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.