C(sp3)–F Bond Activation and Hydrodefluorination of the CF3 Group Catalyzed by a Nickel(II) Hydride Complex: Theoretical Insight into the Mechanism with a Spin-State Change and Two Ion-Pair Intermediates

Hydrodefluorination of trifluoromethylstyrene has been achieved recently through C(sp3)–F bond cleavage using a nickel(II) hydride iso-PmBox complex NiIIH (iso-PmBox = NNN-pincer ligand) as a catalyst. CCSD(T) and DFT calculations unveiled a unique reaction mechanism and a new catalytic function of NiIIH. This hydrodefluorination occurs via unexpected ion-pair intermediates through a two-state reactivity mechanism, which is unusual in the Ni case. The first step is hydride transfer from NiIIH to trifluoromethylstyrene to afford an ion-pair adduct composed of a cationic nickel(II) complex and an α-alkyl anion on the closed-shell singlet state. Concerted styrene insertion into the NiII–H bond hardly occurs because of the rigidity of the planar iso-PmBox ligand. The next step is single-electron transfer from the α-alkyl anion to the nickel(II) complex with a spin-state change to the triplet state. After formation of a nickel(II) alkyl complex, the C(sp3)–F bond cleavage occurs on the triplet state to yield a gem-difluoromethylstyrene (product) as a product with the formation of a nickel(II) fluoro complex NiIIF. This C–F bond cleavage hardly occurs on the singlet state. The active species is regenerated via fluoride transfer from NiIIF to phenylsilane to afford an ion-pair adduct composed of a cationic nickel(II) complex and an anionic hypervalent Si species, and then a H atom transfers as a hydride from the hypervalent Si species to the nickel(II) complex with a spin-state change from the triplet state to the singlet state. The rate-determining step is the hydride transfer from NiIIH to styrene. The palladium analogue PdIIH cannot catalyze this reaction because of the difficulty in spin-state change. Careful analysis of computational results reveals that the success of this difficult reaction is attributable to the use of the spin-flexible Ni element and the rigid iso-PmBox ligand.