Redox-induced arene hapticity change in a mixed-sandwich rhodium(III/II/I) system: an illustrative mechanistic application of electron-exchange kinetics

Abstract

Abstract : Rate constants for homogeneous self exchange, kh(ex), and electrochemical exchange, ke(ex), have been measured for the first and second reduction steps of (eta 5-c5Me5) (eta 6-C6-Me6) M2+, where M = Rh and Co, in order to assess the manner in which the eta 6 yields eta 4 arene hapticity change observed upon formation of the Rh(I) two-electron reduction product is coupled with electron transfer. The analogous Co(III) / (II) / (I), mixed- sandwich system was chosen for comparison, because, unlike Rh(III)/(II)/(I), the arene retains the eta 6 configuration upon reduction. The kh(ex) values were evaluated in acetone, acetonitrile, and benzonitrile by utilizing the proton NMR line-broadening technique, and the ke(ex) values were obtained in these solvents and also nitrobenzene and propylene carbonate by using phase-selective ac voltammetry at an annealed gold electrode. The corresponding variations in kh(ex) and kh(ex) with the redox couple are shown to be uniformly consistent with the expectations of Marcus theory, indicating that the electrochemical as well as homogeneous-phase exchange kinetics refer to outer-sphere pathways, as desired. Both the Rh(III) / (II) and Rh(II) / (I) couples exhibit smaller rate constants in a given solvent than the corresponding cobalt systems; while these differences are mild for the former couple, the rate constants for Rh(II) / (I) are substantially smaller than for the other reactions.