Physicochemical study of titanium(III) mononuclear, dinuclear, and dimeric polyaminocarboxylates formed in aqueous and frozen aqueous solution

Abstract

Potentiometric and e.s.r. studies have proved the existence of dimeric and dinuclear titanium(III) complexes of some polyaminocarboxylic acids. The ligands studied include ethylenediaminetetra-acetic acid (EDTA), 3,6-dioxaoctane-1,8-diamine-NNN′N′-tetra-acetic acid (EGTA), diethylenetriaminepenta-acetic acid (DTPA), and triethylene-tetra- aminehexa-acetic acid (TTHA). Although they are structurally similar, the nature of their titanium(III) chelates is quite diverse. Potentiometric titration data has shown that titanium(III) will form 2 : 1 and 1 : 1 chelates with EGTA, DTPA, and TTHA which are stable over a wide pH range. E.s.r. measurements at 77 K on solutions containing the dinuclear 2 :1 titanium(III) EGTA, DTPA, or TTHA chelates reveal the presence of ΔM₈=±1 and ΔM₈=±2 transitions arising from a triplet state involving an essentially magnetic dipolar interaction between the two titanium(III) ions in the dinuclear chelates. Similar measurements show that signals are also observed at room temperature which are again presumably due to the triplet state. E.s.r. measurements on solutions containing titanium(III) and EGTA or TTHA in 1 : 1 mole ratio similarly show the presence of ΔM₈=±2 transitions, at significantly different field positions than for the 2 : 1 chelate, indicating the formation of a dimeric species. AΔM₈=±2 signal is not observed in the case of DTPA indicating that the mononuclear chelate is the major species formed under 1 :1 conditions. Solutions containing titanium(III) and EDTA in 1 : 1 mole ratio give rise to ΔM₈=±1 and ΔM₈=±2 signals at 77 K providing unequivocal evidence for the formation of dimeric species. From a survey of e.s.r. spectral data and comparison of results obtained on similar systems, structures have been proposed for the dimeric and dinuclear complexes formed in solution.