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Stabilization and structure of the cis Tautomer of a free-base porphyrin

Stabilization and structure of the cis Tautomer of a free-base porphyrin
Stabilization and structure of the cis Tautomer of a free-base porphyrin

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.

hydrogen bonding, porphyrins, supramolecular chemistry, tautomers, X-ray diffraction
1433-7851
10088-10092
Thomas, Kolle E.
b79e1c79-5c1e-46b5-a4b5-f82bb02a927d
McCormick, Laura J.
f1c2f8cd-adcc-4bbf-9289-0b33a006d2bb
Vazquez-Lima, Hugo
e33ec9ee-1df4-4633-a970-eb464b54aa2f
Ghosh, Abhik
01a311da-a139-43f0-ae29-0efa0e8788f7
Thomas, Kolle E.
b79e1c79-5c1e-46b5-a4b5-f82bb02a927d
McCormick, Laura J.
f1c2f8cd-adcc-4bbf-9289-0b33a006d2bb
Vazquez-Lima, Hugo
e33ec9ee-1df4-4633-a970-eb464b54aa2f
Ghosh, Abhik
01a311da-a139-43f0-ae29-0efa0e8788f7

Thomas, Kolle E., McCormick, Laura J., Vazquez-Lima, Hugo and Ghosh, Abhik (2017) Stabilization and structure of the cis Tautomer of a free-base porphyrin. Angewandte Chemie - International Edition, 56 (34), 10088-10092. (doi:10.1002/anie.201701965).

Record type: Article

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.

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More information

e-pub ahead of print date: 29 March 2017
Published date: 14 August 2017
Keywords: hydrogen bonding, porphyrins, supramolecular chemistry, tautomers, X-ray diffraction

Identifiers

Local EPrints ID: 439015
URI: http://eprints.soton.ac.uk/id/eprint/439015
ISSN: 1433-7851
PURE UUID: 31750673-392a-4087-b187-72469e735890
ORCID for Laura J. McCormick: ORCID iD orcid.org/0000-0002-6634-4717

Catalogue record

Date deposited: 01 Apr 2020 16:30
Last modified: 16 Sep 2021 11:14

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Contributors

Author: Kolle E. Thomas
Author: Hugo Vazquez-Lima
Author: Abhik Ghosh

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