectivity map of 2Ti-Me (left) and molecular structure of 2Ti-CH2SiMe3 (right). Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 4-2 Selected metrical parameters for 2Ti-CH2SiMe3.

Bond Distances (Å) Bond Angles (°) Ti1–N1 2.267(2) N2–Ti1–N8 94.63(6) Ti1–N2 2.175(2) N2–Ti1–N4 87.37(6) Ti1–N4 2.121(2) N4–Ti1–N6 90.58(6) Ti1–N6 2.134(2) N6–Ti1–N8 84.42(6) Ti1–N8 2.197(2) Ti1–C32–Si1 129.5(1) Ti1–C32 2.124(2)

Like the scandium complexes 2Sc-R, these alkyl complexes 2Ti-R are extremely moisture sensitive. The titanium µ-O dimer 3Ti can be synthesized quantitatively by

1 addition of water to 2Ti-R and eliminating an equivalent of alkane. The H NMR spectrum of 3Ti has broadened resonances like 1Ti, and an Evans’ method measurement yields a solution magnetic moment of μeff = 2.53 corresponding to two unpaired electrons (S = 1).

Compound 3Ti crystallizes readily by slow vapour diffusion of pentane into a saturated benzene solution, and the molecular structure is depicted in Figure 4-5. It features equal

Ti–O bonds of 1.8247(6) Å and Ti1–O1–Ti1’ is slightly bent at 167.34(15)° which is consistent with packing effects as seen in 3Sc and 3Sc-Ph. Just like 3Sc-Ph, the phenyl

96 substituted B2Pz4Py ligand titanium µ-O dimer 3Ti-Ph has an almost linear Ti1–O1–Ti1’ angle of 178.98(16)°. Although bond distance and angles around the Ti–O–Ti unit can vary greatly depending on nuclearity of the compounds, 3Ti has similar metrical parameters to other strictly homonuclear titanium µ-O dimers.‡

Figure 4-5 Molecular structure of 3Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 4-3 Selected metrical parameters for 3Ti.

Bond Distances (Å) Bond Angles (°) Ti1–N1 2.284(2) N2–Ti1–N8 98.54(9) Ti1–N2 2.162(2) N2–Ti1–N4 82.62(9) Ti1–N4 2.136(2) N4–Ti1–N6 91.91(9) Ti1–N6 2.123(2) N6–Ti1–N8 84.03(9) Ti1–N8 2.196(2) Ti1–O1–Ti1’ 167.34(15) Ti1–O1 1.8247(6)

‡ From the CCDC accessed on 1 April 2019, the average Ti–O bond distance in 206 hits for µ-O titanium dimers is 1.820 Å and they range from 1.706 to 1.995 Å. The average Ti–O–Ti angle is 170.8° ranging from 150.0 to 180°. 97

The analogous titanium hydride complex [2Ti-H]2 was synthesized directly through a salt metathesis reaction with 1Ti and NaHBEt3 instead of the less efficient β-hydride elimination route (Scheme 4-3). The addition of the borohydride reagent to 1Ti in toluene caused the pink suspension to turn dark red within 15 mins. Subsequent filtration of insoluble NaCl and evaporation of the filtrate in vacuo resulted in a dark red residue. The residue was washed with pentane to remove excess NaHBEt3 and the product was isolated as a maroon solid in 85% yield.

Scheme 4-3 Synthesis of titanium hydride complex [2Ti-H]2.

1 1 Despite [2Ti-H]2 being formally Ti(III) d complex, resonances observed in the H

NMR spectrum was characteristic to that of a diamagnetic complex; peaks are well resolved and coupling constants could be measured (Figure 4-6). This could be due to the high s character of the hydride ligands that allow for spin transfer between metal centers in a dimeric species. Apart from the usual ligand resonances, a broad signal is also observed at –66.36 ppm and was assigned to Ti–H based on integration. Although the IR spectrum of [2Ti-H]2 showed similarities with [2Sc-H]2, a Ti–H stretch could not be confidently

98 assigned as it could be overlapping with other IR bands. Furthermore, attempts to synthesize deuteride [d1-2Ti-H]2 were not successful. No reactivity was observed when 1Ti was treated with LiDBEt3 or in the presence of 12-crown-4, a complexing agent that is selective for the lithium cation. The Signer method of molecular weight determination was performed once gain (outlined in Section 2.4) with azobenzene as the standard.120 Both compounds were dissolved in benzene and the Signer method gave a calculated molecular weight of 1214 g mol-1, which is consistent with the actual molecular weight of 1196.24 g

-1 mol for a dimeric structure (Figure 4-7). Fortunately, dark orange crystals of [2Ti-H]2 could be obtained by layering HMDSO onto a concentrated bromobenzene solution. The molecular structure is depicted in Figure 4-8 in which the hydride was located from the difference map and refined isotropically.

* 5

9 6 8 7 3 4 1 2

1 Figure 4-6 H NMR spectrum of [2Ti-H]2 in C6D6 with inset expansion region of the Ti-H signal. Ligand resonances are assigned with numbers. Solvent residual signal indicated with an asterisk.

Figure 4-7 Signer method for the molecular weight determination of [2Ti-H]2.

Figure 4-8 Molecular structure of [2Ti-H]2. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

100

Table 4-4 Selected metrical parameters for [2Ti-H]2.

Bond Distances (Å) Bond Angles (°) Ti1–N1 2.181(3) N2–Ti1–N8 94.6(1) Ti1–N2 2.185(3) N2–Ti1–N4 87.0(1) Ti1–N4 2.315(3) N4–Ti1–N6 90.4(1) Ti1–N6 2.165(3) N6–Ti1–N8 85.9(1) Ti1–N8 2.354(3) Ti1–H1–Ti1’ 113.1(2) Ti1–H1 1.72(3) Ti1–H1’ 1.80(4) Ti1···Ti1’ 2.938(1)

The bridging Ti–H moiety in [2Ti-H]2 is symmetric, with a Ti–H–Ti’ angle of

113.1(2)° that is comparable with other dimeric Ti(III)–H complexes, while Ti–H bond lengths of 1.72(3) and 1.80(4) Å are found to be slightly shorter than literature averages of

203-205 1.889 and 1.926 Å. However these Ti–H bond lengths in [2Ti-H]2 likens that of a related [[(Me3SiNCH2CH2)2NSiMe3]TiH]2 dimer with Ti–H bond lengths of 1.77(3) and

1.88(3) Å.206 The dimeric diamidoamine titanium hydride complex has the same Ti···Ti’ distance of 2.931(3) Å, both of which are shorter than related titanium(III) hydrides averaging 3.182 Å and could likely imply a weak metal–metal interaction.203-205 Similar to

1 [2Ti-H]2, [[(Me3SiNCH2CH2)2NSiMe3]TiH]2 also has two d titanium(III) centers, yet the

1H NMR spectrum of it has resonances in the normal chemical shift range, with the hydride showing up –35.9 ppm. The effective magnetic moment for [2Ti-H]2 was measured to be

1.93 (Evans’ method, S = ½) which implies about one unpaired electron per titanium dimer.

201 The lower than expected magnetic moment (2.83 µB for two unpaired electrons) and

NMR spectrum of [2Ti-H]2 suggest that there is a thermal equilibrium between the singlet and triplet state. In order to get a better understanding of the electronic structure of [2Ti-

H]2, DFT calculations were performed which showed that the triplet spin state is the ground

101 state with one unpaired electron on each titanium center. The closed-shell singlet state was found to be 7.6 kcal mol-1 higher in energy than the triplet and no significant bonding between the titanium centers was found; the Wiberg bond index for the Ti···Ti interaction was 0.09, consistent with the observed distance of 2.938(1) Å found in the X-ray structure.

The reaction coordinate for the dissociation of [2Ti-H]2 into individual monomers were also calculated from the established triplet ground state and it was found that this

-1 -1 process is disfavoured by 21.3 kcal mol which is lower than 28.3 kcal mol for [2Sc-H]2

(Section 2.4). Interestingly, there is an intermediate on the coordinate for dissociation where the Ti2H2 core is disrupted, but the two (B2Pz4Py)TiH fragments remain associated through relatively strong π stacking interactions between the pyrazolyl rings of the two monomers (Figure 4-9). This species is quite close in energy to the ground state dimer, being only 3.3 kcal mol-1 higher in energy. Inclusion of dispersion forces raises this relative energy to 8.9 kcal mol-1, indicating that these forces play a strong role in stabilizing the dimeric structure of the compound. Thus, this raises the overall barrier to dissociation of

-1 [2Ti-H]2 to 46.4 kcal mol when these dispersion forces are taken into consideration. The low energy barrier to this intermediate suggests that it could be thermally populated and therefore this loosely associated dimer can exist as a triplet or as an open shell singlet, depending on whether the spins of the electrons on each Ti center are parallel or opposed.

The latter state cannot be described using DFT approaches and therefore multireference calculations (complete active space self-consistent field (CASSCF) in this case) were carried out. Interestingly, the two spin states are found to be degenerated so that they can mix, leading to a multi-reference ground state. This may explain the lower effective magnetic moment found for solutions of [2Ti-H]2 via the Evans’ method.

102

Figure 4-9 Geometry optimized structures of the dissociation of [2Ti-H]2 into the monomers proceeding through a loosely associated dimer (middle).

Unsurprisingly the reaction profile for the insertion of CO2 into the Ti–H bond mirrors that of the related scandium hydride system. It was calculated that the barrier to

-1 insertion is only 3.3 kcal mol starting from the van der Waals complex between 2Ti-H and

1 CO2, which results in a κ formate complex through a highly exothermic pathway (–43.1

-1 kcal mol downhill, Figure 4-10). Despite facile insertion of CO2 into the Ti–H bond, reactivity of [2Ti-H]2 with CO2 was not observed experimentally even when heated to 80

°C for several days seemingly due to the high energy barrier for dissociation into the highly reactive monomer. The titanium formate complex 4Ti-H was instead synthesized through a salt elimination route starting from 1Ti and sodium formate in 90% yield, and an effective magnetic moment of 1.93 (S = ½) was measured by Evans’ method. Although 4Ti-H was not structurally characterized by X-ray crystallography, the FT-IR spectrum of it features strong intensity bands at 1673 and 1244 cm-1 corresponding to the asymmetric and symmetric stretching vibrations of the CO2 unit respectively, consistent with other

163,164 -1 monodentate titanium formate complexes. The larger Δν of 429 cm in 4Ti-H is also indicative of a monodentate formate moiety, in contrast to the bidentate formato scandium

103

-1 complex 4Sc-H which has νasym(CO2) and νsym(CO2) at 1566 and 1377 cm respectively

(Figure 4-11).207,208 A larger Δν indicates that the C–O single bond and C=O double bond are relatively discrete as expected for a κ1(O) formate. Alternatively, a smaller Δν can be

2 viewed as electron delocalization within a κ (O,O) CO2 moiety.

Figure 4-10 DFT calculated reaction profile for the insertion of CO2 into the Ti–H bond of 2Ti-H. ΔG values given in parentheses.

104

1673 cm-1 1245 cm-1 1566 cm-1 1377 cm-1

Figure 4-11 Superimposed FT-IR spectra of 4Ti-H and 4Sc-H (left) with expansion of the C–O region showing the effect of denticity on bond stretches (right).

4.4 Generation of Cationic Titanium Complexes

4.4.1 Titanium Formatoborate Complex

As the synthesis of various B2Pz4Py titanium complexes has been well established as described above, we wanted to investigate the possibility of using the analogous cationic titanium system as a potential catalyst for the hydrosilylation of CO2 by extending the

Sc(III) chemistry to it. Perhaps the lower Lewis acidity of titanium in comparison to scandium, and the presence of a d electron could impart differences in the stability of these compounds or coordination chemistry, thus affecting its overall reactivity.

The preparation of titanium formatoborate complex 8Ti-H follows closely to that of

8Sc-H (Scheme 4-4). When a toluene solution of Et3SiH and B(C6F5)3 was added to 1Ti, complete dissolution of the sparingly soluble pink titanium chloro complex occurred within a few minutes to yield a dark brown-yellow solution. Subsequent evaporation of the

105 reaction solvent in vacuo, trituration with pentane and filtration led to the isolation of 7Ti as an olive green solid in 79% yield.

Scheme 4-4 Synthesis of 8Ti-H with B(C6F5)3-activated hydrosilane in the presence of CO2.

훿표푏푠푒푟푣푒푑 = 훿푑푖푎푚푎푔푛푒푡푖푐 + 훿푖푠표푡푟표푝푖푐

훿푖푠표푡푟표푝푖푐 = 훿푐표푛푡푎푐푡 + 훿푑푖푝표푙푎푟

Equation 4-1 Contribution of unpaired electron-nucleus interactions that affects the observed NMR spectrum of a paramagnetic molecule in solution.

1 Although resonances for 7Ti in the H NMR spectrum are paramagnetically-shifted due to the Ti(III) center, every signal could be easily identified and integrated (Figure

1 4-12). H resonances for the meta and para pyridyl protons in 7Ti are significantly shifted

3 and they are located at 12.89 and –4.27 ppm respectively with a JHH coupling constant of

7.3 Hz which is typical around pyridine rings.169 The observed NMR spectrum arises from both diamagnetic and paramagnetic shifts (Equation 4-1). The presence of molecular paramagnetism due to unpaired electron-nucleus interactions results in an isotropic shift

(δisotropic). The isotropic shift is governed by two main interactions: (1) Fermi contact interaction which relates to the extend of metal-ligand bonding and location of unpaired spin density on the ligands (δcontact); (2) intramolecular dipole-dipole interaction between

106 electron magnetic moments localized on the metal and nuclear magnetic moments localized on the ligands (δdipolar). The pronounced effect on the observed chemical shifts of the pyridyl protons likely indicates a greater Fermi contact interaction in which unpaired spin density is located on the pyridine ring, whereas the dipole-dipole interaction that produces dipolar contribution to the isotropic shift has a lesser impact. Since the latter is a through- space type of interaction, both pyrazolyl and pyridyl protons should observe comparable dipolar couplings.209 The broadened B–H resonance was observed at 4.42 ppm due to quadrupolar 11B nucleus (I = 3/2), while the borate anion is at –24.1 ppm in the 11B{1H}

NMR spectrum, comparable to –23.9 ppm observed in 7Sc. The structure was tentatively assigned as the µ-chloride scandium dimer on the basis that exposure of 7Ti to 1 atm of

1 CO2 results in the formation of 1Ti and a new set of ligand signals in the H NMR spectrum and its bulk purity of the sample was established through elemental analysis.

1 Figure 4-12 H NMR spectra of 7Ti (top) and 8Ti-H (bottom) in C6D6. Solvent residual signal indicated with an asterisk.

107

Treatment of the reaction mixture of 7Ti, outlined in Scheme 4-4, with 1 atm of CO2 causes an immediate colour change from dark brown-yellow to bright orange, and the formatoborate complex 8Ti-H can be isolated as a light orange solid in 81% yield from a

1 one-pot reaction starting from 1Ti. Most noticeably, H resonances for the meta and para pyridyl protons have shifted to 7.05 and 1.95 ppm, and are identified by integration and the

3 1 JHH coupling constant of 7.3 Hz (Figure 4-12). The hydridoborate H resonance in 7Ti at

4.47 ppm disappears and a new peak was observed at –18.08 ppm corresponding to the formatoborate C–H proton. A small downfield shift in the 11B{1H} NMR to –16.6 ppm was observed which suggests that the boron center is influenced by the paramagnetic Ti(III) center, in contrast to related paramagnetic decamethyl titano- and vanadocenium formatoborate complexes that have boron resonances ≈ –1 ppm.210 An increase in the

19 1 chemical shift difference between m- and p-C6F5 resonances in the F{ H} NMR spectra

(Δδm,p) from 2.5 to 6.6 ppm in 7Ti and 8Ti-H respectively also indicates the change of the hydridoborate anion to formatoborate.134,140,146,148,210

Scheme 4-5 Hydrosilylation of formate moiety starting from 8Ti-H, and the suggested deactivation pathway to form [2Ti-H]2.

108

During preliminary reactivity investigation, 8Ti-H was initially treated with an excess of Et3SiH (100 eq.) in the presence of 1 eq. of B(C6F5)3 and the formation of a titanium hydridoborate complex 6Ti was observed based on the characteristic trio of peaks

19 1 corresponding to the o-/m-/p-C6F5 resonances (Δδm,p = 2.9 ppm) in the F{ H} NMR spectrum (Scheme 4-5). This involves the dissociation of free B(C6F5)3 from 8Ti-H to form a neutral titanium formate complex. The formate moiety then reacts with the B(C6F5)3- activated hydrosilane, to form triethylsilyl formate and the corresponding hydridoborate complex. However, triethylsilyl formate was not observed in the 1H NMR as it could have been further hydrosilylated into (Et3SiO)2CH2, Et3SiOCH3 and ultimately CH4 which would be present in the headspace of the sealed NMR tube. To complicate things even further, proton resonances related to these Ti(III) complexes involved in the catalytic cycle were not observed, and only organic compounds could be detected in the 1H NMR spectrum. Unsurprisingly when the reaction mixture of 8Ti-H was placed under similar reaction conditions as 8Sc-H outlined in Section 2.6.3 (1 mol% of 8Ti-H, 50 °C under 1 atm of CO2), a small peak for CH4 was observed once again however majority of Et3SiH remained unreacted. It was originally thought that the lack of reactivity could be due to a similar deactivation pathway as that experienced by 8Sc-H, in which intramolecular hydride ligand transfer from boron to titanium in 6Ti occurs to form 2Ti-H and B(C6F5)3 (Scheme

4-5). However, formation of the analogous titanium triethylsiloxide complex is unlikely due to the high energy cost required for the dissociation of [2Ti-H]2, thus hindering the addition of the reactive Ti–H bond across the O=C bond of triethylsilyl formate.

Furthermore, the characteristic dark red colour formed when 2Ti-H is dissolved in solution was not observed thus suggesting that this deactivation pathway is unlikely.

109

1 Figure 4-13 H NMR spectrum of 6Ti in C6D6 with coloured dots to indicate assignment of ligand resonances. Solvent residual signal indicated with an asterisk.

It is worthy to note that 2Ti-H reacts with B(C6F5)3 to form the corresponding titanium hydridoborate complex 6Ti. Heating a solution of 2Ti-H with one equivalent of

B(C6F5)3 at 80 °C results in the dark red solution turning dark orange. Clean conversion to

1 the paramagnetic product 6Ti was observed in the H NMR spectrum in which the Ti–H resonance in 2Ti-H at –66.36 ppm has disappeared (Figure 4-13). Although the H–B(C6F5)3 proton resonance could not be observed likely due to peak broadening by the quadrupolar

11B nucleus (I = 3/2), the borate anion appears as a broad singlet at –23.8 ppm in the 11B

19 1 NMR spectrum. Additionally the F{ H} NMR spectrum shows three broad o-/m-/p-C6F5

- 134 resonances with Δδm,p = 2.5 ppm, thus supporting the presence of a [HB(C6F5)3] anion.

