Tungsten(VI) complexes with citric acid (H4cit). Structural characterisation of Na6[{WO2(cit)}2O]·10H2O

Abstract

The complex anion [W₂O₅(cit)₂]^6– was obtained by crystallisation from an equimolar aqueous solution of sodium tungstate and citric acid (H₄cit). It has been characterised by elemental analysis, IR and ¹H and ¹³C NMR spectroscopy. The IR spectrum is consistent with a monooxo-bridged dinuclear structure as revealed by a single-crystal X-ray diffraction study of Na₆[W₂O₅(cit)₂]·10H₂O. The crystals are monoclinic, space group P2₁/a, a= 17.708(3), b= 9.908(2) and c= 9.018(2)Å, β= 91.57(2)°, R= 0.025 for 3101 observed [I/σ(I) 2.5] reflections. The complex anion contains a O₂WOWO₂ core with the bridging oxo group lying at a crystallographic centre of symmetry (W–O_b–W 180°). Each citrate ligand is three co-ordinated to one tungsten atom through the deprotonated hydroxy, α-carboxylate, and one β-carboxylate group. Principal dimensions are: W–O_b 1.893(1), (W = O_t)_av 1.757(2), W–O_hydroxy 1.958(2), W–O_α-carboxy 2.195(3) and W–O_β-carboxy 2.289(2)Å. All methylene and β-carboxylate groups become magnetically equivalent upon redissolution in water as a result of the dissociation of the less strongly bonded β-carboxylate group.