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Supramolecular stabilization of metastable tautomers in solution and solid state

This work presents a successful application of recently reported supramolecular strategy for stabilization of metastable tautomers in cocrystals to monocomponent non-heterocyclic tautomeric solids. The quantum-chemical computations and solution studies show that the investigated Schiff base molecule, derived from 3-methoxysalycilaldehyde and 2-amino-3-hydroxypyridine (ap), is far more stable as the enol tautomer. In the solid state however, in all three obtained polymorphic forms it existed solely as the keto tautomer, in each case stabilized by unexpected hydrogen bonding pattern. The computations have shown that hydrogen bonding of investigated Schiff base with the suitable molecules shifts the tautomeric equilibrium to the less stable keto form. The extremes to which supramolecular stabilization can lead are demonstrated by the two polymorphs of molecular complexes of Schiff base with ap. The molecules of both constituents of molecular complexes are present as metastable tautomers (keto anion and protonated pyridine, respectively), stabilizing each other via a very strong hydrogen bond. All obtained solid forms proved stable on various solid-state and solvent mediated methods used to establish their relative thermodynamic stabilities and possible interconversion conditions.

tautomerism ; Schiff base ; keto-enol ; supramolecular stabilization ; metastable tautomers

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