Lanthanide ions coordination sites

Various substitution studies (171-173) were conducted in the early stages of research on these new oxide superconductors. One most dramatic result was the facile substitution of other (magnetic) lanthanide ions for yttrium in the VUI-coordinated site of the structure. The incorporation of these magnetic ions had no effect on the superconductivity nor the Tc of the material— quite astounding, since the presence of magnetic ions in superconductors was previously believed to destroy the phenomenon entirely Table 13 presents several examples of such substituted compounds. 

Subsequently, Chadwick and Williams (17) have pointed out that the above approaches are rather naive since no one fixed location for the lanthanide ion should be assumed. The LIS values derived from Yb(dpm)3 and 5cr-cholestan-3/ -ol have been corrected at carbons close to the coordination site for a complex formation shift by using the LIS produced by the diamagnetic La(dpm)3 compound. Given this correction, the Yb ion is computer positioned by assuming an equal population of sites symmetrically disposed about the -0-C(3)-H(3) plane. 

The discussed examples clearly demonstrate the importance of azamacrocycles as structural element to create supramolecular function. Their rigid structure, the basicity and transition metal-ion coordinating ability make them suitable as scaffolds and binding sites. Lanthanide chemosensors containing azamacrocyclic ligands have already reached applications in medical diagnostics. Other applications of azamacrocyclic systems with supramolecular functions, particularly in biochemistry, will follow. 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

Upon complexation with a lanthanide ion, these complexes may form square antiprism or twisted square antiprism (TS APR) structures with a vacant coordination site in the cap position, which is assumed to be occupied by a solvent molecule. Just as in the chelated complexes described previously, two distinct types of chiral stereochemistry are present. In analogy with OC-6 species, the sense of rotation of the pendant arms is denoted as A or A depending upon if the arms rotate clockwise (A) or counterclockwise (A) as one proceeds down the direction of the C4 axis. There is also chirality (or helicity) associated with the nonplanar 12-membered ring. If one looks along the skew-line connecting the coordinated nitrogens, the carbon atoms... 

Axial symmetry in trimetallic lanthanide complexes requires the location of the metal ions along the molecular threefold or fourfold axes. Since the terminal coordination sites are different from the central coordination site for symmetry reasons, two different crystal-field parameters 2terminal ancj /)2ccntral must be considered. Equation (61) holds for the general case of three (n = 3) magnetically non-coupled lanthanide metal ions packed along the symmetry axis and eqs. (62)-(64) can be used for homotrimetallic axial complexes. To the best of our knowledge, only one partial study of the NMR data for a >3-symmetrical axial trimetallic complex has been reported (Bocquet et al., 2002 Floquet et al., 2003 see sect. 5.1.2). The Dih-symmetrical complexes [R3(L16-3H)2(OH2)6]3+ do not fit the requirements for axial symmetry since the metal ions are located on mirror planes and not on the threefold axis, but... 

As discussed early in this chapter, quantum confinement has little effect on the localized electronic levels of lanthanide ions doped in insulating nanocrystals. But when the particle size becomes very small and approaches to a few nanometers, some exceptions may be observed. The change of lanthanide energy level structure in very small nanocrystals (1-10 nm) is due to a different local environment around the lanthanide ion that leads to a drastic change in bond length and coordination number. Lanthanide luminescence from the new sites generated in nanoparticles can be found experimentally. The most typical case is that observed in nanofilms ofEu Y203 with a thickness of 1 nm, which exhibits a completely different emission behavior from that of thick films (100-500 nm) (Bar et al., 2003). 

Lanthanide chemistry with Schiff bases is quite extensive and numerous acyclic, cyclic, monometallic and polymetallic (4f-4f, 4f-5f, 4f-nd) complexes have been synthesized and studied, in particular with compartmental ligands, owing to the ability of the latter to bind two or more metal ions in close proximity. Asymmetrization of these ligands also provides important diversification of the coordinating sites (Vigato and Tamburini, 2004). On the other... 

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