Molecular structures of free-base corroles: nonplanarity, chirality, and enantiomerization

The molecular structures of free-base corroles are illustrative of a variety of bonded and nonbonded interactions including aromaticity, intra- as well as intermolecular hydrogen bonding, steric interactions among multiple NH hydrogens within a congested central cavity, and the effects of peripheral substituents. Against this backdrop, an X-ray structure of 2,3,7,8,12,13,17,18-octabromo-5,10,15-tris(pentafluorophenyl)corrole, H3[Br8TPFPCor], corresponding to a specific tautomer, has been found to exhibit the strongest nonplanar distortions observed to date for any free-base corrole structure. Two adjacent N-protonated pyrrole rings are tilted with respect to each other by approximately 97.7°, while the remainder of the molecule is comparatively planar. Dispersion-corrected DFT calculations were undertaken to investigate to what extent the strong nonplanar distortions can be attributed to steric effects of the peripheral substituents. For meso-triphenylcorrole, DFT calculations revealed nonplanar distortions that are only marginally less pronounced than those found for H3(Br8TPFPCor). A survey of X-ray structures of sterically unhindered corroles also uncovered additional examples of rather strong nonplanar distortions. Detailed potential energy calculations as a function of different saddling dihedrals also emphasized the softness of the distortions. Because of nonplanar distortions, free-base corrole structures are chiral. For H3[Br8TPFPCor], DFT calculations led to an estimate of 15 kcal/mol (0.67 eV) as the activation barrier for enantiomerization of the free-base structures, which is significantly higher than the barrier for NH tautomerism calculated for this molecule, about 5 kcal/mol (0.2 eV). In summary, steric crowding of the internal NH hydrogens appears to provide the main driving force for nonplanar distortions of meso-triarylcorroles; the presence of additional β-substituents adds marginally to this impetus.