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

2-[(2H-Indazol-3-yl)methylene]-1H-indene-1,3(2H)-dione 6 and (E)-2-[(2H-indazol-3-yl)methylene]-2,3-dihydro-1H-inden-1-one 7 have been synthesized. In the crystal, the NH hydrogen atom of 6 is disordered between the N(1) and N(2) atoms with the population ratio of 0.69:0.31. Molecule 7 crystallizes in two tautomeric polymorphs: 7-1H tautomer (yellow) and 7-2H tautomer (red). Both 6 and 7 form centrosymmetric dimers in the crystal with the monomeric units linked by C═O···H···N bifurcated hydrogen bonds in 6 and N-H···N hydrogen bonds in 7. According to ¹H and ¹³C NMR data, in DMSO-d₆ solution, the 6-1H tautomer predominates, whereas in less polar CDCl₃ or CD₂Cl₂, the 6-2H tautomer is stabilized by a strong N-H···O═C intramolecular hydrogen bond. Compound 7 in dimethyl sulfoxide (DMSO) or ethanol solutions exists in the form of 7-1H and 7-2H tautomers. On the example of the 7-2H tautomer, it was shown for the first time that the 2H 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 2H → 1H tautomerization occurs. As follows from density functional theory calculations, the high stability of the 2H form in solution is due to the formation of centrosymmetric dimers, which are more stable than the corresponding dimers of the 1H tautomer.