Kinetics and mechanism of oxygen atom transfer from nitro compounds mediated by nickel(0) complexes

Article abstract of DOI:10.1021/ic50203a050

(Phosphine)nickel(0) complexes NiL₄ react readily with a variety of aliphatic and aromatic nitro compounds RNO₂ to afford the corresponding nitrosonickel(0) complexes (RNO)NiL₂ and the phosphine oxide in high yields. Kinetic studies show that the coordinatively unsaturated NiL₃, formed by phosphine dissociation, is involved in the rate-limiting reaction with RNO₂. The polar effects of the nitro compounds as measured by the Taft σ* parameters for alkyl groups and the Hammett σ constants for aryl groups correlate directly with their reactivity in oxygen atom transfer. A rate-limiting transition state which is highly polar is also indicated by the linear relationship between the second-order rate constants (i.e., log k_e) and the electrochemical reduction potential E of the nitro compound with a slope of 7, closely approaching the theoretical limit predicted by Marcus' theory for outer-sphere electron transfer to afford the ion pair [L₃Ni^I,RNO₂^-·]. Collapse of such a species to a cyclic intermediate would allow the direct elimination of phosphine oxide and concomitant formation of the nitrosonickel(0) complex.