Proton-coupled electron transfer between 4-cyanophenol and photoexcited rhenium(I) complexes with different protonatable sites.

Two rhenium(I) tricarbonyl diimine complexes, one of them with a 2,2'-bipyrazine (bpz) and a pyridine (py) ligand in addition to the carbonyls (Re(bpz)(CO)(3)(py)), and one tricarbonyl complex with a 2,2'-bipyridine (bpy) and a 1,4-pyrazine (pz) ligand (Re(bpy)(CO)(3)(pz)) were synthesized, and their photochemistry with 4-cyanophenol in acetonitrile solution was explored. Metal-to-ligand charge transfer (MLCT) excitation occurs toward the protonatable bpz ligand in the Re(bpz)(CO)(3)(py) complex while in the Re(bpy)(CO)(3)(pz) complex the same type of excitation promotes an electron away from the protonatable pz ligand. This study aimed to explore how this difference in electronic excited-state structure affects the rates and the reaction mechanism for photoinduced proton-coupled electron transfer (PCET) between 4-cyanophenol and the two rhenium(I) complexes. Transient absorption spectroscopy provides clear evidence for PCET reaction products, and significant H/D kinetic isotope effects are observed in some of the luminescence quenching experiments. Concerted proton-electron transfer is likely to play an important role in both cases, but a reaction sequence of proton transfer and electron transfer steps cannot be fully excluded for the 4-cyanophenol/Re(bpz)(CO)(3)(py) reaction couple. Interestingly, the rate constants for bimolecular excited-state quenching are on the same order of magnitude for both rhenium(I) complexes.