DFT studies of trans and cis influences in the homolysis of the Co-C bond in models of the alkylcobalamins.
Résumé
Density functional theory (DFT) calculations (BP86/6-31+G(d,p)) and an analysis of the electron density using Bader's quantum theory of atoms in molecules (QTAIM) are used to explore factors that influence the bond dissociation energy (BDE) of the Co-C bond in models for the cofactor in the coenzyme B12-dependent enzymes. An increase in the basicity of L in [L-Co(III)(corrin)-CH3](n+), L = NH3, NH2(-), and NH(2-), causes an elongation of the trans Co-C bond, but this does not necessarily cause the BDE to decrease. The bond between the metal and the N-donor of L, Co-Nα, usually becomes shorter after Co-C homolysis as the resulting five-coordinate product permits the metal ion to move toward L. This contraction increases with the basicity of L and stabilizes the five-coordinate product. The BDE is found to correlate well with two variables, the basicity of L and the difference in the Co-Nα bond length between the five-coordinate product and the six-coordinate ground state. When L is a naturally occurring amino acid or a model for its metal-coordinating side chain, the BDE is found to be moderately dependent on L and decrease with an increase in the softness of the donor atom of L. Sulfides produce a BDE < 30 kcal mol(-1), whereas neutral alcohol donors produce a stronger Co-C bond with a BDE of 34-35 kcal mol(-1). All other ligands are associated with a trans Co-C bond that is almost invariant in strength and with a BDE of 31-33 kcal mol(-1). Models of the type [H3N-Co(III)(N4)-CH3](n+), where N4 = bis(dimethylglyoxime), porphyrin, corrin, and corrole, show that the nature of the tetraaza equatorial ligand can change BDE values by over 8 kcal mol(-1); the BDE when N4 = bis(dimethylglyoxime) is significantly larger than for the other three systems, among which differences in BDE are quite small (2.4 kcal mol(-1)). The differential stabilization of the five-coordinate product by the shrinking of the Co-Nα bond (in corrin and in corrole) or its elongation (in porphyrin and in bis(dimethylglyoxime)) is an important factor in determining the BDE of these systems. Corrin has the longest and weakest Co-C bond; this, together with a significant contraction of the Co-Nα after homolysis, is likely to be the origin of its relatively low BDE.