Lawrence Berkeley National Laboratory
Stabilization and Structure of the cis Tautomer of a Free-Base Porphyrin
- Author(s): Thomas, KE
- McCormick, LJ
- Vazquez-Lima, H
- Ghosh, A
- et al.
Published Web Locationhttps://doi.org/10.1002/anie.201701965
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Single-crystal X-ray analysis of the β-heptakis(trifluoromethyl)-meso-tetrakis(p-fluorophenyl)porphyrin, H2[(CF3)7TpFPP], has revealed the first example of a stable cis tautomer of a free-base porphyrin, the long-postulated intermediate of porphyrin tautomerism. The stability of the unique molecule appears to reflect a dual origin: a strongly saddled porphyrin skeleton, which alleviates electrostatic repulsion between the two NH protons, and two polarization-enhanced, transannular N−H⋅⋅⋅O−H⋅⋅⋅N hydrogen bond chains, each involving a molecule of water. DFT calculations suggest that the observed tautomer has a lower energy than the alternative, doubly hydrated trans tautomer by some 8.3 kcal mol−1. A fascinating prospect thus exists that H2[(CF3)7TpFPP]⋅2 H2O and cognate structures may act as supramolecular synthons, which, given their chirality, may even be amenable to resolution into optically pure enantiomers.