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

A cobalt(II) fluoride complex, [(LCo)-Co-tBu(mu-F)](2) [L-tBu = 2,2,6,6-tetramethyl-3,5-bis(2,6-diisopropylphenylimido)-hept-4-yl], was synthesized from (LCo)-Co-tBu using Me3SnF via homolytic cleavage of the Sn-F bond. (LCo)-Co-tBu also performed the overall binuclear oxidative addition of fluorinated arenes to give [(LCo)-Co-tBu(mu-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(1), followed by rapid reduction of the cobalt(III) aryl fluoride intermediate by a second molecule of (LCo)-Co-tBu. [(LCo)-Co-tBu(mu-F)](2) also reacts with Et3SiH to give the hydride complex [(LCo)-Co-tBu(mu-H)](2). This hydride complex has low reactivity toward alkenes and N-2, in contrast to an earlier report.