Reactions of Square-Planar d⁸ Pincer Complexes with Oxygen and Hydrogen

Reactions of palladium pincer complexes with molecular oxygen and hydrogen are described for homogeneous epoxidation catalysis. The proposed three-step catalytic cycle utilizes uncommon transformations that are tailored for a late-transition metal pincer complex: O2 insertion into a metal-hydride bond, O-atom transfer, and hydrogenolysis of the resulting metal hydroperoxide. Insertion reactions of O2 into palladium hydride bonds are described and a hydrogen atom abstraction (HAA) mechanism is proposed. Hydrogenolysis reactions of palladium hydroxide and alkoxide species are also described. Mechanistic investigations suggest a four-centered transition state between the palladium hydroxide and molecular hydrogen. Insights from both experimental and computational results are used to suggest optimal catalyst structure for the aforementioned transformations.