Insights into the spin-forbidden reaction between L2Pd0 and molecular oxygen.

The reaction between dioxygen and palladium(0) is a key step in palladium-catalyzed aerobic oxidation reactions. The spin-forbidden reaction between (en)Pd and O2 has been analyzed by spin-unrestricted density functional methods and shown to proceed by a stepwise process involving (1) formation of a triplet eta1-superoxo-PdI adduct, (2) spin-crossover from the triplet to the singlet surface, and (3) collapse of the singlet eta1-superoxo-PdI adduct into an eta2-peroxo-PdII complex. Delocalization of spin density from triplet O2 onto the palladium center in the first step reduces the exchange interaction between the unpaired spins and facilitates crossover from the triplet to singlet surface.