Electronic structural comparison of the reactions of dioxygen and alkenes with nitrogen-chelated palladium(0).

The reaction of molecular oxygen with palladium(0) centers is a key step in Pd-catalyzed aerobic oxidation reactions. The present study provides a density functional theory (DFT) computational analysis of the mechanism and electronic structural features of the reversible, associative exchange between O(2) and ethylene at an ethylenediamine (en)-coordinated palladium(0) center. Salient features of the mechanism include: (1) the near thermoneutrality of the O(2)-alkene exchange reaction, consistent with experimentally observed reversible exchange between O(2) and alkenes at well-defined Pd centers, (2) end-on activation of triplet O(2) at an apical site of the trigonal Pd(0) center, resulting in formation of a Pd(I)(η(1)-superoxide) species, (3) rearrangement of the Pd(I)(η(1)-superoxide) species into a pseudo-octahedral (en)Pd(η(2)-O(2))(η(2)-C(2)H(4)) species with concomitant crossing from the triplet to singlet energy surfaces, and (4) release of alkene from an axial face of (en)Pd(II)(η(2)-peroxo) with a geometry in which the alkene leaves with an end-on trajectory (involving an interaction of the Pd d(z(2)) and alkene π* orbitals). This study highlights the similar reactivity and reaction pathways of alkenes and O(2) with an electron-rich metal center, despite the different ground-state electronic configurations of these molecules (closed-shell singlet and open-shell triplet, respectively).