Hydrogen bonding-controlled assemblies of polymeric and octanuclear tungsten citrates

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 1700 cm−1 for protonated carboxylic acid. 13C NMR spectra show the coordination of α-alkoxy and α-carboxy groups of citrate ligand in both complexes.

Graphical abstractTwo polymeric tungsten citrate complexes have been synthesized and characterized. The structural analysis indicates that changing the pH value leads to the different structures. The uncoordinated β-carboxy groups of citrate ligand are of different degree of protonation and form intermolecular hydrogen bonds leading to a polymeric chain structure of the two complexes.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Two new tungsten citrate complexes were synthesized from aqueous solution. ► The reaction shows that the pH value plays a key role. ► H-bond formed by terminal carboxylates plays an important role in the structures. ► 13C NMR spectra show the coordination of citrate to W in both complexes.