DOTA analogues with a phosphinate-iminodiacetate pendant arm: modification of the complex formation rate with a strongly chelating pendantElectronic supplementary information (ESI) available: NMR spectra, detailed description and results of potentiometry, distribution diagrams, tables and figures related to solid-state structures, and UV-Vis spectra of the complexes. CCDC 1482406 and 1482407. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt01797a

The new ligand H 6 do3aP 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 H 4 dota and iminodiacetic acid themselves, respectively, so, macrocyclic and iminodiacetate units behave independently. The formation kinetics of the Ce( iii )-H 6 do3aP 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 )-H 4 dota 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 [Cu 4 (do3aP ida )(OH)(H 2 O) 4 ]Cl·7.5H 2 O. The formation of highly stable out-of-cage complexes slows down the transfer of the metal ion into the cavity of the macrocyclic ligand.