Molecular and crystal structure of benzohydroxamic acid and its ring-substituted derivatives

Crystal structure of benzohydroxamic acid and its ring-substituted derivati ves 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 grou p 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, th e dihedral angle of the two planes is 46.1 degrees which corresponds to the intramolecular van der Waals contact of the ortho-substituents. In other c ompounds, the tilt originates from intermolecular hydrogen bonding and vari es between 3.5 and 22.0 degrees; four crystallographically independent mole cules present in the structure of benzohydroxamic acid also differ signific antly in this tilt, as well as three independent molecules of the 4-nitro d erivative 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 N H and O=C, bonded network in the crystal is of three different types. In un substituted acid, its 4-Me, 4-Cl and 3-Cl derivative, the molecules are ass embled into hydrogen-bonded layers stacked loosely along the largest cell p arameter. 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-n itro compound is quite unique, consisting of pseudohexagonal, partly interp enetrating stacks. in several cases, the hydrogen bonding is supported by K -interaction of the aromatic rings.