2-Methylthio-imidazolins: a rare case of different tautomeric forms in solid state and in solution

13C CPMAS NMR as a Tool for Full Structural Description of 2-Phenyl Substituted Imidazoles That Overcomes the Effects of Fast Tautomerization

Tautomerization of 2-phenylimidazolecarbaldehydes has not been studied in detail so far, although this process is a well-known phenomenon for imidazole derivatives. That is why we focus our study on a series of 2-phenylimidazolecarbaldehydes and their parent alcohols that were synthesized and studied by detailed 1H and 13C NMR in solution and in the solid state. The apparent problem is that the fast tautomerization impedes the full structural description of the compounds by conventional 13C NMR measurements. Indeed, the 13C NMR spectra in solution exhibit poor resolution, and in most cases, signals from the imidazole ring are not detectable. To avoid this problem, we used 13C CP-MAS NMR as an alternative spectroscopic method for unambiguous spectroscopic characterization of the studied series of 2-phenylimidazoles. The data were analyzed in combination with quantum chemical DFT-GIAO methods by considering the tautomerization process and the intermolecular interactions. The DFT (B3LYP/6-31G(d,p)) calculations allowed to identify and suggest the preferred tautomer in the gas phase and in DMSO solvent, which for alcohols are (2-phenyl-1H-imidazol-4-yl)methanol and its analogs, and for the aldehydes are the 2-phenyl-1H-imidazole-5-carbaldehydes. The gas-phase calculated energy differences between the two possible tautomeric forms are in the range 0.645-1.415 kcal/mol for the alcohols and 2.510-3.059 kcal/mol for the aldehydes. In the DMSO solvent, however, for all compounds, the calculated energy differences go below 1.20 kcal/mol. These data suggest that both tautomeric forms of the studied 2-phenylimidazoles can be present in solution at room temperature. Our data from detailed 2D NMR measurements in the solid state (1H-13C HETCOR and 1H-1H double-quantum coherence MAS NMR) suggested that also in the solid state both tautomers coexist in different crystalline domains. This fact does not obscure the 13C CP-MAS NMR spectra of the studied 2-phenyl substituted imidazoles and suggests this spectroscopic method as a powerful tool for a complete structural description of tautomeric systems with aromatic conjugation.

Combinations of Tautomeric Forms and Neutral-Cationic Forms in the Cocrystals of Sulfamethazine with Carboxylic Acids

The cocrystals of sulfamethazine with different acids, namely, 2-mercaptophenylcarboxylic acid, 2,6-pyridinedicarboxylic acid, 4-(4-hydroxyphenylazo)phenylcarboxylic acid, 3-(4-hydroxyphenyl)propanoic acid, and 4-(phenyl)phenylcarboxylic acid, are studied here. Each has distinct notable supramolecular features. The pyrimidin-2-amine unit of the sulfamethazine provided unique examples of cocrystals in which amidine and imidine forms or neutral and protonated forms of sulfamethazine are observed in 2:2 ratios. Hence, this study provides avenues to explore cocrystals with tautomeric forms together in a cocrystal and also neutral and protonated cocrystal partners as apparent multicomponents in cocrystals. Among the cocrystals, three of them have the amidine form of the sulfamethazine in respective self-assembly. The cocrystal of 2-mercapto-phenylcarboxylic acid with sulfamethazine has the amidine form and it has the distinction of having S-H···π interactions. The cocrystal of sulfamethazine with 2,6-pyridinecarboxylic acid is a rare example of a 1:1 cocrystal of sulfmethazine with dicarboxylic acid. It has methanol molecules as a solvent of crystallization. Sulfamethazine forms a hydrated cocrystal with 4-(4-hydroxyphenylazo)-phenylcarboxylic acid that has conventional R 2 2(8) synthons of amidine hydrogen-bonding with carboxylic acid. The phenolic part of the acid component is anchored to the water molecule and provides a robust self-assembly. The hydrated cocrystal of sulfamethazine with 3-(4-hydroxyphenyl)propanoic acid (2:2 cocrystal) has two independent molecules of sulfamethazine, one in amidine form and the other in imidine form. It has two neutral carboxylic acids anchored through complementary hydrogen bonds and also has two water molecules of crystallization. The cocrystal of sulfamethazine with 4-(phenyl)phenylcarboxylic acid is also a 2:2 cocrystal. It is a di-hydrate, which has a neutral and protonated form of sulfamethazine. The neutral form is hydrogen-bonded to a neutral carboxylic acid, whereas the protonated form is charge-assisted hydrogen-bonded to the corresponding carboxylate anion.