Rhodocenium Monocarboxylic Acid Hexafluoridophosphate and Its Derivatives: Synthesis, Spectroscopy, Structure, and Electrochemistry Markus Jochriem,[A] Larissa A
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EurJIC Full Paper European Journal of Inorganic Chemistry doi.org/10.1002/ejic.202000071 Metallocenes Rhodocenium Monocarboxylic Acid Hexafluoridophosphate and Its Derivatives: Synthesis, Spectroscopy, Structure, and Electrochemistry Markus Jochriem,[a] Larissa A. Casper,[b] Stefan Vanicek,[a,c] Dirk Petersen,[c] Holger Kopacka,[a] Klaus Wurst,[a] Thomas Müller,[d] Rainer F. Winter,*[b] and Benno Bildstein*[a] Abstract: As an extension of our continuing work in metalloce- are reported. Like the parent rhodocenium ion, all new deriva- nium chemistry, we report here on new functionalized rhodoce- tives undergo two chemically consecutive reductions at poten- nium salts. In contrast to isoelectronic cobaltocenium com- tials that depend on the respective ring substituent. The second pounds, rhodium as a 4d metal allows synthetic routes via pre- reduction competes with a rapid reaction of the corresponding functionalized cyclopentadienyl half-sandwich precursors, rhodocenes to their 18 VE dimers. Rate constants for this proc- thereby facilitating access to monofunctionalized rhodocenium ess range from 2 × 103 s–1 to 2.5 × 105 s–1 as estimated from salts containing substituents comprising methyl, trimethylsilyl, digital simulations of experimental voltammograms. Rhodoce- carboxylate and carboxylate ester as well as amide derivatives. nium carboxylic acid (8) constitutes a special case in that proton Synthetic aspects, scope and limitations, as well as spectro- instead of metal reduction is observed at Pt or Au electrodes. scopic (1H/13C-NMR, IR, HR-MS), and structural (XRD) properties Introduction we were able to introduce valuable new synthetic protocols in cobaltocenium chemistry[1] that are currently used to access After almost 70 years of research in metallocene chemistry, still chemically interesting cobaltocenium compounds.[2] Our moti- relatively little is known about cobaltocenium, rhodocenium vation for studying cobaltocenium compounds in general stems and iridocenium salts that are isoelectronic and as stable and from their unique combination of advantageous properties like redox responsive as their ubiquitous ferrocene congeners. The air-stability, reversible redox chemistry, high polarity with con- main reason for the underdeveloped chemistry of these metal- comitant high solubility in polar solvents, and a strongly elec- locenium salts is their intrinsic cationic nature, making it quite tron-withdrawing character, thereby opening up new applica- difficult to access functionalized derivatives by standard chemi- tions in redox catalysis, bioinorganic chemistry, ligand design cal reactions developed for arenes. However, in recent years and coordination chemistry.[2] In comparison to cobaltocenium chemistry, rhodocenium chemistry is even less developed.[3] In fact, only three monosub- [a] Institute of General, Inorganic and Theoretical Chemistry, stituted rhodocenium compounds are presently known. First, University of Innsbruck, Sheats and co-workers[4] reported in 1983 the synthesis of rho- Center for Chemistry and Biomedicine, Innrain 80-82, 6020 Innsbruck, Austria docenium monocarboxylic acid hexafluoridophosphate in 20 % E-mail: [email protected] isolated yield by a non-chemoselective statistical approach, tak- https://www.uibk.ac.at/aatc/ ing advantage of the differences in solubility of unsubstituted [b] Department of Chemistry, University of Konstanz, rhodocenium, rhodocenium mono- and rhodocenium dicarbox- Universitätsstrasse 10, 784557 Konstanz, Germany E-mail: [email protected] ylic acid salts. No further chemistry of rhodocenium mono- https://www.chemie.uni-konstanz.de/winter/ carboxylic acid hexafluoridophosphate was described. Second, [c] Department of Chemistry, University of Oslo, Andre and co-workers[5] published in 1990 the preparative Sem Sælands vei 26, 0315, Oslo Norway HPLC separation of ferrocenylrhodocenium from other metallo- [d] Institute of Organic Chemistry, University of Innsbruck, Center for Chemistry and Biomedicine, cenylrhodocenium salts. The latter were again obtained by a [6] Innrain 80-82 6020 Innsbruck, Austria statistical protocol. Third, Yan and co-workers reported in Supporting information and ORCID(s) from the author(s) for this article are 2015 a hydroxyethyltriazolylrhodocenium salt obtained from available on the WWW under https://doi.org/10.1002/ejic.202000071. (η4-ethynylcyclopentadiene)(η5-cyclopentadienyl)rhodium(I) by © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. · azide-alkyne cycloaddition followed by oxidation with