Characterization of an activated iridium water splitting catalyst using infrared photodissociation of H ₂ tagged ions

We report the vibrational predissociation spectra of two related organometallic half-sandwich iridium species which have been recently reported as activated intermediates in the context of homogenous water oxidation. These compounds are extracted from solution into a cryogenic photofragmentation mass spectrometer and "tagged" with weakly bound H ₂ molecules that do not significantly perturb the intrinsic structures of the ions. The resulting spectra display very sharp (∼5 cm ^-1), well-resolved bands that provide a stringent test for electronic structure calculations, and are accurately recovered by harmonic predictions for the bare species. The spectra reveal subtle distortions of the ligand structure when a solvent molecule (acetonitrile) is directly coordinated with the metal center.