PENTAMETHYLCYCLOPENTADIENYL TRANSITION-METAL COMPLEXES. ELECTROCHEMISTRY OF TRANSITION-METAL pi -COMPLEXES. 7. CYCLOPENTADIENYL(ARENE)COBALT CATIONS: PREPARATION, ELECTROCHEMICAL REDUCTION, AND SPECTROSCOPIC INVESTIGATION OF THE PARAMAGNETIC D7 MONOCATIONS.

Article abstract of DOI:10.1021/ja00327a014

A novel halogen/arene exchange reaction starting from left bracket Co(pmco)X//2 right bracket //2 (pmcp equals eta **5-1,2,3,4,5-pentamethylcyclopentadienyl, X equals Cl, Br) makes accessible the Co(pmcp)(arene)**2** plus 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** plus and Co(pmcp)(arene)**2**30 (cp equals eta **5-cyclopentadienyl, pmcp equals eta **5-pentamethylcyclopentadienyl; arene: bz equals eta **6-benzene, mes equals eta **6-mesitylene, pmbz equals eta **5-cyclopentadienyl, and hmbz equals // eta **6-hexamethylbenzene) salts (BF//4** minus , PF//6** minus ) in propylene carbonate revealed two reversible one-electron reduction steps of nearly constant separation for each compound ( plus 0. 4 to minus 0. 12 v and minus 0. 56 to minus 1. 1 vs. SCE respectively). These results are compared to the reduction potentials determined for the two-step reduction of Co(hmbz)//2**2** plus 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.