Electronic Structure of Gallium, Copper, and Nickel Complexes of Corrole. High-Valent Transition Metal Centers versus Noninnocent Ligands

Abstract

Using regular nonlocal density functional theory (DFT) as well as combined DFT and configuration interaction calculations, we have carried out a first theoretical study of the electronic structure of metallocorroles. The valence orbital energy spectra and the calculated electronic absorption spectrum of (Cor)Ga (Cor3- = corrolato), a prototype non-transition-metal corrole, are qualitatively similar to those of a metalloporphyrin such as zinc porphyrin. The "four-orbital model" holds well for corroles. The a2 and b1 HOMOs of (Cor)Ga are crude analogues of the well-known a1u and a2u porphyrin HOMOs, respectively. Thus, as in the case of porphyrins, there are two nearly equienergetic π-cation radical states for corroles. DFT also appears to provide a good description of the stabilization of high-valent transition-metal centers and of ligand noninnocence, two intertwined and central themes in metallocorrole chemistry. The calculated ground state of (Cor)Cu is a diamagnetic d8 Cu(III) state, with Cu(II) π-cation radical states only slightly higher in energy, which faithfully mirrors the experimental scenario. In contrast, there are no known Cu(III) porphyrin complexes. For (Cor)Ni, low-spin Ni(II) π-cation radical states are significantly lower in energy than a Ni(III) state, again consistent with experiment, reflecting the favorable energetics of d8 square planar complexes. The various optimized geometries reveal significant, characteristic structural changes accompanying the formation of A2- and B1-type corrole π-cation radicals. We predict that the resonance Raman spectra of metallocorroles should reflect these structural features and, thereby, assist in the assignment of valence tautomeric states of transition-metal corrole complexes.