ferroce- This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any nium hexafluoridophosphate. This rhodocenium-functionalized medium, provided the original work is properly cited. alcohol was converted to norbornene and methacrylate mono- Eur. J. Inorg. Chem. 2020, 1300–1310 1300 © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2-dj3lcqan4inp3 EurJIC Full Paper European Journal of Inorganic Chemistry doi.org/10.1002/ejic.202000071 mers and subsequently polymerized to the corresponding side- be applied for rhodocenium compounds, due to differences in chain functionalized redox-responsive polymers. either reactivity or solubility of cobaltocenium/rhodocenium In this contribution, we aim at developing rhodocenium salts. Specifically, while the main target of this study, rhodoce- chemistry a bit further, in part based on our experience in nium monocarboxylic acid hexafluoridophosphate, could be cobaltocenium chemistry,[1,2] and disclose here a first chemo- generated by a nucleophilic addition/hydride removal/side- selective synthesis of rhodocenium monocarboxylic acid hexa- chain oxidation sequence that was recently developed in our fluoridophosphate which is a key synthon for other monosub- group for the successful synthesis of its cobaltocenium ana- stituted rhodocenium salts accessible by standard carboxylic logue,[1a] its purification from side products and isolation acid reactivity. The structural, spectroscopic and electrochemi- proved not feasible despite many efforts. Similarly, direct amin- cal properties of these new rhodocenium derivatives are com- ation of rhodocenium salts by a vicarious nucleophilic substitu- pared with those of their cobaltocenium congeners. tion gave only inseparable product mixtures, in contrast to cor- responding cobaltocenium salts.[1c] Therefore alternative syn- thetic strategies are needed for new monofunctionalized Results and Discussion rhodocenium compounds. In this context, an advantageous fea- ture of rhodium chemistry is the much higher stability of half- Synthesis sandwich rhodium(III) halide complexes[7] in comparison to [8] In general, rhodocenium chemistry is somewhat different from their labile cobalt(III) analogues. As we will see in the follow- cobaltocenium chemistry, as is often observed when comparing ing, these rhodium(III) species are valuable synthons for our the reactivities and stabilities of analogous compounds of 3d vs. purposes (Scheme 1). 4d/5d metals in a homologous group of elements. Successful Freshly prepared thallium methylcyclopentadienide (Cp′Tl) synthetic routes to cobaltocenium compounds cannot simply was reacted in dry tetrahydrofuran under an atmosphere of ar- Scheme 1. Synthesis of compounds. Eur. J. Inorg. Chem. 2020, 1300–1310 www.eurjic.org 1301 © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim EurJIC Full Paper European Journal of Inorganic Chemistry doi.org/10.1002/ejic.202000071 gon with chlorido(cyclooctadiene)rhodium(I) dimer (1) to afford carboxylato ligand, similar to its analogous cobalt congener.[2d] (cyclooctadiene)(methylcyclopentadienyl)Rh(I) (2) as an air-sta- Derivatives of monocarboxylic acid hexafluoridophosphate (8) ble yellow oil. Without further purification, oxidation of 2 with were synthesized by standard organic transformations. Quanti- diiodine gave di(methylcyclopentadienyl)-dirhodium(III) tetra- tative conversion of 8 to rhodoceniumcarboxylic acid chloride iodide (3) in a satisfactory overall yield of 67 %. Reaction of 3 hexafluoridophosphate (10) was performed in refluxing thionyl with two equivalents of sodium cyclopentadienide (NaCp) fol- chloride, followed by esterification with methanol to methoxy- lowed by precipitation of the product with aqueous hexafluoro- carbonylrhodocenium hexafluoridophosphate (11) or amide phosphoric acid afforded methylrhodocenium hexafluorido- formation with aqueous ammonia to rhodoceniumcarboxylic phosphate (4) in 77 % isolated yield as an air-stable white acid amide hexafluoridophosphate (12), respectively. A major powder. In a similar manner, trimethylsilylrhodocenium hexa- target compound of this work was aminorhodocenium hexa- fluoridophosphate (7) was obtained from 1 by treatment with fluoridophosphate (15), due to the high synthetic potential of lithium trimethylsilylcyclopentadenide via the silylated Rh(I)/ the amino functionality. In analogy to cobaltocenium chemis- Rh(III) complexes 5/6 in 66 % overall yield. Starting from lithium try,[1b,4] the azide derivative 13 was prepared from 10 with trimethylstannylcyclopentadienide, an analogous reaction se- sodium azide. Conversion of compound 13 to 15 in refluxing quence was performed aiming at trimethylstannylrhodocenium concentrated sulfuric acid by classic Curtius rearrangement hexafluoridophosphate,