The dissociation of molecular hydrogen, a crucial step in hydrogenation reactions, is not favorable on gold surfaces. The concept of ‘frustrated Lewis pairs’ (FLPs) was introduced a decade ago to explain the activation of small molecules, such as H2, through an ‘encounter complex’ where a Lewis acid (LA) and Lewis base (LB) are in close proximity, but prevented from forming a strong classical adduct. The heterolytic cleavage of H2 occurred at room temperature with FLPs of phosphorus and boron compounds. The concept has expanded rapidly from main group-FLP to metal-based FLP and surface FLP, in which the metal-ligand or metal-support interface promotes the heterolytic cleavage of H2 producing a hydride (delivered to a metal center) and the proton is incorporated into the ligand (e.g., ligand protonation). We recently proposed that heterolytic dissociation of H2 is promoted by amine-ligands adsorbed on gold surfaces. The heterolytic cleavage of H2 contrasts with the homolytic cleavage of H2 commonly observed on metal surfaces of traditional heterogeneous catalysts (platinum group metals), but opens up the possibility to explore less active metals. Based on experimental and theoretical studies we proposed a mechanism based on the heterolytic H2 activation on the gold-ligand interface. We were able to disclose a remarkable promotional effect of N-containing ligands with two heteroatoms as a new ligand-based strategy to boost gold activity. Experimentally, excess of amine (100 eq.) was necessary to achieve reasonable reaction rates. In continuation to these studies, we will present here a screening of phosphorous-based ligands. To our surprise, some P-ligands (mainly alkyl phosphine and alkyl phosphite) were active for alkyne hydrogenation with gold, using only 1 eq. (based on the metal amount), but aryl phosphines were inactive. Mechanistic investigations using DFT have unrevealed the effect of P-ligands, regarding the structural influence (cone angle, buried volume and remote steric hindrance) on the catalytic activity, and demonstrated a novel mechanism for H2 activation on gold catalysis. After that, the versatile protocol was applied for the hydrogenation of many functional groups such as alkyne-to-alkene and epoxide-to-alkene under mild conditions, taking advantage of the high selectivity of gold.

Proposed mechanism for liquid-phase hydrogenations on gold nanoparticle catalysts by activation of H2 with P-ligands

ACS Spring Meeting 2021

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