Intramolecular N–H⋯O hydrogen bonding assisted by resonance. III. Structural studies of 1-ketone-2-arylhydrazone derivatives

The crystal and molecular structures of the following 1-ketone-2-arylhydrazones have been determined: (1) 4-(phenylhydrazono)-3-methyl-4H-isoxazol-5-one, C10H9N3O2, M(r) = 203.2, monoclinic, C2/c, a = 11.100 (3), b = 7.960 (2), c = 21.744 (4) Angstrom, beta = 95.82 (2)degrees, V = 1911.3 (8) Angstrom(3), Z = 8, D-x = 1.41 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.096 mm(-1), F(000) = 848, T = 295 K, R = 0.040 for 1737 observed reflections; (2) 4-(4-nitrophenylhydrazono)-3-methyl-4H-isoxazol-5-one, C10H8N4O4, M(r) = 248.2, monoclinic, P2(1)/c, a = 7.046 (2), b = 7.774 (1), c = 20.917 (4) Angstrom, beta = 98.09 (2)degrees, V = 1134.3 (4) Angstrom(3), Z = 4, D-x = 1.45 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.108 mm(-1), F(000) = 512, T = 295 K, R = 0.048 for 1599 observed reflections; (3) 4-(2-chlorophenylhydrazono)-3-methyl-4H-isoxazol-5-one, C10H8ClN3O2, M(r) = 237.6, orthorhombic, Pca2(1), a = 18.324 (2), b = 5.771 (1), c = 20.016 (2) Angstrom, V = 2116.6 (6) Angstrom(3), Z = 8, D-x = 1.49 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.345 mm(-1), F(000) = 976, T = 295 K, R = 0.044 for 1359 observed reflections: (4) 4-(4-nitrophenylhydrazono)-3-methyl-1-phenyl-4H-pyrazol-5-one, C16H13N5O3, M(r) = 323.3, monoclinic, P2(1)/n, a = 12.817 (1), b = 7.595 (5), c = 15.652 (3) Angstrom,beta = 95.74 (1)degrees, V = 1516 (1) Angstrom(3), Z = 4, D-x = 1.42 Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom, mu = 0.096 mm(-1), F(000) = 672, T = 295 K, R = 0.045 for 1672 observed reflections. All molecules are planar and present pi-delocalization within the ketohydrazone moiety. These conditions should favour the formation of a strong intramolecular N-H...O hydrogen bond assisted by resonance, but in the present case all compounds display hydrogen bonds which can be considered weak or very weak. An analysis of the bond angles suggests that the discrepancy is to be imputed to the fact that the heterocyclic five-membered rings introduce a geometrical constraint which hinders the strengthening of the hydrogen bond. Spectroscopic data, nu(NH) IR frequencies and delta(NH) NMR chemical shifts are in agreement with the hydrogen-bonding magnitude measured by N...O distances. The steric impossibility of obtaining short N...O distances gives rise to the formation of other intermolecular interactions, relevant to the crystal packing, such as bifurcated N-H...O hydrogen bonds, C-H...O short contacts and pi-pi interactions.