Dioxygen Activation by Non-Adiabatic Oxidative Addition to a Single Metal Center

A chromium(I) dinitrogen complex reacts rapidly with O-2 to form the mononuclear dioxo complex [Tp(tBu, Me)Cr(V)(O)(2)] (Tp(tBu, Me) = hydrotris(3-tert-butyl-5-methylpyrazolyl) borate), whereas the analogous reaction with sulfur stops at the persulfido complex [Tp(tBu, Me)Cr(III)(S-2)]. The transformation of the putative peroxo intermediate [Tp(tBu, Me)Cr(III)(O-2)] (S=3/2) into [Tp(tBu, Me)Cr(V)(O)(2)] (S=1/2) is spin-forbidden. The minimum-energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex [(Tp(tBu, Me)Cr)(2)(mu-O)(2)] exists, mechanistic experiments suggest that O-2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface.