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

For homogeneous mononuclear ruthenium water oxidation catalysts, the Ru-O2 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)(O2)](2+) complex (tpy=2,2':6',2''-terpyridine; bpy=2,2'-bipyridine) are presented. The complex [Ru(tpy)(bpy)(O2)](2+), formed by gas-phase reaction of [Ru(tpy)(bpy)](2+) with molecular O2, 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 (18) O2 substitution. The band frequency and intensity indicate that the O2 moiety binds to the Ru center in a side-on, bidentate manner. Comparisons with DFT calculations highlight the shortcomings of the B3LYP functional in properly depicting the Ru-O2 interaction.