The Structure of Pyrazoles in the Solid State:  A Combined CPMAS, NMR, and Crystallographic Study

Abstract

The structures of six N-unsubstituted pyrazoles, three already known and three newly synthesized, have been studied by a combination of X-ray crystallography, multinuclear NMR (solution and solid state), and density functional theory (DFT) calculations. In those cases where crystal structure and CPMAS NMR were available, the agreement was almost perfect, allowing a prediction of the tautomer (with certitude) and the tetrameric structure (with high probability) in the case of 5-isopropyl-3-phenyl-1Hpyrazole without knowing the X-ray structure. In the case of the 5-(2-benzylphenyl)-3-trifluoromethyl-1H-pyrazole above represented, the DFT calculations at the B3LYP/6-31G** level justify the great stability of this tautomer by the presence of an intramolecular N-H‚‚‚π interaction, present in solution.

Pyrazole Complexes and Supramolecular Chemistry

“…For N-H pyrazoles with bulky substituents at the 3-and 5-positions, dimers, tetramers, or hexamers were found. [11] This secondary structure, in turn, explains some surprising trends in melting points: for instance, the melting points of 4-methylpyrazole and 4,5-dimethylpyrazole are lower than that of pyrazole. Now, unlike N-CH 3 pyrazoles, these three compounds possess N-H groups, and their molecules engage in N-H···N hydrogen bonding.…”

“…As a consequence, it occurs in pyrazoles that crystallize by forming trimers, such as 3,5-dimethylpyrazole, but is not observed in catemers. [11] Pyrazole Complexes: Synthesis and Stability Azole complexes are almost invariably prepared by substitution reactions of the free azole with a suitable precursor containing a coordinative unsaturation or a ligand that can be displaced easily under the reaction conditions. In a few cases, pyrazole complexes were prepared by the substitution reaction of a preformed pyrazolate salt with a metal complex such as a halide complex.…”

Crystal structure of 3-(furan-2-yl)-5-(4-methoxyphenyl)-1H-pyrazole, C14H12N2O2

“…Recently, the proton transfer in the solid state has been communicated for this group of materials [2]. Pyrazoles crystallize with four hydrogen bonding patterns like: dimers, trimers, tetramers and catemers throngh N-H···Nh ydrogen interactions using both Natoms of the pyrazole [3]. The crystal structure analysis shows that proton of the pyrazole group is resided at the nitrogen atom N1.…”

Crystal structure of 3-(furan-2-yl)-5-(4-phenylphenyl)-1H-pyrazole — methanol (1:1), C19H14N2O · CH3OH

“…Pyrazoles which crystallized with four hydrogen-bonding patterns: dimers, trimers, tetramers and catemers throngh N−H···Nh ydrogen interactions using both N atoms of the pyrazole, have received much more interesting in their crystals [3,4]. The crystal structure analysis shows that the title molecules contain two independent pyrazole molecules and two MeOH molecules in the asymmetric unit (figure, top).…”