New lanthanide-cholesteryl ester complexes: spectroscopic evidence of their non-mesogenic character

Lanthanide (EU3+, Nd3+, Sm3+) [thenoyltrifluoroacetonate(TTA)](3)-cholester yl nonanoate and tetradecanoate complexes were studied by IR and H-1, C-13 and F-19 NMR spectroscopy. The H-1 NMR signals of these paramagnetic comple xes were fully assigned by means of 2D NMR and relaxation techniques. The c oordination geometry was deduced from experimental data by a computer simul ation which showed that the ligand requires a similar to 180 degrees twist Of the C-28-O-3 bond of the ester in a linear conformation to avoid steric repulsions with the TTA ligands. Substantial lanthanide-induced shifts (LIS ) were observed for Eu3+ and Nd3+ complexes whereas Sm3+ caused smaller Del ta delta values. A series of chemical shift determinations led to the deter mination of the association constants of these complexes (K approximate to 22M(-1)). The deduced structure allowed us to explain the loss of mesogenic character of these complexes compared with the free calamitic cholesteryl esters. Copyright (C) 2001 John Wiley & Sons, Ltd.