Biliverdine-Based Metalloradicals:  Sterically Enhanced Noninnocence

This is a first density functional theory survey of transition-metal biliverdines (Blv), where we have chosen to focus on key Mn, Fe, Co, and Cu complexes. According to the calculations, the complexes are invariably noninnocent, featuring Blv^•2- ligand radicals. In this, biliverdine complexes resemble metallocorroles, but the parallels are only approximate. Briefly, metallobiliverdines exhibit a much greater tendency to adopt noninnocent electronic structures than analogous metallocorroles. The O···O nonbonded contacts in biliverdines apparently preclude the formation of short metal−N bonds that, in turn, could stabilize high-valent metal ions. Thus, while most copper corroles (Cor) exhibit diamagnetic Cu^III ground states, copper biliverdines are clearly Cu^IIBlv^•2- species. In the same spirit, while chloroiron corroles are best described as Fe^III(S = ³/₂)Cor^•2-, the analogous biliverdine derivative seems best described as Fe^III(S = ⁵/₂)Blv^•2-, i.e., featuring a high-spin Fe^III center with long (>2.0 Å) Fe−N bond distances. Overall, the results highlight the important role that steric effects may play in modulating the electronic structures and the potentially noninnocent nature of transition-metal complexes.