The reaction proceeded was likely due to disruption of the π stacking interactions between the pyrazolyl rings in the presence of perfluorophenyl rings. The addition of B(C6F5)3 aids in the dissociation of [2Ti-H]2 which then allows for hydride abstraction to occur.

Subsequent exposure of 6Ti to an atmosphere of CO2 immediately converts it to the titanium formatoborate complex 8Ti-H likely due cooperation of the Lewis acidic titanium and the

- nucleophilic hydride in the [HB(C6F5)3] anion in activating CO2. Although these stepwise

110 reactions suggest that catalytic hydrosilylation of CO2 could be possible, preliminary work showed that 8Ti-H is not a particularly useful catalyst. Therefore, it would be useful to isolate any off-cycle products in order to gain more insight into the reactivity of the complexes involved in the catalytic cycle.

4.4.2 Titanium Triflimide

Akin to the protonolysis of scandium alkyl complexes with a strong Brønsted acid

HNTf2 (Section 3.3), a more efficient route to generating these cationic titanium complexes

211-213 was investigated via halide abstraction from 1Ti with Me3SiNTf2. Treatment of 1Ti with one equivalent of Me3SiNTf2 causes an immediate formation of a clear orange solution, which upon evaporation yields the titanium triflimide complex 11Ti as a beige solid in 73% yield. An Evans’ method measurement yields a solution magnetic moment of

μeff = 1.92 with one unpaired electron (S = ½).

Scheme 4-6 Synthesis of 11Ti by halide abstraction.

111

1 19 1 Figure 4-14 H (left) and F{ H} (right) NMR spectra of 11Ti in C6D6. Solvent residual signal indicated with an asterisk.

Similar to the previously discussed cationic titanium hydridoborate and

1 formatoborate complexes, the H NMR spectrum of 11Ti also exhibit well resolved resonances that are paramagnetically shifted (Figure 4-14). A doublet and triplet at 8.61

3 and –1.06 ppm with JHH coupling constant of 7.3 Hz are assigned as the meta and para pyridyl protons respectively. While the three pyrazolyl proton resonances are observed as broad singlets due to close proximity to the paramagnetic Ti(III) center, the aromatic para-

3 tolyl resonances appear as well defined doublets ( JHH = 7.5 Hz) at 7.59 and 7.26 ppm. A boron chemical shift for the B2Pz4Py ligand was observed at –20.2 ppm and a singlet appeared at –74.4 ppm for the triflimide anion in the 19F{1H} NMR spectrum. While it was originally thought that the triflimide anion was bound κ2(O,O) to titanium due to equivalent

–CF3 groups like in the case for 11Sc, the molecular structure of 11Ti shows that it is instead bound through κ1(O) (Figure 4-16). This suggests the possibility of fluxional anion coordination to titanium in the solution state, from one oxygen atom to another oxygen atom bonded that is bonded to a different sulfur atom.214 19F{1H} NMR spectrum obtained for 11Ti at variable temperatures showed significant broadening of the triflimide resonance

112 when cooled to –80 °C (Figure 4-15). Although two distinct peaks for the κ1(O)-triflimide anion were not observed, the variable temperature NMR spectra hints that coalescence point would be around –80 °C and that fluxional behaviour is prominent even at low temperatures. The difference in denticity between 11Sc and 11Ti could be attributed to titanium’s lower oxophilicity and smaller ionic radius which causes it to protrude out less from the plane defined by the four pyrazolyl nitrogens. While the κ2(O,O) coordination mode has been observed on titanium systems, the slightly tucked in titanium center in 11Ti would therefore be unable to accommodate the bidentate triflimide anion.179,214

25 °C

10 °C

-5 °C

-20 °C

-35 °C

-50 °C

-65 °C

-80 °C

19 1 Figure 4-15 F{ H} NMR spectrum of the triflimide region of 11Ti in Tol-d8 at variable temperatures.

113

Titanium triflimide complex 11Ti has a Ti–O bond length of 2.058(2) Å, which is typical of a Ti–O σ bond.179 Differences in bond lengths within the triflimide anion also indicate the absence of delocalization of the negative charge across O–S–N–S–O that one would expect for a κ2(O,O) chelate. The S1–O1 bond length is longer at 1.458(2) Å compared to the remaining S–O bond length which remain short (average 1.413(5) Å). The

N10–S1 bond length is also shorter than N10–S2 at 1.533(3) and 1.584(3) Å respectively, which is suggestive of the structure of 12Ti shown in Scheme 4-6.

Figure 4-16 Molecular structure of 11Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 4-5 Selected metrical parameters for 11Ti.

Bond Distances (Å) Bond Angles (°) Ti1–N1 2.143(2) N2–Ti1–N8 95.76(7) Ti1–N2 2.147(2) N2–Ti1–N4 86.72(7) Ti1–N4 2.086(2) N4–Ti1–N6 92.22(7) Ti1–N6 2.077(2) N6–Ti1–N8 84.92(7) Ti1–N8 2.155(2) N1–Ti1–O1 174.26(7) Ti1–O1 2.058(2) S1–O1 1.458(2) N10–S1 1.533(3) N10–S2 1.584(3)

114

4.5 Titanium Peroxo and Terminal Oxo Complexes

Previously discussed reactivity of scandium and titanium alkyl complexes have shown that they are easily hydrolyzed with the addition of water to form the corresponding metal hydroxo intermediate, which undergoes subsequent condensation to form the µ-O dimer 3M. While 3M in general is highly stable, 3Ti is easily oxidized to its more stable +4 oxidation state. Exposure of a solution of paramagnetic 3Ti in C6D6 to an atmosphere of O2 led to the formation of new diamagnetic peaks in the 1H NMR spectrum, and an orange precipitate formed when the solution was left to stand overnight. Evaporation of the reaction mixture and reconstituting it in a more polar solvent revealed the presence of two

2 distinct compounds that were identified to be the side-on η -peroxo (13Ti) and terminal oxo

(14Ti) titanium complexes (Scheme 4-7 and Figure 4-17).

Scheme 4-7 Oxidation of 3Ti with O2.

115

1 Figure 4-17 H NMR spectrum of the reaction mixture of 3Ti and 1 atm O2 in CDCl3. Peak assignment indicated by coloured dots while solvent residual signal indicated with an asterisk.

Scheme 4-8 Synthesis of 13Ti by oxidation with H2O2.

The assignment of these ligand resonances was aided by synthesizing the two complexes independently. The addition of 30% w/v H2O2 to a solution of 1Ti in THF causes the light orange solution to darken over a few minutes (Scheme 4-8). Evaporation of the reaction mixture in vacuo after 2 h and recrystallization of the crude product yielded dark orange crystals of the titanium peroxo complex 13Ti. The molecular structure of 13Ti of C2v symmetry is depicted in Figure 4-18 and it features identical Ti1–O1 and Ti1–O2 bond lengths at 1.851(3) and 1.843(4) Å respectively. The O1–O2 bond distance of 1.450(5) Å

116

2- is longer than that of 1.21 Å for O2, and is consistent with the assignment as an O2 moiety

2 ‡ in other side-on η -peroxo titanium complexes. The UV–vis spectrum of the 13Ti complex in dichloromethane (DCM) shows a strong absorption band at 264 nm (ε = 14100 M-1 cm-

1 ) attributed to the O2→Ti ligand-to-metal charge-transfer band which is consistent with a

0 215-217 d electronic configuration of a Ti(IV) center. The IR spectrum of 13Ti also displays the expected vibrational bands for side-on coordinated peroxo ligand: a ν(O–O) stretching

-1 -1 216-218 frequency at 911 cm and a νsym(Ti–O) at 598 cm .

Figure 4-18 Molecular structure of 13Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 4-6 Selected metrical parameters for 13Ti.

Bond Distances (Å) Bond Angles (°) Ti1–N1 2.303(3) N2–Ti1–N2’ 93.56(12) Ti1–N2 2.159(2) N2–Ti1–N4 85.25(9) Ti1–N4 2.121(2) N4–Ti1–N4’ 90.96(12) Ti1–O1 1.851(3) Ti1–O1–O2 66.58(19) Ti1–O2 1.843(4) Ti1–O2–O1 67.2(2) O1–O2 1.450(5)

‡ 2 From the CCDC accessed on 12 April 2019, the average Ti–O bond distance in 92 hits for Ti–η (O2) is 1.870 Å. The bond lengths range from 2.162 to 2.499 Å. The average O–O bond distance is 1.464 Å, ranging from 1.419 to 1.487 Å. 117

It was envisioned that the targeted terminal oxo titanium complex 14Ti could be obtained by reacting 13Ti with an oxygen atom acceptor (Scheme 4-9). Stoichiometric oxidation of triphenylphosphine (PPh3) was achieved when reacted with 13Ti at 65 °C over

3 days; the slow reactivity likely due to steric effects from the approach of PPh3 to the peroxo moiety.219,220 A new distinct set of ligand resonances was observed in the 1H NMR spectrum as the reaction progressed, together with a sharp singlet at 27.0 ppm in the

31P{1H} for the formation of triphenylphosphine oxide. Subsequent evaporation of the reaction mixture in vacuo and washing the solid residue with Et2O yielded 14Ti as a pale- yellow solid in 73% yield.

II Scheme 4-9 Synthesis of 14Ti by oxidation from a Ti species or by oxygen atom abstraction from 13Ti.

Alternatively, 14Ti can be synthesized from a more efficient route starting from the titanium chloro precursor complex (Scheme 4-9). 1Ti is first reduced with KC8 in THF to generate in situ a reactive Ti(II) species, likely with a molecule of THF coordinated to

103,104,108 titanium trans to the pyridyl group as seen in other (B2Pz4Py)M(II) complexes.

Subsequent removal of graphite by filtration and oxidation by treating it with iodosobenzene (PhIO), an oxo transfer reagent, at 65 °C affords the corresponding terminal oxo complex. Orange crystals of the said compound were obtained by slow evaporation of a saturated solution of 14Ti in DCM and the molecular structure which is monomeric is

118 depicted in Figure 4-19. In comparison to 13Ti, the Ti–O bond length in 14Ti has shortened to 1.645(3) Å which is characteristic of a titanium terminal oxo bond,‡ and this is accompanied by the lengthening of Ti1–N1 from 2.303(3) Å to 2.371(4) Å due to a stronger

2 trans influence for a terminal oxo group compared to an η -peroxo ligand. Similarly, 14Ti features a strong ligand-to-metal charge-transfer absorption band at 267 nm (ε = 10400 M-

1 cm-1) in the UV–vis spectrum and a ν(Ti=O) stretching frequency was observed at 950 cm-1.

Figure 4-19 Molecular structure of 14Ti. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 4-7 Selected metrical parameters for 14Ti.

Bond Distances (Å) Bond Angles (°) Ti1–N1 2.371(4) N2–Ti1–N2’ 95.75(14) Ti1–N2 2.143(3) N2–Ti1–N4 84.16(10) Ti1–N4 2.114(3) N4–Ti1–N4’ 91.79(15) Ti1–O1 1.645(4)

‡ From the CCDC accessed on 12 April 2019, the average Ti=O bond distance in 65 hits is 1.647 Å, ranging from 1.611 to 1.688 Å. 119

4.6 Summary and Conclusions

Titanium(III) complexes of the B2Pz4Py ligand were prepared and compared to the analogous Sc(III) systems. The hydrido derivative 2Ti-H dimeric ground state is favoured by London dispersion forces and π stacking interactions between pyrazolyl rings of the two monomers. This added stabilization results in a greater barrier for dissociation into the reactive monomer, which is inaccessible at 80 °C thus resulting in no observable reactivity with CO2. Hydride abstraction can be achieved by reacting 2Ti-H with B(C6F5)3 to afford the titanium hydridoborate complex 6Ti. It further reacts with CO2 to afford the formatoborate complex 8Ti-H, both of which are proposed intermediates in the hydrosilylation of CO2. Preliminary studies show that activity of 8Ti-H mirrors that of the scandium analogue and investigation of deactivation pathways are ongoing. Differences in ligand coordination were also observed in titanium formate and triflimide complexes. Air and moisture stable Ti(IV) peroxo and terminal oxo were also prepared starting from the titanium chloro precursor 1Ti or from in situ generated reactive Ti(II) species, and were shown to be discrete monomeric compounds.

120

Chapter 5: Thesis Summary and Future Directions

5.1 Thesis Summary

Research in organometallic chemistry have provided much contribution and advances in the field of small molecule activation and the production of value-added chemicals. While mechanisms for these small molecule transformations have been greatly studied, reactive intermediates are usually proposed to fit these mechanisms. The reactivity of these intermediates can often be tuned and stabilized through proper ligand design, with one such approach being the utilization of bulky ancillary ligands that result in molecular species that have a single site of reactivity.

Coordination chemistry of the recently developed B2Pz4Py ligand has been expanded towards the early transition metals particularly scandium and titanium. Its ligation to scandium through salt elimination to afford an octahedral scandium chloro precursor complex 1Sc was described in Chapter 2. A series of highly robust and thermally stable scandium alkyl derivatives were synthesized that did not undergo sigma bond metathesis reactions. Instead, the scandium hydride complex was synthesized by reacting

1Sc with NaHBEt3 and it was shown to exist as a dimeric species by DFT calculations, molecular weight measurements and DOSY NMR. CO2 insertion into scandium alkyl and hydride bonds occur at elevated temperatures, with the latter due to prior dissociation into the reactive monomeric scandium hydride species. Ionic scandium hydridoborate and formatoborate complexes were also synthesized and its application towards catalytic

121 hydrosilylation of CO2 was explored, although the deactivation pathway that forms a scandium siloxide complex shuts down its catalytic activity.

Chapter 3 focuses on attempts to isolate reactive anionic terminal oxo and neutral hydroxo scandium species by taking advantage of the sterically bulky pentadentate ligand.

Trapping experiments showed the formation of an anionic terminal oxo scandium species in-situ upon deprotonation of a scandium formate complex. The highly basic oxo moiety rapidly scavenges for any sources of protons to form the corresponding hydroxo species that quickly condenses to form the more thermodynamically stable µ-O dimer. Other derivatives like a neutral dihydroxo or cationic hydroxo scandium complex can be obtained via partial hydrolysis of the B2Pz4Py ligand framework.

Finally, Chapter 4 explores the reactivity of the analogous titanium complexes supported by the pentadentate ligand. Synthesis of these compounds followed established procedures that were employed in Chapter 2. While there could be some differences in the coordination chemistry around titanium, e.g. titanium formate and triflimide complexes, reactivity of these complexes mostly followed the analogous scandium compounds. The titanium hydride complex [2Ti-H]2, while structurally similar to [2Sc-H]2, does not react with CO2 to form the formate complex seemingly due to the high energy barrier for dissociation into the reactive monomeric species. The titanium alkyl complexes were likewise moisture sensitive and quickly reacted to form the µ-O dimer. The dimeric species can also be oxidized in the presence of O2 to form a 1:1 mixture of peroxo and terminal oxo complexes, both of which were independently synthesized and are discussed within.

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5.2 Future Directions

5.2.1 Mixed Metal Oxides

Homobimetallic µ-O scandium or titanium dimer (3Sc or 3Ti) is a common byproduct formed in the presence of adventitious water. However, when two different metal centers are present in this molecular compound, electronics now differ from one side to another. This alters the Lewis acidity of the heterobimetallic species and they exhibit different chemical reactivity compared to the homobimetallic analogs. This leads to unique properties like enhanced polymerization properties due to the cooperative effect between the two metal centers.182 These molecular heterobimetallic oxo species are generally synthesized through dealkyation or deamination of organometallic or metal–amide reagents when reacted with metal–hydroxyl compounds (Scheme 5-1).181 A variety of well- defined M–OH complexes can be prepared by controlled hydrolysis routes which are subsequently deprotonated by a Brønsted basic organometallic or metal–amide reagent.221-

225 Alternatively, heterobimetallic oxo-bridged complexes can be made via adduct formation from a donor M=O moiety to a Lewis acidic metal center.226

Scheme 5-1 General synthesis of mixed metal oxides through dealkylation or deamination pathways.

The dealkylation route would be the ideal route to pursue in the synthesis of these mixed metal oxides since scandium and cobalt hydroxo and alkyls supported by the

123

104,198 B2Pz4Py ligand framework have been well established. As presented in Chapter 2, organoscandium complexes 2Sc-R would constitute as one of the reagents as it was shown that the scandium hydroxo complex 2Sc-OH readily condenses to form the µ-O dimer 3Sc.

104 Treatment of 2Sc-CH2SiMe3 with an equivalent of cobalt hydroxo complex 2Co-OH in toluene resulted in the quantitative formation of the Sc-µ-O-Co complex 3ScCo and tetramethylsilane (TMS) (Scheme 5-2).227

Scheme 5-2 Synthesis of oxo-bridged heterobimetallic 3ScCo and its contributing resonance structures via dealkylation of 2Sc-CH2SiMe3.

DFT computations using the B3PW91 functional were performed to calculate the energy of the system of 3ScCo under different spin multiplicities. The closed shell singlet structure (Sc3+–O–Co3+ low spin) was found to have the lowest energy and is 11.4 kcal mol-1 lower in energy than the triplet state (Sc2+–O–Co4+ low spin), and 11.7 kcal mol-1 lower than the quintet state (Sc2+–O–Co4+ high spin). Both scandium and cobalt are

124 trivalent and diamagnetic in the ground state which results in two distinct sets of ligand

1 resonances for the B2Pz4Py ligand as observed in the H NMR spectrum. Furthermore the

HOMO-2 and HOMO-4 molecular orbitals were observed to have π-type bonding interactions between the scandium and oxygen in which lone pairs of electrons on oxygen donate electron density into the empty scandium d orbitals, whereas the interaction is antibonding with cobalt (Figure 5-1). Therefore, bonding in the ground state system can be more accurately described as (ScIII)‒ =O→(CoIII)+ (Scheme 5-2). The Sc–O and Co–O bond lengths were also calculated to be 1.84 and 1.86 Å respectively which is consistent with actual bond distances of 1.837(3) and 1.866(3) Å as determined by X-ray crystallography.

Figure 5-1 DFT calculated HOMO-2 and HOMO-4 orbitals of the ground state of 3ScCo showing π bonding interaction between scandium and oxygen.

The bonding interaction in 3ScCo is reflective of a masked anionic scandium terminal oxo complex that was previously discussed in Section 3.2. The nucleophilic character of the scandium oxo moiety is still evident as the addition of an equivalent of Me3SiBr to

3ScCo resulted in the immediate formation of scandium trimethylsiloxide 9Sc and ligated

125 cobalt bromide.104 This also suggests that the Co–O is weak and could homolytically or heterolytically cleave with the addition of a H· or H+ source, thus leading to the formation of the scandium hydroxo complex 2Sc-OH. However this would likely immediately condense to form the thermodynamically stable scandium µ-O dimer 3Sc.

