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Titanium(IV) Complexes with N,N′-Dialkyl-2,3-dihydroxyterephthalamides and 1-Hydroxy-2(1H)-pyridinone as Siderophore and Tunichrome Analogues

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journal contribution
posted on 16.11.2009, 00:00 by Ritika Uppal, Hayley P. Israel, Christopher D. Incarvito, Ann M. Valentine
The aqueous chemistry of Ti(IV) with biological ligands siderophores and tunichromes is modeled by using N,N′-dialkyl-2,3-dihydroxyterephthalamides (alTAMs), analogues of catecholamide-containing biomolecules, and 1-hydroxy-2(1H)-pyridinone (1,2-HOPO), an analogue of hydroxamate-containing biomolecules. Both types of ligands stabilize Ti(IV) with respect to hydrolytic precipitation, and afford tractable complexes. Complexes with the methyl derivative of alTAM, meTAM, are characterized by using mass spectrometry and UV/vis spectroscopy. Complexes with etTAM are characterized by the same techniques as well as X-ray crystallography, cyclic voltammetry, and spectropotentiomeric titration. The ESI mass spectra of these complexes in water show both 1:2 and 1:3 metal/ligand species. The X-ray crystal structure of a 1:2 complex, K2[Ti(etTAM)2(OCH3)2]·2CH3OH (1), is reported. The midpoint potential for reduction of 1 dissolved in solution is −0.98 V. A structure for a 1:3 Ti/etTAM species, Na2[Ti(etTAM)3] demonstrates the coordination and connectivity in that complex. Spectropotentiometric titrations in water reveal three metal-containing species in solution between pH 3 and 10. 1,2-HOPO supports Ti(IV) complexes that are stable and soluble in aqueous solution. The bis-HOPO complex [Ti(1,2-HOPO)2(OCH3)2] (5) was characterized by X-ray crystallography and by mass spectrometry in solution, and the tris-HOPO dimer [(1,2-HOPO)3TiOTi(1,2-HOPO)3] (6) was characterized by X-ray crystallography. Taken together, these experiments explore the characteristics of complexes that may form between siderophores and tunichromes with Ti(IV) in biology and in the environment, and guide efforts toward new, well characterized aqueous Ti(IV) complexes. By revealing the identities and some characteristics of complexes that form under a variety of conditions, these studies further our understanding of the complicated nature of aqueous titanium coordination chemistry.