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

Lanthanide (Eu3+, Nd3+, Sm3+) [thenoyltrifluoroacetonate(TTA)]3‐cholesteryl nonanoate and tetradecanoate complexes were studied by IR and 1H, 13C and 19F NMR spectroscopy. The 1H NMR signals of these paramagnetic complexes were fully assigned by means of 2D NMR and relaxation techniques. The coordination geometry was deduced from experimental data by a computer simulation which showed that the ligand requires a ∼180° twist of the C28—O3 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 Δδ values. A series of chemical shift determinations led to the determination of the association constants of these complexes (K ≈ 22 M−1). The deduced structure allowed us to explain the loss of mesogenic character of these complexes compared with the free calamitic cholesteryl esters. Copyright © 2001 John Wiley & Sons, Ltd.