The analogous Ti-µ-O-Co complex 3TiCo was also studied by DFT calculations to gain some insight into the bonding interaction of the Ti–O–Co unit. Different spin states were considered and the quartet spin state (Ti4+–O–Co2+ high spin) was found to be the ground state. This is 20.4 kcal mol-1 more stable than the doublet spin state (Ti3+–O–Co3+ high spin) and 35.5 kcal mol-1 more stable than the sextet spin state structure (Ti3+–O–Co3+ high spin). Since there is a change in oxidation state of the quartet ground state, the system

IV II is better described as Ti =O→Co . 3TiCo would be synthesized in a similar fashion to 3ScCo in which a titanium alkyl complex 2Ti-R is reacted with a cobalt hydroxo complex 2Co-OH.

As the ground state structure is indicative of a classical Lewis acid-base adduct, it is expected that the product from the dealkylation reaction between 2Ti-R and 2Co-OH would be the same as treating the titanium terminal oxo complex 14Ti with a neutral B2Pz4Py cobalt(II) complex.104

5.2.2 Anionic Terminal Imido Scandium Complexes

In the pursue for anionic metal–ligand multiple bonds, isolation of the anionic terminal oxo scandium species [K][Sc=O] could pose as a challenging task due to its high basicity and high tendency to deprotonate compounds with acidic protons. Hence a more viable approach would be to target the terminal imido moiety ([NR]2-) as tuning of the R

126 substituent would aid in stabilization and eventual isolation of the compound. Examples of anionic rare-earth imido complexes still remain rare and they are typically synthesized by the deprotonation of an appropriate arylamido complex. Early work by this method on

Sm228 and Yb229 led to the generation of bridging imido complexes. More recently, Schelter and co-workers reported a monomeric cerium imido complex that was stabilized by a tripodal, anionic hydroxylaminato ligand framework, TriNOx3-, and an alkali metal.230 The stability of this complex was attributed to the presence of an electron withdrawing 3,5- bis(trifluoromethyl)phenyl substituent which reduces electron density at the imido nitrogen. Therefore, it is envisioned that deprotonation of a scandium amido complex supported by the B2Pz4Py ligand framework could result in the formation of an anionic scandium imido complex.

Scandium amido complexes can be synthesized from a salt metathesis reaction between the chloro complex 1Sc and a lithium arylamide; the latter can which be prepared by simple deprotonation of the corresponding aniline with nBuLi (Scheme 5-3).

Additionally, an emerging group of borylamidos, NHBR2, was found to form stronger

Sc=N bonds upon deprotonation than the arylimido counterparts due to the strong σ- and

π-donating nature of the borylimido ligand,231 thereby providing additional stabilizing factors in contrast to the arylimides.

Scheme 5-3 Synthesis of aryl (15Sc) or borylamido (16Sc) scandium complexes.

127

Treatment of 1Sc with a variety of lithium amides afford the amido complexes as colourless or pale yellow solids in moderate to good yield; the lower yield is attributed to their high solubility in pentane due to the presence of solubilizing alkyl groups. Although a variety of amido complexes were synthesized, only the 4-tert-butylphenyl and 1,2-C2H2 linked borylamine have been chosen as representative examples of aryl and borylamido complexes respectively (Scheme 5-3). The pattern of ligand resonances for all of these scandium amido compounds is indicative of C2v symmetry and is similar to the previously scandium alkyl compounds. The solid state structures of 15Sc and 16Sc were determined by

X-ray crystallography (Figure 5-2) and the former shows that the 4-tert-butylphenyl group aligns up in the “narrow grove” (between N2–Sc1–N2’). The barrier to rotation to place the amido group in the “wide groove” has to be low since an averaged structure was observed in solution at room temperature by 1H NMR spectroscopy, similar to that of the scandium alkyl complexes. Sc–N bond lengths and Sc–N–X (X = C or B) bond angles are consistent with other scandium amido complexes (Table 5-1) and presence of the sterically bulky boryl group in 16Sc results in a Sc–N–B bond angle of 157.6(3)°. The obtuse angle coupled with the short Sc···H distance of 2.16(5) Å suggest that there is significant α hydrogen agostic interactions in the solid state.79 This is in accordance to systems involved in N(pπ)→Sc(dπ) donation suggesting the possibility of forming an imide.

128

Table 5-1 Selected metrical parameters for 15Sc (top) and 16Sc (bottom).

Compound Bond Distances (Å) Bond Angles (°) 15Sc Sc1–N1 2.251(3) N2–Sc1–N4 95.78(8) Sc1–N2 2.221(2) N2–Sc1–N2’ 81.77(11) Sc1–N4 2.233(2) N4–Sc1–N4’ 83.58(11) Sc1–N6 2.064(3) Sc1–N6–C22 141.7(3) Sc1···H6A 2.45(3) 16Sc Sc1–N1 2.334(4) N2–Sc1–N8 100.51(12) Sc1–N2 2.223(3) N2–Sc1–N4 82.52(12) Sc1–N4 2.215(4) N4–Sc1–N6 92.60(12) Sc1–N6 2.181(3) N6–Sc1–N8 80.47(12) Sc1–N8 2.237(3) Sc1–N12–B3 157.6(3) Sc1–N10 2.032(4) Sc1···H10A 2.16(5)

Although the next step to forming a scandium imido complex would simply be the deprotonation of the amido group, choice of base and solvent would be important in terms of stabilizing the resulting imido moiety. Common bases like KHMDS or KH could be

129 used which would result in the coordination of the potassium cation with the Sc=N bond thereby stabilizing it. Ethereal solvents like THF, Et2O or DME could also be used to further stabilize alkali metal cation, and the addition of a crown ether to encapsulate the cation could potentially lead to a charge-separated salt of the imido complex. Furthermore, the formation of rare earth terminal oxo complexes was shown to occur between a methylidene84 or imido complex232 and benzophenone. Therefore generating the scandium imido species would present an alternate route to accessing the sought after anionic scandium terminal oxo complex.

5.2.3 Small Molecule Activation and Functionalization

5.2.3.1 Hydrazine Derivatives from N2

The field of low-valent early transition metals has flourished in recent years due to their application in bond activation and functionalization of substrates in catalysis.233 These complexes are often highly reducing and are able to activate molecules which is facilitated by oxidation of the metal and formation of strong metal–ligand bonds. One area of great interest is the conversion of dinitrogen into value-added N-containing organic compounds from a fundamental and practical approach,234 and one such transformation is via C–N

235-243 bond forming processes mediated by N2–transition metals. Hydrazine derivatives, which may contain up to four C–N bonds on the N–N core, are primary targets for direct

244 conversion of N2 and are often proposed as important synthetic intermediates. The formation of pure organic compounds through hydrazine derivatives with N2 as the nitrogen source have so far been limited to Ti,245 Zr,246 Hf,247,248 Mo249 and W.250

130

Recently, Zhang, Xi and co-workers reported the first rare-earth dinitrogen complex that was highly efficient in converting N2 into hydrazine derivatives from carbon- based electrophile.251 A chloride-bridged discandium complex supported by Cp* and

3- amidinate ligands was shown to form the (N2) -bridged complex upon reduction with excess potassium while under an atmosphere of N2 (Scheme 5-4). Subsequent treatment

2- with methyl triflate affords the (N2Me2) -bridged discandium complex, which further reacts with carbon-based electrophiles to produce the hydrazine derivative and regenerating the chloro precursor complex in the process.

Scheme 5-4 Scandium-mediated synthesis cycle for the formation of hydrazine derivatives with (E = carbon-based electrophile).251

II As previously discussed in Section 4.5, the reactive (B2Pz4Py)Ti species can be generated in situ by reducing the titanium chloro complex 1Ti with KC8. This reactive species can subsequently be oxidized by O2 or an oxygen atom transfer reagent to form the peroxo (13Ti) or terminal oxo complex (14Ti) respectively. By extension of this to the

II scandium system, a dark brown solution of the reduced scandium species (B2Pz4Py)Sc can be generated in situ when 1Sc is treated with one equivalent of KC8. Subsequent

131 addition of an atmosphere of dinitrogen resulted in a colour change back to colourless

(Scheme 5-5).

II Scheme 5-5 Synthesis of side-on dinitrogen complex 17Sc from a reduced Sc species.

The structure of the product 17Sc, as determined by X-ray crystallography, was found to be a side-on dinitrogen bridging discandium complex (Figure 5-3). The N10–

2- N10’ distance of 1.259(6) Å in 17Sc is in the double bond range consistent with an (N=N) ligand (Table 5-2).252 The side-on bridging motif was first observed by the reduction of

110 [(C5Me4H)2Sc][(μ-Ph)BPh3] with KC8 under an atmosphere of N2, or by the direct

253 reaction of [(C5Me4H)3Sc] with N2. More recently, an end-on dinitrogen bridging

254 complex was prepared via reduction of Sc[N(SiMe3)2]3 under N2. The Sc2N2 unit in 17Sc is slightly bent in which the dihedral angle between the planes defined by Sc1–N10–N10’ and Sc1’–N10–N10’ from each asymmetric unit is 15.95°.

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Figure 5-3 Molecular structure of 17Sc. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

Table 5-2 Selected metrical parameters for 17Sc.

Bond Distances (Å) Bond Angles (°) Sc1–N1 2.362(3) N2–Sc1–N8 102.2(1) Sc1–N2 2.275(3) N2–Sc1–N4 81.5(1) Sc1–N4 2.196(2) N4–Sc1–N6 91.6(1) Sc1–N6 2.220(3) N6–Sc1–N8 80.5(1) Sc1–N8 2.217(3) N10–Sc1– N10’ 36.6(2) Sc1–N10 1.988(4) Sc1– N10’ 2.015(3) N10– N10’ 1.259(6)

Subsequent reactions with regards to functionalization of the N2 core would entail the reduction of 17Sc for example with potassium. The resulting complex from N2

3- + activation to (N2) would be counterbalance by K ion potentially through an inner-sphere coordination to the N2 core although the addition of an equivalent of cryptand could aid in the stabilization of this complex. Crystallization and analysis of this reduced species by X-

133 ray crystallography would also provide important metrical data to support the proposed

3- (N2) ion as the N–N bond distance should be between a N=N double bond and a N–N

3- 255-257 single bond; ranging around 1.36–1.41 Å in (N2) -rare-earth complexes. Although

3- the next step would be to investigate the reactivity of the (N2) discandium complex with

2- a variety of electrophiles, the formation of the side-on coordinated bent [N2R2] fragment, similar to that in Scheme 5-4, could be a potential challenge due to the presence of the

1 1 sterically demanding B2Pz4Py ligand. Instead, the bridging substituted hydrazido µ2,η ,η -

258 N2R2 discandium complex would be the more favourable product. Although Zhang, Xi and co-workers showed that addition of another electrophile (carbon based or Brønsted acid) would result in the regeneration of the chloro-bridged scandium dimer and the elimination of the hydrazine derivative, their work was based off a one-pot reaction in which stoichiometric amount of reagents were added sequentially (Scheme 5-4).251 This could potentially present a compatibility issue between electrophiles in the pursue of

1 2 1 2 catalytic synthesis of substituted hydrazine derivatives N2R 2R 2 where R and R are two different electrophiles. Furthermore, presence of excess reducing agent could react with reactive intermediates and electrophiles could react with the scandium chloro precursor complex, both of which would result in the shutting down of the catalytic cycle.

5.2.3.2 Transition Metal Oxos in Frustrated Lewis Pairs Systems

Gilbert Lewis’ concept of acids and bases is key in coordination chemistry in which strong adducts are usually formed between the electron-pair-donor (Lewis base) and electron-pair-acceptor (Lewis acid). It was recognized that introduction of sterically bulky

134 groups on the Lewis pairs hinders strong adduct formation and the functionalities are spatially separated. These compounds exhibit ambiphilic behavior and have led to the

259 260 261,262 development of catalysts in small molecule activations like H2, CO2 and N2.

Although most of the initial research on FLPs was focused on main group elements, there has been a recent shift towards using transition metal compound as the Lewis base component of FLPs.263-266 In particular, utilization of the M=O moiety in molecular oxo

267 complexes as the Lewis base with sterically encumbered Lewis acid B(C6F5)3.

Scheme 5-6 Synthesis of an oxorhenium Lewis acid-base adduct with B(C6F5)3 and its equilibrium to form a reactive FLP. Mes = 1,3,5-trimethylphenyl.268

Ison and co-workers recently reported an oxorhenium complex supported by a diamidopyridine ligand that was able to activate H2 to form the corresponding protonated rhenium oxo group and a hydridoborate anion (Scheme 5-6).268 The FLP complex, which was thermally generated above ≈50 °C, was found to be able to hydrogenate a variety of terminal and internal as well as cyclic olefins when it was subjected to 3.5 bar of H2 at 100

°C. Mechanistic and computational data showed that the reaction proceeded via initial activation of the olefin by B(C6F5)3 to form an alkylborate, thereby generating a carbocation at the olefin, which coordinates to the nucleophilic rhenium oxo. Subsequent

135 heterolytic cleavage of H2 occurs to form the rhenium hydroxonium cation and alkylborate anion. The acidic hydroxyl group then protonates the alkylborate to generate the product.

Scheme 5-7 Synthesis of Lewis acid-base adduct between 14Ti and B(C6F5)3 and subsequent activation of H2.

Therefore, it would be interesting to investigate the viability of employing the

B2Pz4Py titanium oxo complex 14Ti for small molecule activation in an FLP type system.

Generation of the classic Lewis acid-base adduct is expected to proceed upon treatment of

14Ti with B(C6F5)3 (Scheme 5-7). Unlike the oxorhenium complex which has an adjacent methyl group (vide supra), the single site of reactivity at the apical position in 14Ti·B(C6F5-

)3 would be beneficial in controlling its reactivity. Ison reported that substitution of this alkyl group to a hydride, aryl or benzoyl group has a pronounced impact on olefin insertion rates.268,269 Additionally other reactivity pathways which could complicate the system are possible, for example olefin insertion into the M–H bond or abstraction of the hydride or alkyl group with B(C6F5)3.

The Lewis acid-base adduct 14Ti·B(C6F5)3 would then be treated with H2 to form the titanium hydroxonium hydridoborate complex. Monitoring reaction progress by NMR spectroscopy would be particularly useful as diagnostic peaks in the 1H NMR such as the

136

11 hydroxonium proton O–H or quartet for the H–B(C6F5)3 proton due to coupling to the B nucleus (I = 3/2) would indicate formation of the ionic product. Likewise, a doublet for H–

11 19 B(C6F5)3 in the B NMR and a small Δδm,p of about 3 ppm in the F NMR would indicate the presence of the HB(C6F5)3 anion. Subsequently the ion pair complex would be treated with a variety of olefins while under an atmosphere of H2 to test its viability to hydrogenate these substrates. While strong Lewis acids such B(C6F5)3 promote H2 activation to form the ion pair complex, subsequent hydride transfer to the olefin could be unfavoured.270

Hence several boranes of different Lewis acidity could be tested to fine tune electronics and steric demands of this system to facilitate catalytic activity.

Aside from olefins, CO2 could also be activated to form the corresponding titanium hydroxonium formatoborate complex via hydride transfer from the hydridoborate anion, similar to that in Section 4.4.1. The addition of Et3SiH and a slight excess of B(C6F5)3 could catalyzed the hydrosilylation of CO2 into various products such as Et3SiOC(O)H,

(Et3SiO)2CH2, Et3SiOCH3 and CH4. The hydridoborate anion is reformed and it can

147 activate CO2 once more in the presence of CO2. The use of this FLP type system could be useful in contrast to the previously discussed B2Pz4Py scandium and titanium formatoborate complexes (8Sc-H and 8Ti-H in Schemes 2-10 and 4-5 respectively) as it avoids the formation of [B2Pz4PyTi][HB(C6F5)3] 6Ti. Deactivation pathways like the reversible hydride transfer back to titanium to form B2Pz4PyTiH 2Ti-H and free B(C6F5)3 as previously shown would not occur, therefore the titanium hydroxonium hydridoborate complex could remain active in the hydrosilylation of CO2.

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Chapter 6: Experimental Details

6.1 General Considerations

Manipulation and storage of all oxygen and moisture sensitive materials was performed under an argon atmosphere in a MBRAUN glove box. Reactions were performed on a double manifold high vacuum line fitted with an OxisorBW scrubber (Matheson Gas products) argon purification cartridge, using standard techniques. Glassware was stored in a 135 °C oven prior to immediate transfer to the glovebox antechamber or assembly on the vacuum line and evacuated while hot.

Tetrahydrofuran, toluene, n-pentane and diethylether were dried and purified using a

Grubbs/Down purification system,271 and stored in evacuated 500 mL thick-walled vessels over sodium/benzophenone ketal. 1,4-Dioxane, benzene-d6, tetrahydrofuran-d8 and toluene-d8 were dried and stored over sodium/benzophenone ketal. Bromobenzene-d5, dichloromethane-d2 and hexamethyldisiloxane was dried and stored over calcium hydride.

All dried solvents were degassed and vacuum transferred prior to use into thick-walled glass vessels for storage over activated molecular sieves (4 Å). Chloroform-d, dimethyl sulfoxide-d6 and hexanes were purchased from Sigma-Aldrich and used as received.

103 104 272 273 Li[Ph-Pz4B2PyH], Li[tol-Pz4B2PyH], ScCl3(THF)3, LiCH2SiMe3,

63 n 274 275 276 276 277 LiCH2SiMe2Ph, Li Pr, LiDBEt3, KOSiMe3, KOSiEt3, LiNHB(NDippCH)2

278 and KC8 were prepared according to literature procedures. All other chemicals were purchased from Sigma-Aldrich and used as received. Solutions of MeLi (1.6 M in

Et2O), NaHBEt3 (1.0 M in toluene) and LiDBEt3 were evaporated in vacuo and stored in a

138 glove box freezer at -35 ºC. LiHMDS and B(C6F5)3 were sublimed prior to use. CO2

(Coleman Instrument grade, 99.99%) was purchased from Air Liquide and used as received.

Nuclear magnetic resonance spectroscopy experiments including 1H, 2H, 11B{1H},

13C{1H}, 19F{1H}, 31P, 31P{1H}, 1H-1H COSY, 1H-13C HMBC, 1H-13C HSQC, 1H-1H

NOESY and DOSY were performed on Bruker-400, Ascend-500 or Avance-600 spectrometers. For DOSY experiments, the gradient amplitude was varied from 2% to 95% with an optimized δ (gradient pulse length) of 2600 µs and a Δ (diffusion time) of 0.075 s.

