Oxidation of syn-dirhodium complexes of cyclohepta- and cyclooctatriene: influence of carbon-hydrogen bond activation in multiple electron-transfer reactions

Abstract

The oxidation of dinuclear complexes Cp{sub 2}Rh{sub 2}({mu}-C{sub 7}H{sub 8}) (3) and Cp{sub 2}Rh{sub 2}({mu}-C{sub 8}H{sub 10}) (6) has been examined in dichloromethane and acetone solutions. Complex 3 undergoes one-electron oxidation to 3{sup +} (E{degrees} = -0.04 V vs SCE), which is further oxidized to highly unstable 3{sup 2+} at E{sub p} = +0.34 V (CH{sub 2}Cl{sub 2}). Bulk electrolysis of 3 gave a mixture of the tropylium complex [Cp{sub 2}Rh{sub 2}({mu}-C{sub 7}H{sub 7})]{sup +} (4) and the {mu}-hyrido complex [Cp{sub 2}Rh{sub 2}({mu}-H)({mu}-C{sub 7}H{sub 8})]{sup +} (5), the relative amounts depending on the electrolysis potential. Complex 6 gave the persistent cation radical 6{sup +} (E{degrees} = +0.14 V) and underwent oxidation to a transient dication (E{degrees} = +0.45 V). Both 6{sup +} and 6{sup 2+} eventually form complex 7, characterized in situ by NMR spectroscopy as [Cp{sub 2}Rh{sub 2}({mu}-(1,2,5,6-{eta}:2-{sigma}, 3-4-{eta})-C{sub 8}H{sub 9})]{sup +}, in which one of the sp{sup 2} ring carbons has lost an H atom and become {sigma} bonded to a Rh atom. Complex 7 was reduced back to 6 electrochemically. The one-electron processes seen for these syn-dinuclear complexes, in contrast with the two-electron processes observed for anti analogues, and the differences are rationalized in terms of themore » type of structural rearrangements available to the one-electron intermediates in the two cases. The one-electron intermediates with syn-dinuclear metals enjoy greater thermodynamic stability compared to their anti counterparts since C-H bond cleavage is required to produce multielectron processes in the former. 36 refs., 4 figs., 2 tabs.« less