title: Titanium(IV) citrate speciation and structure under environmentally and biologically relevant conditions.
abstract: 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 beta-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 (lambda(max) approximately 250-310 nm with epsilon approximately 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(C(6)H(4)O(7))(3)].17H(2)O (1) (or Na(8)[Ti(cit)(3)].17H(2)O) that crystallizes from aqueous solution at pH 7-8 is reported. Compound 1 crystallizes in the triclinic space group P, with a = 11.634(2) Angstroms, b = 13.223(3) Angstroms, c = 13.291(3) Angstroms, V = 1982.9(7) Angstroms(3), and Z = 2.
authors.affiliation: Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, USA.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Collins
authors.forename: Ritika
authors.initials: R
authors.lastname: Uppal
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Incarvito
authors.forename: Ann M
authors.initials: AM
authors.lastname: Valentine
mesh_terms: D002951:Citrates / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D014025:Titanium / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D002951:Citrates; D009942:Organometallic Compounds; D014867:Water; D014025:Titanium