Molecular and Crystal Structure of Benzohydroxamic Acid and Its Ring-Substituted Derivatives

Crystal structure of benzohydroxamic acid and its ring-substituted derivatives RC6H4C(O)NHOH (R = 4-Me, 4-NO2, 4-Cl, 3-Cl and 2-Cl) was determined by single-crystal X-ray diffraction. In all the compounds, the hydroxamic group is in the planar amide form and the structures differ mainly in the tilt of the aromatic and hydroxamic acid planes. For the 2-chloro derivative, the dihedral angle of the two planes is 46.1° which corresponds to the intramolecular van der Waals contact of the ortho-substituents. In other compounds, the tilt originates from intermolecular hydrogen bonding and varies between 3.5 and 22.0°; four crystallographically independent molecules present in the structure of benzohydroxamic acid also differ significantly in this tilt, as well as three independent molecules of the 4-nitro derivative do. Although there are only two types of hydrogen bonding in all the compounds, a short one between OH and O-N and a second longer between NH and O=C, bonded network in the crystal is of three different types. In unsubstituted acid, its 4-Me, 4-Cl and 3-Cl derivative, the molecules are assembled into hydrogen-bonded layers stacked loosely along the largest cell parameter. As a result of the large tilt of the molecular planes in the 2-Cl compound, its molecules are linked into chains with unusual, strongly bent orientation of the aromatic groups. The self-assembly of the remaining 4-nitro compound is quite unique, consisting of pseudohexagonal, partly interpenetrating stacks. In several cases, the hydrogen bonding is supported by π-interaction of the aromatic rings.