Electrochemical Study of Manganese and Rhenium Arene Complexes (C6R6)M(CO)3+ (R = Me, Et)

An electrochemical study of the reduction or (eta(6)-C6R6)M(CO)(3)(+) (R = Me, Et; M = Mn, Re) in dichloromethane or acetonitrile at platinum electrodes is reported. X-ray structures of the manganese and rhenium C6Et6 complexes show that the metal is well shielded from nucleophilic attack. With rhenium as the metal, a chemically reversible two-electron reduction is observed on the CV time scale to yield ring-slipped (eta(4)-C6R6)Re(CO)(3)(-) complexes, which can be directly observed at room temperature via bulk electrolysis. With manganese, the initial one-electron reduction product can react chemically to yield eta(5)-ring-coupled complexes or can dissociate CO and dimerize to [(C6R6)Mn(CO)(2)](2) or, at low temperatures, undergo a second electron addition to the eta(4)-ring-slipped product. The dimerization reactions can be inhibited by immobilizing the precursor (eta(6)-C6Me6)Mn(CO)(3)(+) onto a glassy carbon electrode and performing the CV in water solvent. Regardless of the metal or the R substituent, the second electron addition is characterized by very slow heterogeneous charge transfer and by an E-2(o) value that is positive of E-1(o). The reactivity of (eta(6)-C6R6)M(CO)(3)(+) toward associative attack by acetonitrile is in the order C6Me6 >> C6Et6 and Re >> Mn. Conversely, the neutral 19-electron radicals obtained by one-electron addition react by CO dissociation in the opposite order: C6Et6 >> C6Me6 and Mn >> Re.