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.

We report here an electrochemical and optical spectroscopic study of new Fe(IV) and Mn(IV) meso-triarylcorrole complexes. The complexes studied are three Fe(IV)Cl, three Mn(IV)Cl, and three dimeric Fe(IV)OFe(IV) meso-tris(p-X-phenyl)corrole complexes, where X = CH3, H, and CF3. The first oxidation potentials of the Fe(IV)Cl and Mn(IV)Cl corrole complexes are considerably higher than those of the corresponding Fe(IV) corrole μ-oxo dimers, suggesting that the corrole ligands in the chloride complexes are already oxidized to a radical-like state. This is consistent with the suggestion by Walker and co-workers (ref ) that the iron center in an (octaalkylcorrolato)FeIVCl complex is best described as intermediate spin (S = 3/2) and that it is antiferromagnetically coupled to a corrole π-radical. We have attempted to clarify the nature of this antiferromagnetic coupling by means of DFT calculations and propose that it results from an metal(d z 2)-corrole(“b1”) orbital interaction. In contrast, the corrole ligand in the Fe(IV) corrole μ-oxo dimers does not seem to have radical character. The optical spectra of the Fe(IV)Cl and Mn(IV)Cl corrole derivatives exhibit distinctive split Soret bands, one arm of which is strongly substituent sensitive. This behavior contrasts with that of free-base corroles and porphyrins and of typical metalloporphyrins whose optical spectra are relatively substituent-insensitive. We qualitatively assign this substituent-sensitive feature to a transition with significant ligand-to-metal charge-transfer character.

It is commonly believed that nonplanar distortions of the porphyrin skeleton bring about significant red shifts in the electronic absorption spectra. However, based on absorption spectroscopy and semiempirical AM1 studies of meso-tetrakis(perfluoroalkyl)porphyrins, DiMagno and co-workers have challenged this notion.Here we present density functional theory based configuration interaction singles calculations which uphold the traditional view. Both ruffling and saddling deformations do bring about significant red shifts in the electronic spectra. Nonplanarity-induced destabilization of the porphyrin HOMOs appears to be the principal cause of these red shifts.