Polymorphism in uranyl chelates. Part 3. The crystal and molecular structure of the β form of bis(1,1,1,5,5,5-hexafluoropentane-2,4-dionato)dioxo(trimethyl phosphate)uranium(VI)

Abstract

The crystal structure of the title compound β-[UO₂(hfpd)₂{PO(OMe)₃}] has been determined, and is triciinic, space group P, with dimensions a= 12.352(8), b= 12.013(8), c= 11.984(8)Å and α= 105.7(2), β= 109.3(1), γ= 119.2(2)°. The structure is molecular with a packing completely different from that in the α form. The uranium atom is co-ordinated by seven oxygen atoms to form a pentagonal bipyramid. The hfpd planes are only slightly tilted with respect to the pentagonal plane, giving a nearly planar molecule, in contrast to the α form where the molecule is boat-shaped. There is no positional disorder in the CF₃ and PO(OMe)₃ groups in the β form, again in contrast to the α form. There are repulsive O–O, O–F, and F–F interactions between the molecules, while weak hydrogen-bonded interactions of the type H ⋯ O and H ⋯ F play a cohesive role. The different i.r. uranyl-stretching frequencies for the α and β forms are explained by the differences in their uranyl environments.