1 13 1 All H and C{ H} NMR spectra were internally referenced relative to Si(CH3)4 using residual solvent protons and naturally abundant 13C resonances for all deuterated solvents.124 2H NMR spectra were referenced to the residual deuterium signal of benzene or toluene. NMR spectra were processed and analyzed with MestReNova (v. 9.0.1-13254).

NMR spectra are presented with the following format: chemical shift (ppm) (multiplicity, n 11 13 1 19 1 JXY = coupling constant (Hz), integration, assignment). B, C{ H} and F{ H} spectra have integration omitted from the reported format.

X-ray crystallographic analyses for all other compounds were performed by Drs.

Chris Gendy, Denis Spasyuk, Benjamin Gelfand, Jian-Bin Li and Wenhua Bi on either a

Nonius Kappa CCD diffractometer using graphite-monochromated Mo Kα radiation or a

Bruker Smart APEX II three-circle diffractometer using Cu Kα radiation. Crystals were coated in Fomblin Y HVAC 140/13 oil. More details on individual structures can be found in Appendix A.

Single crystal X-ray diffraction (SC-XRD) data for compound 2Ti-H was collected using the Canadian Macromolecular Crystallography Facility CMCF-BM beamline at the

139

Canadian Light Source (CLS).279 CMCF-BM is a bending magnet beamline equipped with a Si (111) double crystal monochromator, Rayonix MX300HE CCD detector and MD2 microdiffractometer equipped with Mini Kappa Goniometer Head. Data for compound 2Ti-

H was collected at 18.000 keV (0.689 Å) and 19.000 keV (0.653) respectively using a single crystal. All data was collected at low temperature (100 K). Cell refinement and data reduction were performed using XDS.280 An empirical absorption correction, based on the multiple measurements of equivalent reflections, and merging of data was performed using

SADABS.281 Data conversion from XDS file format to SADABS file format was performed using XDS2SAD.282 The space group was confirmed by XPREP routine.283,284

Elemental analyses were performed by Johnson Li using a Perkin Elmer Model

2400 Series II analyser at the Instrumentation Facility of the Department of Chemistry,

University of Calgary. Solution high-resolution mass spectrometry (APCI-MS) measurements were performed by Wade White on samples prepared in the glove box in a gas tight syringe. Infrared spectra were collected on a Nicolet Avatar FT-IR spectrometer, and samples were prepared either as a KBr pellet or thin film by evaporation of a benzene solution on AgCl plates.

6.2 Experimental Procedures

General synthesis of scandium and titanium chloro complexes (1R)

A 100 mL round bottom flask was charged with Li[R-Pz4B2PyH] (R = Ph or p-tol) and ca. 30 mL of THF. LiHMDS was subsequently added as a solid to the white mixture.

The mixture was stirred at 25 ºC for 15 mins which resulted in a colourless solution.

140

MCl3(THF)3 (M = Sc or Ti) was added as a solid to a stirred solution of the deprotonated ligand. The mixture was stirred at room temperature for 24 h upon which precipitation of the product occurred over time. The mixture was filtered through a coarse porosity frit, and the solid residue was subsequently washed with 10 mL of a 3:1 pentane:THF solution and

10 mL of pentane. The product was dried in vacuo.

Synthesis of Ph-Pz4B2PyScCl (1Sc-Ph)

Li[Ph-Pz4B2PyH] (1.526 g, 2.884 mmol) was

deprotonated by LiHMDS (0.483 g, 2.887 mmol) and

subsequently reacted with ScCl3(THF)3 (1.060 g, 2.883

mmol) to yield the product as a white solid (1.452 g,

2.414 mmol, 84%). Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 1Sc-Ph in benzene.

1 3 H NMR (400 MHz, THF-d8) δ 8.34 (d, JHH = 2.2 Hz, 4H, H8), 7.88 (m, 4H, H5), 7.65 (d,

3 3 JHH = 2.4 Hz, 4H, H10), 7.51 – 7.44 (m, 4H, H6), 7.44 – 7.37 (m, 2H, H7), 7.29 (dd, JHH

3 13 1 = 8.7, 6.7 Hz, 1H, H3), 7.23 – 7.19 (m, 2H, H2), 6.23 (t, JHH = 2.2 Hz, 4H, H9). C{ H}

NMR (101 MHz, THF-d8) δ 172.2 (C1), 144.2 (C4), 142.6 (C8), 136.9 (C10), 136.2 (C5),

11 136.0 (C3), 128.8 (C2), 128.6 (C6), 128.0 (C7), 105.2 (C9). B NMR (128 MHz, THF-d8)

δ -2.53. HRMS (APCI): Calcd m/z 602.1739 (M+H)+, found m/z 602.1736 (M+H)+.

Elemental Analysis: Calcd. (%) for C29H25B2ClN9Sc: C, 57.90; H, 4.19; N, 20.95. Found:

C, 56.01; H, 4.55; N, 19.61.

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Synthesis of Tol-Pz4B2PyScCl (1Sc)

Li[Tol-Pz4B2PyH] (1.963 g, 3.523 mmol) was

deprotonated by LiHMDS (0.590 g, 3.526 mmol)

and subsequently reacted with ScCl3(THF)3 (1.295

g, 3.523 mmol) to yield the product as a white solid

(1.828 g, 2.903 mmol, 82%). Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 1Sc in benzene.

1 3 3 H NMR (500 MHz, C6D6) δ 8.14 (d, JHH = 2.2 Hz, 4H, H9), 7.75 (d, JHH = 7.9 Hz, 4H,

3 3 H5), 7.55 (d, JHH = 2.2 Hz, 4H, H11), 7.21 (d, JHH = 7.9, 1.3 Hz, 6H, H6 and H2), 6.67

3 3 13 1 (t, JHH = 7.8 Hz, 1H, H3), 5.79 (t, JHH = 2.2 Hz, 4H, H10), 2.33 (s, 6H, H8). C{ H}

NMR (126 MHz, C6D6) δ 171.4, (C1) 141.8 (C9), 140.3 (C4), 136.9 (C7), 136.3 (C11),

135.8 (C5), 135.4 (C3), 129.1 (C6), 128.5 (C2), 104.9 (C10), 21.4 (C8). 11B NMR (161

+ MHz, C6D6) δ -0.01. HRMS (APCI): Calcd m/z 630.2052 (M+H) , found m/z 630.2081

+ (M+H) . Elemental Analysis: Calcd. (%) for C31H29B2ClN9Sc: C, 59.13; H, 4.64; N, 20.02.

Found: C, 59.42; H, 4.82; N, 19.33.

Synthesis of Ph-Pz4B2PyTiCl (1Ti-Ph)

Li[Ph-Pz4B2PyH] (1.436 g, 2.714 mmol) was

deprotonated by LiHMDS (0.545 g, 3.257 mmol) and

subsequently reacted with TiCl3(THF)3 (0.965 g, 2.604

mmol) to yield the product as a pink solid (0.940 g, 1.555

142 mmol, 60%). Single crystals suitable for X-ray diffraction were obtained by layering cyclohexane onto a concentrated solution of 1Ti-Ph in THF.

1 H NMR (500 MHz, THF-d8) δ 7.88 (br. s, 4H), 7.54 (br. s, 4H), 7.38 (br. s, 2H), 6.46 (br. s, 1H), 5.59 (br. s, 4H), 5.23 (br. s, 2H), 2.85 (br. s, 4H), -9.80 (br. s, 4H). 11B NMR (161

13 MHz, THF-d8) δ -8.12. No observable resonances in C NMR spectrum. µeff: 1.50 (Gouy

Balance). Elemental Analysis: Calcd. (%) for C29H25B2ClN9Ti: C, 57.62; H, 4.17; N, 20.85.

Found: C, 57.42; H, 4.46; N, 20.20.

Synthesis of Tol-Pz4B2PyTiCl (1Ti)

Li[Tol-Pz4B2PyH] (1.409 g, 2.528 mmol) was

deprotonated by LiHMDS (0.423 g, 2.528 mmol)

and subsequently reacted with TiCl3(THF)3 (0.937 g,

2.528 mmol) to yield the product as a pink solid

(1.255 g, 1.984 mmol, 78%). Single crystals suitable for X-ray diffraction were obtained from a hot concentrated solution of 1Ti in benzene.

1 H NMR (500 MHz, C6D6) δ 7.75 (br s, 4H), 7.29 (br s, 4H), 5.72 (br s, 1H), 5.19 (br s,

11 6H), 2.67 (br s, 4H), 2.42 (s, 6H), -9.98 (br s, 4H). B NMR (161 MHz, C6D6) δ -12.24.

13 No observable resonances in C NMR spectrum. µeff: 1.34 (500 MHz, THF-H8). Elemental

Analysis: Calcd. (%) for C31H29B2ClN9Ti: C, 58.86; H, 4.62; N, 19.93. Found: C, 58.97;

H, 4.84; N, 19.15.

143

General synthesis of scandium alkyl complexes (2Sc-R)

A 20 mL scintillation vial was charged with 1Sc and 5 mL of toluene was added.

The alkyl lithium was weighed separately in a 1 dram vial and dissolved in 2 mL of toluene.

The solution of alkyl lithium was added dropwise into a stirred solution of 1Sc. The reaction mixture was left to stir for 24 hours at room temperature after which it was filtered through a 0.1 µm PTFE syringe filter to remove LiCl. The filtrate was transferred into a 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield a colourless to pale yellow residue. The residue was triturated and sonicated with 20 mL of pentane. The suspension was then filtered through a medium porosity frit, washed with 2x3 mL of pentane and dried in vacuo.

Synthesis of Tol-Pz4B2PyScCH2SiMe3 (2Sc-CH2SiMe3)

1Sc (115 mg, 0.183 mmol) was reacted with

LiCH2SiMe3 (18 mg, 0.191 mmol, 1.05 equiv.) to

yield 2-CH2SiMe3 as a pale yellow solid (68 mg,

0.100 mmol, 55%). Single crystals suitable for X-

ray diffraction were obtained by layering HMDSO onto a concentrated solution of 2Sc-CH2SiMe3 in benzene.

1 3 3 H NMR (500 MHz, C6D6) δ 8.10 (d, JHH = 2.2 Hz, 4H, H9), 7.78 (d, JHH = 7.8 Hz, 4H,

3 3 3 H5), 7.45 (d, JHH = 2.2 Hz, 4H, H11), 7.39 (d, JHH = 7.8 Hz, 2H, H2), 7.20 (d, JHH = 7.7

3 3 Hz, 4H, H6), 6.72 (t, JHH = 7.8 Hz, 1H, H3), 5.85 (t, JHH = 2.2 Hz, 4H, H10), 2.32 (s, 6H,

13 1 H8), 0.71 (s, 2H, H12), 0.21 (s, 9H, H13). C{ H} NMR (126 MHz, C6D6) δ 172.2 (C1),

144

141.0 (C9), 140.7 (C4), 136.9 (C7), 136.5 (C11), 136.2 (C5), 134.5 (C3), 129.0 (C6), 127.4

11 (C2), 104.6 (C10), 42.3 (C12), 21.4 (C8), 3.8 (C13). B NMR (161 MHz, C6D6) δ -0.01.

Elemental Analysis: Calcd. (%) for C35H40B2N9ScSi: C, 61.69; H, 5.92; N, 18.50. Found:

C, 60.74; H, 5.86; N, 18.10.

Synthesis of Tol-Pz4B2PyScCH2SiMe2Ph (2Sc-CH2SiMe2Ph)

1Sc (88 mg, 0.140 mmol) was reacted with

LiCH2SiMe2Ph (23 mg, 0.147 mmol, 1.05 equiv.)

to yield 2Sc-CH2SiMe2Ph as a pale yellow solid (80

mg, 0.108 mmol, 77%).

1 3 H NMR (500 MHz, C6D6) δ 7.91 (d, JHH = 2.0 Hz,

4H, H9), 7.80 – 7.75 (m, 6H, H5 and H15), 7.44 (d,

3 3 JHH = 2.2 Hz, 4H, H11), 7.38 (d, JHH = 7.8 Hz, 2H, H2), 7.34 (m, 2H, H16), 7.30 – 7.26

3 3 3 (m, 1H, H17), 7.20 (d, JHH = 7.7 Hz, 4H, H6), 6.70 (t, JHH = 7.7 Hz, 1H, H3), 5.82 (t, JHH

= 2.2 Hz, 4H, H10), 2.32 (s, 6H, H8), 0.87 (s, 2H, H12), 0.29 (s, 6H, H13).13C{1H} NMR

(126 MHz, C6D6) δ 172.2 (C1), 146.2 (C14), 141.1 (C9), 140.6 (C4), 136.9 (C7), 136.5

(C11), 136.2 (C5), 134.5 (C3), 134.2 (C15), 129.0 (C6), 128.2 (C17), 128.0 (C16), 127.3

11 (C2), 104.6 (C10), 38.3 (C12), 21.4 (C8), 2.3 (C13). B NMR (161 MHz, C6D6) δ -0.05.

Elemental Analysis: Calcd. (%) for C40H42B2N9ScSi: C, 64.62; H, 5.69; N, 16.96. Found:

C, 64.16; H, 5.41; N, 16.31.

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i i Synthesis of Tol-Pz4B2PySc Bu (2Sc- Bu)

i 1Sc (62 mg, 0.098 mmol) was reacted with Li Bu (7

i mg, 0.109 mmol, 1.11 equiv.) to yield 2Sc- Bu as a

pale yellow solid (40 mg, 0.060 mmol, 61%).

1 3 H NMR (500 MHz, C6D6) δ 8.09 (d, JHH = 2.1 Hz,

3 4H, H9), 7.77 (d, JHH = 7.7 Hz, 4H, H5), 7.45 (d,

3 3 3 JHH = 2.3 Hz, 4H, H11), 7.39 (d, JHH = 7.7 Hz, 2H, H2), 7.20 (d, JHH = 7.7 Hz, 4H, H6),

3 3 3 6.73 (t, JHH = 7.8 Hz, 1H, H3), 5.86 (t, JHH = 2.2 Hz, 4H, H10), 2.50 (sept, JHH = 6.5 Hz,

3 3 1H, H13), 2.32 (s, 6H, H8), 1.19 (d, JHH = 6.4 Hz, 6H, H14), 1.17 (d, JHH = 6.7 Hz, 2H,

13 1 H12). C{ H} NMR (126 MHz, C6D6) δ 172.3 (C1), 140.9 (C4), 140.7 (C9), 136.8 (C7),

136.5 (C11), 136.2 (C5), 134.4 (C3), 129.0 (C6), 127.3 (C2), 104.6 (C10), 64.6 (C12), 31.9

11 (C13), 29.6 (C14), 21.4 (C8). B NMR (161 MHz, C6D6) δ 0.08. Elemental Analysis:

Calcd. (%) for C35H38B2N9Sc: C, 64.54; H, 5.88; N, 19.35. Found: C, 57.13; H, 4.71; N,

17.58.

n n Synthesis of Tol-Pz4B2PySc Pr (2Sc- Pr)

n 1Sc (79 mg, 0.125 mmol) was reacted with Li Pr (7

n mg, 0.140 mmol, 1.12 equiv.) to yield 2Sc- Pr as an

off-white solid (66 mg, 0.101 mmol, 81%).

1 3 H NMR (500 MHz, C6D6) δ 8.05 (d, JHH = 2.1 Hz,

3 4H, H9), 7.78 (d, JHH = 7.9 Hz, 4H, H5), 7.45 (d,

3 3 3 JHH = 2.3 Hz, 4H, H11), 7.39 (d, JHH = 7.8 Hz, 2H, H2), 7.20 (d, JHH = 7.7 Hz, 4H, H6),

146

3 3 6.74 (t, JHH = 7.8 Hz, 1H, H3), 5.85 (t, JHH = 2.2 Hz, 4H, H10), 2.32 (s, 6H, H8), 2.06

3 13 1 (m, 2H, H13), 1.27 (t, JHH = 7.1 Hz, 3H, H14), 1.15 (m, 2H, H12). C{ H} NMR (126

MHz, C6D6) δ 172.3 (C1), 140.8 (C4), 140.6 (C9), 136.8 (C7), 136.4 (C11), 136.2 (C5),

134.5 (C3), 129.0 (C6), 127.3 (C2), 104.7 (C10), 55.0 (C12), 25.4 (C13), 22.5 (C14), 21.4

11 (C8). B NMR (161 MHz, C6D6) δ -0.01. Elemental Analysis: Calcd. (%) for

C34H36B2N9Sc: C, 64.08; H, 5.69; N, 19.78. Found: C, 62.73; H, 5.64; N, 19.05.

Synthesis of Tol-Pz4B2PyScCH3 (2Sc-Me)

1Sc (133 mg, 0.211 mmol) was reacted with MeLi

(5 mg, 0.227 mmol, 1.08 equiv.) to yield 2Sc-Me as

a white solid (120 mg, 0.197 mmol, 93%). Single

crystals suitable for X-ray diffraction were obtained

by slow vapour diffusion of pentane onto a concentrated solution of 2Sc-Me in benzene.

1 3 3 H NMR (400 MHz, C6D6) δ 8.03 (d, JHH = 2.1 Hz, 4H, H9), 7.80 (d, JHH = 7.8 Hz, 4H,

3 3 3 H5), 7.50 (d, JHH = 2.3 Hz, 4H, H11), 7.34 (d, JHH = 7.8 Hz, 2H, H2), 7.21 (d, JHH = 7.7

3 3 Hz, 4H, H6), 6.74 (t, JHH = 7.8 Hz, 1H, H3), 5.84 (t, JHH = 2.2 Hz, 4H, H10), 2.33 (s, 6H,

13 1 H8), 0.58 (s, 3H, H12). C{ H} NMR (101 MHz, C6D6) δ 172.1 (C1), 140.7 (C9), 139.4

(C4), 136.8 (C7), 136.3 (C11), 136.1 (C5), 134.6 (C3), 129.0 (C6), 127.6 (C2), 104.7

11 (C10), 26.4 (C12), 21.4 (C8). B NMR (161 MHz, C6D6) δ 0.12. Elemental Analysis:

Calcd. (%) for C32H32B2N9Sc: C, 63.09; H, 5.29; N, 20.69. Found: C, 63.60; H, 5.14; N,

19.68.

147

Synthesis of Tol-Pz4B2PyScH (2Sc-H)

A 20 mL scintillation vial was charged with 1Sc (113

mg, 0.179 mmol) and 5 mL of toluene was added.

NaHBEt3 (24 mg, 0.197 mmol, 1.10 equiv.) was

weighed separately in a 1 dram vial and dissolved

in 2 mL of toluene. The solution of NaHBEt3 was

added dropwise into a stirred solution 1Sc. The

reaction mixture was left to stir for 90 mins at room

temperature after which it was filtered through a 0.1

µm PTFE syringe filter to remove NaCl. The filtrate was transferred into a 50 mL thick- walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield a colourless residue. The residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (85 mg, 0.143 mmol, 80%).

