Lanthanide bimetallic catalyst

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. 

Patents have been reported for a series of bimetallic metallocene catalysts based on lanthanide and group IV metals. The bridging ligand is a substituted ethyl-linked fluorenyl indenyl bearing a substituent of varying length [26]. The complexes are reported to act as good olefin polymerization catalysts in the presence of MAO.52,53... 

A similar catalyst system was also effective for asymmetric ring opening of meso-aziridines with TMSN3. The reaction provided a direct method for the synthesis of optically active 1,2-diamines. In the reaction, bimetallic lanthanide... 

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 ... 

Lanthanides in combination with transition metals have been shown to have a positive effect in promoting heterogeneous catalytic reactions. The bimetallic Yb—Pd catalyst obtained from the precursor (pMF)i0Yb2 Pd(CN)4]3 K on a titania surface offers improved performance over a palladium-only catalyst for the reduction of NO by CH4 in the presence of 02.99 100 The structure, shown in Figure 6, consists of two inverted parallel zigzag chains that are connected through the lanthanide atoms by trans-bridging [Pd(CN)4]2- anions.101... 

Figure 6 Bimetallic Yb—Pd catalyst obtained from the precursor (DMF)1oYb2[Pd(CN)4]3

The first report on the coordination polymerisation of epoxide, leading to a stereoregular (isotactic) polymer, concerned the polymerisation of propylene oxide in the presence of a ferric chloride-propylene oxide catalyst the respective patent appeared in 1955 [13]. In this catalyst, which is referred to as the Pruitt Baggett adduct of the general formula Cl(C3H60)vFe(Cl)(0C3H6),CI, two substituents of the alcoholate type formed by the addition of propylene oxide to Fe Cl bonds and one chlorine atom at the iron atom are present [14]. A few years later, various types of catalyst effective for stereoselective polymerisation of propylene oxide were found and developed aluminium isopropoxide-zinc chloride [15], dialkylzinc-water [16], dialkylzinc alcohol [16], trialkylalumi-nium water [17] and trialkylaluminium-water acetylacetone [18] and trialkyla-luminium lanthanide triacetylacetonate H20 [19]. Other important catalysts for the stereoselective polymerisation of propylene oxide, such as bimetallic /1-oxoalkoxides of the [(R0)2A10]2Zn type, were obtained by condensation of zinc acetate with aluminium isopropoxide in a 1 2 molar ratio of reactants [20-22]. 

These complexes are the first examples of multifunctional catalysts and demonstrate impressively the opportunities that can reside with the as yet hardly investigated bimetallic catalysis. The concept described here is not limited to lanthanides but has been further extended to main group metals such as gallium [31] or aluminum [32]. In addition, this work should be an incentive for the investigation of other metal-binaphthyl complexes to find out whether polynuclear species play a role in catalytic processes there as well. For example, the preparation of ti-tanium-BINOL complexes takes place in the presence of alkali metals [molecular sieve ( )]. A leading contribution in this direction has been made by Kaufmann et al, as early as 1990 [33], It was proven that the reaction of (5)-la with monobromoborane dimethyl sulfide leads exclusively to a binuclear, propeller-like borate compound. This compound was found to catalyze the Diels-Alder reaction of cyclopentadiene and methacrolein with excellent exo-stereoselectivity and enantioselectivity in accordance with the empirical rule for carbonyl compounds which has been presented earlier. 

The d-f heteronuclear or lanthanide-transition metal (abbreviated as Ln-M) complexes attract interest from both academic and industry because of the challenge for their synthesis, the novelty of their structures, and their potential application as advanced materials, such as molecular or nano magnets,bimetallic catalysts, and sensors. The complexes can be assigned to three categories based on the nature of the Ln-M interaction (a) complexes with direct Ln-M bonding, (b) complexes with Ln-M interactions bridged by ligands, and (c) the complexes with ionically associated Ln-coordination units and M-coordination units. Most of the d-f heteronuclear complexes of carboxylic acids reported so far are found with type (b) structure, and very few of them are of structure type (c). The lanthanides and the transition metals in these complexes are far away from each other, and no direct Ln-M interactions have been observed. 

Perovskites of the type LnMO, (Ln = lanthanide and M = transition metal) may offer interesting features as precursors for supported metal catalysts. For example, careful reduction can be carried out in order to produce a finely dispersed transition metal over the sesquioxides Ln,0,. Also, the flexibility in the perovskite composition allows the preparation of compounds of the type LnM ., M 0, (M and M = different transition metals) or Ln., A,MO, (A - for example an alkaline or alkaline earth metal), which show the unique possibility as precursor of producing well dispersed bimetallic catalyst or doped metal catalyst. 

Since these initial publications, the considerable efforts of Shibasaki and coworkers have lead to lanthanide element-binaphtholato complexes as being the most developed and potent of asymmetric phospho-aldol catalysts. From their early work on catalysis in the nitro-aldol reaction [28], Shibasaki and co-workers have published widely and in considerable detail on the use of hetero-bimetallic catalysis, developing the field to such a degree that enzyme-like comparisons have been made. Much of the success of the Shibasaki team in hetero-bimetallic catalysis has been reviewed recently [29], the key to which has been the delineation of the solid state structures of the catalytic precursors via single crystal X-ray diffraction studies [30] (Fig. 1) and the development of improved synthetic routes to this class of heterobimetallic system (Scheme 12) which emphasise the important role of added water [31]. 

The first step in RNA hydrolysis is the intramolecular nucleophilic attack of the phosphorus atom by the 2 -OH of ribose. This step is activated by the coordination of the phosphodiester linkage in RNA to the lanthanide(III) ion in the bimetallic cluster [R 2(OH)2] , since the electrons are withdrawn by the metal ion from the phosphorus atom. This electron withdrawal promotes the electrophilicity of the P atom, although it is not so drastic as the effect achieved by the Ce(IV) in DNA hydrolysis (cf. sect. 5). Furthermore, the hydroxide ion bound to another lanthanide(III) ion in the bimetallic cluster functions as a general base catalyst, and enhances the electrophilicity of the 2 -OH by removing its proton. Alternatively, the 2 -OH is directly coordinated to this metal ion, and its dissociation to alkoxide ion is facilitated. In this way, both the nucleophilic center (the oxygen in the 2 -OH) and the electrophilic center (the phosphorus atom) are simultaneously activated by the bimetallic cluster, and thus the intramolecular nucleophilic attack proceeds efficiently. 

The breakdown of the resultant pentacoordinated intermediate is rate limiting, since the first step is an intramolecular reaction facilitated by a favorable activation entropy term. In this step, the metal-bound water functions as a general acid catalyst. The water bound to the lanthanide(III) ions has a pA a in the range 8-9, which should be further decreased in the bimetallic clusters since the second trivalent ion should further withdraw electrons from the... 

Bimetallic catalysts (lanthanide halides or 3 diketonates and alkyl-aluminium or lithium derivatives) have been reported. SmCp is also a catalyst for the trimerization of acetylenes to substituted benzenes and... 

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... 

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