A structural study of 4-aminoantipyrine and six of its Schiff base derivatives

Six derivatives of 4-amino-1,5-dimethyl-2-phenyl-2,3-dihydro-1H-pyrazol-3-one (4-aminoantipyrine), C11H13N3O, (I), have been synthesized and structurally characterized to investigate the changes in the observed hydrogen-bonding motifs compared to the original 4-aminoantipyrine. The derivatives were synthesized from the reactions of 4-aminoantipyrine with various aldehyde-, ketone- and ester-containing molecules, producing (Z)-methyl 3-[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)amino]but-2-enoate, C16H19N3O3, (II), (Z)-ethyl 3-[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)amino]but-2-enoate, C17H21N3O3, (III), ethyl 2-[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)amino]cyclohex-1-enecarboxylate, C20H25N3O3, (IV), (Z)-ethyl 3-[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)amino]-3-phenylacrylate, C22H23N3O3, (V), 2-cyano-N-(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)acetamide, C14H14N4O2, (VI), and (E)-methyl 4-{[(1,5-dimethyl-3-oxo-2-phenyl-2,3-dihydro-1H-pyrazol-4-yl)amino]methyl}benzoate, C20H19N3O3, (VII). The asymmetric units of all these compounds have one molecule on a general position. The hydrogen bonding in (I) forms chains of molecules via intermolecular N-H...O hydrogen bonds around a crystallographic sixfold screw axis. In contrast, the formation of enamines for all derived compounds except (VII) favours the formation of a six-membered intramolecular N-H...O hydrogen-bonded ring in (II)-(V) and an intermolecular N-H...O hydrogen bond in (VI), whereas there is an intramolecular C-H...O hydrogen bond in the structure of imine (VII). All the reported compounds, except for (II), feature π-π interactions, while C-H...π interactions are observed in (II), C-H...O interactions are observed in (I), (III), (V) and (VI), and a C-O...π interaction is observed in (II).