Monomeric gold hydrides for carbon dioxide reduction: ligand effect on the reactivity

We analyzed the ligand electronic effect in the reaction between a [LAu(I)H]^0/- hydride species and CO₂, leading to a coordinated formate [LAu(HCOO)]^0/-. We explored 20 different ligands, such as carbenes, phosphines and others, carefully selected to cover a wide range of electron-donor and -acceptor properties. We included in the study the only ligand, an NHC-coordinated diphosphene, that, thus far, experimentally demonstrated facile and reversible reaction between the monomeric gold(I) hydride and carbon dioxide. We elucidated the previously unknown reaction mechanism, which resulted to be concerted and common to all the ligands: the gold-hydrogen bond attacks the carbon atom of CO₂ with one oxygen atom coordinating to the gold center. A correlation between the ligand σ donor ability, which affects the electron density at the reactive site, and the kinetic activation barriers of the reaction has been found. This systematic study offers useful guidelines for the rational design of new ligands for this reaction, while suggesting a few promising and experimentally accessible potential candidates for the stoichiometric or catalytic CO₂ activation.