title: Stabilization and Structure of the cis Tautomer of a Free-Base Porphyrin.
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.
authors.affiliation: Department of Chemistry, UiT-, The Arctic University of Norway, 9037, Tromsø, Norway.
authors.forename: Kolle E
authors.initials: KE
authors.lastname: Thomas
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720-8229, USA.
authors.forename: Laura J
authors.initials: LJ
authors.lastname: McCormick
authors.affiliation: Department of Chemistry, UiT-, The Arctic University of Norway, 9037, Tromsø, Norway.
authors.forename: Hugo
authors.initials: H
authors.lastname: Vazquez-Lima
authors.affiliation: Department of Chemistry, UiT-, The Arctic University of Norway, 9037, Tromsø, Norway.
authors.forename: Abhik
authors.identifier: 0000-0003-1161-6364
authors.initials: A
authors.lastname: Ghosh
keywords: X-ray diffraction; hydrogen bonding; porphyrins; supramolecular chemistry; tautomers