CO₂ Conversion to formates catalyzed by iridium(III) catalysts precursors with proton responsive OH and NH electron-rich tetrazole ligands

Recent efforts in addressing the environmental problems have involved using CO₂ as a cheap and nontoxic C1 source. Iridium catalysts with bidentate ligands are excellent catalysts for CO₂, especially if these complexes possess proton-responsive OH or NH groups. Here-in we report the synthesis of novel Ir half-sandwich complexes with N^N bidentate tetrazolyl ligands. Serendipitous deprotection of methoxy groups resulted in N^N bidentate ligands bearing OH groups. The complexes were evaluated for CO₂ hydrogenation, for which the roles of steric bulk or the presence of electronic effects influence their catalytic activity in CO₂ hydrogenation. The complexes are highly active for CO₂ hydrogenations with around 4.3 mmol of formate produced. The presence of proton responsive groups on the catalysts was found to steer the mechanistic cycle away from using a bridged Ir-H-Ir intermediate before forming catalytically active species. In addition, these catalysts were found to hydrogenate CO₂ in the presence of bicarbonate ions selectively.