Synthesis, properties, and crystal structures of new mono- and homo-binuclear uranyl(VI) complexes with compartmental Schiff bases

Abstract

Mono-and homo-binuclear uranyl(VI) complexes of the type [UO₂(H₂L¹)]·solv, [(UO₂)₂(L¹)(solv)], [UO₂(H₂L²)]·solv, and [(UO₂)₂(L²)(solv)], where solv is a co-ordinatingsolvent[H₂O, dimethylformamide (dmf), or dimethyl sulphoxide (dmso)] and H₄L¹ and H₄L² are the potentially heptadentate dinucleating ligands derived by the condensation of 2,3-dihydroxybenzaldehyde with 1,5-diamino-3-azapentane or 1,5-diamino-3-thiapentane, have been prepared by a template procedure or by reaction of the preformed ligands with [UO₂(CH₃CO₂)₂]·2H₂O and characterized by i.r., ¹H and ¹³C n.m.r. spectroscopic and X-ray diffraction techniques. The crystal structures of [UO₂(H₂L¹)]·dmf, [(UO₂)₂(L¹)(dmso)], and [(UO₂)₂(L¹)(dmf)] have been determined by X-ray crystallography and refined to conventional R values of 0.044, 0.054, and 0.039 respectively: [UO₂(H₂L¹)]·dmf is monoclinic, space group P2₁/n, with a= 21.155(5), b= 11.833(8), c= 9.679(8)Å, β= 102.34(3)°, Z= 4; [(UO₂)₂(L¹)(dmso)] is monoclinic, space group P2₁/c, with a= 14.985(5), b= 16.536(5), c= 19.956(5)Å, β= 90.13(3)°, Z= 8; [(UO₂)₂(L¹)(dmf)] is monoclinic, space group P2₁/c, with a= 9.285(6), b= 16.187(5), c= 17.544(5)Å, β= 104.56(3)°, and Z= 4. In [UO₂(H₂L¹)]·dmf the ligand behaves as a quinquedentate dianionic and, using the inner co-ordination chamber, binds equatorially to UO₂²⁺ leading to seven-co-ordinated uranium in a distorted bipyramidal co-ordination geometry. A dmf molecule is hydrogen bonded to the phenolic oxygens of the ligand. Selected bond distances for this compound are U–O(uranyl) 1.79 (mean), U–O(ligand) 2.235 (mean), and U–N (mean) 2.59 Å. In [(UO₂)₂(L¹)(dmso)] and [(UO₂)₂(L¹)(dmf)] the tetra-anionic chelating ligand co-ordinates the inner UO₂²⁺ as in [UO₂(H₂L¹)]·dmf, while the outer UO₂²⁺ is co-ordinated by four oxygen atoms of the dinucleating ligand and by the oxygen atom of the solvent molecule. Structural details of the binuclear complexes are comparable. Comparison of the structures in the solid and in solution revealed some conformational differences.