Computational study of the double C–Cl bond activation of dichloromethane and phosphine alkylation at [CoCl(PR₃)₃]

Density functional theory calculations have been employed to model the double C–Cl bond activation of CH₂Cl₂ at [CoCl(PR₃)₃] to give [CoCl₃(CH₂PR₃)(PR₃)₂]. Calculations incorporating dichloromethane solution (PCM approach) on a [CoCl(PMe₃)₃] model system showed the two C–Cl cleavage steps to involve different mechanisms. The first C–Cl cleavage step occurs on the triplet surface and proceeds via Cl abstraction with a barrier of 19.1 kcal mol⁻¹. Radical recombination would then give singlet mer, trans-[CoCl₂(CH₂Cl)(PMe₃)₃] with an overall free energy change of +1.8 kcal mol⁻¹. Alternative C–Cl activation processes based on nucleophilic attack by the Co centre at dichloromethane with loss of Cl⁻ have significantly higher barriers. The second C–Cl cleavage occurs via nucleophilic attack of PMe₃ at the CH₂Cl ligand with formation of a new P–C bond and displacement of Cl⁻. This may either occur in an intermolecular fashion (after prior PMe₃ dissociation) or intramolecularly. Both processes have similar barriers of ca. 12 kcal mol⁻¹. The comproportionation of [CoCl₃(CH₂PMe₃)(PMe₃)₂] with [CoCl(PMe₃)₃] to give [CoCl₂(CH₂PMe₃)(PMe₃)], [CoCl₂(PMe₃)₂] and 2 PMe₃ is computed to be strongly exergonic, consistent with the observation of this process in analogous experimental systems.