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, [HIr I (CO)(PPh 3 ) 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.