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 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.