Kinetics of formation and dissociation of lanthanide(iii) complexes with the 13-membered macrocyclic ligand TRITA4−

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Abstract
The tetraazamacrocyclic ligand TRITA4− is intermediate in size between the widely studied and medically used 12-membered DOTA4− and the 14-membered TETA4−. The kinetic inertness of GdTRITA− was characterized by the rates of exchange reactions with Zn2+ and Eu3+. In the Zn2+ exchange, a second order [H+] dependence was found for the pseudo-first-order rate constant (k0 = (4.2 ± 0.5) × 10−7 s−1; k′ = (3.5 ± 0.3) × 10−1 M−1s−1, k″ = (1.4 ± 0.4) × 103 M−2s−1). In the Eu3+ exchange, at pH 2+ species. At physiological pH, the kinetic inertness of GdTRITA− is considerably lower than that of GdDOTA− (t1/2 = 444 h (25 °C) vs. 3.8 × 105 h (37 °C), respectively). However, GdTRITA− is still kinetically more inert than GdDTPA2−, the most commonly used MRI contrast agent (t1/2 = 127 h). The formation reactions of LnTRITA− complexes (Ln = Ce, Gd and Yb) proceed via the rapid formation of a diprotonated intermediate and its subsequent deprotonation and rearrangement in a slow, OH− catalyzed process. The stability of the LnH2TRITA* intermediates (log KLnH2L* = 3.1–3.9) is lower than that of the DOTA-analogues. The rate constants of the OH− catalyzed step increase with decreasing lanthanide ion size, and are about twice as high as for DOTA-complexes.

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