C(sp3)–C(sp3) Reductive Elimination from (Phenoxyimine)Cobalt(III)(CH3)2(PMe3)2 Complexes

A series of six-coordinate, idealized octahedral phenoxyimine (FI)–cobalt(III) dimethyl bis(trimethylphosphine) complexes was synthesized and characterized by NMR spectroscopy and single-crystal X-ray diffraction. The thermal stability of the parent Ph-FI complex was evaluated at 60 °C in benzene-d₆, and a 13(1):1 ratio of ethane to methane was observed. The major detectable cobalt product of this reaction was the bis(chelate) cobalt derivative (Ph-FI)₂Co formed by disproportionation of the (FI)cobalt(I) product following ethane reductive elimination. Addition of excess PMe₃ inhibited C(sp³)–C(sp³) reductive elimination, consistent with phosphine dissociation preceding C–C bond formation from a five-coordinate (FI)cobalt(III) dimethyl intermediate. The reductive elimination of substituted (R-FI)cobalt(III) dimethyl bis(trimethylphosphine) compounds was evaluated in acetonitrile-d₃, where ligands bearing electron-donating aniline substituents underwent reductive elimination the fastest and electron-withdrawing substituents the slowest. These data support a buildup of positive charge in the rate-limiting step, consistent with the formation of a more electropositive five-coordinate cobalt center prior to rate-limiting C–C reductive elimination. Attempted synthesis of a cobalt(III) dimethyl complex bearing a sterically hindered FI ligand with a tert-butyl substituent ortho to the phenol led exclusively to the corresponding bis(chelate) cobalt derivative, whose formation was rationalized by steric destabilization of pre–reductive elimination intermediates.