A new dinuclear Ti(IV)-peroxo-citrate complex from aqueous solutions. Synthetic, structural, and spectroscopic studies in relevance to aqueous titanium(IV)-peroxo-citrate speciation

The wide use of titanium in applied materials has prompted pertinent studies targeting the requisite chemistry of that metal's biological interactions. In order to understand such interactions as well as the requisite titanium aqueous speciation, we launched investigations on the synthesis and spectroscopic and structural characterization of Ti(IV) species with the physiological citric acid. Aqueous reactions of TiCl₄ with citric acid in the presence of H₂O₂ and neutralizing ammonia afforded expediently the red crystalline material (NH₄)₄[Ti₂(O ₂)₂(C₆H ₄O₇)₂]·2H₂O (1). Complex 1 was further characterized by UV-vis, FT-IR, FT- and laser-Raman, NMR, and finally by X-ray crystallography. Compound 1 crystallizes in the monoclinic space group P2₁/n, with a = 10.360(4) Å, b = 10.226(4) Å, c = 11.478(6) Å, β = 107.99(2)°, V = 1156.6(9) ų, and Z = 2. The X-ray structure of 1 reveals a dinuclear anionic complex containing a Ti^IV₂O₂ core. In that central unit, two fully deprotonated citrate ligands are coordinated to the metal ions through their carboxylate moieties in a monodentate fashion. The central alkoxides serve as bridges to the two titanium ions. Also attached to the Ti^IV₂O₂ core are two peroxo ligands each bound in a side-on fashion to the respective metal ions. NH₄⁺ ions neutralize the 4- charge of the anion in 1, further contributing to the stability of the derived lattice through H-bond formation. The structural similarities and differences with congener vanadium-(V)-peroxo-citrate complexes may point out potential implications in the chemistry of titanium with physiological ligands, when the former is present in a biologically relevant medium.