Pyridine-<i>N</i>-oxide Analogues of DOTA and Their Gadolinium(III) Complexes Endowed with a Fast Water Exchange on the Square-Antiprismatic Isomer

Two macrocyclic ligands derived from H4dota containing three acetate pendant arms and one 2-methylpyridine-N-oxide coordinating unit were synthesized. The ligand H3do3apyNO (H3L1, 10-[(1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) contains an unsubstituted pyridine-N-oxide ring; the ligand H4do3apyNO-C (H4L2, 10-[(4-carboxy-1-oxidopyridin-2-yl)methyl]-1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) is functionalized with a carboxylic group in the 4 position of the pyridine ring to allow attachment to other molecules. The ligands form octadentate (N4O4 environment) LnIII complexes, with one water molecule completing the coordination sphere in a capping position. The complexes are present in solution exclusively as square-antiprismatic isomers over the whole lanthanide series. The introduction of the carboxylic group to the pyridine-N-oxide unit in H4L2 has no significant effect on the hydration number (q = 1) and the water exchange rate of the [Gd(H2O)(L2)]− complex compared to the parent [Gd(H2O)(L1)] complex (water residence times: τM = 39 ns for [Gd(H2O)(L1)] and τM = 34 ns for [Gd(H2O)(L2)]− at 298 K).