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.