| Abstract: | 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. |
