Hydrogen bonding-controlled assemblies of polymeric and octanuclear tungsten citrates

Abstract

Water-soluble polymeric tungsten citrate (Him)10n(NH4)2n[(WO2)2O(Hcit)2]3n·10nH2O (1) and octanuclear tungsten citrate (Him)8(NH4)13{[(WO2)2O(Hcit)(cit)H(cit)(Hcit)O(WO2)2][(WO2)2O(cit)2]2}·14H2O (2) (H4cit=citric acid, im=imidazole), were obtained from aqueous solution. They were characterized by elemental analyses, NMR and IR spectra, thermogravimetric (TG) analyses and X-ray structural analyses. The basic units of the two complexes contain a dimeric tungsten citrate in different protonated forms, where tungsten atom is six-coordinated in an approximately octahedral geometry. Each citrate ligand uses its α-alkoxy, α-carboxy and one β-carboxy group to act as a tridentate ligand, while the other β-carboxy or carboxylic acid group is uncoordinated. The presences of the protonated/deprotonated terminal carboxylates and their participation in hydrogen-bonding interactions play an important role in the overall structures. In complex 1, a hexanuclear tungsten citrate species [(WO2)2O(Hcit)2]312− is linked by strong H-bonding between β-carboxylic acid group and terminal oxygen [2.587(6)Å], which extends into one-dimensional polymeric chain. In complex 2, there exist two types of strong hydrogen bonds between β-carboxy and carboxylic acid groups [2.467(8), 2.516(6)Å], forming an octanuclear species {[(WO2)2O(Hcit)(cit)H(cit)(Hcit)O(WO2)2][(WO2)2O(cit)2]2}21−. This is supported by the absence of IR band around 1700cm−1 for protonated carboxylic acid. 13C NMR spectra show the coordination of α-alkoxy and α-carboxy groups of citrate ligand in both complexes.