1 3 H NMR (400 MHz, C6D6) δ 9.17 (br s, 1H, ScH), 8.10 (d, JHH = 2.1 Hz, 4H, H9), 7.86

3 3 3 (d, JHH = 7.7 Hz, 4H, H5), 7.53 (d, JHH = 2.3 Hz, 4H, H11), 7.47 (d, JHH = 7.8 Hz, 2H,

3 3 3 H2), 7.22 (d, JHH = 7.7 Hz, 4H, H6), 6.77 (t, JHH = 7.7 Hz, 1H, H3), 5.75 (t, JHH = 2.2

13 1 Hz, 4H, H10), 2.33 (s, 6H, H8). C{ H} NMR (101 MHz, C6D6) δ 172.4 (C1), 142.0 (C9),

141.6 (C4), 136.6 (C7), 136.3 (C5), 136.1 (C11), 134.0 (C3), 128.9 (C6), 127.4 (C2), 104.4

11 (C10), 21.4 (C8). B NMR (161 MHz, C6D6) δ -1.01. HRMS (APCI): Calcd. m/z 594.2286

+ + M , found m/z 594.2288 M . Elemental Analysis: Calcd. (%) for C31H30B2N9Sc: C, 62.55;

H, 5.08; N, 21.18. Found: C, 62.77; H, 5.21; N, 20.81.

148

Synthesis of Tol-Pz4B2PyScD (d1-2Sc-H)

Same synthetic procedure as 2Sc-H but used LiDBEt3 instead of NaHBEt3.

1Sc (104 mg, 0.165 mmol) was reacted with LiDBEt3 (18 mg, 0.170 mmol, 1.03 equiv.) to yield d1-2Sc-H as a white solid (53 mg, 0.089 mmol, 54%).

2 H NMR (77 MHz, C6H6) δ 9.16.

Synthesis of Tol-Pz4B2PyScOScPz4B2Py-Tol (3Sc)

Method A: A preweighed 25 mL thick-walled glass

vessel equipped with a Kontes PTFE valve plug was

charged with 2Sc-CH2SiMe3 (72 mg, 0.106 mmol)

and dissolved in 10 mL of toluene. 10 µL of

degassed deionized water was added to the solution

under a positive flow of argon. The solution was stirred for 30 mins at room temperature before removing all solvents in vacuo. The residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (59 mg, 0.049 mmol, 93%). Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 3Sc in benzene.

Method B: A J-Young NMR tube was charged with 2Sc-H (10 mg, 0.017 mmol) and dissolved in C6D6. The colourless solution was degassed by freeze-pump-thaw at -78 ºC

149 and backfilled with 1 atm of N2O. The solution was heated to 80 ºC for 18 hours to ensure

1 complete formation of 3Sc. Quantitative conversion by H NMR.

1 3 3 H NMR (500 MHz, C6D6) δ 7.94 (d, JHH = 7.9 Hz, 4H, H5), 7.84 (d, JHH = 2.0 Hz, 4H,

3 3 H9), 7.56 – 7.50 (m, 6H, H2 and H11), 7.23 (d, JHH = 7.8 Hz, 4H, H6), 6.84 (t, JHH = 7.8

3 13 1 Hz, 1H, H3), 5.81 (t, JHH = 2.1 Hz, 4H, H10), 2.32 (s, 6H, H8). C{ H} NMR (126 MHz,

C6D6) δ 172.6 (C1), 141.6 (C9), 141.5 (C4), 136. 7 (C7), 136.4 (C11), 136.3 (C5), 134.0

11 (C3), 129.0 (C6), 127.0 (C2), 104.2 (C10), 21.4 (C8). B NMR (161 MHz, C6D6) δ -0.43.

HRMS (APCI): Calcd. m/z 1205.4604 (M+H)+, found m/z 1205.4585 (M+H)+. Elemental

Analysis: Calcd. (%) for C62H58B4N18OSc: C, 61.83; H, 4.85; N, 20.93. Found: C, 61.21;

H, 4.97; N, 20.47.

Synthesis of Tol-Pz4B2PyScOOCH (4Sc-H)

Method A: A J-Young NMR tube was charged with

2Sc-H (10 mg, 0.017 mmol) and dissolved in C6D6.

The colourless solution was degassed by freeze-

pump-thaw at -78 ºC and backfilled with 1 atm of

CO2. The solution was heated to 80 ºC for 18 hours to ensure complete formation of 4Sc-H. Quantitative conversion by NMR.

Method B: A 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug was charged with both 1Sc (158 mg, 0.251 mmol) and HCOONa (18 mg, 0.265 mmol, 1.05 equiv.). The apparatus was connected to the vacuum line and 20 mL of 1,4-dioxane was vacuum transferred to the solid mixture at -78 ºC. The cloudy white mixture was heated to

150

95 ºC for 3 days after which the solvent was removed in vacuo. The residue was extracted with 20 mL toluene and filtered through a 0.1 µm PTFE syringe filter to remove NaCl and excess HCOONa. The filtrate was transferred into a 50 mL thick-walled glass vessel and subsequently evaporated in vacuo to yield a colourless residue. The residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (135 mg, 0.211 mmol, 84%). Single crystals suitable for X- ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 4Sc-H in benzene.

1 3 H NMR (500 MHz, C6D6) δ 8.60 (s, 1H, H12), 8.40 (d, JHH = 2.1 Hz, 4H, H9), 7.82 (d,

3 3 3 JHH = 7.9 Hz, 4H, H5), 7.60 (d, JHH = 7.8 Hz, 2H, H2), 7.39 (d, JHH = 2.3 Hz, 4H, H11),

3 3 3 7.21 (d, JHH = 7.7 Hz, 4H, H6), 6.79 (t, JHH = 7.7 Hz, 1H, H3), 5.83 (t, JHH = 2.2 Hz, 4H,

13 1 H10), 2.33 (s, 6H, H8). C{ H} NMR (101 MHz, C6D6) δ 178.1 (C12), 173.3 (C1), 142.1

(C9), 140.9 (C4), 136.8 (C7), 136.5 (C11), 136.3 (C5), 134.4 (C3), 129.0 (C6), 127.3 (C2),

11 -1 104.3 (C10), 21.4 (C8). B NMR (161 MHz, C6D6) δ -0.43. νC-O: 1566 cm . HRMS

(APCI): Calcd m/z 640.2340 (M+H)+, found m/z 640.2311 (M+H)+. Elemental Analysis:

Calcd. (%) for C32H30B2N9O2Sc: C, 60.13; H, 4.73; N, 19.72. Found: C, 61.39; H, 5.37; N,

18.63.

151

Synthesis of Tol-Pz4B2PyScOAc (4Sc-Me)

Method A: A J-Young NMR tube was charged with

2Sc-Me (10 mg, 0.017 mmol) and dissolved in C6D6.

The colourless solution was degassed by freeze-

pump-thaw at -78 ºC and backfilled with 1 atm of

CO2. The solution was heated to 70 ºC for 2 hours

to ensure complete formation of 4Sc-Me.

Quantitative conversion by NMR.

Method B: A 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug was charged with both 1Sc (125 mg, 0.199 mmol) and NaOAc (17 mg, 0.207 mmol, 1.04 equiv.). The apparatus was connected to the vacuum line and 20 mL of THF was vacuum transferred to the solid mixture at -78 ºC. The cloudy white mixture was heated to 60 ºC for 24 hours after which the solvent was removed in vacuo. The residue was extracted with

20 mL toluene and filtered through a 0.1 µm PTFE syringe filter to remove NaCl and excess

NaOAc. The filtrate was transferred into a 50 mL thick-walled glass vessel and subsequently evaporated in vacuo to yield a colourless residue. The residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (98 mg, 0.150 mmol, 76%). Single crystals suitable for X- ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 4Sc-Me in benzene.

1 3 3 H NMR (500 MHz, C6D6) δ 8.47 (d, JHH = 2.1 Hz, 4H, H9), 7.85 (d, JHH = 7.7 Hz, 4H,

3 3 3 H5), 7.63 (d, JHH = 7.7 Hz, 2H, H2), 7.41 (d, JHH = 2.3 Hz, 4H, H11), 7.21 (d, JHH = 7.7

152

3 3 Hz, 4H, H6), 6.81 (t, JHH = 7.7 Hz, 1H, H3), 5.86 (t, JHH = 2.2 Hz, 4H, H10), 2.33 (s, 6H,

13 1 H8), 1.82 (s, 3H, H13). C{ H} NMR (101 MHz, C6D6) δ 189.1 (C12), 173.4 (C1), 142.1

(C9), 141.0 (C4), 136.7 (C7), 136.4 (C11), 136.3 (C5), 134.3 (C3), 129.0 (C6), 127.3 (C2),

11 -1 104.3 (C10), 22.4 (C13), 21.4 (C8). B NMR (161 MHz, C6D6) δ -0.29. νC-O: 1535 cm .

Elemental Analysis: Calcd. (%) for C32H30B2N9O2Sc: C, 60.67; H, 4.94; N, 19.30. Found:

C, 61.83; H, 5.36; N, 18.05.

Synthesis of [Tol-Pz4B2PySc][MeB(C6F5)3] (5Sc)

A 50 mL thick-walled glass vessel equipped with a Kontes

PTFE valve plug was charged with 2Sc-Me (63 mg, 0.103

mmol) and dissolved in 5 mL of toluene. A solution of

B(C6F5)3 (53 mg, 0.103 mmol, 1.0 equiv.) in 3 mL of toluene

was added dropwise into a stirred solution of 2Sc-Me. The

colourless solution was stirred for 30 mins and solvent was evaporated in vacuo. The colourless residue triturated and sonicated with 20 mL of pentane.

The white suspension was then filtered through a medium porosity frit and washed with

2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (102 mg,

0.0910 mmol, 88%).

1 3 3 H NMR (500 MHz, C6D6) δ 8.03 (d, JHH = 2.3 Hz, 4H, H9), 7.48 (d, JHH = 8.0 Hz, 4H,

3 3 H5), 7.33 (d, JHH = 2.4 Hz, 4H, H11), 7.17 – 7.12 (m, 6H, H2 and H6), 6.53 (t, JHH = 7.8

3 Hz, 1H, H3), 5.68 (t, JHH = 2.3 Hz, 4H, H10), 2.36 (br s, 3H, H12), 2.30 (s, 6H, H8).

13 1 1 - C{ H} NMR (126 MHz, C6D6) δ 170.64 (C1), 148.89 (dm, JCF = 235 Hz, [HB(C6F5)3]

153

1 - ), 140.94 (C9), 139.35 (dm, JCF = 237 Hz, [MeB(C6F5)3] ), 138.01 (C11), 137.85 (C7),

1 - 137.48 (dm, JCF = 235 Hz, [MeB(C6F5)3] ), 137.09 (C3), 135.52 (C5), 129.39 (C6), 128.70

- 11 (C2), 124.17 ([MeB(C6F5)3] ), 105.94 (C10), 25.36 (C12), 21.30 (C8). B NMR (161 MHz,

- 19 1 C6D6) δ -0.39 (Pz4B2Py), -14.17 ([MeB(C6F5)3] ). F{ H} NMR (471 MHz, C6D6) δ -

134.44 (o-F), -161.00 (p-F), -165.62 (o-F).

Synthesis of [Tol-Pz4B2PySc][HB(C6F5)3] (6Sc)

A 50 mL thick-walled glass vessel equipped

with a Kontes PTFE valve plug was charged

with 2Sc-H (57 mg, 0.0958 mmol), B(C6F5)3 (49

mg, 0.0958 mmol, 1.0 equiv.) and 10 mL of toluene. The reaction mixture was heated to 70 ºC for 1 hour upon which the cloudy white mixture turned clear. The solvent was removed in vacuo to yield a colourless residue. The residue was triturated and sonicated with 20 mL of pentane twice. The white suspension was then filtered through a medium porosity frit and washed with 2x20 mL of pentane. The pentane filtrate was stored at -35 °C and single crystals suitable for X-ray diffraction formed overnight. The mother liquor was decanted and evaporated in vacuo to yield more product as a white solid (30 mg, 0.0271 mmol, 28%).

1 3 3 H NMR (500 MHz, C6D6) δ 7.95 (d, JHH = 2.3 Hz, 4H, H9), 7.59 (d, JHH = 7.9 Hz, 4H,

3 3 H5), 7.33 – 7.27 (m, 6H, H2 and H11), 7.21 (d, JHH = 7.7 Hz, 4H, H6), 6.58 (t, JHH = 7.8

3 1 - Hz, 1H, H3), 5.65 (t, JHH = 2.3 Hz, 4H, H10), 4.00 (q, JHB = 75.7 Hz, 1H, [HB(C6F5)3] ),

13 1 1 2.32 (s, 6H, H8). C{ H} NMR (126 MHz, C6D6) δ 171.18 (C1), 148.93 (dm, JCF = 232

154

- 1 - Hz, [HB(C6F5)3] ), 140.99 (C9), 139.41 (dm, JCF = 234 Hz, [HB(C6F5)3] ), 138.37 (C4),

1 - 137.86 (C11), 137.81 (C7), 137.43 (dm, JCF = 236 Hz, [HB(C6F5)3] ), 136.32 (C3), 135.68

- (C5), 129.40 (C6), 127.94 (C2), 121.85 ([HB(C6F5)3] ), 105.38 (C10), 21.32 (C8). C4 not

11 1 observed. B NMR (161 MHz, C6D6) δ -0.07 (Pz4B2Py), -19.97 (d, JHB = 76.2 Hz,

- 19 1 3 [HB(C6F5)3] ). F{ H} NMR (471 MHz, C6D6) δ -132.81 (d, JFF = 25.0 Hz, o-F), -160.41

(p-F), -164.34 (m-F). Elemental Analysis: Calcd. (%) for C49H30B3F15N9Sc: C, 53.15; H,

2.73; N, 11.39. Found: C, 52.80; H, 2.91; N, 11.26.

Synthesis of [Tol-Pz4B2PyScClScPz4B2Py-Tol][HB(C6F5)3] (7Sc)

A 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug was charged with 1Sc (81 mg, 0.129 mmol) and 15 mL of toluene was added. A solution of B(C6F5)3

(101 mg, 0.135 mmol, 1.05 equiv.) and Et3SiH (17 mg, 0.146 mmol, 1.14 equiv.) dissolved in 5 mL of toluene was added to the reaction mixture. The cloudy white suspension immediately turned into a clear solution. The reaction mixture was stirred overnight after which the solvent was removed in vacuo. The resulting residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and

155 dried in vacuo (104 mg, 0.0599 mmol, 93%). Single crystals suitable for X-ray diffraction were obtained from a hot concentrated solution of 7Sc in benzene.

1 3 3 H NMR (500 MHz, C6D6) δ 7.71 (d, JHH = 7.9 Hz, 4H, H5), 7.50 (d, JHH = 2.4 Hz, 4H,

3 3 3 H9), 7.40 (d, JHH = 7.9 Hz, 2H, H2), 7.32 (d, JHH = 2.3 Hz, 4H, H11), 7.29 (d, JHH = 7.8

3 3 Hz, 4H, H6), 6.73 (t, JHH = 7.9 Hz, 1H, H3), 5.83 (t, JHH = 2.3 Hz, 4H, H10), 4.55 (d,

1 13 1 JHB = 117.4 Hz, 1H, HB(C6F5)3), 2.36 (s, 6H, H8). C{ H} NMR (126 MHz, C6D6) δ

1 - 1 171.16 (C1), 149.23 (dm, JCF = 236 Hz, [HB(C6F5)3] ), 140.56 (C11), 138.47 (dm, JCF =

- 1 - 240 Hz, [HB(C6F5)3] ), 138.39 (C9), 138.17 (C7), 137.16 (dm, JCF = 234 Hz, [HB(C6F5)3]

- ), 136.64 (C3), 135.60 (C5), 129.55 (C6), 105.78 (C10), 21.31 (C8). C4 and [HB(i-C6F5)3]

11 1 not observed. B NMR (161 MHz, C6D6) δ 0.18 (Pz4B2Py), -23.91 (d, JHB = 86.1 Hz,

- 19 1 [HB(C6F5)3] ). F{ H} NMR (471 MHz, C6D6) δ -133.21 (o-F), -165.19 (p-F), -167.76 (m-

F). Elemental Analysis: Calcd. (%) for C80H59B5ClF15N18Sc2: C, 55.32; H, 3.42; N, 14.52.

Found: C, 54.99; H, 3.49; N, 13.73.

Synthesis of [Tol-Pz4B2PySc][HCOOB(C6F5)3] (8Sc-H)

Method A: A J Young NMR tube was charged with

4Sc-H (11 mg, 0.017 mmol) and dissolved in C6D6.

A solution of B(C6F5)3 (8.8 mg, 0.017 mmol) in 0.1

mL of C6D6 was added to the J Young NMR tube.

Immediate and quantitative conversion by 1H NMR.

Method B: A 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug was charged with 1Sc (118 mg, 0.187 mmol) and

156

15 mL of toluene was added. A solution of B(C6F5)3 (101 mg, 0.197 mmol, 1.05 equiv.) and Et3SiH (24 mg, 0.210 mmol, 1.10 equiv.) dissolved in 5 mL of toluene was added to the reaction mixture. The cloudy white suspension immediately turned into a clear solution.

The apparatus was connected to the vacuum line and the solution was degassed by freeze- pump-thaw at -196 ºC and backfilled with 1 atm of CO2. The reaction mixture was stirred overnight after which the solvent was removed in vacuo. The resulting residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (164 mg, 0.142 mmol, 76%). Single crystals suitable for X-ray diffraction were obtained by layering HMDSO onto a concentrated solution of 8Sc-H in benzene.

1 3 H NMR (500 MHz, C6D6) δ 8.12 (s, 1H, H12), 7.83 (d, JHH = 2.2 Hz, 4H, H9), 7.62 (d,

3 3 3 JHH = 7.8 Hz, 4H, H5), 7.46 (d, JHH = 2.3 Hz, 4H, H11), 7.21 (d, JHH = 7.8 Hz, 4H, H6),

3 3 3 7.16 (d, JHH = 7.8 Hz, 2H, H2), 6.62 (t, JHH = 7.9 Hz, 1H, H3), 5.76 (t, JHH = 2.3 Hz,

13 1 4H, H10), 2.33 (s, 6H, H8). C{ H} NMR (126 MHz, C6D6) δ 172.9 (C12), 170.8 (C1),

1 - 1 - 148.5 (dm, JCF = 239 Hz, [HCO2B(C6F5)3] ), 140.6 (dm, JCF = 251 Hz, [HCO2B(C6F5)3]

1 - ), 140.2 (C9), 138.8 (C4), 137.7 (dm, JCF = 251 Hz, [HCO2B(C6F5)3] ), 137.5 (C7), 137.3

- (C11), 136.5 (C3), 135.6 (C5), 129.3 (C6), 128.8 (C2), 118.4 ([HCO2B(C6F5)3] ), 105.4

11 19 (C10), 21.3 (C8). B NMR (161 MHz, C6D6) δ 0.01. F NMR (471 MHz, C6D6) δ -135.08

3 3 3 (d, JFF = 24.4 Hz, o-F), -157.63 (t, JFF = 20.8 Hz, p-F), -164.56 (t, JFF = 22.8 Hz, m-F).

