Anti-Spin-Delocalization Effect in Co−C Bond Dissociation Enthalpies

Abstract

The homolytic Co−C bond dissociation enthalpy (BDE) is central to the understanding of organocobalt-mediated reactions in the areas of both bioinorganic chemistry and transition-metal catalysis. However, the determination of the Co−C BDEs still remains a difficult task using either an experimental or theoretical approach. Here we investigate how to use the density functional theory method to accurately calculate the Co−C BDEs by testing a number of functionals. It is found that the recently developed TPSS/LANL2DZ+p method can reproduce 28 experimental Co−C BDEs within a precision of ca. 2.2 kcal/mol. Equipped with this useful tool, we next examined the effects of the in-plane ligands on the Co−C BDEs in a systematic fashion for the first time. It is found that the in-plane ligands can vary the Co−C BDEs by ca. 10 kcal/mol. Across different in-plane ligands the Co−C BDEs are found to exhibit a strong, negative correlation with the spin densities at the cobalt atoms after the homolysis. This observation is not consistent with the conventional chemical intuition that delocalization of the spin of a free radical through the hyperconjugation interactions should stabilize the radical and, thereby, weaken the chemical bond that undergoes homolysis. We name this unexpected finding the anti-spin-delocalization effect. Further analyses of the molecular orbitals and atomic charges indicate that (1) the in-plane ligands can reduce the Co spin density through hyperconjugation with their empty antibonding π* orbitals and (2) the in-plane ligands can also stabilize the Co−C starting material through ionic interactions by attracting electrons from Co. Both the stabilization effects are determined by the electronegativity of the in-plane ligands. Thus the origin for the anti-spin-delocalization effect is proposed to be that the stabilization effect of the in-plane ligands is larger for the starting material than for the radical.