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.

钛(IV)柠檬酸盐复合物（(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），以及Ba3(NH4)7[Ti(cit)3H3(cit)3Ti]·15H2O（4）（H4cit = 柠檬酸）均已从不同反离子配位的钛(IV)柠檬酸盐溶液中纯化分离。通过元素分析、红外光谱以及1H核磁共振和13C核磁共振光谱对这些分离出的复合物进行了表征。钛(IV)柠檬酸盐复合物的形成主要取决于溶液的pH值，例如，铵钛(IV)柠檬酸盐1的形成pH范围为1.0−2.8，铵铁钛(IV)柠檬酸盐2为2.5−3.5，二钡钛(IV)柠檬酸盐3为2.8−4.0，而铵钡钛(IV)柠檬酸盐4则为5.0−6.0。X射线结构分析揭示了复合物2−4具有三种不同的质子化形式的二齿柠檬酸根阴离子，它们通过带有负电荷的α-烷氧基和α-羧基氧原子与钛(IV)原子螯合。这与α-烷氧基和α-羧基所承载的碳原子在13C核磁共振光谱中出现的较大化学位移相一致。复合物3和4中钡原子的典型配位模式为六配位，包含三个柠檬酸根的α-烷氧基和三个β-羧基。复合物2、3和4中的β-羧酸与β-羧基之间的强氢键[复合物2为2.634(8) Å，复合物3为2.464(7) Å，复合物4为2.467(7) Å]可能是稳定柠檬酸盐复合物的关键因素。复合物3的分解在1000 °C下会产生纯净的二钡钛酸相和混合相的二钡钛酸与钡钛酸。