The Direct Oxidative Addition of O2 to a Mononuclear Cr(I) Complex Is Spin Forbidden

Mononuclear chromium­(I) alkyne complex (i-Pr2Ph)2nacnacCr­(η2-C2(SiMe3)2) (1) reacts rapidly with dioxygen to yield chromium­(V) dioxo species (i-Pr2Ph)2nacnacCr­(O)2 (2). The mechanism of this oxygen cleavage has been studied experimentally and computationally. Isotope labeling studies rule out a direct four-electron oxidative addition of O2 to one chromium atom, which involves a spin-forbidden transformation. Instead, the reaction likely proceeds via an unsymmetric binuclear chromium bis­(μ-oxo) complex. The latter has been independently prepared and structurally characterized. Its reactivity with O2 is consistent with the proposed mechanism.