Lanthanide complexes macrocyclic polyethers

Complexes of alcohols like methanol, ethanol, 2-propanol and n-butanol (116-122), and ethers like Diox (47,120,123-125) and THF (126-128) have been prepared. The bonding between these ligands and the metal ions is considered to be very weak. In recent years, complexes of the lanthanides with a few macrocyclic polyethers have been reported. Cassol et al. (129) have prepared the complexes of benzo-15-crown-5 and dibenzo-18-crown-6 with lanthanide nitrates and isothiocyanates. King and Heckley (130) have also reported the complexes of these ligands with lanthanide nitrates. The heavier lanthanide nitrate complexes of dibenzo-18-crown-6... 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Lanthanide complexes with macrocyclic polyethers [64] have been obtained by mixing solutions of the ligand and solution of a hydrated lanthanide salt [65]. 

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

Structural investigation into the steric control of polyether complexation in the lanthanide series - macrocyclic 18-crown-6 versus acyclic pentaethylene glycol, R. D. Rogers, A. N. Rollins, R. D. Etzenhouser, E. J. Voss and C. B. Bauer, Inorg. Chem., 1993, 32, 3451. 

Ligand replacements for which kinetic data have been reported include [Ni(dgen)] + plus edta (parallel dissociative and associative paths), [Pb(edta)] plus R-( —)-pdta, polyether complexes of lead(n) plus nitrogen macrocycles, metal(ii)-oxine complexes plus edta, and a variety of analogous reactions involving lanthanide and actinide complexes (see Chapter 10). Kinetic data are also available for ligand exchange between trien and tetren complexes of cad-mium(ii) and the edta complex of copper(ii). ... 

A non-template synthesis of the 1 + 1 macrocycle H2LI77O derived from condensation of the (2-hydroxybenzaldehyde-3-yl)-0-(CH2CH20)3-(3-yl-2-hydroxy-benzaldehyde) and 1,2-diaminoethane in acetonitrile has been reported and its structure confirmed by X-ray diffraction [125], Direct complexation with LaCls-7H2O in methanol yields [La(H2L1770)(H2O)4]Cl3. Single crystal X-ray analysis revealed that lanthanum is 10-coordinated, like barium in the complexes shown in Figures 8-7, 8-8 and 8-10. It should be mentioned that lanthanide(III) ions have not been employed so far as templates for the construction of related macrocycles, in which a softer donor site is combined with a hard polyether binding site. 

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