-1 νC-O: 1614 cm . Elemental Analysis: Calcd. (%) for C50H30B3F15N9O2Sc: C, 52.17; H,

2.63; N, 10.95. Found: C, 52.34; H, 2.73; N, 10.76.

157

Synthesis of [Tol-Pz4B2PySc][MeCOOB(C6F5)3] (8Sc-Me)

A 50 mL thick-walled glass vessel equipped with a

Kontes PTFE valve plug was charged with 4Sc-Me

(49 mg, 0.075 mmol) and B(C6F5)3 (40 mg, 0.078

mmol, 1.04 equiv.). 10 mL was added to dissolve

the solids to give a colourless solution. The solution

was stirred for 1 h after which the solvent was

removed in vacuo. The resulting residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (83 mg, 0.071 mmol, 95%).

1 3 3 H NMR (500 MHz, C6D6) δ 7.65 (d, JHH = 8.0 Hz, 4H, H5), 7.59 (d, JHH = 2.2 Hz, 4H,

3 3 3 H9), 7.36 (d, JHH = 2.3 Hz, 4H, H11), 7.28 (d, JHH = 7.8 Hz, 2H, H2), 7.20 (d, JHH = 7.7

3 3 Hz, 4H, H6), 6.64 (t, JHH = 7.8 Hz, 1H, H3), 5.73 (t, JHH = 2.3 Hz, 4H, H10), 2.32 (s, 6H,

13 1 H8), 1.59 (s, 3H, H13). C{ H} NMR (126 MHz, C6D6) δ 183.1 (C12), 171.3 (C1), 148.6

1 - 1 - (dm, JCF = 240 Hz, [MeCO2B(C6F5)3] ), 140.2 (dm, JCF = 251 Hz, [MeCO2B(C6F5)3] ),

1 - 139.9 (C9), 138.7 (C4), 137.5 (dm, JCF = 262 Hz, [MeCO2B(C6F5)3] ), 137.6 (C7), 137.5

- (C11), 136.0 (C3), 135.7 (C5), 129.4 (C6), 128.2 (C2), 119.5 ([MeCO2B(C6F5)3] ), 105.4

11 19 (C10), 24,3 (C13), 21.3 (C8). B NMR (161 MHz, C6D6) δ -0.20. F NMR (471 MHz,

3 3 3 C6D 6) δ -135.45 (d, JFF = 22.2 Hz, o-F), -158.54 (t, JFF = 20.9 Hz, p-F), -165.24 (t, JFF

-1 = 20.9 Hz, m-F). νC-O: 1571 cm . Elemental Analysis: Calcd. (%) for C51H32B3F15N9O2Sc:

C, 52.57; H, 2.77; N, 10.82. Found: C, 52.25; H, 3.05; N, 10.51.

158

General synthesis of scandium siloxide complexes (9Sc-R)

A 20 mL scintillation vial was charged with 1Sc and 5 mL of toluene was added.

KOSiR3 (R = Me or Et) was weighed separately in a 1 dram vial and dissolved in 2 mL of toluene. The solution of KOSiR3 was added dropwise into a stirred solution of 1Sc. The reaction mixture was left to stir for 18 hours at room temperature after which it was filtered through a 0.1 µm PTFE syringe filter to remove KCl. The filtrate was transferred into a 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield a colourless residue. The residue was triturated and sonicated with 20 mL of pentane twice. The suspension was then filtered through a medium porosity frit, washed with 2x3 mL of pentane and dried in vacuo.

Synthesis of Tol-Pz4B2PyScOSiMe3 (9Sc-Me)

1Sc (119 mg, 0.189 mmol) was reacted with

KOSiMe3 (25 mg, 0.195 mmol, 1.03 equiv.) to yield

9Sc-Me as a white solid (75 mg, 0.110 mmol, 58%).

1 3 H NMR (500 MHz, C6D6) δ 7.98 (d, JHH = 2.0 Hz,

3 4H, H9), 7.79 (d, JHH = 7.8 Hz, 4H, H5), 7.57 (d,

3 3 3 JHH = 2.3 Hz, 4H, H11), 7.32 (d, JHH = 7.8 Hz, 2H, H2), 7.21 (d, JHH = 7.7 Hz, 4H, H6),

3 3 6.74 (t, JHH = 7.8 Hz, 1H, H3), 5.87 (t, JHH = 2.2 Hz, 4H, H10), 2.33 (s, 6H, H8), 0.26 (s,

13 1 9H, H12). C{ H} NMR (126 MHz, C6D6) δ 171.9 (C1), 141.1 (C4), 140.6 (C9), 136.7

(C7), 136.5 (C11), 136.0 (C5), 134.7 (C3), 129.0 (C6), 128.3 (C2), 104.6 (C10), 21.4 (C8),

11 3.2 (C12). B NMR (161 MHz, C6D6) δ 0.05. HRMS (APCI): Calcd m/z 684.2787

159

(M+H)+, found m/z 684.2770 (M+H)+. Elemental Analysis: Calcd. (%) for

C34H38B2N9OScSi: C, 59.76; H, 5.60; N, 18.45. Found: C, 59.30; H, 5.69; N, 17.72.

Synthesis of Tol-Pz4B2PyScOSiEt3 (9Sc-Et)

1Sc (61 mg, 0.0969 mmol) was reacted with

KOSiEt3 (17 mg, 0.0998 mmol, 1.03 equiv.) to yield

9Sc-Et as a white solid (36 mg, 0.0496 mmol, 51%).

Single crystals suitable for X-ray diffraction were

obtained from a concentrated solution of 9Sc-Et in

pentane stored at -35 °C.

1 3 3 H NMR (500 MHz, C6D6) δ 8.03 (d, JHH = 2.0 Hz, 4H, H9), 7.79 (d, JHH = 7.9 Hz, 4H,

3 3 3 H5), 7.56 (d, JHH = 2.3 Hz, 4H, H11), 7.34 (d, JHH = 7.7 Hz, 2H, H2), 7.20 (d, JHH = 7.7

3 3 Hz, 4H, H6), 6.74 (t, JHH = 7.8 Hz, 1H, H3), 5.89 (t, JHH = 2.2 Hz, 4H, H10), 2.32 (s, 6H,

3 3 13 1 H8), 1.03 (t, JHH = 7.9 Hz, 9H, H13), 0.73 (q, JHH = 7.9 Hz, 6H, H12). C{ H} NMR

(126 MHz, C6D6) δ 172.1 (C1), 141.1 (C4), 140.6 (C9), 136.7 (C7), 136.2 (C11), 136.1

(C5), 134.6 (C3), 129.0 (C6), 128.4 (C2), 104.5 (C10), 21.4 (C8), 7.9 (C12), 7.8 (C13). 11B

+ NMR (161 MHz, C6D6) δ -0.19. HRMS (APCI): Calcd m/z 726.3256 (M+H) , found m/z

+ 726.3227 (M+H) . Elemental Analysis: Calcd. (%) for C37H44B2N9OScSi: C, 61.26; H,

6.11; N, 17.38. Found: C, 61.21; H, 6.05; N, 17.15.

160

Synthesis of Tol-Pz4B2PyScOH (2Sc-OH)

A vial charged with solid 2Sc-CH2SiMe3 (20 mg)

was brought out of the box, uncapped and exposed

to air for 10 minutes before it was brought back into

the box. 1H NMR analysis indicated that ≈35% of

the resulting compound is comprised of 3Sc.

1 3 H NMR (500 MHz, C6D6) δ 7.89 (d, JHH = 7.7 Hz,

3 4H, H5), 7.69 (d, JHH = 2.2 Hz, 4H, H9), 7.55 –

3 7.51 (m, 6H, H2 and H11), 7.22 (d, JHH = 7.8 Hz,

3 3 4H, H6), 6.79 (t, JHH = 7.7 Hz, 1H, H3), 5.73 (t, JHH = 2.2 Hz, 4H, H10), 5.47 (s, 1H,

-1 ScOH), 2.32 (s, 6H, H8). νO-H: 3716 cm .

Synthesis of Tol-Pz4B2PyScNTf2 (11Sc)

A 20 mL scintillation vial was charged with 1Sc (66

mg, 0.105 mmol) and 5 mL of toluene was added.

Me3SiNTf2 (37 mg, 0.105 mmol, 1.00 equiv.) was

weighed separately in a 1 dram vial and dissolved in

2 mL of toluene. The solution of Me3SiNTf2 was

added dropwise into a stirred solution of 1Sc. The reaction mixture was left to stir for 1 h at room temperature during which the white suspension turned clear with some oily residue on the sides of the vial. The reaction mixture was filtered through a 0.1 µm PTFE syringe filter and the filtrate was transferred into a 50

161 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield a colourless residue. The residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a white solid and dried in vacuo (70 mg, 0.080 mmol, 76%). Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 11Sc in toluene at -35 °C.

1 3 3 H NMR (500 MHz, C6D6) δ 8.25 (d, JHH = 2.3 Hz, 4H, H9), 7.58 (d, JHH = 7.8 Hz, 4H,

3 3 H5), 7.37 (d, JHH = 2.3 Hz, 4H, H11), 7.29 (d, JHH = 7.8 Hz, 2H, H2), 7.17 (d, J = 8.5 Hz,

3 3 4H, H6), 6.64 (t, JHH = 7.8 Hz, 1H, H3), 5.75 (t, JHH = 2.3 Hz, 4H, H10), 2.31 (s, 6H,

13 1 H8). C{ H} NMR (126 MHz, C6D6) δ 171.67 (C1), 141.93 (C9), 139.43 (C4), 137.25

(overlapping, C7 and C11), 135.83 (C5), 135.72 (C3), 129.15 (C6), 128.1 (C2), 120.0 (q,

1 11 19 1 JCF = 320 Hz, C12), 105.22 (C10), 21.35 (C8). B NMR (161 MHz, C6D6) δ 0.08. F{ H}

NMR (471 MHz, C6D6) δ -79.25. Elemental Analysis: Calcd. (%) for

C33H29B2F6N10O4S2Sc: C, 45.33; H, 3.34; N, 16.02. Found: C, 45.20; H, 3.32; N, 15.09.

General synthesis of scandium amido complexes (15Sc and 16Sc)

A 20 mL scintillation vial was charged with 1Sc and 5 mL of toluene was added.

The lithium amide was weighed separately in a 1 dram vial and dissolved in 2 mL of toluene. The solution of lithium amide was added dropwise into a stirred solution of 1Sc.

The reaction mixture was left to stir for 24 hours at room temperature after which it was filtered through a 0.1 µm PTFE syringe filter to remove LiCl. The filtrate was transferred

162 into a 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield a pale yellow residue. The residue was triturated and sonicated with 20 mL of pentane. The suspension was then filtered through a medium porosity frit, washed with 2x3 mL of pentane and dried in vacuo.

t Synthesis of Tol-Pz4B2PyScNHPh(4- Bu) (15Sc)

1Sc (145 mg, 0.230 mmol) was reacted with

LiNHPh(4-tBu) (36 mg, 0.232 mmol, 1.01 equiv.)

to yield 15Sc as a pale yellow solid (132 mg, 0.178

mmol, 77%).

1 H NMR (500 MHz, C6D6) δ 7.85 – 7.81 (m, 8H,

3 H5 and H9), 7.53 (d, JHH = 2.3 Hz, 4H, H11), 7.43

3 3 3 (d, JHH = 7.8 Hz, 2H, H2), 7.22 (d, JHH = 7.7 Hz, 4H, H6), 7.05 (d, JHH = 8.6 Hz, 2H,

3 3 H14), 6.77 (t, JHH = 7.8 Hz, 1H, H3), 6.42 (d, JHH = 8.5 Hz, 2H, H15), 6.08 (s, 1H, H12),

3 11 5.82 (t, JHH = 2.2 Hz, 4H, H10), 2.33 (s, 6H, H8), 1.28 (s, 9H, H18). B NMR (161 MHz,

C6D6) δ 0.44.

163

Synthesis of Tol-Pz4B2PyScNHB(NDippCH)2 (16Sc)

1Sc (0.091 mg, 0.145 mmol) was reacted with

LiNHB(NDippCH)2 (59 mg, 0.147 mmol, 1.01

equiv.) to yield 16Sc as a pale yellow solid (127 mg,

0.127 mmol, 88%).

1 3 H NMR (500 MHz, C6D6) δ 7.80 (d, JHH = 7.9

3 Hz, 4H, H5), 7.58 (d, JHH = 2.1 Hz, 4H, H9), 7.43

3 3 (d, JHH = 7.8 Hz, 2H, H2), 7.37 (d, JHH = 2.3 Hz,

3 3 3 4H, H11), 7.21 (d, JHH = 7.8 Hz, 4H, H6), 7.19 (t, JHH = 7.7 Hz, 2H, H17), 7.11 (d, JHH

3 3 = 7.7 Hz, 4H, H16), 6.73 (t, JHH = 7.8 Hz, 1H, H3), 6.09 (s, 2H, H13), 5.77 (t, JHH = 2.2

3 Hz, 4H, H10), 4.10 (s, 1H, H12), 3.56 (sept, JHH = 6.9 Hz, 4H, H18), 2.32 (s, 6H, H8),

3 3 11 1.26 (d, JHH = 6.8 Hz, 12H, H19), 1.08 (d, JHH = 6.9 Hz, 12H, H20). B NMR (161 MHz,

13 1 C6D6) δ 24.68 (boryl), -0.51 (borate). C{ H} NMR (126 MHz, C6D6) δ 172.49 (C1),

147.13 (C17), 141.22 (C14), 140.99 (C4), 140.63 (C9), 136.75 (C7), 136.18 (C5), 136.04

(C11), 134.35 (C3), 129.00 (C6), 127.24 (C2), 126.91 (C15), 124.05 (C16), 117.31 (C13),

104.55 (C10), 28.59 (C18), 25.43 (C19), 23.06 (C20), 21.38 (C8).

164

Synthesis of Tol-Pz4B2PyTiCH3 (2Ti-Me)

A 50 mL thick-walled glass vessel equipped with a

Kontes PTFE valve plug was charged with 1Ti (51

mg, 0.081 mmol) and MeLi (2 mg, 0.091 mmol).

The apparatus was connected to the vacuum line and 20 mL of Et2O was vacuum transferred to the solid mixture at -78 ºC. The reaction mixture was left to stir at room temperature for 18 h, upon which the cloudy pink mixture turned translucent orange. The solvent was evaporated in vacuo and the residue was extracted with 20 mL toluene and filtered through a 0.1 µm PTFE syringe filter to remove

LiCl and excess MeLi. The filtrate was transferred into a 50 mL thick-walled glass vessel and subsequently evaporated in vacuo to yield an orange residue. The residue was triturated and sonicated with 20 mL of pentane. The orange suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as an orange solid and dried in vacuo (39 mg, 0.064 mmol, 79%). Single crystals suitable for X- ray diffraction were obtained by layering HMDSO onto a concentrated solution of 2Ti-Me in benzene.

1 H NMR (500 MHz, C6D6) δ 7.90 (br. s), 7.28 (br. s), 6.54 (br. s), 6.35 (br. s), 2.41 (br. s),

11 0.69 (br. s), -10.80 (br. s). B NMR (161 MHz, C6D6) δ -15.39. No observable resonances

13 in C NMR spectrum. µeff: 1.77 (500 MHz, C6D6). Elemental Analysis: Calcd. (%) for

C32H32B2N9Ti: C, 62.79; H, 5.27; N, 20.59. Found: C, 63.04; H, 5.10; N, 19.13.

165

Synthesis of Tol-Pz4B2PyTiCH2SiMe3 (2Ti-CH2SiMe3)

A 20 mL scintillation vial was charged with 1Ti (131

mg, 0.207 mmol) and 5 mL of toluene was added.

LiCH2SiMe3 (20 mg, 0.212 mmol) was weighed

separately in a 1 dram vial and dissolved in 2 mL of toluene. The solution of alkyl lithium was added dropwise into a stirred solution of 1Ti. The reaction mixture was left to stir for 24 hours at room temperature after which it was filtered through a 0.1 µm PTFE syringe filter to remove LiCl. The filtrate was transferred into a 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield an orange residue. The residue was triturated and sonicated with 20 mL of pentane. The orange suspension was then filtered through a medium porosity frit, washed with 2x3 mL of pentane. The product was isolated as an orange solid and dried in vacuo (125 mg, 0.183 mmol, 88%). Single crystals suitable for X-ray diffraction were obtained by layering HMDSO onto a concentrated solution of 2Ti-CH2SiMe3 in benzene.

1 H NMR (500 MHz, C6D6) δ 7.90 (br. s), 7.32 (br. s), 7.27 (br. s), 6.68 (br. s), 6.46 (br. s),

11 2.40 (br. s), 1.62 (br. s), -0.83 (br. s), -9.47 (br. s). B NMR (161 MHz, C6D6) δ -16.09.

13 No observable resonances in C NMR spectrum. µeff: 2.04 (500 MHz, C6D6). Elemental

Analysis: Calcd. (%) for C35H40B2N9SiTi: C, 61.43; H, 5.89; N, 18.42. Found: C, 61.91;

H, 6.11; N, 18.09.

166

Synthesis of Tol-Pz4B2PyTiH (2Ti-H)

A 20 mL scintillation vial was charged with 1Ti (95

mg, 0.150 mmol) and 5 mL of toluene was added.

NaHBEt3 (19 mg, 0.156 mmol, 1.04 equiv.) was

weighed separately in a 1 dram vial and dissolved

in 2 mL of toluene. The solution of NaHBEt3 was

added dropwise into a stirred solution of 1Ti. The

reaction mixture was left to stir for 18 h at room

temperature during which the pink suspension turned dark red. The reaction mixture was filtered through a 0.1 µm PTFE syringe filter to remove NaCl. The filtrate was transferred into a 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield a dark red residue. The residue was triturated and sonicated with 20 mL of pentane. The red suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a maroon solid and dried in vacuo (75 mg, 0.125 mmol, 83%). Single crystals suitable for X-ray diffraction were obtained by layering

HMDSO onto a concentrated solution of 2Ti-H in bromobenzene.

1 3 H NMR (500 MHz, C6D6) δ 7.93 (d, JHH = 7.4 Hz, 4H, H5), 7.64 (s, 4H, H9), 7.57 (d,

3 3 3 JHH = 2.2 Hz, 4H, H11), 7.30 (d, JHH = 7.7 Hz, 4H, H6), 6.83 (d, JHH = 7.6 Hz, 2H, H2),

3 6.59 (s, 4H, H10), 5.91 (t, JHH = 7.5 Hz, 1H, H3), 2.39 (s, 6H, H8), -66.36 (s, 1H, TiH).

