2-Indazole Tautomers Stabilized by Intra- and Intermolecular Hydrogen Bonds.

2-[(2-Indazol-3-yl)methylene]-1-indene-1,3(2)-dione and ()-2-[(2-indazol-3-yl)methylene]-2,3-dihydro-1-inden-1-one have been synthesized. In the crystal, the NH hydrogen atom of is disordered between the N(1) and N(2) atoms with the population ratio of 0.69:0.31. Molecule crystallizes in two tautomeric polymorphs: -1 tautomer (yellow) and -2 tautomer (red). Both and form centrosymmetric dimers in the crystal with the monomeric units linked by C═O···H···N bifurcated hydrogen bonds in and N-H···N hydrogen bonds in . According to H and C NMR data, in DMSO- solution, the -1 tautomer predominates, whereas in less polar CDCl or CDCl, the -2 tautomer is stabilized by a strong N-H···O═C intramolecular hydrogen bond. Compound in dimethyl sulfoxide (DMSO) or ethanol solutions exists in the form of -1 and -2 tautomers. On the example of the -2 tautomer, it was shown for the first time that the 2 tautomers of 3-substituted indazoles can be stabilized by an intermolecular hydrogen bond and may remain in aprotic solvents almost indefinitely. However, in the open air or in water, fast 2 → 1 tautomerization occurs. As follows from density functional theory calculations, the high stability of the 2 form in solution is due to the formation of centrosymmetric dimers, which are more stable than the corresponding dimers of the 1 tautomer.