Ligand-Based Reduction and Electron-Transfer-Induced Tub-to-Chair Isomerization of Cyclooctatetraene Ligand in Rh(η5-C5Ph5)(η4-C8H8)

Abstract

The electrochemical reduction of Rh(η5-C5Ph5)(η4-C8H8), 1, has been studied in THF/0.1 M [NBu4]A, where A = [PF6]- or [CF3SO3]-. Cyclic, differential pulse, and square-wave voltammetry establish that 1 exists as two isomers having tub-shaped (1,5-bonded) or chair-shaped (1,3-bonded) cyclooctatetraene (COT) ligands. The former is the major isomer at room temperature. One-electron reduction of 1,5-1 (Epc ≈ −2.7 V vs ferrocene) results in rapid isomerization to the chair isomer. Electrolysis produces [1,3-1]- (E1/2 = −2.37 V), which has ESR spectra consistent with a SOMO that is largely (COT) ligand-based. Equilibrium and rate constants are estimated for a square scheme describing the combined electron-transfer/isomerization sequence. The tub isomer is more highly favored at room temperature for Rh(η5-C5Ph5)(COT) than for the previously studied Co(η5-C5H5)(COT). Because the redox behavior of 1 closely parallels that of the cobalt analogues, the present result strengthens evidence for the decidedly different electronic structures of nominally isoelectronic Co-group versus Ni-group η4-COT compounds. Formally d9 complexes of the former are classified as highly delocalized “18 + δ” complexes, whereas the latter are more traditional 19-electron systems.