Proton-Coupled Electron Transfer from the Hydride Perspective: Resolving Metal Oxidation and Protonation in a Hydridocarbonyl Complex by 2D-IR Spectroelectrochemistry

Proton-coupled electron transfer (PCET) is a fundamental
process
in energy conversion and catalysis, yet direct spectroscopic characterization
of metal hydride intermediates remains challenging. Here, we employ
ultrafast two-dimensional infrared spectroelectrochemistry (2D-IR-SEC)
to investigate the structural and vibrational dynamics of a model
hydridocarbonyl complex, [HIrI(CO)­(PPh3)3] (H1), during sequential oxidation and PCET
reactions. The strong anharmonic coupling between the Ir–H
and Ir­(CO) stretching modes enables simultaneous monitoring
of the metal oxidation state and hydride protonation state in real
time. Spectroelectrochemical studies reveal that the first oxidation
reversibly generates a stable 17-electron species. The second oxidation,
in contrast, triggers irreversible PCET-driven deprotonation. 2D-IR-SEC
spectra of the singly oxidized species show motional narrowing of
the band with a stronger Ir–H character, reflecting altered
solvation dynamics upon oxidation. This work thus establishes 2D-IR-SEC
as a powerful tool for resolving coupled electron and proton transfer
events in metal hydrides, with implications for the design of PCET-mediated
catalysts.