Unveiling Temperature-Dependent Ligand Gating in Atomically Precise Pd4S Catalysts for Selective Alkyne Hydrogenation.

The dynamic behavior of ligands on the surface of metal nanoclusters significantly influences catalytic activity and reaction performance. However, further progress has been impeded by the inherent instability of homogeneous catalysts and the limitations of available characterization techniques. Here, we developed a synthetic strategy based on the slow release of a sulfur source, successfully preparing a sulfur-stabilized, atomically precise [Pd4S(dtbbpy)4(μ-CO)2]2+ ([Pd4S]2+) nanocluster (dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This cluster serves as a stable model catalyst for the homogeneous selective hydrogenation of alkynes. Experimental results revealed that, owing to differences in steric hindrance and adsorption energy among various alkynes, the [Pd4S]2+ exhibits differentiated selectivity toward substrates such as phenylacetylene, 2-octyne, and 2,5-dimethyl-3-hexyne-2,5-diol. Combined experimental and density functional theory (DFT) studies indicate that the alkyne substrate accesses the active site by competitively displacing the CO ligand and undergoes activation on the [Pd4S]2+ center via a side-on adsorption mode. Variable-temperature NMR and molecular dynamics (MD) simulations uncovered a temperature-dependent dynamic behavior of the [Pd4S]2+ surface ligands, wherein the amplitude of ligand motion varies with temperature, thereby gating substrate access to the active center. Based on these findings, we achieved precise control over the reactivity and selective catalysis of alkynes with different steric profiles through temperature regulation.