On the Question of Redox-Induced Haptotropic Rearrangements in the Electrochemical Reduction of η6-Triolefin Chromium Complexes Having Nonaromatic Ligands

The electrochemical reduction of two types of Cr(η6-triolefin)(CO)3 complexes has been studied by voltammetry and by electrolysis. In contrast to Cr(η6-arene)(CO)3 compounds, the bicyclic ligand in Cr(η4:η2-C9H8Ph2)(CO)3 (1a) and the fulvene-type ligand in Cr(η6-C5H4CPh2)(CO)3 (2) are nonaromatic and display two separate one-electron reductions to the nominally 19e and 20e complexes:  E1/2(1) = −2.00 V and E1/2(2) = −2.20 V for 1a, E1/2(1) = −1.40 V and E1/2(2) = −2.10 V for 2 in THF/0.1 M [NBu4][PF6] (potentials vs ferrocene/ferrocenium). On the basis of the IR and ESR data of 1a- and on the redox behavior when one or more phenyl groups in 1a are replaced by a SiMe3 group, 1a- is viewed as a delocalized metal−ligand radical that is likely to retain the η4:η2 hapticity of the bicyclic ligand. In contrast, an η6/η5 haptotropic rearrangement is very fast and perhaps concomitant with the 2/2- electron-transfer process. The rearrangement lends thermodynamic stability to the monoanion 2-, manifested as an incr...