Structure and bonding in octahedral uranium(IV) complexes of the type UX4·2L (X = halogen, L = unidentate, neutral oxygen donor). Part 1. The crystal structures of tetrabromobis(NN′-dimethyl-NN′-diphenylurea)uranium(IV) and α- and β-tetrachlorobis(NN′-dimethyl-NN′-diphenylurea)uranium(IV)

Abstract

The crystal structures of three complexes of uranium(IV) halides with the carbonyl donor ligand NN′-dimethyl-NN′-diphenylurea (ddu) have been determined from three-dimensional X-ray data, and refined by full-matrix least-squares methods: UBr₄·2ddu (1) and two polymorphic forms of UCl₄·2ddu, labelled α(2) and β(3). For (1) the crystals are orthorhombic, space group Fddd(final R= 0.052), for (2) monoclinic, space group P2₁/n(final R= 0.073), and for (3) orthorhombic, space group Pna2₁(final R= 0.052). In each complex the co-ordination geometry of uranium is distorted octahedral, with the halogen atoms at the corners of a puckered square. The high molecular symmetry (222) of UBr₄·2ddu is shown to arise from the crowded ddu ligand (which can assume two-fold symmetry), coupled with the short U–O bonding distance and large atomic diameter of bromine. The molecules in α- and β-UCl₄·2ddu are devoid of symmetry and their conformations are completely dissimilar. The U–Br distance is 2.771(2)Å, and the U–Cl distances are 2.604(8)Å in (2) and 2.606(9)Å in (3); U–O is 2.217(10)Å in (1), 2.252(14)Å in (2), and 2.268(18)Å in (3).