Structures of NH-pyrazoles bearing only C-methyl substituents: 4-methylpyrazole is a hydrogen-bonded trimer in the solid (100 K)

Abstract

The X-ray molecular structure of 4-methylpyrazole (3) at 100 K has been determined. The compound crystallizes as a trimer with the NwH protons localized. 4-Methylpyrazole belongs to the series of eight NH-pyrazoles bearing only H and substituents on the carbon atoms. There are two pairs of tautomers so the number of pyrazoles CH 3 existing in solution is only six. The thermal behavior of the six pyrazoles was investigated by di †erential scanning calorimetry and their melting points analyzed on the basis of their solid state structures and number and position of C-methyl substituents.

A Series of Novel N,N‐Donor Ligands with Binaphthyl Backbones

“…We found seven hydrogenbonded and structurally characterized pyrazole trimers in the CCDC. [31] The N···N distances are in a range between 2.82 and 3.02 Å, which confirms the presence of a rather strong N···H-N interaction (2.85 Å) in [(R)-2] 3 as already revealed by the chemical shift of the NH proton.…”

Pyrazole Complexes and Supramolecular Chemistry

“…This interaction is an important contribution to the melting point of imidazoles and pyrazoles: whereas at room temperature pyrazole and imidazole are solids, N-methylpyrazole and N-methylimidazole are, like pyridine, liquids. [10] The two different nitrogen atoms are in the 1,2-positions (vicinal) in pyrazoles, and in the 1,3-positions (alternate) in imidazoles. An important consequence is that only open, chain-like structures, termed catemers, are possible for imidazoles.…”

Hydrogen bonding lights up overtones in pyrazoles

“…1. As a consequence, planar C 3h pyrazole trimers or winding chains are often observed in the crystalline solids, [9][10][11] unless sterical constraints induced by bulky ring substitution enforce C 2h dimer or strongly folded tetramer formation. Concerted and stepwise proton tunneling in cyclic pyrazole oligomers is a very sensitive probe of this subtle interplay between cooperative bonding, hydrogen bond strain, and sterical hindrance.…”