Heterobimetallic lanthanide complexes

The first catalytic asymmetric aza-Henry reaction appears to have been reported by Shibasaki in 1999, as part of a general research program focused on heterobimetallic lanthanide complexes and their application in asymmetric catalysis [190]. In the event, a 1 1 3 mixture of KOt-Bu, Yb(Oi-Pr)3, and (i )-BINOL afforded an active catalyst suggested to have the structure 302 (Equation 32) [191]. This catalyst was shown to promote enantioselective additions of nitromethane to N-phosphinoylarylimines, including 300, to provide the corresponding products, such as 303, in 79 % yield and 91 % ee. 

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. 

Aryloxide (phenolate) ligands provide rigid and versatile ancillary ligand sets in order to study the multifunctional reactivity of alkylaluminum and alkylmagnesium reagents toward Ln-OR moieties. Several types of symmetrically substituted phenolate ligands — summarized in Chart 4 — were employed for the synthesis of a variety of heterobimetallic lanthanide(III) and lanthanide(II) metal complexes. Alkylation reactions revealed the preferred... 

Shibasaki, M. and Groger, H. (1999) Chiral heterobimetallic lanthanoid complexes highly efficient multifunctional catalysts for the asymmetric formation of CC CO and CP bonds. Topics in Organometallic Chemistry, 2 (Lanthanides) 199-232. 

Fig. 8 Structures of half-sandwich heterobimetallic lanthanide/aluminuni complexes 54-56...

One of the earliest indications that the heterobimetallic [iW]cBER+UNi catalytic mechanism may exist came from the work on the addition of lanthanide complexes to unmodified cobalt-catalysed alkene hydroformylation [73]. Upon addition of the lanthanide complexes, a dramatic rate increase was observed. Lanthanides are known to readily generate highly reactive hydrides, and this would support the supposition that lanthanide hydrides are attacking the acyl cobalt complex RCOCo (CO)3/RCOCo(CO)4. In situ spectroscopic analysis was not available/reported. 

One of the more reactive and selective catalysts of this type involves a bifunctional catalyst containing an alkali metal cation and an anionic lanthanide complex resulting from addition of excess binolate with lanthanide halides. Such catalysts have been used in asymmetric nitroaidol (Henry) reactions of ketones. Heterobimetallic Li-La alkoxo complexes (Figure 4.15) catalyzed these reactions with particularly high enantioselectivity. ... 

Lanthanide amides Ln[N(SiMe3)2]3()tt-Cl)Li(THF)3 or Ln[N(SiMe3)2]3 have been reported to be efficient catalysts for amidation of aldehydes with amines under mild conditions without the use of peroxide and base. But this kind of catalyst is not suitable for the amidation of aldehydes with secondary cycUc amines. Heterobimetallic lanthanide/alkali metal complexes stabilized by phenolate ligand are new classes of bimetallic catalysts for amidation of... 

Metallotropic rearrangement, in mercury tri-azenide complexes, 30 41 Metals, see also Heterobimetallics specific element Transition metal complex alkoxides, 15 159-297 of actinides, 15 290-293 of alkali metals, 15 260-263 of alkaline earths, 15 264-266 of aluminium, 15 266-272 of beryllium, 15 264-266 double type, 15 293-294 of gallium, 15 266-272 of lanthanides, 15 290-293 of magnesium, 15 264-266 properties of, 15 260 of transition metals, 15 272-290 trialkylsilyloxides, 15 295-297 of zinc, 15 264-266... 

The assembling tendency of donorfunctionalized ligands was investigated in detail in heterobimetallic complexes derived from hydroxypyridine and the anion of 2-pyrrolidone. The diversity of structural types is controlled inter alia by solvent, kind of 6-substitution of the pyridine ring, type of additional counterions and work up procedures. LnxCuy-complexes derived from hydroxypyridine exhibit Ln Cu ratios of 1 3, 2 2, 2 4, 2 8, 4 4 and 8 12 [141-144]. The anion of 2-pyrrolidine forms polymeric macrocyclic chain structures of type LnHg2 n and Ln2Hg3 n dependent on the size of the lanthanide element [145]. 

Homo- and heterobimetallic complexes of the lanthanides are routinely observed with the large calix[8]arene [191]. However, this extended highly functionalized cavity can also accommodate just one lanthanide ion as shown in [Eu(H6L)(N03XDMF)4] 3DMF (H8L = p-tert-butylcalix[8]arene, Table 13) [192]. Only two adjacent phenolic OH groups were deprotonated according to... 

The relevance of Ln/Al heterobimetallic complexes for the emulation of zirconocene-based polymerization catalysis [13-15] was later on also stressed by the Lanthanide Ziegler-Natta Model [16]. Accordingly, lanthanidocene alkyl complexes were not only successfully employed for clarifying major initiation, propagation, and termination steps (Scheme 3) [17,18]. 

Lanthanide halide complexes free of coordinated Lewis bases, such as alcohols, phosphates, amines, dimethylsulfoxide, or THF, suffer from low solubilities in non-coordinating solvents. Therefore, catalytic systems based on LnCl3 generally require preformation or aging in order to reach maximum activities. In contrast, lanthanide tetrahalogenoaluminate complexes are soluble in aromatic solvents. Such simple Ln/Al heterobimetallic halide... 

Although heterobimetallic complexes with alkylated rare-earth metal centers were proposed to promote 1,3-diene polymerization via an allyl insertion mechanism, details of the polymerization mechanism and of the structure of the catalytically active center(s) are rare [58,83,118-125]. Moreover, until now, the interaction of the cationizing chloride-donating reagent with alkylated rare-earth metal centers is not well-understood. Lanthanide carboxylate complexes, which are used in the industrial-scale polymerization of butadiene and isoprene, are generally derived from octanoic, versatic, and... 

Attempts to structurally characterize the generated Ln(III)/Al hetero-bimetallic complexes were not successful. Addition of coordinating solvents, such as THF or pyridine, afforded complex mixtures from which only lanthanide(III) chloride donor-adducts crystallized. The presence of reactive ethyl groups was confirmed by the reaction with D20 (generating CH3CH2D) [134], The catalytic relevance of such heterobimetallic complexes was confirmed by the quantitative conversion of a 7500-fold excess of iso-prene into polyisoprene within 5-10 minutes at ambient temperature, after the addition of one equivalent of i-Bu3Al cocatalyst. 

Structural evidence for the formation of alkylated lanthanide metal centers in the ternary catalyst mixtures stems predominantly from ferf.bulanolalc complexes [178-181]. Fully characterized heterobimetallic zzeopentanolate analogues were synthesized in recently [ 142],... 

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