13 1 C{ H} NMR (126 MHz, C6D6) δ 174.5 (C1), 160.9 (C9), 141.3 (C4), 137.4 (C11), 136.7

(C7), 136.2 (C5), 134.4 (C3), 129.0 (C6), 124.1 (C2), 105.9 (C10), 21.5 (C8). 11B NMR

167

(161 MHz, C6D6) δ -2.51. µeff: 1.93 (500 MHz, C6D6). Elemental Analysis: Calcd. (%) for

C31H30B2N9Ti: C, 62.25; H, 5.06; N, 21.08. Found: C, 61.62; H, 5.44; N, 20.31.

Synthesis of Tol-Pz4B2PyTiOTiPz4B2Py-Tol (3Ti)

A preweighed 25 mL thick-walled glass vessel

equipped with a Kontes PTFE valve plug was

charged with 2Ti-CH2SiMe3 (21 mg, 0.030 mmol)

and dissolved in 10 mL of toluene. 10 µL of

degassed deionized water was added to the solution

under a positive flow of argon. The solution was stirred for 30 mins at room temperature before removing all solvents in vacuo. The residue was triturated and sonicated with 20 mL of pentane. The orange suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as an orange solid and dried in vacuo (16 mg, 0.013 mmol, 87%). Single crystals suitable for X-ray diffraction were obtained by slow vapour diffusion of pentane onto a concentrated solution of 3Ti in benzene.

1 H NMR (500 MHz, C6D6) δ 7.82 (br. s), 7.32 (br. s), 7.26 (br. s), 6.35 (br. s), 5.15 (br. s),

11 13 2.40 (br. s), -0.08 (br. s). No observable resonances in B and C NMR spectra. µeff: 2.53

(500 MHz, THF-H8). Elemental Analysis: Calcd. (%) for C62H58B4N18OTi2: C, 61.53; H,

4.83; N, 20.83. Found: C, 60.67; H, 4.64; N, 19.96.

168

Synthesis of Tol-Pz4B2PyTiOOCH (4Ti-H)

A 50 mL thick-walled glass vessel equipped with a

Kontes PTFE valve plug was charged with both 1Ti

(93 mg, 0.147 mmol) and HCOONa (10 mg, 0.147

mmol). The apparatus was connected to the vacuum

line and 20 mL of THF was vacuum transferred to the solid mixture at -78 ºC. The cloudy pink mixture was heated to 65 ºC for 5 days, upon which the cloudy pink mixture turned translucent orange. The solvent was evaporated in vacuo and the residue was extracted with 20 mL toluene and filtered through a 0.1 µm

PTFE syringe filter to remove NaCl and excess HCOONa. The filtrate was transferred into a 50 mL thick-walled glass vessel and subsequently evaporated in vacuo to yield an orange residue. The residue was triturated and sonicated with 20 mL of pentane. The orange suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as an orange solid and dried in vacuo (85 mg, 0.132 mmol, 90%).

1 H NMR (500 MHz, C6D6) δ 7.69 (br. s), 7.26 (br. s), 5.35 (br. s), 5.13 (br. s), 2.41 (br. s),

11 1.82 (br. s), -10.76 (br. s). B NMR (161 MHz, C6D6) δ -10.54. No observable resonances

13 -1 in C NMR spectrum. νC-O: 1673 cm . µeff: 1.41 (500 MHz, THF-H8). Elemental Analysis:

Calcd. (%) for C32H30B2N9O2Ti: C, 59.85; H, 4.71; N, 19.63. Found: C, 60.93; H, 4.48; N,

18.82.

169

Synthesis of [Tol-Pz4B2PyTi][HB(C6F5)3] (6Ti)

A J-Young NMR tube was charged with both

2Ti-H (10 mg, 0.0167 mmol) and B(C6F5)3 (9

mg, 0.0176 mmol, 1.05 equiv.), and dissolved

in 0.6 mL of C6D6. The reaction mixture was heated to 80 °C for 18 h which resulted in a colour change from dark red to dark orange.

1 3 3 H NMR (500 MHz, C6D6) δ 7.62 (d, JHH = 7.2 Hz, 4H, H5), 7.34 (d, JHH = 6.2 Hz, 4H,

3 H6), 6.70 (s, 4H, H9), 5.07 (d, JHH = 6.6 Hz, 2H, H2), 4.34 (s, 5H, H3 and H11), 2.44 (s,

- 11 6H, H8), -8.16 (s, 4H, H10). [HB(C6F5)3] not observed. B NMR (161 MHz, C6D6) δ -

- 19 23.81 (br. s, [HB(C6F5)3] ). Pz4B2Py not observed. F NMR (471 MHz, C6D6) δ -132.71

(o-F), -164.10 (p-F), -166.52 (m-F).

Synthesis of [Tol-Pz4B2PyTiClTiPz4B2Py-Tol][HB(C6F5)3] (7Ti)

A 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug was charged with 1Ti (81 mg, 0.129 mmol) and 15 mL of toluene was added. A solution of B(C6F5)3

(101 mg, 0.135 mmol, 1.05 equiv.) and Et3SiH (17 mg, 0.146 mmol, 1.14 equiv.) dissolved in 5 mL of toluene was added to the reaction mixture. The cloudy pink suspension

170 immediately turned into a dark brown-green solution. The reaction mixture was stirred overnight after which the solvent was removed in vacuo. The resulting residue was triturated and sonicated with 20 mL of pentane. The green suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as an olive green solid and dried in vacuo (104 mg, 0.0599 mmol, 93%).

1 3 3 H NMR (500 MHz, C6D6) δ 12.89 (d, JHH = 7.3 Hz, 2H, H2), 7.81 (d, JHH = 7.2 Hz, 4H,

3 H5), 7.38 (d, JHH = 7.0 Hz, 4H, H6), 7.05 (s, 4H, H9), 3.83 (s, 4H, H11), 2.40 (s, 6H, H8),

3 - 13 1 -4.11 (s, 4H, H10), -4.26 (t, JHH = 7.5 Hz, 1H, H3). [HB(C6F5)3] not observed. C{ H}

1 - NMR (126 MHz, C6D6) δ 149.23 (d, JCF = 236 Hz, [HB(C6F5)3] ), 148.52 (C11), 143.73

1 - (C4), 138.15 (C7), 137.23 (d, JCF = 244 Hz, [HB(C6F5)3] ), 134.89 (C5), 129.92 (C6),

- 11 94.96 (C2), 59.76 (C9), 21.36 (C8). C1, C3, C10 and two [HB(C6F5)3] not observed. B

- 19 1 NMR (161 MHz, C6D6) δ -24.20 (br. s, [HB(C6F5)3] ). Pz4B2Py not observed. F{ H}

NMR (471 MHz, C6D6) δ -132.92 (o-F), -164.82 (p-F), -167.33 (m-F). µeff: 2.88 (500 MHz,

C6D6). Elemental Analysis: Calcd. (%) for C80H59B5ClF15N18Ti2: C, 55.14; H, 3.41; N,

14.47. Found: C, 54.93; H, 3.58; N, 13.74.

171

Synthesis of [Tol-Pz4B2PyTi][HCOOB(C6F5)3] (8Ti-H)

A 50 mL thick-walled glass vessel equipped with a

Kontes PTFE valve plug was charged with 1Ti (118

mg, 0.187 mmol) and 15 mL of toluene was added.

A solution of B(C6F5)3 (101 mg, 0.197 mmol, 1.05

equiv.) and Et3SiH (24 mg, 0.210 mmol, 1.10

equiv.) dissolved in 5 mL of toluene was added to the reaction mixture. The cloudy pink suspension immediately turned into a dark brown- green solution. The apparatus was connected to the vacuum line and the solution was degassed by freeze-pump-thaw at -196 ºC and backfilled with 1 atm of CO2. The solution turned orange within a few minutes. The reaction mixture was stirred overnight after which the solvent was removed in vacuo. The resulting residue was triturated and sonicated with

20 mL of pentane. The orange suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as an orange solid and dried in vacuo (164 mg, 0.142 mmol, 76%).

1 3 3 H NMR (500 MHz, C6D6) δ 7.60 (d, JHH = 7.5 Hz, 4H, H5), 7.26 (d, JHH = 7.0 Hz, 4H,

3 H6), 7.05 (d, JHH = 7.4 Hz, 2H, H2), 6.63 (s, 4H, H9), 4.07 (s, 4H, H11), 2.39 (s, 6H, H8),

3 13 1 1.95 (t, JHH = 7.3 Hz, 1H, H3), -6.76 (s, 4H, H10), -18.08 (s, 1H, H12). C{ H} NMR

1 (126 MHz, C6D6) δ 192.08 (C12), 160.89 (C11), 148.21 (dm, JCF = 241 Hz,

- 1 - [HCO2B(C6F5)3] ), 143.54 (C4), 140.59 (dm, JCF = 247 Hz, [HCO2B(C6F5)3] ), 137.74

1 - (C7), 137.70 (dm, JCF = 246 Hz, [HCO2B(C6F5)3] ) 136.61 (C5), 129.44 (C6), 122.57

- ([HCO2B(C6F5)3] ), 110.37 (C10), 93.24 (C2), 56.56 (C9), 21.26 (C8). C3 not observed.

11 - 19 1 B NMR (161 MHz, C6D6) δ -0.99 (Pz4B2Py), -16.61 ([HCO2B(C6F5)3] ). F{ H} NMR

172

3 3 (471 MHz, C6D6) δ -132.03 (d, JFF = 23.7 Hz, o-F), -157.49 (t, JFF = 21.5 Hz, p-F), -

3 -1 164.08 (t, JFF = 22.8 Hz, m-F). νC-O: 1609 cm . µeff: 2.08 (500 MHz, C6D6). Elemental

Analysis: Calcd. (%) for C50H30B3F15N9O2Ti: C, 52.03; H, 2.62; N, 10.92. Found: C, 52.37;

H, 2.87; N, 10.71.

Synthesis of Tol-Pz4B2PyTiNTf2 (11Ti)

A 20 mL scintillation vial was charged with 1Ti (146

mg, 0.231 mmol) and 5 mL of toluene was added.

Me3SiNTf2 (82 mg, 0.232 mmol, 1.01 equiv.) was

weighed separately in a 1 dram vial and dissolved

in 2 mL of toluene. The solution of Me3SiNTf2 was

added dropwise into a stirred solution of 1Ti. The reaction mixture was left to stir for 1 h at room temperature during which the pink suspension turned clear orange with some white precipitate. The reaction mixture was filtered through a 0.1 µm PTFE syringe filter and the filtrate was transferred into a 50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug and subsequently evaporated in vacuo to yield an orange residue. The residue was triturated and sonicated with 20 mL of pentane. The white suspension was then filtered through a medium porosity frit and washed with 2x3 mL of pentane. The product was isolated as a beige solid and dried in vacuo (148 mg, 0.169 mmol, 73%). Single crystals suitable for X-ray diffraction were obtained by layering HMDSO onto a concentrated solution of 11Ti in bromobenzene.

173

1 3 3 H NMR (500 MHz, C6D6) δ 8.61 (d, JHH = 7.5 Hz, 2H, H2), 7.59 (d, JHH = 7.6 Hz, 4H,

3 H5), 7.26 (d, JHH = 7.3 Hz, 4H, H6), 6.80 (s, 4H, H9), 4.35 (s, 4H, H11), 2.38 (s, 6H, H8),

3 13 1 -1.06 (t, JHH = 7.5 Hz, 1H, H3), -5.32 (s, 4H, H10). C{ H} NMR (126 MHz, C6D6) δ

205.85 (C3), 155.12 (C11), 144.33 (C4), 137.57 (C7), 136.12 (C5), 129.43 (C6), 126.62

1 11 (q, JCF = 320 Hz, C12), 86.33 (C2), 57.92 (C9), 21.29 (C8). C1 and C10 not observed. B

19 1 NMR (161 MHz, C6D6) δ -20.23. F{ H} NMR (471 MHz, C6D6) δ -74.44. µeff: 1.92 (500

MHz, C6D6). Elemental Analysis: Calcd. (%) for C33H29B2F6N10O4S2Ti: C, 45.18; H, 3.33;

N, 15.97. Found: C, 45.18; H, 3.48; N, 15.60.

Synthesis of Tol-Pz4B2PyTiO2 (13Ti)

Method A: 1Ti (49 mg, 0.0775 mmol) was weighed

out in a 50 mL round bottom flask equipped with a

180º glass joint adapter. The apparatus was

connected to the vacuum line and 20 mL of THF was vacuum transferred to the solid mixture at -78 ºC. The cloudy pink mixture was warmed up to room temperature after which 30% w/w H2O2 (20 μL, 0.176 mmol, 2.28 equiv.) was added dropwise under a positive flow of Ar. The mixture immediately turned orange with slight precipitation over time. The reaction mixture of stirred for 2 h then filtered through a 0.1 µm PTFE syringe filter in air. The dark orange filtrate was evaporated in vacuo, extracted with 20 mL DCM and filtered once more. The filtrate was concentrated to ca. 2 mL in a 20 mL scintillation vial and layered with hexanes. Orange crystals of 13Ti was isolated by decanting off the mother liquor and drying in vacuo (26 mg, 0.0413 mmol,

53%).

174

Method B: A 20 mL scintillation vial was charged with 1Ti (90 mg, 0.142 mmol) and 5 mL of THF was added. Solid addition of KC8 (20 mg, 0.148 mmol, 1.04 equiv.) to the vial caused the pink suspension to immediately turn dark brown. The solution was stirred for 5 mins then filtered through a 0.1 µm PTFE syringe filter. The filtrate was transferred to a

50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug. The solution was degassed by freeze-pump-thaw at -196 ºC and backfilled with 1 atm of O2 at room temperature. The solution turned orange after a few minutes and was left to stir for 2 h. The reaction mixture was evaporated in vacuo to give the product (83 mg, 0.132 mmol, 93%) as an orange solid. Single crystals suitable for X-ray diffraction were obtained layering hexanes onto a concentrated solution of 13Ti in DCM over 3 days.

1 3 3 H NMR (500 MHz, CD2Cl2) δ 8.13 (d, JHH = 2.2 Hz, 4H, H9), 7.78 (d, JHH = 7.9 Hz,

3 3 3 4H, H5), 7.72 (d, JHH = 2.4 Hz, 4H, H11), 7.34 (d, JHH = 7.6 Hz, 4H, H6), 7.29 (t, JHH =

3 3 7.7 Hz, 1H, H3), 7.21 (d, JHH = 7.7 Hz, 2H, H2), 6.26 (t, JHH = 2.3 Hz, 4H, H10), 2.45 (s,

13 1 6H, H8). C{ H} NMR (126 MHz, CD2Cl2) δ 170.69 (C1), 141.64 (C9), 139.07 (C4),

137.43 (C7), 136.02 (C11), 135.57 (C5), 135.32 (C3), 129.13 (C6), 128.23 (C2), 105.24

11 (C10), 21.38 (C8). B NMR (161 MHz, CD2Cl2) δ -0.74. HRMS (APCI): Calcd m/z

+ + -1 -1 630.2183 (M+H) , found m/z 630.2163 (M+H) . νO-O: 911 cm ; νTi-O: 598 cm . Elemental

Analysis: Calcd. (%) for C31H29B2N9O2Ti: C, 59.18; H, 4.65; N, 20.04. Found: C, 59.05;

H, 4.80; N, 17.67.

175

Synthesis of Tol-Pz4B2PyTiO2 (14Ti)

Method A: A 50 mL thick-walled glass vessel

equipped with a Kontes PTFE valve plug was

charged with 13Ti (27 mg, 0.0429 mmol) and PPh3

(12 mg, 0.0458 mmol, 1.07 equiv.). 10 mL of THF was added and the reaction mixture was stirred for 5 days at 65 °C. The solvent was evaporated in vacuo and the residue was triturated and sonicated with 20 mL of Et2O. The orange suspension was then filtered through a medium porosity frit and washed with 2x3 mL of Et2O. The product was isolated as a light orange solid and dried in vacuo (19 mg,

0.0310 mmol, 73%).

Method B: A 20 mL scintillation vial was charged with 1Ti (81 mg, 0.128 mmol) and 5 mL of THF was added. Solid addition of KC8 (20 mg, 0.148 mmol, 1.16 equiv.) to the vial caused the pink suspension to immediately turn dark brown. The solution was stirred for 5 mins then filtered through a 0.1 µm PTFE syringe filter. The filtrate was transferred to a

50 mL thick-walled glass vessel equipped with a Kontes PTFE valve plug along with PhIO

(29 mg, 0.132 mmol, 1.03 equiv.). The solution was left to stir for 18 h at 60 °C, upon which the solution turned orange. The reaction mixture was evaporated in vacuo, extracted with 5 mL of DCM and filtered through a 0.1 µm PTFE syringe filter. The extract was evaporated in vacuo and the residue was triturated and sonicated with 20 mL of pentane.

The orange suspension was then filtered through a medium porosity frit and washed with

2x3 mL of pentane. The product was isolated as a light orange solid and dried in vacuo (45 mg, 0.0734 mmol, 57%). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a concentrated solution of 14Ti in DCM.

176

1 3 3 H NMR (400 MHz, CDCl3) δ 8.52 (d, JHH = 2.1 Hz, 4H, H9), 7.80 (d, JHH = 7.8 Hz, 4H,

3 3 3 H5), 7.49 (d, JHH = 7.7 Hz, 2H, H2), 7.42 (d, JHH = 2.3 Hz, 4H, H11), 7.33 (d, JHH = 7.7

3 3 Hz, 4H, H6), 7.27 (t, JHH = 7.7 Hz, 1H, H3), 6.25 (t, JHH = 2.3 Hz, 4H, H10), 2.47 (s, 6H,

13 1 H8). C{ H} NMR (101 MHz, CDCl3) δ 170.98 (C1), 142.25 (C9), 137.27 (C7), 136.15

(C11), 135.82 (C5), 134.00 (C3), 139.22 (C4) 128.92 (C6), 126.40 (C2), 104.68 (C10),

11 -1 21.45 (C8). B NMR (128 MHz, CDCl3) δ -0.71. νTi-O: 950 cm . Elemental Analysis:

Calcd. (%) for C31H29B2N9OTi: C, 60.73; H, 4.77; N, 20.56. Found: C, 57.66; H, 5.05; N,

15.99.

