Pentamethylcyclopentadienyl Transition-Metal Complexes. Electrochemistry of Transition-Metal π-Complexes. 7. Cyclopentadienyl(arene)cobalt Cations: Preparation, Electrochemical Reduction, and Spectroscopic Investigation of the Paramagnetic d < sup > 7 < /sup > Monocations

Abstract

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. © 1984, American Chemical Society. All rights reserved.