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

A cobalt­(II) fluoride complex, [LtBuCo­(μ-F)]2 [LtBu = 2,2,6,6-tetramethyl-3,5-bis­(2,6-diisopropylphenylimido)­hept-4-yl], was synthesized from LtBuCo using Me3SnF via homolytic cleavage of the Sn–F bond. LtBuCo also performed the overall binuclear oxidative addition of fluorinated arenes to give [LtBuCo­(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 LtBuCo. [LtBuCo­(μ-F)]2 also reacts with Et3SiH to give the hydride complex [LtBuCo­(μ-H)]2. This hydride complex has low reactivity toward alkenes and N2, in contrast to an earlier report.