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

Abstract

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.-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—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) Å, N—H...N 169 (3)° 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) Å for non-resonant N—H...N bonds. It is shown that this is due to the internal π-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 Å, 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 15N-CP/MAS solid-state NMR.