Free-Base Corrole Anion

Free-base corroles have long been known to be acidic, readily undergoing deprotonation by mild bases and in polar solvents. The conjugate base, however, has not been structurally characterized until now. Presented here is a first crystal structure of a free-base corrole anion, derived from tris(p-cyanophenyl)corrole, as the tetrabuylammonium salt. The low-temperature (100 K) structure reveals localized hydrogens on a pair of opposite pyrrole nitrogens. DFT calculations identify such a structure as the global minimum but also point to two cis tautomers only 4-7 kcal/mol above the ground state. In terms of free energy, however, the cis tautomers are above or essentially flush with the trans-to-cis barrier so the cis tautomers are unlikely to exist or be observed as true intermediates. Thus, the hydrogen bond within each dipyrrin unit on either side of the molecular pseudo-C-2 axis through C-10 (i.e., between pyrrole rings A and B or between C and D) qualifies as or closely approaches a low-barrier hydrogen bond. Proton migration across the pseudo-C-2 axis entails much higher activation energies >20 kcal/mol, reflecting the relative rigidity of the molecule along the C-1-C-19 pyrrole-pyrrole linkage.

Automated summary

The crystal structure of a free-base corrole anion derived from tris(p-cyanophenyl)corrole has been determined for the first time. The structure exhibits localized hydrogens on a pair of opposite pyrrole nitrogens, indicating the presence of low-barrier hydrogen bonds. This finding has significant implications for understanding the acid-base properties and reaction mechanisms of free-base corroles.