Three new 10-membered-ring triaza tricarboxylate ligands have been synthesized and the formations of their Gd3+ complexes examined. The ligands 1,4,7-triazacyclodecane-N,N′,N″-triacetate (DETA3-), 9-methyl-1,4,7-triazacyclodecane-N,N′,N″-triacetate (MeDETA3-), and 9,9-dimethyl-1,4,7-triazacyclodecane-N,N′,N″-triacetate (Me2DETA3-) have an unusually high first protonation constant (log K1 ≈ 14-15), and evidence is presented for the formation of a strongly hydrogen-bonded proton between the two nitrogens that share the propylene bridge in the monoprotonated forms of these chelates. Exchange of the hydrogen-bonded proton is slow below 0 °C, and its 1H NMR signal has been detected. Complexation with Gd3+ is also slow, preventing determination of metal ion-ligand stabilities by potentiometry. Conditional stability constants for Gd(DETA), Gd(MeDETA), and Gd-(Me2DETA) were measured by using a proton relaxivity technique and compared with similar data for the 9-membered-ring macrocyclic complex Gd(NOTA). The rates of formation of Gd(DETA) and Gd(MeDETA) are similar, while Gd(Me2DETA) forms more slowly. The rate-determining step of complexation is proton loss and rearrangement of a monoprotonated intermediate, formed in a fast preequilibrium reaction. Dissociation of the complexes takes place both in a spontaneous fashion and through a proton-assisted pathway. The rate of both processes decreases in the order Gd(Me2DETA) ≫ Gd(NOTA) > Gd(DETA) > Gd(MeDETA). An increase in ring size from 9 to 10 and substitution of a methyl group onto a ring carbon of the ligand lead to increased kinetic stabilities of the complexes. The substitution of two methyl groups onto a ring carbon, however, results in a significant decrease in both the thermodynamic and kinetic stabilities of the resulting Gd3+ complex.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry