Spatially Confined Construction of Ultrasmall Pd Clusters Within Nitro-Bonded Covalent Organic Frameworks for Efficient Alkyne Semihydrogenation.

Confinement of metal species in porous supports is an effective strategy to optimize hydrogenation performance ascribing to tunable nanopore environments. However, only focusing on the electronic structure modulation for metal species has limited the design of improved catalysts. Herein, spatial confinement strategy is reported for constructing ultrasmall metal clusters in nitro-bonded COF (M@TpPa-NO, M = Pd, Pt, Ru, Rh, Ir). Thereinto, Pd@TpPa-NO can achieve efficient co-catalytic alkyne semi-hydrogenation by the organic nitro units and the Pd clusters, with an outstanding phenylacetylene hydrogenation activity of TOF = 13756 h and a high 94% styrene selectivity under 25 °C and 1 bar H. In situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculations confirm that the H dissociation occurs at Pd clusters and the nitro groups accept spilled H atoms for subsequent semi-hydrogenation. The facile styrene desorption from TpPa-NO support contributes to a high semi-hydrogenation selectivity. This work provides new perspectives for designing efficient catalysts with overcoming the activity-selectivity trade-off in selective hydrogenation reactions.