INTRAMOLECULAR N-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O HYDROGEN-BONDING ASSISTED BY RESONANCE .3. STRUCTURAL STUDIES OF 1-KETONE-2-ARYLHYDRAZONE DERIVATIVES

The crystal and molecular structures of the following 1-ketone-2-arylh ydrazones have been determined: (1) 4-(phenylhydrazono)-3-methyl-4H-is oxazol-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 alph a) = 0.71069 Angstrom, mu = 0.096 mm(-1), F(000) = 848, T = 295 K, R = 0.040 for 1737 observed reflections; (2) -nitrophenylhydrazono)-3-met hyl-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), lambd a(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) chlorophenylhydra zono)-3-methyl-4H-isoxazol-5-one, C10H8ClN3O2, M(r) = 237.6, orthorhom bic, 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 al pha) = 0.71069 Angstrom, mu = 0.345 mm(-1), F(000) = 976, T = 295 K, R = 0.044 for 1359 observed reflections: (4) enylhydrazono)-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), lamb da(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 pl anar and present pi-delocalization within the ketohydrazone moiety. Th ese conditions should favour the formation of a strong intramolecular N-H...O hydrogen bond assisted by resonance, but in the present case a ll compounds display hydrogen bonds which can be considered weak or ve ry 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 delt a(NH) NMR chemical shifts are in agreement with the hydrogen-bonding m agnitude measured by N...O distances. The steric impossibility of obta ining short N...O distances gives rise to the formation of other inter molecular interactions, relevant to the crystal packing, such as bifur cated N-H...O hydrogen bonds, C-H...O short contacts and pi-pi interac tions.