Titanium(IV) Citrate Speciation and Structure under Environmentally and Biologically Relevant Conditions

The water-soluble complexes of Ti(IV) with citrate are of interest in environmental, biological, and materials chemistry. The aqueous solution speciation is revealed by spectropotentiometric titration. From pH 3−8, given at least three equivalents of ligand, 3:1 citrate/titanium complexes predominate in solution with successive deprotonation of dangling carboxylates as the pH increases. In this range and under these conditions, hydroxo- or oxo-metal species are not supported by the data. At ligand/metal ratios between 1:1 and 3:1, the data are difficult to fit, and are consistent with the formation of such hydroxo- or oxo- species. Stability constants for observed species are tabulated, featuring log β-values of 9.18 for the 1:1 complex [Ti(Hcit)]⁺, and 16.99, 20.41, 16.11, and 4.07 for the 3:1 complexes [Ti(H₂cit)₃]^2-, [Ti(H₂cit)(Hcit)₂]^4-, [Ti(Hcit)₂(cit)]^6-, and [Ti(cit)₃]^8-, respectively (citric acid = H₄cit). Optical spectra for the species are reported. The complexes exhibit similar yet distinct spectra, featuring putative citrate-to-Ti(IV) charge-transfer absorptions (λ_max ≈ 250−310 nm with ε ≈ 5000−7000 M^-1 cm^-1). The prevailing 3:1 citrate/titanium ratio in solution is supported by electrospray mass spectrometry data. The X-ray crystal structure of a fully deprotonated tris-citrate complex Na₈[Ti(C₆H₄O₇)₃]·17H₂O (1) (or Na₈[Ti(cit)₃]·17H₂O) that crystallizes from aqueous solution at pH 7−8 is reported. Compound 1 crystallizes in the triclinic space group P1̄, with a = 11.634(2) Å, b = 13.223(3) Å, c = 13.291(3) Å, V = 1982.9(7) ų, and Z = 2.