Speciation of water-soluble titanium citrate: Synthesis, structural, spectroscopic properties and biological relevance

Titanium(IV) citrate complexes with different anions Na-3[Ti(H(2)cit)(2)(Hcit)] center dot 9H(2)O (1), K-4[Ti(H(2)cit)(Hcit)2] center dot 4H(2)O (2), K-5[Ti(Hcit)(3)] center dot 4H(2)O (3) and Na-7[TiH(cit)(3)] center dot 18H(2)O (4) (H(4)Cit = citric acid) were isolated in pure forms from the solutions of titanate and citrate at various pH values. X-ray structural analyses revealed the presence of a monomeric tricitrato titanium unit in the four complexes. Each Ti(IV) ion is coordinated octahedrally by the three citrate ligands in different protonated forms. The citrate ligand chelates bidentately to the titanium ion through its negatively charged alpha-alkoxy and alpha-carboxy groups. This is consistent with the large downfield C-13 NMR shifts for the carbon atoms bearing the alpha-alkoxy and alpha-carboxy groups. The very strong hydrogen-bonds existing in the protonated and deprotonated beta-carboxy groups may be the key factor for the stabilization of the titanium citrate complexes. When the pH value is lower than 7.0, C-13 NMR spectra of 1:3 Ti:citrate solutions are similar to those of the titanium citrate complexes isolated at the corresponding pH values. The dissociation of free citrate increases with the rise of pH value. However, C-13 NMR spectra of 1:3 Ti:citrate solutions indicate that there may exist different citrate titanium species when the pH value is higher than 7.0. (C) 2006 Published by Elsevier Ltd.