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Stabilization and Structure of the cis Tautomer of a Free‐Base Porphyrin
Author(s) -
Thomas Kolle E.,
McCormick Laura J.,
VazquezLima Hugo,
Ghosh Abhik
Publication year - 2017
Publication title -
angewandte chemie
Language(s) - English
Resource type - Journals
eISSN - 1521-3757
pISSN - 0044-8249
DOI - 10.1002/ange.201701965
Subject(s) - tautomer , porphyrin , chemistry , synthon , supramolecular chemistry , free base , molecule , hydrogen bond , stereochemistry , crystallography , chirality (physics) , enantiomer , crystal structure , photochemistry , organic chemistry , salt (chemistry) , nambu–jona lasinio model , chiral symmetry breaking , physics , quantum mechanics , quark
Single‐crystal X‐ray analysis of the β‐heptakis(trifluoromethyl)‐ meso ‐tetrakis( p ‐fluorophenyl)porphyrin, H 2 [(CF 3 ) 7 TpFPP], 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 H 2 [(CF 3 ) 7 TpFPP]⋅2 H 2 O and cognate structures may act as supramolecular synthons, which, given their chirality, may even be amenable to resolution into optically pure enantiomers.