Self-assembly of NH-pyrazoles via intermolecular N-H...N hydrogen bond

The crystal structures of two NH-pyrazole derivatives forming intermolecular N-H ... N 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.-C12H13N2+.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-H ... N hydrogen bonds in molecular crystals. A limited number of compounds form chains or dimers via homonuclear N+-H ... N positive-charge-assisted hydrogen bonds, typical of proton sponges, which can be remarkably short [e.g. N ... N 2.714(3), N-H 1.09(3), H ... N 1.63(3) Angstrom, N-H ... N 169(3)degrees in (P2)]. Most pyrazoles, however, pack via neutral N-H ... N 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-H ... N bond with respect to the accepted mean value N ... N 2.97(10) Angstrom for non-resonant N-H ... N 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 N ... N distances as short as 2.82 Angstrom, identical C-C and C-N distances in the two halves of the pyrazole molecule, and typical phenomena of N-H N dynamical proton disorder, detectable by N-15-CP/MAS solid-state NMR.