Proton-Coupled Electron
Transfer between 4-Cyanophenol
and Photoexcited Rhenium(I) Complexes with Different Protonatable
Sites
- Publication date
- Publisher
Abstract
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)<sub>3</sub>(py)]<sup>+</sup>), and
one tricarbonyl complex with a 2,2′-bipyridine (bpy) and a
1,4-pyrazine (pz) ligand ([Re(bpy)(CO)<sub>3</sub>(pz)]<sup>+</sup>) 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)<sub>3</sub>(py)]<sup>+</sup> complex while in the [Re(bpy)(CO)<sub>3</sub>(pz)]<sup>+</sup> 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)<sub>3</sub>(py)]<sup>+</sup> reaction couple. Interestingly, the rate
constants for bimolecular excited-state quenching are on the same
order of magnitude for both rhenium(I) complexes
