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 degrees C in benzene-d(6), 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)(2)Co formed by disproportionation of the (FI)cobalt(I) product following ethane reductive elimination. Addition of excess PMe3 inhibited C(sp(3))-C(sp(3)) 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-d(3), 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.