Ir−Sn Tetrazole Complexes with Proton-Responsive P−OH Groups and an Ir−Ir Tetrazole Complex for CO₂ Hydrogenation

Bimetallic catalytic systems, especially heterogeneous bimetallic catalytic systems, have been shown to be effective catalysts for CO₂ hydrogenation as a result of synergistic effects. Here-in we report the synthesis of bimetallic Ir−Ir, Ir−Sn and a novel five-membered heteroatomic Ir−Sn tetrazole complex. These complexes were obtained as a result of the heterolysis of P−S bond in the ligands during complexation. These complexes were evaluated for catalytic activity in homogeneous CO₂ hydrogenation where they displayed excellent catalytic activity generating formate selectively. The mechanism of action for the Ir−Sn catalysts for CO₂ hydrogenation indicates metal−metal bond breakage prior to formation of catalytically active species. However, this metal−metal bond breakage did not result in the decomposition of the catalysts. In addition, these bimetallic complexes were observed to effectively hydrogenate CO₂ directly, or hydrogenate bicarbonate ions formed in solution during the catalytic cycle. For Ir−Ir catalyst, only one metal center was found to be directly involved in CO₂ hydrogenation with the second iridium center primarily responsible as an electron source.