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

Titanium(IV) citrate complexes (NH₄)₂[Ti(H₂cit)₃]·3H₂O (<b>1</b>), (NH₄)₅[Fe(H₂O)₆][Ti(H₂cit)₃(Hcit)₃Ti]·3H₂O (<b>2</b>), Ba₂[Ti(H₂cit)(Hcit)₂]·8H₂O (<b>3</b>), and Ba₃(NH₄)₇[Ti(cit)₃H₃(cit)₃Ti]·15H₂O (<b>4</b>) (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 <b>1</b>, pH 2.5−3.5 for ammonium iron titanium(IV) citrate <b>2</b>, pH 2.8−4.0 for dibarium titanium(IV) citrate <b>3</b>, and pH 5.0−6.0 for ammonium barium titanium(IV) citrate <b>4</b>. X-ray structural analyses revealed that complexes <b>2</b>−<b>4</b> 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 <b>3</b> and <b>4 </b>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 <b>2</b>, 2.464(7) Å for complex <b>3</b>, and 2.467(7) Å for complex <b>4</b>] may be the key factor for the stabilization of the citrate complexes. The decomposition of complex <b>3</b> results in the formation of a pure dibarium titanate phase and <b>4 </b>for the mixed phases of dibarium titanate and barium titanate at 1000 °C.