Iridium and Palladium CO₂Hydrogenation in Water by Catalyst Precursors with Electron-Rich Tetrazole Ligands

Hydrogenation of CO2 to formate serves as a means of H2 storage. By utilizing robust, electron-rich and bulky bidentate tetrazolylpyridyl ligands, we have prepared CO2 hydrogenation iridium(III) and palladium(II) catalyst precursors (3-7) that operate in water as a solvent. In situ 1H NMR spectroscopic data obtained when the hydrogenation catalyst is 5 detect that a hydrido-bridged binuclear iridium intermediate compound is formed prior to the formation of the catalytically active iridium hydride species. This hydrido-bridged iridium intermediate is observable by in situ 1H NMR spectroscopy even after extended periods of storage. In addition, we demonstrate that product formation in the CO2 hydrogenation, catalyzed by these tetrazolylpyridyl iridium catalysts, proceeds via either CO2 hydrogenation or NaHCO3 reduction.