Competing Pathways in the photo-Proton-Coupled Electron Transfer Reduction of fac-[Re(bpy)(CO)3(4,4′-bpy]+* by Hydroquinone

The emitting metal-to-ligand charge transfer (MLCT) excited state of fac-[ReI(bpy)(CO)3(4,4′-bpy)]+ (1) (bpy is 2,2′-bipyridine, 4,4′-bpy is 4,4′-bipyridine), [ReII(bpy–•)(CO)3(4,4′-bpy)]+*, is reductively quenched by 1,4-hydroquinone (H2Q) in CH3CN at 23 ± 2 °C by competing pathways to give a common electron–proton-transfer intermediate. In one pathway, electron transfer (ET) quenching occurs to give ReI(bpy–•)(CO)3(4,4′-bpy)]0 with k = (1.8 ± 0.2) × 109 M–1 s–1, followed by proton transfer from H2Q to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+. Protonation triggers intramolecular bpy•– → 4,4′-bpyH+ electron transfer. In the second pathway, preassociation occurs between the ground state and H2Q at high concentrations. Subsequent Re → bpy MLCT excitation of the adduct is followed by electron–proton transfer from H2Q in concert with intramolecular bpy•– → 4,4′-bpyH+ electron transfer to give [ReI(bpy)(CO)3(4,4′-bpyH•)]+ with k = (1.0 ± 0.4) × 109 s–1 in 3:1 CH3CN/H2O.