DFT Study of Co−C Bond Cleavage in the Neutral and One-Electron-Reduced Alkyl−Cobalt(III) Phthalocyanines

Density functional theory (DFT) has been applied to the analysis of the structural and electronic properties of the alkyl−cobalt(III) phthalocyanine complexes, [CoIIIPc]−R (Pc = phthalocyanine, R = Me or Et), and their pyridine adducts. The BP86/6-31G(d) level of theory shows good reliability for the optimized axial bond lengths and bond dissociation energies (BDEs). The mechanism of the reductive cleavage was probed for the [CoIIIPc]−Me complex which is known as a highly effective methyl group donor. In the present analysis, which follows a recent study on the reductive Co−C bond cleavage in methylcobalamin (J. Phys. Chem. B 2007, 111, 7638−7645), it is demonstrated that addition of an electron and formation of the π-anion radical [CoIII(Pc)]−Me- significantly lowers the energetic barrier required for homolytic Co−C bond dissociation. Such BDE lowering in [CoIII(Pc)]−Me- arises from the involvement of two electronic states:  upon electron addition, a quasi-degenerate π*Pc state is initially formed, but when the cobalt−carbon bond is stretched, the unpaired electron moves to a σ*Co-C state and the final cleavage involves the three-electron (σ)2(σ*) bond. As in corrin complexes, the π*Pc−σ*Co-C states crossing does not take place at the equilibrium geometry of [CoIII(Pc)]−Me- but only when the Co−C bond is stretched to ∼2.3 Å. The DFT computed Co−C BDE of 23.3 kcal/mol in the one-electron-reduced phthalocyanine species, [CoIII(Pc)]−Me-, is lowered by ∼37% compared to the neutral Py−[CoIIIPc]−Me complex where BDE = 36.8 kcal/mol. A similar comparison for the corrin-containing complexes shows that a DFT computed BDE of 20.4 kcal/mol for [CoIII(corrin)]−Me leads to ∼45% bond strength reduction, in comparison to 37.0 kcal/mol for Im−[CoIII(corrin)]−Me+. These results suggest some preference by the alkylcorrinoids for the reductive cleavage mechanism.