Bimetallic lanthanide complexes

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand [L58]3- have been synthesized and structurally characterized (Fig. 15), and their photophysical properties studied (90,91). The bimetallic cryptates only form with the lanthanides from gadolinium to lutetium due to the lanthanide contraction. The triplet energy of the ligand (ca. 16,500 cm-1) is too low to populate the terbium excited state. The aqueous lifetime of the emission from the europium complex is less than 0.5 ms, due in part to the coordination of a solvent molecule in solution. A recent development is the study of d-f heterobimetallic complexes of this ligand (92) the Zn-Ln complexes show improved photophysical properties over the homobinuclear and mononuclear complexes, although only data in acetonitrile have been reported to date. 

Application of the model-free methods to axial bimetallic lanthanide complexes with sterically rigid ligands... 

Mono- and bimetallic lanthanide complexes of the tren-based macrobicyclic Schiff base ligand have been synthesized and... 

In a polynuclear or polymetallic lanthanide complex, additional processes become feasible. Figure 3 is a simplified scheme that shows the energy transfer pathways available in a bimetallic lanthanide complex singlet-mediated energy transfer and competitive non-radiative quenching pathways are not shown in the scheme to facilitate interpretation. Not only can both lanthanide ions be sensitized by the excited states of the chromophore, but energy transfer between lanthanide ions is also possible where appropriate spectral overlap exists between the two lanthanide ions. 

The review begins with a selective summary of the history of organome-taUic and reductive rare-earth chemistry as an introduction to bimetallic lanthanide complexes bridged by ligands derived from arenes and to complexes of ferrocene-based diamide ligands, a major subtheme in the chapter. Detailed discussions then follow on complexes derived from fused rings and their reactivity with phosphorus and on complexes derived from biphenyl and stilbene as substrates, with emphasis on ferrocene-based diamide complexes. 

Fig. 54. Axial coordinates considered in bimetallic axial lanthanide complexes (adapted from Rigault et al. (2000a)).

Calix[ ]arenes are a family of macrocycles prepared by condensation reactions between n /v/ra-substituted phenols and n formaldehyde molecules under either base or acid catalysis. Different sizes of the macrocycles can be obtained (n = 4-20) (Stewart and Gutsche, 1999) depending on the exact experimental conditions, which were mastered in the 1960 s (Gutsche, 1998), but the most common receptors are those with n =4,6,8 (macrocycles with an odd number of phenol units are more difficult to synthesize). We use here the simplified nomenclature in which the number of phenolic units is indicated between square brackets and para substituents are listed first.4 Calixarenes, which can be easily derivatized both on the para positions of the phenolic units and on the hydroxyl groups, have been primarily developed for catalytic processes and as biomimics, but it was soon realized that they can also easily encapsulate metal ions and the first complexes with d-transition metal ions were isolated in the mid-1980 s (Olmstead et al., 1985). Jack Harrowfield characterized the first lanthanide complex with a calixarene in 1987, a bimetallic europium complex with p-terf-butylcalix[8]arene (Furphy etal., 1987). 

Divalent organolanthanide complexes can also initiate MMA polymerization. A divalent lanthanide complex, as a single-electron transfer reagent, can readily react with the monomer to generate a radical anion species, which subsequently couples into a bimetallic trivalent lanthanide enolate intermediate, which is the active center. Therefore, divalent organolanthanide complexes serve as bisinitiators for MMA polymerization [160]. 

FIGURE 75 Schematic representation of energy absorption, emission, and dissipation processes in a bimetallic (R R ) lanthanide complex. F, fluorescence P, phosphorescence et, energy transfer r, radiative nr, nonradiative. 

Novak and Boffa, in studying lanthanide complexes, observed an unusual facile organometaUic electron transfer process takes place that generates in situ bimetallic lanthanide(lll) initiators for polymerizations of methacrylates [239]. They concluded that methyl methacrylate polymerizations initiated by the Cp 2Sm complexes occur through reductive dimerizations of methyl methacrylate molecules to form bisinitiators that consists of two samarium(lll) enolates joined through their double bond terminally [239]. Their conclusion is based on the tendency of Cp 2Sm complexes to reductively couple unsaturated molecules ... 

Scheme 13.32 Structures of ligands and schematic depiction of the subunit structures in bimetallic or polymetallic lanthanide complexes.

Catalytic reactions using lanthanide Lewis acids are reviewed in this chapter. In the lanthanide Lewis acid catalysis, characteristic properties of lanthanide Lewis acids, such as tolerance to moisture and selective activation of imines over aldehydes, were utilized. Catalytic asymmetric reactions with lanthanide Lewis acids are growing rapidly. Especially, the bimetallic lanthanide catalysts enabled various transformations, which are difficult with a simple Lewis acid catalyst. Suitable design of aggregated lanthanide complexes is important for further development in this field. 

The use of lanthanide complexes in asymmetric catalysis was pioneered by Danishefsky s group with the hetero-Diels-Alder reaction,and their utility as chiral Lewis acid catalysts was shown by Kobayashi. The Brpnsted base character of lanthanide-alkoxides has been used by Shibasaki for aldol reactions, cyanosilylation of aldehydes and nitroaldol reactions.The combination of Lewis acid and Brpnsted base properties of lanthanide complexes has been exploited in particular by Shibasaki for bifunctional asymmetric catalysis. These bimetallic lanthanide-main-group BINOL complexes are synthesized according to the following routes ... 

Evans, W. J., Ulibarri, T. A., Ziller, J. W. Reactivity of (C5Mes)2Sm with aryl-substituted alkenes Synthesis and structure of a bimetallic styrene complex that contains an tj -arene lanthanide interaction./. A/w. Chem. 5 oc., 112,219-223 (1990). 

Theoretical approaches have been extensively used to predict the nature of the ground state of several Yb(II) complexes, using the multireference CASSCF method. However, this computational approach is hardly tractable for bimetallic complexes and reactivity studies involving divalent lanthanide complexes. There was thus a need for a simplified approach but without, if possible, too much loss of precision. 

Coordination chemistry can be used to prepare arrays from stable building blocks. Metathesis of Re(Bpy)(CO)3Cl followed by treatment with lanthanide complexes of 4-picolylD03A, TriazolylD03A or triazole bearing DOTA monoamides yields a series of d-f hybrid complexes (Fig. 9.17, a—c). The Gd-Re bimetallic systems combine long-lived... 

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

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