Mononuclear Titanium(IV)−Citrate Complexes from Aqueous Solutions: pH-Specific Synthesis and Structural and Spectroscopic Studies in Relevance to Aqueous Titanium(IV)−Citrate Speciation

Titanium is a metal frequently employed in a plethora of materials supporting medical applications. In an effort to
comprehend the involvement of titanium in requisite biological interactions with physiological ligands, synthetic
efforts were launched targeting aqueous soluble species of Ti(IV). To this end, aqueous reactions of TiCl4 with
citric acid afforded expediently, under pH-specific conditions, the colorless crystalline materials Na6[Ti(C6H4.5O7)2(C6H5O7)]·16H2O (1) and Na3(NH4)3[Ti(C6H4.5O7)2(C6H5O7)]·9H2O (2). Complexes 1 and 2 were characterized by
elemental analysis, FT-IR, 13C-MAS solid state and solution NMR, cyclic voltammetry, and X-ray crystallography.
1 crystallizes in the triclinic space group P1̄, with a = 15.511(9) Å, b = 15.58(1) Å, c = 9.848(5) Å, α = 85.35(2)°, β = 76.53(2)°, γ = 61.97(2)°, V = 2042(2) Å3, and Z = 2. 2 crystallizes in the triclinic space group P1̄, with
a = 12.437(5) Å, b = 12.440(5) Å, c = 12.041(5) Å, α = 83.08(2)°, β = 81.43(2)°, γ = 67.45(2)°, V = 1697(2)
Å3, and Z = 2. The X-ray structures of 1 and 2 reveal the presence of a mononuclear complex, with Ti(IV)
coordinated to three citrate ligands in a distorted octahedral geometry around Ti(IV). The citrates employ their
central alkoxide and carboxylate groups to bind Ti(V), while the terminal carboxylates stay away from the TiIVO6
core. Worth noting in 1 and 2 is the similar mode of coordination but variable degree of protonation of the bound
citrates, with the locus of (de)protonation being the noncoordinating terminal carboxylates. As a result, this work
suggests the presence of a number of different Ti(IV)−citrate species of the same nuclearity and coordination
geometry as a function of pH. This is consistent with the so far existing pool of mononuclear Ti(IV)−citrate species
and provides a logical account of the aqueous speciation in the requisite binary system. Such information is vital
in trying to delineate the interactions of soluble and bioavailable Ti(IV) forms promoting biological interactions in
humans. To this end, chemical properties, structural attributes, and speciation links to potential ensuing biological
effects are dwelled on.