Rhodocenium Monocarboxylic Acid Hexafluoridophosphate and Its Derivatives: Synthesis, Spectroscopy, Structure, and Electrochemistry

Abstract

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 (1H/13C‐NMR, IR, HR‐MS), and structural (XRD) properties are reported. Like the parent rhodocenium ion, all new derivatives undergo two chemically consecutive reductions at potentials that depend on the respective ring substituent. The second reduction competes with a rapid reaction of the corresponding rhodocenes to their 18 VE dimers. Rate constants for this process range from 2 × 103 s–1 to 2.5 × 105 s–1 as estimated from digital simulations of experimental voltammograms. Rhodocenium carboxylic acid (8) constitutes a special case in that proton instead of metal reduction is observed at Pt or Au electrodes.