A Heterogeneous Metal-Ligand Bifunctional Catalyst for Chemoselective Hydrogenation of α,β-Unsaturated Aldehydes.

The concept of metal-ligand bifunctional catalysts has been extensively explored in homogeneous catalysis, yet it has not been reported in conventional heterogeneous catalysis. Inspired by the outer-sphere hydrogenation mechanism in homogeneous metal-ligand bifunctional catalysts, we here find that an Ir single-atom catalyst with a predominantly Ir1-P4 coordination structure behaves as a heterogeneous metal-ligand bifunctional catalyst exclusively favoring the hydrogenation of the C═O bond over the C═C bond in α,β-unsaturated aldehydes─an industrially important but highly challenging transformation. The hydrogenation process is revealed to proceed through a Noyori-type transition state during the 1,2-addition pathway at the Ir-P bifunctional sites, as evidenced by combining the in situ two-dimensional 1H-1H spin diffusion and 1H-31P heteronuclear correlation nuclear magnetic resonance spectroscopy, apparent reaction order test, kinetic isotope effect analysis, and density functional theory calculations. This work demonstrates a new type of heterogeneous metal-ligand bifunctional catalyst for the chemoselective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols and sheds light on the hydrogenation mechanism.