Characterization of the Oxygen Binding Motif in a Ruthenium Water Oxidation Catalyst by Vibrational Spectroscopy

For homogeneous mononuclear ruthenium water oxidation catalysts, the Ru-O-2 complex plays a crucial role in the rate determining step of the catalytic cycle, but the exact nature of this complex is unclear. Herein, the infrared spectra of the [Ru(tpy)(bpy)(O-2)](2+) complex (tpy=2,2:6,2-terpyridine; bpy=2,2-bipyridine) are presented. The complex [Ru(tpy)(bpy)(O-2)](2+), formed by gas-phase reaction of [Ru(tpy)(bpy)](2+) with molecular O-2, was isolated by using mass spectrometry and was directly probed by cryogenic ion IR predissociation spectroscopy. Well-resolved spectral features enable a clear identification of the O-O stretch using O-18(2) substitution. The band frequency and intensity indicate that the O-2 moiety binds to the Rucenter in a side-on, bidentate manner. Comparisons with DFT calculations highlight the shortcomings of the B3LYP functional in properly depicting the Ru-O-2 interaction.