Reactivity of electrochemically generated rhenium (II) tricarbonyl α-diimine complexes:: A reinvestigation of the oxidation of luminescent Re(CO)3(α-diimine)Cl and related compounds

The oxidative electrochemistry of luminescent rhenium (I) complexes of the type Re(CO)(3)(LL)Cl, 1, and Re(CO)(3)(LL)Br, 2, where LL is an alpha-diimine, was re-examined in acetonitrile. These compounds undergo metal-based one-electron oxidations, the products of which undergo rapid chemical reaction. Cyclic voltammetry results imply that the electrogenerated rhenium (II) species 1(+) and 2(+) disproportionate, yielding [Re(CO)(3)(LL)(CH3CN)](+), 7, and additional products. Double potential step chronocoulometry, experiments confirm that 1(+) and 2(+) react via second-order processes and, furthermore, indicate that the rate of disproportionation is influenced by the basicity and steric requirements of the alpha-diimine ligands. The simultaneous generation of rhenium (I) and (III) carbonyl products was detected upon the bulk oxidation of 1 using infrared spectroelectrochemistry, The rhenium (III) products are assigned as [Re(CO)(3)(LL)Cl-2](+), 5; an inner-sphere electron-transfer mechanism of the disproportionation is proposed on the basis of the apparent chloride transfer. Chemically irreversible two-electron reduction of 5 yields 1 and Cl-. No direct spectroscopic evidence was obtained for the generation of rhenium (III) tricarbonyl bromide disproportionation products, [Re(CO)(3)(LL)Br-2](+), 6; this is attributed to their relatively rapid decomposition to 7 and dibromine. In addition, the 17-electron radical cations, 7(+), were successfully characterized using infrared spectroelectrochemistry.