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pH-Dependent Isolations and Spectroscopic, Structural, and Thermal Studies of Titanium Citrate Complexes

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posted on 2004-10-04, 00:00 authored by Yuan-Fu Deng, Zhao-Hui Zhou, Hui-Lin Wan
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 24 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 shifts of the 13C NMR spectra for the carbon atoms bearing the α-alkoxyl and α-carboxyl groups. The typical coordination modes of the barium atoms in complexes 3 and 4 are six-coordinated, with three α-alkoxyl groups and three β-carboxyl groups of citrate ions. The strong hydrogen bonding between the β-carboxylic acid and the β-carboxyl groups [2.634(8) Å for complex 2, 2.464(7) Å for complex 3, and 2.467(7) Å 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 °C.

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