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

Titanium(IV) citrate complexes (NH4)(2)[Ti(H(2)cit)(3)].3H(2)O (1), (NH4)(5)[Fe(H2O)(6)][Ti(H(2)cit)(3)(Hcit)(3)Ti].3H(2)O (2), Ba-2[Ti(H(2)cit)(Hcit)(2)].8H(2)O (3), and Ba-3(NH4)(7)[Ti(cit)(3)H-3(cit)(3)Ti].15H(2)O (4) (H(4)cit = 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 H-1 NMR and C-13 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 alpha-alkoxyl and alpha-carboxyl oxygen atoms. This is consistent with the large downfield shifts of the C-13 NMR spectra for the carbon atoms bearing the alpha-alkoxyl and alpha-carboxyl groups. The typical coordination modes of the barium atoms in complexes 3 and 4 are six-coordinated, with three alpha-alkoxyl groups and three beta-carboxyl groups of citrate ions. The strong hydrogen bonding between the beta-carboxylic acid and the beta-carboxyl groups [2.634(8) Angstrom for complex 2, 2.464(7) Angstrom for complex 3, and 2.467(7) Angstrom for complex 4] may be the key factor for the stabilization of the citrate complexes. The decomposition of complex 3 results in the formation of a pure dibarium titanate phase and 4 for the mixed phases of dibarium titanate and barium titanate at 1000 degreesC.