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

The new ligand H6 do3aPida 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 H4 dota and iminodiacetic acid themselves, respectively, so, macrocyclic and iminodiacetate units behave independently. The formation kinetics of the Ce(iii)-H6 do3aPida 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)-H4 dota system. The kinetic inertness of the [Ce(do3aPida )]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 (do3aPida )(OH)(H2 O)4 ]Cl·7.5H2 O.