Mechanistic Study of Oxy Insertion into Nickel–Carbon Bonds with Nitrous Oxide

Abstract

Transition-metal-mediated oxy insertion into metal–carbon bonds is useful for the development of catalytic cycles for selective hydrocarbon oxidation. However, there are few bona fide examples of net oxy insertion with transition-metal complexes. An extremely rare example of a 3d metal mediating oxy insertion into metal–carbon bonds is a series of NiII alkyl complexes reacting with nitrous oxide (N2O) reported by Hillhouse and co-workers; however, the mechanism was never fully elucidated. A computational study has been performed on bipyridyl nickel metallacycles that form nickel alkoxides upon reaction with N2O to attain insight into future catalyst design for oxygen atom transfer reactions. Two possible mechanisms are explored. Of the two pathways, the computations suggest that the preferred mechanism proceeds through a Ni–oxyl intermediate followed by alkyl migration of the nickel–carbon bond to form an alkoxide. Oxyl formation was found to be the rate-determining step, with a free energy barrier of 29.4 kcal/mol for bpyNiII(cyclo-(CH2)4). Complexes that contain sp2-hybridized molecules at the β-carbon site within the metallacycle ring do not undergo oxy insertion due to elevated barriers. While exploring insertion with another oxidant, namely pyridine N-oxide, we found that N2O is critical for net oxy insertion with this complex due to the substantial thermodynamic advantage of N2 expulsion. Reaction with pyridine N-oxide necessitated expulsion of a “worse” leaving group, resulting in much higher barriers (ΔG⧧ = 49.7 kcal/mol) for the oxyl formation step.