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2-[(2 H -Indazol-3-yl)methylene]-1 H -indene-1,3(2 H )-dione 6 and ( E )-2-[(2 H -indazol-3-yl)methylene]-2,3-dihydro-1 H -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 -1 H tautomer (yellow) and 7 -2 H 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 1 H and 13 C NMR data, in DMSO- d 6 solution, the 6 -1 H tautomer predominates, whereas in less polar CDCl 3 or CD 2 Cl 2 , the 6 -2 H 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 -1 H and 7 -2 H tautomers. On the example of the 7 -2 H tautomer, it was shown for the first time that the 2 H 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 H → 1 H tautomerization occurs. As follows from density functional theory calculations, the high stability of the 2 H form in solution is due to the formation of centrosymmetric dimers, which are more stable than the corresponding dimers of the 1 H tautomer.

A series of 7-hydroxy-2-methylidene-2,3-dihydro-1H-inden-1-ones with 2-pyrrolyl (3), 4-dimethylaminophenyl (4), 4-nitrophenyl (5), and carboxyl group (6) as substituents at the exocyclic double bond was synthesized in the form of the E-isomers (4-6) or predominantly as the Z-isomer (3) which in solution is converted to the E-isomer. The synthesized compounds and their model analogues were studied by NMR spectroscopy, X-ray analysis, and MP2 theoretical calculations. The E-isomers having intramolecular O-H···O═C hydrogen bond are converted by UV irradiation to the Z-isomers having bifurcated O-H···O···H-X hydrogen bond. Unexpected shortening (and, thus, strengthening) of the O-H···O═C component of the bifurcated hydrogen bond upon the formation of the C═O···H-X hydrogen bond was found experimentally, proved theoretically (MP2), and explained by a roundabout interaction of the H-donor (HX) and H-acceptor (C═O) via the system of conjugated bonds.

An unexpected substitution of the anionic chelating ligands at the M II centre by a neutral tripodal ligand has been observed in the reaction of Mn II , Co II , Ni II and Cu II hexafluoroacetylacetonates (hfac) with tris(2-pyridyl)phosphine (Py 3 P) or its oxide (Py 3 P = O). The nature of the metal ion in M(hfac) 2 and the M/L ratio determine the degree of substitution of hfac-anions (partial vs. total) and therefore, the structure of the complex formed (scorpionate vs. bis-scorpionate ones, respectively). Hence, the reaction of the ligands with [Cu(hfac) 2 (H 2 O) 2 ] in an equimolar ratio affords scorpionate [Cu(N,N',N''-Py 3 P = X)(O,O'-hfac)(O-hfac)], wherein one hfac-ligand chelates metal, while the other hfac acts as an O-monodentate one. Using the two equivalents of Py 3 P in this reaction leads to [Cu(N,N',N''-Py 3 P) 2 ](hfac) 2 , which contains a bis-scorpionate cation [Cu(Py 3 P) 2 ] 2+ and two noncoordinated hfac-anions. [Co(hfac) 2 (H 2 O) 2 ] and [Ni(hfac) 2 (H 2 O) 2 ], regardless of the M/L molar ratio, react with Py 3 P = O to give cationic scorpionates [M(N,N',N''-Py 3 P = O)(O,O'-hfac)(H 2 O)](hfac), in which one hfac-anion is noncoordinated. In contrast, [Mn(hfac) 2 (H 2 O) 2 ], on interaction with Py 3 P, results in the cationic complex [Mn(N,N',N''-Py 3 P) 2 ][Mn(hfac) 3 ] 2 bearing a bis-scorpionate cation [Mn(Py 3 P) 2 ] 2+ and two [Mn(hfac) 3 ] 2 - counterions. The synthesized scorpionates have been characterized by X-ray diffractometry, cyclic voltammetry, SQUID magnetometry, FT-IR and UV-Vis techniques.