pH- and mol-ratio dependent tungsten(VI)–citrate speciation from aqueous solutions: syntheses, spectroscopic properties and crystal structures

Investigation of the aqueous coordination chemistry for citrate and tungsten(VI) resulted in the isolation of three new monomeric and dimeric tungsten(VI) citrate NaK3[W2O5(Hcit)2]·4H2O (1), (Hphen)3[WO2(H2cit)(Hcit)]·6H2O (2) and K4[WO3(cit)]·2H2O (3), (H4citcitric acid). The three complexes have been characterized by elemental analyses, IR and NMR spectroscopies. The IR and NMR spectra are consistent with monomeric species or a monooxo-bridged dinuclear structure as revealed by single crystal X-ray diffraction study. The dimeric anion of complex 1 contains a bent (O2W)O(WO2) core with an angle of WObW 162.3(3)°. Each citrate ligand is tridentately coordinated to one tungsten atom through the α-alkoxyl, α-carboxyl, and one β-carboxyl group, making each metal atom six-coordinate and forming a mesomer with configuration of ΔSΛR. The mononuclear tungstate 2 consists of a cis-dioxo partially protonated citrato tungstate(VI) anion and three phenanthrolinium cations. The W(VI) atom is coordinated by two acid oxo groups and two bidentate citrate ligands through α-alkoxyl and α-carboxyl groups, while the other two β-carboxyl or carboxylic groups remain free. The tungsten atom in the complex 3 forms an octahedral coordination with three fac-oxo groups and one tridentate citrate, in which the later is coordinated through the α-alkoxyl and α-carboxyl groups, and much more weakly by one of the two terminal β-carboxyl groups [2.348(4) Å]. Interconversion of citrato tungstate(VI) with pH value and the molar ratio of the reactant are discussed.