Catalytic Hydrogenation Dominated by Concerted Hydrogen Tunneling at Room Temperature.

Tunneling control of chemical reactions is treasured as the third reactivity paradigm, next to kinetic and thermodynamic control. However, reports on the successful observation and mechanistic insight into quantum tunneling in conventional heterogeneous catalysis are limited. By using an atomically dispersed palladium catalyst, we now demonstrate room-temperature catalytic hydrogenation dominated by concerted triple hydrogen tunneling. While a large kinetic isotope effect value of ∼2440 is observed in the benzyl aldehyde hydrogenation when both H2 and solvent (CH3OH) are deuterated, the use of protic solvent is important to achieve enhanced catalysis. Systematic investigations reveal that, with a protic solvent molecule situated between the catalytic site and aldehyde, the formation of a local hydrogen bond network helps to induce the concerted triple hydrogen tunneling, namely, that two protons transfer from the ligand on the catalytic site to the mediated solvent and the oxygen of CO on aldehyde, respectively, and the other transfers from Pd on the catalytic site to the carbon of CO on aldehyde. With the width and height of the potential energy barrier alterable by protic solvents, the hydrogen tunneling probability can be regulated by solvents. Furthermore, far-infrared irradiation is found to enhance the hydrogenation rate.