Theoretical investigation on the elusive structure–activity relationship of bioinspired high-valence nickel–halogen complexes in oxidative fluorination reactions

Abstract

Very recently, bioinspired high-valence metal-halogen complexes have been proven to be competent oxidants in the C–H bond activation and heteroatom dihalogenation reactions. However, the structure–activity relationship of such active species and the reaction mechanisms of oxidations mediated by these oxidants are still elusive. In this study, density functional theory (DFT) calculations were performed to systematically study the oxidizing ability of the high-valence Ni^III–X (X = F and Cl) complexes Et₄N[Ni^III(Cl/F)(L)], (1_Cl/F, Et = ethyl, L = N,N‘-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide), such as the reaction mechanism of fluorination of 1,4-cyclohexadiene (CHD) by 1_F in the presence of AgF and the reaction mechanism of difluorination of triphenyl phosphine (PPh₃) by 1_F. All calculated results fit well with the experiments and present new mechanistic findings. The C–H bond activation by the high-valence nickel(III)–halogen complexes was found to proceed via a hydrogen-atom transfer (HAT) mechanism by analysis of the molecular orbitals of the transition states. C–H bond activation by 1_F takes a Ni–F–H angle of ca. 180°, whereas that by 1_Cl takes an angle of ca. 120° on the transition states. These results indicate that the exchange-enhanced reactivity is responsible for the dramatic oxidative difference between these two oxidants. The role of AgF in C–H fluorination of CHD by 1_F is proposed to act as a Lewis acid adduct, AgF-binding Ni(III)–fluorine complex 1_F–Ag–F, which acts both as an oxidant in C–H bond activation and as a fluorine donor in the fluorination step. A cooperative oxidation mechanism involving two 1_F oxidants was proposed for the difluorination of PPh₃ by 1_F. These theoretical findings will enrich the knowledge of high-valence metal-halogen chemistry and play a positive role in the rational design of new catalysts.