Monocationic μ‑Diborolyl Triple-Decker Complexes [CpCo(μ-1,3‑C3B2Me5)M(ring)]+: Synthesis, Structures, and Electrochemistry

Cationic triple-decker complexes with a bridging diborolyl ligand, [CpCo­(μ-1,3-C3B2Me5)­M­(ring)]+ (M­(ring) = CoCp (2a), CoCp* (2b), RhCp (3a), RhCp* (3b), IrCp (4a), IrCp* (4b), Ru­(C6H6) (5a), Ru­(p-MeC6H4Pri) (5b), Ru­(C6Me6) (5c), Ru­(η6-cycloheptatriene) (6)), were synthesized by reaction of CpCo­(μ-1,3-C3B2Me5)Tl with [M­(ring)­Hal2]2. The structures of 2aBPh4, 2bPF6, 4aPF6, 5aOTf, and 5cPF6 were determined by X-ray diffraction. The electron-transfer ability of the complexes has been ascertained by electrochemical and spectroelectrochemical techniques. In general, they are able to shuttle reversibly in the sequence 2+/+/0/–, plausibly affording completely delocalized mixed-valence derivatives. DFT calculations revealed structural changes accompanying redox processes and satisfactorily predicted the potentials for the first reduction and first oxidation.