STRUCTURE AND DYNAMICS OF LANTHANOID TETRAAZACYCLODODECANETETRAACETATE DOTA COMPLEXES IN SOLUTION

In aqueous solution lanthanoid DOTA complexes exist as a mixture of two stereoisomers, and their structures have been determined from the dipolar parts of the observed H-1-NMR chemical shifts for EuDOTA and YbDOTA. The relative concentration of the DELTA(deltadeltadeltadelta) isomer decreases with decreasing ionic radius. For EuDOTA a 1: 4 mixture is observed, and the main isomer has a DELTA(lambdalambdalambdalambda) structure which is also found in the solid state. Two independent intramolecular isomerizations were detected by H-1-NMR Exchange Spectroscopy (EXSY). The faster process is the rotation of the four acetate substituents around the C-N bonds [DELTA(lambdalambdalambdalambda) --> LAMBDA(lambdalambdalambdalambda) isomerization]. The cyclodecane ring inversion is slower [DELTA(lambdalambdalambdalambda) --> DELTA(deltadeltadeltadelta) isomerization]. The intramolecular racemization of the DELTA(lambdalambdalambdalambda) main isomer into its LAMBDA(deltadeltadeltadelta) enantiomer includes two successive isomerizations in a DELTA(lambdalambdalambdalambda) --> LAMBDA(lambdalambdalambdalambda) --> LAMBDA(deltadeltadeltadelta) sequence and is the slowest process.