Spectropotentiometric determination of aqueous stability constants for complexation of titanium(IV) by the bio-relevant ligands citrate and L-ascorbate.

摘要

Aqueous metal-ligand stability constants were long determined by potentiometric titration alone. In cases where the complexation is associated with an electronic transition of appropriate energy, however, UV/vis spectroscopy can be used to monitor the transition, and software based on least-squares fitting can treat both data types simultaneously as a function of added titrant. Several other complementary analytical tools can confirm or reject proposed speciation models generated from the stability constants. Although developed for specific metal-ligand systems, this methodology should find wide applicability due to the rigorous standards of analysis that were incorporated into its development and validation, and to the growing use of stability constants in bioinorganic analytical chemistry.; The first chapter reports the streamlined procedure and apparatus developed for the spectropotentiometric determination of aqueous metal-ligand stability constants in an inert atmosphere. The methodology is specifically designed for the experimentally difficult case of a hydrolysis-prone metal in conjunction with a ligand that can produce mono-, bis-, or tris-complexes. Many bio- and environmentally-relevant metal-ligand systems are of this type.; In the second chapter, spectropotentiometric methods are used to describe a series of pH-dependent titanium(IV) coordination complexes of L-ascorbic acid. Important aspects of the aqueous interactions in this system are investigated, including ligand binding mode, pH-dependent metal-ligand stoichiometry, and metal ion-promoted hydrolysis. Stability constants are determined for all abundant aqueous complexes. In the range pH 3∼10, the ascorbate ligand coordinates by bidentate chelation of the dianion to the titanium(IV) ion via proton displacement. A pH-dependent metal-ligand binding motif is developed that is consistent with the stepwise binding of three ascorbate ligands followed by metal ion-promoted hydrolysis, stepwise ligand dissociation, and the concomitant binding of hydroxide ion. Results from electrospray mass spectrometry, a modified Job's analysis, and kinetics measurements support the speciation.; The water-soluble complexes of Ti(IV) with citrate are characterized in the third chapter. The aqueous 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. Stability constants for observed species are tabulated, and optical spectra are reported.