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 α-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 α-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)Cl2]+, 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)Br2]+, 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