DOTA analogues with a phosphinate-iminodiacetate pendant arm: modification of the complex formation rate with a strongly chelating pendant

Abstract

The new ligand H₆do3aP^ida combines the macrocyclic DOTA-like cavity and the open-chain iminodiacetate group connected through a coordinating phosphinate spacer. Its acid–base and coordination properties in solution were studied by potentiometry. Thermodynamic coordination characteristics of both chelating units are similar to those reported for H₄dota and iminodiacetic acid themselves, respectively, so, macrocyclic and iminodiacetate units behave independently. The formation kinetics of the Ce(III)–H₆do3aP^ida complex was studied by UV-Vis spectrophotometry. Various out-of-cage intermediates were identified with 1 : 1, 1 : 2 and 2 : 1 ligand-to-metal ratios. The presence of the strongly coordinating iminodiacetate group significantly slows down the metal ion transfer into the macrocyclic cavity and, so, the formation of the in-cage complex is two orders of magnitude slower than that reported for the Ce(III)–H₄dota system. The kinetic inertness of the [Ce(do3aP^ida)]^3– complex towards acid-assisted dissociation is comparable to that of the [Ce(dota)]⁻ complex. The coordination modes of the ligand are demonstrated in the solid-state structure of [Cu₄(do3aP^ida)(OH)(H₂O)₄]Cl·7.5H₂O.