Titanium(IV) citrate speciation and structure under environmentally and biologically relevant conditions

The water-soluble complexes of Tj(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 P-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(H(2)Cit)(3)](2-), [Ti(H(2)Cit)(HCit)(2)](4-), [Ti(HCit)(2)(Cit)](6-), and [Ti(Cit)(3)](8-), respectively (citric acid = H(4)cit)- Optical spectra for the species are reported. The complexes exhibit similar yet distinct spectra, featuring putative citrate-to-Ti(IV) charge-transfer absorptions (gimel(max) approximate to 250-310 nm with epsilon approximate to 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 Na-8[Ti(C6H4O7)(3)]center dot 17H(2)O (1) (or Na-8[Ti(cit)(3)]center dot 17H(2)O) that crystallizes from aqueous solution at pH 7-8 is reported. Compound 1 crystallizes in the triclinic space group P1, with a = 11.634(2) angstrom, b = 13.223(3) angstrom, c = 13.291(3) angstrom, V = 1982.9(7) angstrom(3), and Z = 2.