Angewandte Chemie - International Edition
Angewandte Chemie - International Edition
5 years ago

Stabilization and Structure of the cis Tautomer of a Free-Base Porphyrin

Stabilization and Structure of the cis Tautomer of a Free-Base Porphyrin
Laura J. McCormick, Kolle E. Thomas, Abhik Ghosh, Hugo Vazquez-Lima
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. White whale of porphyrin researchers: The elusive cis tautomer of a free-base porphyrin has been isolated and structurally characterized for the first time. X-ray diffraction data show the presence of an unusual cis geometry. DFT calculations confirm the greater stability of this structure relative to the more commonly observed trans geometry.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201701965

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