As an extension of our continuing work in metallocenium chemistry, we report here on new functionalized rhodocenium salts. In contrast to isoelectronic cobaltocenium compounds, rhodium as a 4d metal allows synthetic routes via prefunctionalized cyclopentadienyl half-sandwich precursors, thereby facilitating access to monofunctionalized rhodocenium salts containing substituents comprising methyl, trimethylsilyl, carboxylate and carboxylate ester as well as amide derivatives. Synthetic aspects, scope and limitations, as well as spectroscopic ( 1 H/ 13 C-NMR, IR, HR-MS), and structural (XRD) properties IntroductionAfter almost 70 years of research in metallocene chemistry, still relatively little is known about cobaltocenium, rhodocenium and iridocenium salts that are isoelectronic and as stable and redox responsive as their ubiquitous ferrocene congeners. The main reason for the underdeveloped chemistry of these metallocenium salts is their intrinsic cationic nature, making it quite difficult to access functionalized derivatives by standard chemical reactions developed for arenes. However, in recent years [a] mers and subsequently polymerized to the corresponding sidechain functionalized redox-responsive polymers.In this contribution, we aim at developing rhodocenium chemistry a bit further, in part based on our experience in cobaltocenium chemistry, [1,2] and disclose here a first chemoselective synthesis of rhodocenium monocarboxylic acid hexafluoridophosphate which is a key synthon for other monosubstituted rhodocenium salts accessible by standard carboxylic acid reactivity. The structural, spectroscopic and electrochemical properties of these new rhodocenium derivatives are compared with those of their cobaltocenium congeners. Results and Discussion SynthesisIn general, rhodocenium chemistry is somewhat different from cobaltocenium chemistry, as is often observed when comparing the reactivities and stabilities of analogous compounds of 3d vs. 4d/5d metals in a homologous group of elements. Successful synthetic routes to cobaltocenium compounds cannot simply Scheme 1. Synthesis of compounds.

Thermal or photochemical metal-centered cycloaddition reactions of azidocobaltocenium hexafluoridophosphate or azidoferrocene with (cyclooctadiene)(cyclopentadienyl)cobalt(I) afforded the first metallocenyl-substituted tetrazene cyclopentadienyl cobalt complexes together with azocobaltocenium or azoferrocene as side products. The trimetallic CpCo compounds are highly conjugated, colored, and redox-active metallo-aromatic compounds, as shown by their spectroscopic, structural, and electrochemical properties. The CpCo-tetrazenido complex with two terminally appended cobaltocene units catalyzes electrochemical proton reduction from acetic acid at a mild overpotential (0.35 V). Replacing cobaltocene with ferrocene moieties rendered the complex inactive toward catalysis.

In this communication we report a convenient, as short as possible synthesis of aminocobaltocenium hexafluoridophosphate, a very useful compound for further functionalization in cobaltocenium chemistry. Via vicarious nucleophilic substitution of hydrogen of cobaltocenium hexafluoridophosphate with 1,1,1-trimethylhydrazinium iodide as nucleophile bearing its own leaving group, a one-step amination of cobaltocenium in 50% isolated yield is possible, a major improvement over the standard multistep procedure involving common Curtius rearrangement chemistry.

A series of non-symmetric 1,3-di(alkyloxy)imidazolium salts were synthesized by stepwise alkylation of 1-hydroxyimidazole-3-oxide. The quaternary salts were subsequently functionalized by bromination in 2-position. A 2-azidoimidazolium salt and an imidazoline-2-thione were prepared exemplarily. Crystal structures of two 2-bromo-1-alkyloxy-3-methyloxyimidazolium tribromides and a mercury(II)-thione complex have been determined by X-ray diffraction.

Two asymmetrically substituted cobaltocenium carb­oxy­lic acid compounds were synthesized and their crystal structures determined. Both crystallize as hydrates and exhibit an extended hydrogen-bonding network.