Low-Coordinate Cobalt Fluoride Complexes: Synthesis, Reactions, and Production from C–F Activation Reactions

A cobalt­(II) fluoride complex, [L^tBuCo­(μ-F)]₂ [L^tBu = 2,2,6,6-tetramethyl-3,5-bis­(2,6-diisopropylphenylimido)­hept-4-yl], was synthesized from L^tBuCo using Me₃SnF via homolytic cleavage of the Sn–F bond. L^tBuCo also performed the overall binuclear oxidative addition of fluorinated arenes to give [L^tBuCo­(μ-F)]₂ and a cobalt­(II) aryl complex of the corresponding fluorobenzene substrate in a 1:2 molar ratio. The C–F activation reaction has a first-order rate dependence on both cobalt and fluorobenzene concentrations. The rate is increased by meta-fluoride substituents, and slowed by ortho-fluoride substituents, suggesting electronic and steric influences on the transition state, respectively. The data are most consistent with a mechanism beginning with rate-limiting oxidative addition of the aryl fluoride to cobalt­(I), followed by rapid reduction of the cobalt­(III) aryl fluoride intermediate by a second molecule of L^tBuCo. [L^tBuCo­(μ-F)]₂ also reacts with Et₃SiH to give the hydride complex [L^tBuCo­(μ-H)]₂. This hydride complex has low reactivity toward alkenes and N₂, in contrast to an earlier report.