Crystal structure and solution dynamics of the lutetium(III) chelate of DOTA

The X-ray structure of Na[Lu(DOTA)(H₂O)]·4H₂O consists of (-1) complex anions balanced by sodium cations. The nine-coordinate Lu(III) is linked to the four nitrogen and to four oxygen atoms of the macrocyclic ligand DOTA in a square antiprismatic arrangement with a coordinated water molecule in capping position. In aqueous solution the complex is present under two isomeric forms in the relative ratio of 10:1, as evaluated from the low temperature limiting ¹³C NMR spectrum. The variable temperature behavior is consistent with the presence of two dynamic processes. One exchanges enantiomeric pairs for both isomers and involves concerted conformational changes of the ethylenic groups of the macrocyclic ring and rotation of the acetate arms. The other corresponds to the interconversion between the two isomers. Complete line-shape analysis of the variable-temperature ¹³C NMR spectra of Lu-DOTA allowed a quantitative evaluation of the two processes. The enantiomerization is slower than the isomerization: the corresponding calculated free energies of activation are 65.9 ± 1.2, 62.7 ± 2.0 (exchange major⇄minor) and 58.7 ± 2.1 kJ mol^-1 (exchange minor ⇄ major), respectively.