Elucidation of the Wide Range of Reaction Pathways That Accompany the Electrochemical Oxidation of cis,mer-[Mn(CO)(2)(eta(1)-dpm)(eta(2)-dpm)X] (dpm = Ph(2)PCH(2)PPh(2); X = Cl, Br).

Abstract

The electrochemical oxidation of cis,mer-[Mn(CO)(2)(eta(1)-dpm)(eta(2)-dpm)Br] (dpm = Ph(2)PCH(2)PPh(2)), or (cis,mer)(0),()()has been examined in dichloromethane (0.1 M Bu(4)NPF(6)) by voltammetric, bulk electrolytic, in situ and ex situ spectroelectrochemical and simulation techniques. On the voltammetric time scale at 20 degrees C, the neutral 18-electron cis,mer Mn(I) species is oxidized to the corresponding 17-electron cation which at slow scan rates isomerizes to the trans cation. Simulations are consistent with a rate constant of 3.1 +/- 0.3 s(-1) for this isomerization process. Monitoring the reaction by in situ IR spectroscopy at low-temperature enables the identification of the nu(CO) bands of all four species ((cis,mer)(0); (cis,mer)(+); (trans)(0); (trans)(+)) in the resultant square reaction scheme that is operative under these thin layer electrolysis conditions. Additionally, 17-electron cis,fac-[Mn(CO)(2)(eta(1)-dpm)(eta(2)-dpm)Br](+) and its 18-electron (cis,fac)(0) counterpart, generated by a redox-induced catalytic isomerization reaction, are detected and characterized by IR spectroscopy (nu(CO)). Room-temperature bulk oxidative electrolysis experiments reveal that the trans cation, generated in bulk solution from the (cis,mer)(+) and (cis,fac)(+) isomers, slowly ejects bromide with a rate constant of 1.6 x 10(-3) s(-1) to form trans-[Mn(CO)(2)(eta(2)-dpm)(2)](+). The equivalent voltammetry in acetonitrile is complicated by an additional competing kinetic step which is attributed to reaction of this cation with the solvent. However, the major product formed upon oxidation at room temperature is still the trans cation. Less detailed studies on the oxidation of cis,mer-[Mn(CO)(2)(eta(1)-dpm)(eta(2)-dpm)Cl] only show significant differences under conditions of bulk electrolysis after trans-[Mn(CO)(2)(eta(2)-dpm)(2)](2+) is formed via expulsion of Cl(-).