Pd-phosphite and Pd-Pd tetrazolyl paddlewheel complexes as catalysts for CO₂ hydrogenation

Hydrogenation of CO₂ to products such as formic acid has gained attention in recent times as one of the means of addressing the environmental impact of CO₂ as well as H₂ storage. Herein we report the synthesis of a tetrazolyl ligands bearing P–S bonds which undergoes a P–S heterolysis during complexation with [PdCl₂(MeCN)₂] resulting in the accidental synthesis of palladium-tetrazolyl paddlewheel complexes and corresponding palladium phosphite complexes with P-OH bonds. This P–S heterolysis depends on the nature of substituents present on the phosphorus atom present in the ligand since P^N bidentate coordination is possible. These palladium complexes were then evaluated as precatalysts for CO₂ hydrogenation where TONs of around 564 were observed. Presence of proton responsive P-OH groups contribute towards catalytic activity as a result of an assisted H₂ heterolysis. A plausible mechanism of action indicates bicarbonate coordinates to Pd followed by elimination of water which subsequently results in the generation of formate.