Synthesis and Complexation Properties of 1,4,7-Triazacyclononane-1,4,7-tris(2-methylacetate)

A new ligand, l,4, 7-triazacyclononane-l,4, 7-tris(2-methylacetate) (NOTMA) has been prepared and some of its thermodynamic and kinetic complexation properties determined. The highest protonation constant in NOTMA was somewhat higher than the corresponding value in the analogous acetate ligand, NOTA (logK1 = 12.2 versus 11.4). The measured metal ion NOTMA stability constants were logKML = 6.4, 8.8, 12.3, 12.8, 15.5, and 18.6 for Ca2 +, Mg2 +, Cd2 +, Gd3 +, Mn2+ and Zn2 +, respectively. These stability constants were either similar to (Mg2 +, Mn2 +, Zn2+) or smaller (Ca2 +, Cd2 +, Gd3+) than the respective complexes of NOTA, perhaps reflecting steric interactions between the ethylenediamine protons and the methyl groups in the NOTMA complexes formed with metal ions of larger ionic radii. Water proton relaxivity measurements indicate that Gd(NOTMA) has a higher water coordination number than Gd(NOTA). This was confirmed by fluorescence lifetime measurements of Eu(NOTMA) and Eu(NOTA) in H2O and D2O. The mechanism of complex formation and dissociation appears to be similar for NOTA and NOTMA. NOTMA forms complexes with Gd3 + about 2-orders of magnitude more slowly than NOTA (kOH = 5.8 × 105 M-1s-1 versus 7.1 × 107 M-1s-1) while free Gd3+ is released somewhat slower at pH 7 from Gd(NOTMA) than from Gd(NOTA) (kd' (acid catalyzed) = 9.3 × 10-3 M-1s-1 versus 2.3 × 10-2 M-1s-1, respectively).