Solid-state structure and tautomerism of 2-aminotroponimines studied by X-ray crystallography and multinuclear NMR spectroscopy

Structural studies in the solid state by X-ray crystallography and by C-13 and N-15 CPMAS NMR spectroscopy carried out on a series of 2-aminotroponimine derivatives 2-5 has allowed to establish the existence of hydrogen bonding and to determine the most stable tautomer. Almost all the structures reflect the classical double-well potential function for the N-H...N hydrogen bonds. Only in the case of the compound N-(pyrrol-1-yl)-2-(pyrrol-1-ylamino)troponimine (5) the crystal structure shows two independent molecules, one with a classical hydrogen bond and another with either a single-well or a low-barrier hydrogen bond. The structure of this compound is discussed with the use of the solid-state NMR spectroscopic data. 2-Aminotropones, as intermediates to the 2-aminotroponimines, show the oxo-tautomer as the stable form. B3LYP/6-31G* calculations are used to rationalise the experimental results. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).