CRYSTAL-STRUCTURE AND SOLUTION DYNAMICS OF THE LUTETIUM(III) CHELATE OF DOTA

The X-ray structure of Na[Lu(DOTA)(H2O)]. 4H(2)O consists of (-1) comp lex anions balanced by sodium cations. The nine-coordinate Lu(III) is linked to the four nitrogen and to four oxygen atoms of the macrocycli c ligand DOTA in a square antiprismatic arrangement with a coordinated water molecule in capping position. In aqueous solution the complex i s present under two isomeric forms in the relative ratio of 10:1, as e valuated from the low temperature limiting C-13 NMR spectrum. The vari able temperature behavior is consistent with the presence of two dynam ic processes. One exchanges enantiomeric pairs for both isomers and in volves concerted conformational changes of the ethylenic groups of the macrocyclic ring and rotation of the acetate arms. The other correspo nds to the interconversion between the two isomers. Complete line-shap e analysis of the variable-temperature C-13 NMR spectra of LuDOTA allo wed a quantitative evaluation of the two processes. The enantiomerizat ion is slower than the isomerization: the corresponding calculated fre e energies of activation are 65.9 +/- 1.2, 62.7 +/- 2.0 (exchange majo r reversible arrow minor) and 58.7 +/- 2.1 kJ mol(-1) (exchange minor reversible arrow major), respectively.