(14), have been determined by single-crystal X-ray crystallography. In the solid state, the complexes of the lighter lanthanide ions La 3+ −Dy 3+ show a 10-coordinated geometry close to a distorted bicapped antiprism, where the carboxylate pendants are situated alternatively above and below the best plane that contains the nitrogen donor atoms. The complexes of the heavier ions, Ho 3+ −Lu 3+ , have a 9-coordinated geometry close to distorted tricapped trigonal prism, with one of the pendant carboxylate groups uncoordinated. The ligand is in a "twist−fold" conformation, where the twisting of the pyridine units is accompanied by an overall folding of the major ring of the macrocycle so that the pyridine nitrogen atoms and the metal are far from linear. The aqueous solution structures of the complexes were thoroughly characterized, the diamagnetic ones (La 3+ and Lu 3+ ) by their COSY NMR spectra, and the paramagnetic complexes using a linear least-squares fitting of the 1 H LIS (lanthanide-induced shift) and LIR (lanthanide-induced relaxation) data with rhombic magnetic susceptibility tensors. The solution structures obtained for the La 3+ −Dy 3+ complexes (10-coordinate) and for the Tm 3+ −Lu 3+ complexes (9-coordinate) are in very good agreement with the corresponding crystal structures. However, the 10-coordinate structure is still exclusive in solution for the Ho 3+ complex and predominant for the Er 3+ complex.

Schiff-base lateral macrobicycles containing two different binding units, a rigid and unsaturated N(2)X set (X: N, O) and a flexible and cyclic N(2)O(n)() set, linked by two aromatic bridges, have been prepared by reaction of the appropriate bibracchial diamines N,N'-bis(aminobenzyl)-diaza-crown and diformyl precursors in the presence of Ba(II) as templating agent. The expected cryptands do not form in the absence of the cation; the presence of this metal ion is necessary to orient the diamine precursor in a syn conformation. Comparison of the X-ray crystal structures of the barium complex of the bibracchial diamine N,N'-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and the barium complex of the cryptand derived from it incorporating a pyridine unit indicates that the encapsulation of the metal ion is clearly more effective in the case of the cryptand. The coordination of Ba(II) ion to the pyridine nitrogen atom promotes the displacement of the metal into the cavity of the cryptand as well as important changes in the conformation of the crown unit, although the fold of the pendant arms remains practically unchanged. The cryptands reported constitute the first example of a novel family of macrobicycles.

Complexes between the tetrapyridyl pendant-armed macrocyclic ligand (L) and the trivalent lanthanide ions have been synthesized, and structural studies have been made both in the solid state and in aqueous solution. The crystal structures of the La, Ce, Pr, Gd, Tb, Er, and Tm complexes have been determined by single-crystal X-ray crystallography. In the solid state, all the cation complexes show a 10-coordinated geometry close to a distorted bicapped antiprism, with the pyridine pendants situated alternatively above and below the main plane of the macrocycle. The conformations of the two five-membered chelate rings present in the complexes change along the lanthanide series. The La(III) and Ce(III) complexes show a lambdadelta (or deltalambda) conformation, while the complexes of the heavier lanthanide ions present lambdalambda (or deltadelta) conformation. The cationic [Ln(L)]3+ complexes (Ln = La, Pr, Eu, Tb, and Tm) were also characterized by theoretical calculations at the density-functional theory (DFT) B3LYP level. The theoretical calculations predict a stabilization of the lambdalambda (or deltadelta) conformation on decreasing the ionic radius of the Ln(III) ion, in agreement with the experimental evidence. The solution structures show a good agreement with the calculated ones, as demonstrated by paramagnetic NMR measurements (lanthanide induced shifts and relaxation rate enhancements). The 1H NMR spectra indicate an effective D2 symmetry of the complexes in D2O solution. The 1H lanthanide induced shifts (LIS) observed for the Ce(III), Tm(III), and Yb(III) complexes can be fit to a theoretical model assuming that dipolar contributions are dominant for all protons. The resulting calculated values are consistent with highly rhombic magnetic susceptibility tensors with the magnetic axes being coincident with the symmetry axes of the molecule. In contrast with the solid-state structure, the analysis of the LIS data indicates that the Ce(III) complexes present a lambdalambda (or deltadelta) conformation in solution.