Lanthanide centers

With a YbPB catalyst at room temperature, 86% yield and 98% ee were obtained. After extensive optimization of the catalyst, solvent, temperature, pressure, and catalytic loading, 98% yield and 98% ee was achieved using YbPB (5 mol%) at 50°C for 48 h in 1 7 THFitoluene. The active catalyst was isolated its structure is similar to that shown in Scheme 5-46, and a similar mechanism was proposed. Additional spectroscopic studies suggested that complexation of the phosphite to the lanthanide center was a plausible first step, and that the P-C bond is formed by nucleophilic attack of phosphoms on an N-complexed imine [34]. 

The Wyboume crystal field parameters B (f, f), B (d, d), and Bjj(f, d), which describe the interaction due to the presence of the ligands onto the electrons of the lanthanide center. They are deduced from the ligand field energies and wave functions obtained from Kohn—Sham orbitals of restricted DFT calculations within the average of configuration (AOC) reference by placing evenly n — 1 electrons in the 4f orbitals and one electron in the 5d.33... 

The hetero-Diels-Alder cyclization reaction of tra s-l-methoxy-3-trimethylsilyloxy-1,3-butadiene (Si) (= Danishefsky s diene) with benzaldehyde (S ) (Scheme 12.23) [217-221] is a promising reaction for evaluating the catalytic properties of Lewis acidic lanthanide centers, and has enormous potential for asymmetric synthesis of natural products (e.g., monosaccharides) [222-225]. 

Related complexes where two ligands encapsulate the lanthanide center have been reported (38,39). Coordination of a second antenna ligand may increase the luminescence by excluding deactivating solvent molecules to date, however, photophysical studies for these complexes have not been published. A recent paper details ternary complexes with an additional chromophore, antipyrene (antipy)... 

Note that in the case where the Forster mechanism concerns an organic species transferring its energy to the lanthanide center to which it is bound, this is often referred to as the antenna effect and the ligand is sometimes called a sensitiser. Such an effect becomes efficient provided that the organic chromophore possesses a triplet excited state close to but at least 1700 cm-1 above that of the lanthanide emissive state (Parker and Williams, 1996). 

As a consequence monomeric complexes are obtained much more easily. Also, the tendency to bridge lanthanide centers is less distinct and, for example, a small cluster chemistry as it exists for alkoxides, e.g. OiPr [13] and OtBu derivatives [14], is not yet known. However, the Ln-N(amide) bond is less strong than the Ln-O(alkoxide) bond, and even comparable to Ln-C(alkyl) bonds, which has an effect in synthetic chemistry. This has been confirmed by the determination of absolute bond disruption enthalpies D by means of calorimetric titrations for the representative systems Cp Sm X (X = OrBu, D = 82.4 kcalmol-1 NMe2, 48.2 CH(SiMe3)2, 47.0) [15]. 

Pyrrole ligands can form both Ln-N or-bonds and tjs-n-Ln bonds. complexes with sterically less crowded pyrrole ligands [195]. The introduction of sterically demanding groups in a-position as in 2,5-di-fert-butylpyrrole led to a shielding of the nitrogen and subsequent -coordination to the lanthanide center [196]. Additionally, rj1-coordination to a sodium atom is observed in the obtained ate complex. 

In contrast, Nd2(OCHiPr2)6 cannot be deoligomerized by these solvents, including pyridine. This is in accord with the lower nucleophilicity of the siloxide O-atom which is reflected in the less distinct tendency to bridge the oxophilic lanthanide centers [20]. 

The foregoing two sections revealed that lanthanide centers like to expand their coordination sphere by additional adduct formation despite the presence of very bulky ligands such as tritox or OC6H2tBu3-2,4,6. With regard to headwords such as sublimation and homogeneity , these neutral co-ligands are a nuisance and undesired. For example, upon thermal treatment, they often separate off and dimerization occurs along with the formation of a less volatile species [29,78],... 

Transesterification [236] and the ester exchange reaction [237] were reported to be efficiently catalyzed (Eqs. 30 and 31). Either of the exchange reactions are sensitive to steric constraints of the substrates and to metal ion size. For example, transesterification is most applicable to primary alcohols. Increased catalytic activity in the presence of larger lanthanide centers is explained by enhanced coordinative unsaturation and increased basicity of the alkoxide complexes. Strong basicity of the lanthanide isopropoxides is considered to catalyze effectively the transhydrocyanation from acetone cyanohydrin to sev-... 

Strongly chelating ligands provide a sterically rigid ligand frame, a prerequisite for induction of asymmetry at the lanthanide center. Complexes derived from (S)-( — )-BINOL were thoroughly studied in the nitroaldol reaction (Henry reaction, Scheme 27) [250]. 

The photophysical properties of the triple-stranded dimetallic helicates [Ln2(41)3] (Ln=Nd, Sm, Dy, Yb) were investigated in water and D2O solutions by Biinzil and coworkers [64]. Lanthanide-centered luminescence is well sensitized in the triple stranded homodimetallic helicate complexes. The absolute quanmm yield of the ligand-centered luminescence decreases dramatically upon formation of the lanthanide helicates because of a significant enhancement... 

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