pH-dependent isolations and spectroscopic, structural, and thermal studies of titanium citrate complexes.

Titanium(IV) citrate complexes (NH4)2[Ti(H2cit)3]·3H2O (1), (NH4)5[Fe(H2O)6][Ti(H2cit)3(Hcit)3Ti]·3H2O (2), Ba2[Ti(H2cit)(Hcit)2]·8H2O (3), and Ba3(NH4)7[Ti(cit)3H3(cit)3Ti]·15H2O (4) (H4cit = citric acid) were isolated in pure form from the solutions of titanium(IV) citrate with various countercations. The isolated complexes were characterized by elemental analyses, IR spectra, and 1H NMR and 13C NMR spectra. The formation of titanium(IV) citrate complexes depends mainly on the pH of the solutions, that is, pH 1.0−2.8 for the formation of ammonium titanium(IV) citrate 1, pH 2.5−3.5 for ammonium iron titanium(IV) citrate 2, pH 2.8−4.0 for dibarium titanium(IV) citrate 3, and pH 5.0−6.0 for ammonium barium titanium(IV) citrate 4. X-ray structural analyses revealed that complexes 2−4 featured three different protonated forms of bidentate citrate anions that chelate to the titanium(IV) atom through their negatively charged α-alkoxyl and α-carboxyl oxygen atoms. This is consistent with the large downfield shi...