Acta Crystallographica Section B

Structural Science

Volume 55, Part 6 (December 1999)

research papers

Acta Cryst. (1999). B55, 985-993 [ doi:10.1107/S0108768199004966 ]

Self-assembly of NH-pyrazoles via intermolecular N-HN hydrogen bonds

V. Bertolasi, P. Gilli, V. Ferretti, G. Gilli and C. Fernàndez-Castaño

Abstract: The crystal structures of two NH-pyrazole derivatives forming intermolecular N-HN hydrogen bonds are reported: 5-methyl-4-(3-methylpyrazol-5-yl)pyrazol-3-ol, C₈H₁₀N₄O (P1), and 3-methyl-5-dihydro-1H-naphtho[1,2-d]pyrazole hemihydrochloride, C₁₂H₁₂N₂.-C₁₂H₁₃N $_{2}^{+}$ .Cl^- (P2). 26 other structures are surveyed in order to obtain a deeper insight into the ways NH-pyrazoles self-assemble by means of intermolecular N-HN hydrogen bonds in molecular crystals. A limited number of compounds form chains or dimers via homonuclear N⁺-HN positive-charge-assisted hydrogen bonds, typical of proton sponges, which can be remarkably short [e.g. NN 2.714 (3), N-H 1.09 (3), HN 1.63 (3) Å, N-HN 169 (3)° in (P2)]. Most pyrazoles, however, pack via neutral N-HN bonds which are formally assisted by resonance (resonance-assisted hydrogen bond, RAHB) through the N=C-C=C-NH iminoenamine fragment, contained in the ring, giving rise to dimers, trimers, tetramers and infinite chains of pyrazole molecules. Surprisingly, the resonance does not appear to shorten the N-HN bond with respect to the accepted mean value NN 2.97 (10) Å for non-resonant N-HN bonds. It is shown that this is due to the internal [pi] -delocalization of the pyrazole ring, which can be hardly increased by the hydrogen-bond interaction, except in symmetrically 3,5-substituted pyrazoles which display NN distances as short as 2.82 Å, identical C-C and C-N distances in the two halves of the pyrazole molecule, and typical phenomena of N-HN dynamical proton disorder, detectable by ¹⁵N-CP/MAS solid-state NMR.

Keywords: NH-pyrazoles; Hydrogen bonding; Self-assembly.