Polymorphism in uranyl chelate complexes. Part 2. The crystal and molecular structure of the alpha 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 complex α-[UO₂(hfpd)₂{PO(OMe)₃}] has been determined from 1 079 three-dimensional single-crystal (hkl) data collected on a four-circle goniometer with Cu-K_β radiation. The crystals are monoclinic, space group P2₁/m, with a= 10.349(6), b= 21.590(12), c= 5.560(3)Å, and β= 92.01°. R= 0.075. Two bidentate hfpd molecules and one unidentate trimethyl phosphate molecule are co-ordinated to the uranyl group, forming a pentagonal bipyramid of oxygen atoms around the U^VI. The pentagonal ring (four carbonyl oxygen atoms and one trimethyl phosphate oxygen atom) is planar and normal to the linear uranyl group. The hfpd molecules are planar, but tilted by 22.5° to the plane of the pentagonal ring in a boat configuration. The crystal structure is molecular, and the low melting point can be explained by the predominant intermolecular contacts between similarly charged fluorine atoms of CF₃ groups. The thermal-vibratron parameters are large on the periphery of the molecules, approaching liquid-crystal values, and the CF₃ groups have two-fold positional disorder. The trimethyl phosphate group is also disordered. The structure is compared with those of other metal–hfpd complexes and of pentagonal-brpyramidal uranyl complexes.