The structure of 3,5-dimethylpyrazole/carboxylic acids co-crystals

Abstract

Mixtures prepared either by mechanical grinding or by evaporation of equimolar amounts of 3,5-dimethylpyrazole (1) and five carboxylic acids, four benzoic acids (2-5) and a pyrazole-4-carboxylic acid (6), were studied by 13 C and 15 N CPMAS NMR spectroscopy. In the cases corresponding to 1 and 2,4,6-trimethylbenzoic acid (2) or 1 and 2,6-dimethylbenzoic acid (3) the spectrum of the mixture is different from those of its components and we interpret them in terms of co-crystals formation through donor-acceptor hydrogen bonds. The remaining pairs behave as physical mixtures of both components, the spectrum of the mixture being the sum of the individual spectra. The origin of the differences is the much higher acidity of o,o-disubstituted benzoic acids.

Cocrystals of 3,5-Dimethyl-1H-pyrazole and Salicylic Acid: Controlled Formation of Trimers via O−H···N Hydrogen Bonds

“…The experimental powder X-ray diffraction patterns of the samples obtained by mechanochemical synthesis are identical to the calculated powder diffraction patterns in both compounds. 12 For salicylic acid (sa), we also registered its 13 C CPMAS spectrum at 300 K with the following results: COOH 176. moves approximately +10 ppm, and C-2, approximately -6 ppm (without an OH); COOis approximately +5 ppm shifted with respect to the COOH chemical shift value. 12 In Table 4 are gathered the 15 N CPMAS NMR chemical shifts of the two trimers 1 and 2 and the results have been compared with those of dmpz in neutral and acid media (trifluoroacetic acid, TFAA), as well as those obtained for tetramers (dmpz) 2 -(tmb) 2 and (dmpz) 2 (dmb) 2 (Scheme 5).…”

“…12 For salicylic acid (sa), we also registered its 13 C CPMAS spectrum at 300 K with the following results: COOH 176. moves approximately +10 ppm, and C-2, approximately -6 ppm (without an OH); COOis approximately +5 ppm shifted with respect to the COOH chemical shift value. 12 In Table 4 are gathered the 15 N CPMAS NMR chemical shifts of the two trimers 1 and 2 and the results have been compared with those of dmpz in neutral and acid media (trifluoroacetic acid, TFAA), as well as those obtained for tetramers (dmpz) 2 -(tmb) 2 and (dmpz) 2 (dmb) 2 (Scheme 5). The analysis of the data shows that in 1 the dmpz is protonated by the salicylic acid in the formation of the synthon R 2 2 (7).…”

“…15 N NMR. With the five points corresponding to those in the literature (MeNO 2 , NH 3 , pyrrole, pyrazole, pyrazolium) 12,31 we have calculated eq 2.…”

“…We related it to the much higher acidities of the first two, whose experimental pK a s are 3.45 for 2,4,6-trimethylbenzoic acid and 3.35 for 2,6-dimethylbenzoic acid. 12 To assess the utility of these synthons, we present here our studies on the supramolecular reactions between 3,5-dimethyl-1H-pyrazole (dmpz) and salicylic acid (sa). Various techniques could be used to achieve this aim and, in addition to the traditional method involving mixing the two components in a solvent followed by evaporation (slow crystallization), we propose to attempt the preparation of such complexes through the mechanochemical approach, which involves mixing the two components in a mortar with a pestle with or without solvent drops.…”

“…The other carboxylic acids gave rise to physical mixtures of the individual components. We related it to the much higher acidities of the first two, whose experimental p K a s are 3.45 for 2,4,6-trimethylbenzoic acid and 3.35 for 2,6-dimethylbenzoic acid 3 Tetramers of 2,4,6-Trimethylbenzoic Acid ( tmb ) and 2,6-Dimethylbenzoic Acid ( dmb ) with dmpz …”