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.