Lanthanide dimers

The metal-metal distance is then nearly the same as the 0—0 distance. For the lanthanide dimer the bridging unit exists as the structure... 

Other signiflcant complexes of this type, containing one or more lanthanides, are formed by calixarenes (see Calixarenes) Much of this work has been prompted by their potential in separating lanthanides from uranium and other metals. A calixarene ligand with two amide substituents has been synthesised as an extractant for lanthanides dimeric Sm and Eu and monomeric Eu complexes have been prepared. Another calixarene, with four phosphine oxides attached, has been fixed to silica particles and the resulting system has been found to give very efficient extraction of Eu + and Ce + from simulated waste. A calix[4]arene complex of Gd + binds... 

Wang, X., Guo, Y, Li, Y. et al. (2003) Novel polyoxometalate-templated, 3-D supramolecular networks based on lanthanide dimers syndiesis, structure, and fluorescent properties of [Ln2(DNBA)4(DMR)8][Mo60i9] (DNBA= 3,5-dinitrobenzoate). Inorganic Chemistry, 42, 4135 140. 

No appreciable polymerization occurred in concentrated aqueous solutions of La or Nd hydroxonitrilotriacetates but, with the heavier lanthanides, dimerization, via hydroxo bridges, occurred in dilute solution. A series of mixed ligand chelate complexes was obtained with Ho and Er and 5-sulpho-salicylate (ssa) and edta, iV-hydroxyethylethylenediamine-triacetic acid, 1,2-diaminocyclohexanediamine-tetra-acetic acid (YX i.e. [M(Y)(ssa)], and iminodiacetic acetate, hydroxyethyliminodiacetic acid and nitrilotriacetic acid, [M(Y)(ssa)2] with higher carboxylates, no mixed chelate complexes were produced. In the presence of an excess of hydroxyethylethylenediamine triacetic acid (H3A), [Eu(A)(HA)] was formed. Lanthanum(m) formed 1 1 complexes with nitrilotriacetic acid and 8-hydroxyquinoline, or 2-picolinic acid and 8-hydroxyquinoline-5-sulphonic acid. 

Fig. 11. Binding energies of the homonuclear lanthanide dimers. HL and P denote SCF values corrected with correlation contributions calculated with the functionals of Hu and Langreth (1985) and Perdew (1986), respectively. Data taken from Dolg et al. (1992a).

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Bocian, Lindsey and co-workers studied sandwich complex nanocapacitors comprised of porphyrin and phthalocyanine ligands separated by lanthanide metals [133]. A triple-decker sandwich of phthalocyanine-Eu-phthalocyanine-Eu-porphy-rin, with two phenylethynyl linker wires from the porphyrin, potentially has up to nine accessible oxidation states (—4 to +4). SAMs of monomers, dimers, trimers, and oligomers of this sandwich, anchored at one or both ends by thioacetyl groups, gave charge densities up to 10 10 mol cm-2, electron-transfer rates up to 105 electrons s-1, and charge-dissipation half-lives in the 10-50 s range. 

While the above indicates that Ln3 + and transition metal ions in the presence of at least 1 equi. of OR promote the alcoholysis of carboxylate and phosphate esters, sometimes by spectacular amounts, we have not presented evidence about the mechanism for the catalytic reactions. So far, the underlying theme is that the most active forms of the lanthanide ions are the Ln3 + ( OR) forms, either as a monomer (as in the case of Eu3 + ( OCH3)) or as a dimer (as in the case of La2+( OCH3)2). For the transition metal ions the most active forms are those where one face of the... 

Porphyrazines with alkyl or aryl substituents are considerably more soluble than their unsubstituted counterparts (Section III. A). Consequently, various pz isomers with alkyl and aryl substituents, for example, symmetrical M[pz(A4)] and unsym-metrical M[pz(A3B)], have been reported. In particular, the symmetrical species M[pz( A4)] have been used both as vehicles to study the fundamental physical properties of metalated porphyrazines (52) as well as to make double decker or sandwich porphyrazines, cofacial dimers linked with lanthanide metal ions (34), while the unsymmetrical species M[pz(A3B)] have utilized the alkyl-aryl substituents as solubilizing groups and have been applied to all areas of pz chemistry. 

Octapropylporphyrazines, M[pz( -Pr)s], which can be prepared from 4-octyne by the same synthetic route used to prepare octaethylporphyrazines (Scheme 4), have been used to prepare pz sandwich complexes that are cofacial dimers of two porphyrazines linked by a lanthanide ion (34). 

More recently, Aspinah et al. reported the synthesis of a series of lanthanide silsesquioxanes resulting from reactions of 3 with lanthanide tris(silylamides) Ln[N(SiMc3)2]3 (Ln = Y, La, Pr, Eu, Yb). However, single crystals of these materials suitable for X-ray diffraction could not be obtained. The somewhat complicated situation is illustrated in Scheme 25. The lanthanide tris(silylamides) reacted with two-third equivalents of the trisilanol 3 in THF to give the lanthanide silsesquioxanes 85, which are dimeric in solution at 233 K. Reaction of Ln[N(SiMe3)2]3 with one equivalent of 3 in THF resulted in complete conversion of 3 to the trisilylated compound 14, as did the reaction of Ln[N(SiMe3)2]3 with two-third equivalents of 3 in toluene. 

The compounds Ln(C5H5)2Cl also have been made only with the lanthanides above samarium (772). These compounds are stable in the absence of air and moisture, sublime near 200 °C, are insoluble in non-polar solvents, and exhibit room temperature magnetic moments near the free ion values (772, 113). The chloride ion may be replaced by a variety of anions including methoxide, phenoxide, amide and carboxylate. Some of these derivatives are considerably more air-stable than the chloride — the phenoxide is reported to be stable for days in dry air. Despite their apparent stability, little is known about the physical properties of these materials. The methyl-substituted cyclopentadiene complexes are much more soluble in non-polar solvents than the unsubstituted species. Ebulliometric measurements on the bis(methylcyclopentadienyl)lanthanide(III) chlorides indicated the complexes are dimeric in non-coordinating solvents (772). A structmre analysis of the ytterbium member of this series has been completed (714). The crystal and molecular parameters of this and related complexes are compared in Table 5. 

The tris-DPM complexes of the lanthanides from La to Dy is found to belong to the same monoclinic space group P2i/m as the dimeric Pr(DPM)s with z=8 (Table 6). Tb(DPM)s and Dy(DPM)3 also occur as orthorhombic Pmmm or... 

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