Tailoring Pt-Cu Alloy Surfaces to Enhance OH Adsorption for Stereoselective Directed Hydrogenation.

Bimetallic catalysts featuring noble metals alloyed with non-noble metals have been widely employed to tune the catalytic reactivity and selectivity of hydrogenation reactions. The origins of catalytic enhancement for bimetallic nanoparticles compared to their monometallic counterparts often stem from an array of convoluted structural factors including electronic and geometric modifications to the active site ensemble as well as cooperativity arising from the presence of two metal atoms. In this work, we utilize colloidal synthesis of Pt-Cu alloy nanoparticles coupled to chemical and thermal ligand removal methods to tailor the local surface ensemble and oxidation state of the bimetallic catalyst. In doing so, we aim to elucidate the structural and mechanistic origins of stereoselectivity for the OH-directed olefin hydrogenation reaction, a reaction that has important implications in pharmaceutical synthesis. Through detailed surface characterization using CO DRIFTS, kinetic studies on directing and non-directing substrates, and computational modeling, we show that bidentate adsorption of the OH directing group to the Cu site and the olefin to the Pt site accelerates the rate of the directed reaction. Simultaneously, dilution of the Pt ensemble with Cu atoms suppresses the rate of the undirected reaction. These two structural factors combine in a Pt3Cu alloy catalyst to enable hydrogenation turnover frequencies of ~104 h-1 while maintaining a 92:8 diastereomeric ratio for the directed:undirected product. Impressively, the Pt3Cu alloy achieves hydrogenation rates comparable to monometallic Pt while dramatically increasing the diastereoselectivity. The ability for Pt-Cu alloys to accelerate the hydrogenation of an olefin proximal to a directing group through OH adsorption could serve as a general strategy toward chemo- and stereoselective transformations of allylic and homoallylic alcohols.