Structure and conformation of Th(IV)-complexes of ethylenediaminetetraacetic acid and its analogs in aqueous solution

Abstract

The thorium(IV) complexes of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaaceticacid (DTPA), triethylenetetraminehexaacIetic acid (TTHA) and N-hydroxyethylethylenetriaacetic acid (HEDTA) have been studied in aqueous (D₂O) solutions as a function of "a" value (a = moles of base added per mole of ligand or chelate). On the basis of aqueous (D₂O) infrared and proton nuclear magnetic resonance spectra of these Th(IV) complexes, reasonable solution structures are proposed and compared with structures suggested on the basis of previous potentiometric studies. The presence or absence of various nmr splitting patterns has been used to determine qualitatively the lability of individual thorium-ligand bonds. The simple nmr spectra obtained for the Th(IV)-EDTA chelate are indicative of the short lifetime soft the thorium-EDTA bonds. Th(IV) ion begins to hydrolyze to Th(OH)₄ above pD 10 in the Th(IV)-EDTA chelate, whereas, hydrolysis of Th(IV) ion in the more stable Th(IV)-DTPA complex does not occur until pD>12. 3. New potentiometric data gave no evidence for the formation of 1:2 or 2:3 Th(IV)-EDTA complexes when the appropriate molar ratios of metal to ligand were used. Nmr data for inter molecular ligand exchange of 1 to 2 molar ratios of Th(IV) ion to EDTA reveal a slower exchange rate for the mono-protonated free ligand as opposed to the deprotonated free ligand, under the same solution conditions. The slower intermolecular exchange rates exhibited by the more protonated form of EDTA have been interpreted as the ability of a proton on a nitrogen atom to hinder formation of the thorium-nitrogen bond. ...