Synthesis and characterization of some actinoid iminodiacetato-complexes; crystal structure and normal co-ordinate analysis of iminodiacetatodioxouranium(VI)

Abstract

The preparation and the characterization, mainly by i.r. data, of uranium(VI) iminodiacetato-complexes with metal : ligand ratios of 1 : 1 and 1 : 2 and of their pyridine N-oxide adducts are reported. The photochemical preparation and related studies of a uranium(IV) complex with the same ligand are also described. The crystal and molecular structure of the title complex [{UO₂(ida)}_n] has been determined by three-dimensional X-ray data collected by counter techniques. The compound crystallizes in an orthorhombic cell of dimensions a= 10.881 (5), b= 10.132(5), c= 6.907(4)Å, space group Pnma(D¹⁶_2h) and Z= 4. The final R factor for the least-squares refinement is 0.069 for 609 observed reflections. The linear uranyl ion is equatorially surrounded by one ligand, via the two carboxylic oxygen atoms [U–O 2.38(2) and 2.39(2)Å], the imino-nitrogen atom [U–N 2.59(4)Å], and by two other ligands via two symmetry-related oxygen atoms, forming an irregular pentagonal bipyramid. The structure is polymeric with a double system of chains, one linear running along b and the other helicoidal along c; a symmetric bifurcated hydrogen bond joins two adjacent helicoidal systems spaced by a/2. For this complex the fundamental vibrations have been assigned on the basis of a normal co-ordinate analysis and potential-energy distribution obtained by use of a 39-parameter Urey–Bradley force field. The stretching force constant of the U–N(imino) bond is much greater than that of the corresponding U–O(ether) bond in the analogous oxydiacetatodioxouranium(VI) complex (2.48 × 10²versus 1.1 × 10² N m^–1 respectively) and this is attributed to the different covalent character of the two bonds.