177

Appendix A: Crystallographic Data

Table A-1 Crystallographer assignment

Compound Crystallographer 1Sc-Ph Dr. Benjamin Gelfand 1Ti-Ph Dr. Denis Spasyuk 1Sc Dr. Benjamin Gelfand 1Ti Dr. Jian-Bin Li 2Sc-CH2SiMe3 Dr. Chris Gendy 2Sc-Me Dr. Jian-Bin Li 2Ti-CH2SiMe3 Dr. Benjamin Gelfand 2Sc-Ph-OH Dr. Chris Gendy 2Sc-OH Dr. Benjamin Gelfand 2Ti-H Dr. Denis Spasyuk 3Sc-Ph Dr. Wenhua Bi 3Ti-Ph Dr. Wenhua Bi 3Sc Dr. Chris Gendy 3Ti Dr. Benjamin Gelfand 4Sc-H Dr. Benjamin Gelfand 4Sc-Me Dr. Jian-Bin Li 5Sc-THF Dr. Jian-Bin Li 6Sc Dr. Benjamin Gelfand 7Sc Dr. Jian-Bin Li 8Sc-H Dr. Jian-Bin Li 9Sc-Et Dr. Benjamin Gelfand 10Sc Dr. Jian-Bin Li 11Ti Dr. Jian-Bin Li 12Sc Dr. Jian-Bin Li 13Ti Dr. Jian-Bin Li 14Ti Dr. Jian-Bin Li 15Sc Dr. Benjamin Gelfand 16Sc Dr. Jian-Bin Li 17Sc Dr. Chris Gendy

178

Table A-2 Crystal and structure refinement details for 1Sc-Ph, 1Ti-Ph and 1Sc.

1Sc-Ph 1Ti-Ph 1Sc 2(C H B ClN Sc), C H B ClN Ti, C H B ClN Sc, formula 29 25 2 9 29 25 2 9 31 29 2 9 C6H6 0.5(C6H12) 2(C6H6) fw 1281.32 1293.25 785.88 crystal system orthorhombic orthorhombic triclinic space group Pccn Pccn P-1 a (Å) 12.6964(3) 12.6599(3) 10.2171(2) b (Å) 19.4533(4) 19.5796(4) 12.1118(4) c (Å) 24.9385(6) 24.7391(5) 18.2907(4) α (deg) 90 90 103.202(2) β (deg) 90 90 92.4060(10) γ (deg) 90 90 113.9310(10) V (Å3) 6159.5(2) 6132.2(2) 1990.99(9) Z 4 4 2 T (K) 173(2) 173(2) 173(2) Wavelength (Å) 1.54178 1.54178 1.54178 -3 ρcalcd (g∙cm ) 1.382 1.401 1.311 F(000) 2648 2680 820 µ (mm-1) 3.153 3.480 2.539 crystal size, mm3 0.15×0.13×0.09 0.25×0.10×0.02 0.198×0.111×0.053 transmission factors 0.6616 − 0.7475 0.5860 − 0.7536 0.646 – 0.754 θ range (deg) 3.544 − 71.742 3.573 − 66.494 2.510 − 68.228 data/restraints/param 5956/29/406 5380/15/406 7007/0/507 GOF 1.008 0.990 1.024 R1 [I > 2σ(I)] 0.0769 0.0378 0.0790 wR2 [all data] 0.2138 0.1095 0.2498 residual density, e/Å3 0.905 and -0.462 0.704 and -0.490 0.580 and -0.888

179

Table A-3 Crystal and structure refinement details for 1Ti, 2Sc-CH2SiMe3 and 2Sc-Me.

1Ti 2Sc-CH2SiMe3 2Sc-Me C31H29B2ClN9Ti, C32H32B2N9Sc, formula C35H40B2N9ScSi 2(C6H6) 2(C6H6) fw 788.82 681.43 765.46 crystal system triclinic monoclinic triclinic space group P-1 C2/c P-1 a (Å) 10.274(3) 39.5534(15) 10.2075(9) b (Å) 12.047(3) 8.3472(3) 11.9288(10) c (Å) 18.309(5) 27.4331(9) 18.5503(16) α (deg) 104.423(3) 90 104.426(4) β (deg) 92.332(3) 126.713(2) 92.714(5) γ (deg) 112.885(3) 90 112.543(4) V (Å3) 1998.0(9) 7260.7(5) 1994.4(3) Z 2 8 2 T (K) 173(2) 173(2) 173(2) Wavelength (Å) 0.71073 0.71073 0.71073 -3 ρcalcd (g∙cm ) 1.311 1.247 1.275 F(000) 822 2864 804 µ (mm-1) 0.325 0.274 0.229 crystal size, mm3 0.15×0.1×0.07 0.27×0.20×0.16 0.2×0.2×0.2 transmission factors 0.6942 − 0.7455 0.6269 − 0.7456 0.6309 − 0.7461 θ range (deg) 1.916 − 26.000 3.110 − 25.000 3.348 − 26.000 data/restraints/param 7862/0/505 6319/525/468 7825/0/506 GOF 1.008 1.023 1.004 R1 [I > 2σ(I)] 0.0482 0.0403 0.0421 wR2 [all data] 0.1162 0.1135 0.1096 residual density, e/Å3 0.312 and -0.382 0.376 and -0.365 0.436 and -0.401

180

Table A-4 Crystal and structure refinement details for 2Ti-CH2SiMe3, 2Sc-Ph-OH and 2Sc-

OH.

2Ti-CH2SiMe3 2Sc-Ph-OH 2Sc-OH C58H52B4N18O2Sc C62H59.32B4N18O2Sc formula C35H40B2N9SiTi 2 2 fw 684.34 1166.34 1221.76 crystal system monoclinic monoclinic monoclinic space group I12/a1 C2/c P12/c1 a (Å) 32.068(13) 30.6715(7) 17.2004(3) b (Å) 8.309(2) 11.9478(3) 11.8261(2) c (Å) 27.317(7) 15.3903(4) 15.3151(2) α (deg) 90 90 90 β (deg) 96.341(7) 100.5220(10) 104.3460(10) γ (deg) 90 90 90 V (Å3) 7234(4) 5545.0(2) 3018.16(8) Z 8 8 2 T (K) 173 173(2) 173(2) Wavelength (Å) 0.71073 1.54178 1.54178 -3 ρcalcd (g∙cm ) 1.257 1.397 1.344 F(000) 2872 2416 1270.6 µ (mm-1) 0.308 2.607 2.419 0.401×0.324×0.25 crystal size, mm3 0.20×0.10×0.10 0.374×0.121×0.067 2 transmission factors 0.8462 − 0.9457 0.6246 − 0.7535 0.6383 − 0.9155 θ range (deg) 3.328 − 28.282 2.931 − 71.862 2.651 − 70.367 data/restraints/para 8941/0/438 5403/429/383 5706/1/408 m GOF 1.033 1.046 1.034 R1 [I > 2σ(I)] 0.0422 0.0397 0.0425 wR2 [all data] 0.1174 0.1102 0.1194 residual density, e/Å3 0.356 and -0.362 0.380 and -0.331 0.288 and -0.574

181

Table A-5 Crystal and structure refinement details for 2Ti-H, 3Sc-Ph and 3Ti-Ph.

2Ti-H 3Sc-Ph 3Ti-Ph C62H60B4N18Ti2, C58H50B4N18OSc2, formula C58H50B4N18OTi2 C6H5Br C6H6 fw 1353.33 1226.42 577.10 crystal system monoclinic triclinic triclinic space group P2/c P-1 P-1 a (Å) 16.061(3) 13.0211(2) 12.6833(4) b (Å) 12.329(3) 13.5319(2) 13.8963(7) c (Å) 17.176(3) 18.4001(2) 23.0692(8) α (deg) 90 68.4540(10) 102.650(3) β (deg) 98.22(3) 82.9610(10) 101.444(2) γ (deg) 90 85.6880(10) 99.2700(10) V (Å3) 3366.2(12) 2991.23(7) 3798.4(3) Z 2 2 4 T (K) 100 173(2) 173(2) Wavelength (Å) 0.68883 1.54178 1.54178 -3 ρcalcd (g∙cm ) 1.335 1.362 1.009 F(000) 1396 1272 1192 µ (mm-1) 0.816 2.432 2.139 crystal size, mm3 0.05×0.01×0.003 0.50×0.36×0.28 0.25×0.23×0.22 transmission factors 0.0110 − 0.0326 0.5688 − 0.7531 0.6820 − 0.7536 θ range (deg) 1.242 − 24.205 2.595 − 68.732 3.337 − 66.497 data/restraints/param 5778/522/447 10964/0/802 13015/861/748 GOF 1.136 1.022 1.065 R1 [I > 2σ(I)] 0.0616 0.0367 0.0735 wR2 [all data] 0.2175 0.1054 0.2353 residual density, e/Å3 0.788 and -0.511 0.433 and -0.429 1.200 and -0.476

182

Table A-6 Crystal and structure refinement details for 3Sc, 3Ti and 4Sc-H.

3Sc 3Ti 4Sc-H C32H30B2N9O2Sc, formula C62H46B4N18OSc2 C62H58B4N18OTi2 2(C6H6) fw 1204.42 1210.24 795.45 crystal system monoclinic monoclinic triclinic space group P2/c P12/c1 P-1 a (Å) 17.2544(3) 16.8936(14) 12.9337(7) b (Å) 11.7447(2) 11.6920(12) 13.0637(5) c (Å) 15.3640(2) 15.3490(11) 13.3321(7) α (deg) 90 90 67.293(2) β (deg) 104.3550(10) 104.167(3) 82.797(3) γ (deg) 90 90 75.727(2) V (Å3) 3016.27(8) 2939.5(4) 2012.73(17) Z 2 2 2 T (K) 173(2) 173(2) 173(2) Wavelength (Å) 1.54178 0.71073 0.71073 -3 ρcalcd (g∙cm ) 1.326 1.367 1.313 F(000) 1252 1256 832.0 µ (mm-1) 2.400 0.332 0.233 crystal size, mm3 0.21×0.13×0.12 0.234×0.207×0.138 0.522×0.462×0.38 transmission factors 0.6219 − 0.7533 0.6526 − 1.0000 0.7270 − 0.9984 θ range (deg) 2.643 − 67.485 2.715 − 26.730 1.903 − 27.878 data/restraints/param 5307/0/395 6234/0/395 9505/360/617 GOF 1.058 1.020 1.021 R1 [I > 2σ(I)] 0.0471 0.0604 0.0550 wR2 [all data] 0.1355 0.1751 0.1578 residual density, e/Å3 0.487 and -0.305 0.945 and -0.944 0.768 and -0.471

183

Table A-7 Crystal and structure refinement details for 4Sc-Me, 5Sc-THF and 6Sc.

4Sc-Me 5Sc-THF 6Sc C49H30B3F15N9Sc, formula C33H32B2N9O2Sc C54H40B3F15N9OSc 2(C5H12) fw 653.25 1193.34 1251.50 crystal system orthorhombic triclinic triclinic space group P212121 P-1 P-1 a (Å) 13.860(4) 12.552(5) 12.5248(5) b (Å) 14.896(5) 13.862(5) 15.1903(6) c (Å) 15.506(5) 18.123(8) 17.3439(5) α (deg) 90 105.430(11) 96.529(3) β (deg) 90 94.261(12) 108.041(2) γ (deg) 90 115.595(8) 106.308(3) V (Å3) 3201.3(18) 2676(2) 2937.60(19) Z 4 2 2 T (K) 173(2) 173(2) 173 Wavelength (Å) 0.71073 0.71073 1.54178 -3 ρcalcd (g∙cm ) 1.355 1.481 1.415 F(000) 1360 1212 1284 µ (mm-1) 0.277 0.240 1.906 crystal size, mm3 0.13×0.13×0.1 0.300×0.200×0.100 0.396×0.141×0.108 transmission factors 0.5506 − 0.7455 0.6121 − 0.7454 0.6211 − 0.7531 θ range (deg) 2.627 − 24.994 2.285 − 25.014 2.747 − 70.154 data/restraints/param 5646/0/428 9346/0/748 10983/208/890 GOF 1.008 1.007 1.031 R1 [I > 2σ(I)] 0.0646 0.0670 0.0387 wR2 [all data] 0.1470 0.2176 0.1103 residual density, e/Å3 0.263 and -0.350 0.304 and -0.343 0.371 and -0.303

184

Table A-8 Crystal and structure refinement details for 7Sc, 8Sc-H, and 9Sc-Et.

7Sc 8Sc-H 9Sc-Et C62H58B4ClN18Sc2, C50H30B3F15N9O2S formula C37H44B2N9OScSi HBF15, 4(C6H6) c, 2(C3H9O0.5Si) fw 2049.30 1313.60 725.48 crystal system triclinic monoclinic monoclinic space group P-1 C12/c1 P121/c1 a (Å) 17.2598(9) 36.353(4) 12.6144(4) b (Å) 17.7327(10) 12.6079(14) 13.6594(5) c (Å) 19.0548(14) 30.349(3) 21.9450(8) α (deg) 74.561(5) 90 90 β (deg) 77.281(5) 116.768(3) 99.015(2) γ (deg) 61.891(4) 90 90 V (Å3) 4927.6(6) 12419(2) 3734.5(2) Z 2 8 4 T (K) 173(2) 173 173 Wavelength (Å) 1.54178 0.71073 0.71073 -3 ρcalcd (g∙cm ) 1.381 1.405 1.290 F(000) 2104 5360 1528 µ (mm-1) 2.169 0.253 0.273 0.451×0.405×0.37 crystal size, mm3 0.2×0.2×0.1 0.3×0.2×0.1 1 transmission 0.6796 − 0.7536 0.6335 − 0.7455 0.8170 − 1.0000 factors θ range (deg) 2.421 − 72.463 1.503 − 25.000 3.402 − 27.878 data/restraints/para 18582/60/1232 10596/0/820 8888/0/465 m GOF 1.059 1.017 1.021 R1 [I > 2σ(I)] 0.1175 0.0533 0.0434 wR2 [all data] 0.3654 0.1484 0.1119 residual density, 1.624 and -0.758 0.713 and -0.533 0.389 and -0.317 e/Å3

185

Table A-9 Crystal and structure refinement details for 10Sc, 11Ti, and 12Sc.

10Sc 11Ti 12Sc C62H62B4N18O2Sc2 C186H192B12N54O12S C33H29B2F6N10O4S2 formula , 2(C2F6NO4S2), c6, 4.4(C5) Ti 6(C6H6) fw 4039.62 877.30 2253.40 crystal system trigonal monoclinic triclinic space group R-3 P121/c1 P-1 a (Å) 22.1407(3) 18.570(2) 13.8744(10) b (Å) 22.1407(3) 23.927(3) 14.4202(11) c (Å) 38.1488(7) 8.5198(9) 15.9822(12) α (deg) 90 90 64.748(3) β (deg) 90 97.346(5) 73.424(3) γ (deg) 120 90 70.000(3) V (Å3) 16195.5(5) 3754.5(7) 2680.8(4) Z 3 4 1 T (K) 173(2) 173(2) 173(2) Wavelength (Å) 1.54178 0.71073 0.71073 -3 ρcalcd (g∙cm ) 1.243 1.552 1.396 F(000) 6300 1788 1164 µ (mm-1) 2.099 0.423 0.293 crystal size, mm3 0.1×0.07×0.05 0.25×0.21×0.2 0.26×0.2×0.2 transmission 0.6696 − 0.7536 0.5946 − 0.7463 0.6392 − 0.7457 factors θ range (deg) 2.579 − 72.392 2.030 − 25.999 1.951 − 25.997 data/restraints/para 7119/0/426 7372/0/525 10516/72/692 m GOF 1.006 1.027 1.039 R1 [I > 2σ(I)] 0.0537 0.0423 0.0708 wR2 [all data] 0.1714 0.1067 0.2157 residual density, 0.397 and -0.303 0.441 and -0.623 0.753 and -0.866 e/Å3

186

Table A-10 Crystal and structure refinement details for 13Ti, 14Ti, and 15Sc.

13Ti 14Ti 15Sc formula C31H29B2N9O2Ti C31H29B2N9OTi C41H43B2N10Sc fw 629.15 613.15 742.43 crystal system orthorhombic orthorhombic orthorhombic space group Pnma Pnma Pnma a (Å) 8.2398(12) 8.2433(19) 27.4751(7) b (Å) 19.824(3) 19.324(5) 9.8504(3) c (Å) 18.080(2) 18.081(4) 14.1559(4) α (deg) 90 90 90 β (deg) 90 90 90 γ (deg) 90 90 90 V (Å3) 2953.3(7) 2880.2(12) 3831.16(19) Z 4 4 4 T (K) 173(2) 173(2) 173(2) Wavelength (Å) 0.71073 0.71073 1.54178 -3 ρcalcd (g∙cm ) 1.415 1.414 1.287 F(000) 1304 1272 1560 µ (mm-1) 0.337 0.341 1.987 crystal size, mm3 0.15×0.15×0.08 0.2×0.1×0.1 0.276×0.181×0.084 transmission 0.6234 − 0.7461 0.6874 − 0.7455 0.7675 – 0.8837 factors θ range (deg) 3.500 − 25.998 1.542 − 25.996 3.217 − 68.235 data/restraints/para 2980/0/217 2923/0/206 3715/3/291 m GOF 1.027 1.002 1.008 R1 [I > 2σ(I)] 0.0430 0.0532 0.0505 wR2 [all data] 0.1166 0.1742 0.1409 residual density, 0.426 and -0.389 0.725 and -0.485 0.344 and -0.259 e/Å3

187

Table A-11 Crystal and structure refinement details for 16Sc and 17Sc.

16Sc 17Sc formula C57H66B3N12Sc C62H58B4N20Sc2 fw 996.60 1216.44 crystal system triclinic monoclinic space group P-1 P2/c a (Å) 13.2785(8) 17.2472(10) b (Å) 15.6334(9) 11.7287(6) c (Å) 16.0017(9) 15.3089(9) α (deg) 77.880(3) 90 β (deg) 85.286(3) 104.625(4) γ (deg) 76.180(3) 90 V (Å3) 3151.9(3) 2996.5(3) Z 2 2 T (K) 173(2) 173(2) Wavelength (Å) 0.71073 0.71073 -3 ρcalcd (g∙cm ) 1.050 1.348 F(000) 1056 1264 µ (mm-1) 0.159 0.286 crystal size, mm3 0.15×0.14×0.12 0.21×0.11×0.05 transmission factors 0.6710 − 0.7455 0.5805 − 0.7456 θ range (deg) 1.368 − 26.000 2.996 − 25.000 data/restraints/param 12393/0/671 5226/447/399 GOF 1.003 1.010 R1 [I > 2σ(I)] 0.0740 0.0536 wR2 [all data] 0.2375 0.1447 residual density, e/Å3 0.380 and -0.599 0.322 and -0.322

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Appendix B: Copyright Permissions

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Scandium alkyl and hydride complexes supported by a pentadentate diborate ligand: reactions with CO2 and N2O

D. W. Beh, W. E. Piers, I. del Rosal, L. Maron, B. S. Gelfand, C. Gendy and J. Lin, Dalton Trans.,

2018, 47, 13680

DOI: 10.1039/C8DT03313G

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