Pentamethylcyclopentadienyl Transition-Metal Complexes. Electrochemistry of Transition-Metal π-Complexes. 7. Cyclopentadienyl(arene)cobalt Cations: Preparation, Electrochemical Reduction, and Spectroscopic Investigation of the Paramagnetic d⁷Monocations

A novel halogen/arene exchange reaction starting from [Co(pmcp)X₂]₂(pmcp = η⁵-1,2,3,4,5-pentamethylcyclopentadienyl, X = Cl, Br) makes accessible the Co(pmcp)(arene)²⁺dications. A series of these sandwich cations substituted to a varying degree by methyl groups in either ligand ring was prepared. Electrochemical (cyclic voltammetry and polarography at the DME) reduction of Co(cp)(arene)²⁺and Co(pmcp)(arene)²⁺(cp = η⁵-cyclopentadienyl, pmcp = η⁵-pentamethylcyclopentadienyl; arene: bz = η⁶-benzene, mes = η⁶-mesitylene, pmbz = η⁶-pentamethylbenzene, and hmbz = η⁶-hexamethylbenzene) salts (BF₄⁻, PF₆⁻) in propylene carbonate revealed two reversible one-electron reduction steps of nearly constant separation for each compound (+0.4 to −0.12 V and −0.56 to −1.1 V vs. SCE, respectively). These results are compared to the reduction potentials determined for the two-step reduction of Co(hmbz)₂²⁺in the same solvent and to potentials for related sandwich systems in the literature. A regular displacement of both reduction potentials with the number and the position of the methyl substituents at either ring on comparison with the results of an INDO-SCF calculation yields a relation between the electrochemical substituent effect and the bonding of the ligand. Electrochemical and chemical (S₂O₄²⁻, Co(cp)₂) reduction yielded some of the sandwich complexes as the monocations, constituting a novel series of d⁷sandwich systems. Solution shifts from ¹H NMR spectra of the monocations show a pronounced downfield shift of the methyl and a large upfield shift of the ring protons in line with established spin delocalization mechanisms. Low-temperature (4 K) EPR spectra of Co(hmbz)₂²⁺and Co(pmcp)(hmbz)⁺in matrices and frozen solution show orthorhombic distortions much larger than what has been observed hitherto in comparable systems. There is evidence for a considerable degree of covalent bonding in these molecules, similar to the situation in cobaltocene.