Hetero-bimetallic complex

Reactions involving bimetallic catalysts, either homo-dinuclear or hetero-bimetallic complexes, and chemzymes were highlighted by Steinhagen and Helmchen96c in 1996. Some examples are discussed in Chapter 2. Among these examples, Shibasaki s reports have been of particular significance.97 Shibasaki s catalyst is illustrated as 130, which consists of one central metal M1 (La+3, Ba+2, or A1+3), three other metal ions (M2)+ [(M2)+ can be Li+, Na+, or K+], and three bidentated ligands, such as (R)- or (iS )-BINOL. The catalyst exhibits both Lewis acidic properties because of the existence of central metal and the Lewis basic properties because of the presence of the outer metal ions. 

Table 10. Catalytic enantioselective epoxide ring opening with 4-methoxyphenol 105 promoted by gallium hetero-bimetallic complexes in the presence of MS 4A.

T. Iida, N. Yamamoto, N. Matsunaga, H.-G. Woo, M. Shibasaki, Enantioselective Ring Opening of Epoxides with 4-Methoxyphenol Catalyzed by Gallium Hetero-bimetallic Complexes An Efficient Method for the Synthesis of Optically Active 1,2-Diol Monoethers, Angew. Chem Int. Ed. EngL 1998,32 2223-2226. 

While today the construction of such bidentate phosphine ligands with the use of an assembly metal is referred to as supramolecular chemistry, this is not to say that there are no examples in older literature utilizing this principle. Actually, there are many and using as a search term hetero bimetallic complexes (a named coined in the early 1980s) leads us to a plethora of complexes. For instance in complex 29, reported by Rauchfuss in 1982, one would consider copper as the assembly metal and platinum as the potential catalytic metal [70]. 

Partial quantum yields3 of Nd111 and quantum yields of Yb111 luminescence upon ligand excitation (300 nm) in homo-and hetero-bimetallic complexes with p>-fert-butylcalix[8]arene in dmf, at room temperature (Korovin et al., 2000)... 

Shibasaki and coworkers have conducted extensive research on the use of hetero-bimetallic complexes as catalysts for asymmetric synthesis [11]. The reactions are catalyzed by heterobimetallic complexes that function as both a Lewis acid and a Bronsted base. Among these, LaLi3tris(binaphthoxide) catalyst 1 (LLB) was proven to be an effective catalyst in direct asymmetric aldol reactions (Fig. 1) [12]. On the basis of this research, Shibasaki et al. reported the first report of a direct catalytic asymmetric Mannich reaction [13],... 

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. 

The following sub-sections describe the preparation of hetero-bimetallic complexes, and are grouped in terms of the earliest metal present in the complex. 

Another early-late bimetallic complex related to the (71) group can be prepared by connecting the two Cp rings by a carbon bridge. Its reaction with an excess of AlMes affords a doubly bridged hetero bimetallic complex (equation 34). ... 

With respect to intramolecular metal-metal interactions, a series of 1,3-dichalco-gena-[3]ferrocenophanes has been investigated that contain a transition metal complex fragment in the center between the two ring-attached chalcogens (Table 5-5). Hetero-bimetallic complexes of this kind can be considered as either 1,3-dichal-... 

It is difficult to find direct bonding interaction between the ferrocene iron and the second transition metal in the hetero-bimetallic complexes. Only the mono-triphenylphosphine palladium and platinum compounds fc[E2M(PPh3)] (E = O, M = Pd (Scheme 5-14) [79] E = S, M = Pd [268, 269], and Pt [269]) appear to... 

A series of related derivatives with the general formulas TiClra(OR)3 ra((N2C3H3)3BH) (n 1, 2), TiCl(CH2SiMe3)(OR)((N2C3H3i)3BH), and TiCl (0(CI I2)mPPh2)3 ((N2Q,I (3)3BI I), (n 0 2 m = 1-3) have been prepared.2 3,214 The latter series have been employed to prepare the hetero-bimetallic complexes TiCl((N2C3H3)3BH)(0(CH2)raPPh2)2Mo (CO)4.214... 

An extensive series of homo-bimetallic and hetero-bimetallic complexes of the 10-donor, doubly deprotonated ligand Hj-XXXI has been recently obtained (Guerriero et al. 1990). Both the direct complexation of the lanthanide salt(s) with the presynthesized ligand, in the presence of a base, and the metal-templated synthesis from 2,6-diformyl-4-methylphenol and l,8-diamino-3,5-dioxaoctane were found to be suitable for the preparation of the complexes. The homobinuclear complexes were obtained as homogeneous mierocrystals (5 10 pm thick platelets). The heterobi-nuelear eomplexes were microerystalline powders for which all evidence indicated. 

A1 Obaidi, N., Chaudhury, M., Clague, D., Jones, C.J., Pearson, J.C., McCleverty, J.A., and Salam S.S. (1987) Monometallic, homo- and hetero-bimetallic complexes based on redox-active tris(3,5-dimethylpyrazolyl)borato molybdenum and tungsten nitrosyls, part 4. The effects of ligating atom type on reduction potentials of monometallic complexes, J.ChemSoc., Dalton Trans., 1733-6. 

The homogeneous hydroformylation of 1-octene using either of the hetero-bimetallic complexes 30 or 31 as catalysts was studied. The best results were... 

At the end of this section, a special mention should be made of the intensive work performed by Peris on the development of poly-NHC-based hetero-bimetallic complexes. In these complexes, the two metal centers are capable of catalyzing two mechanistically distinct reactions on the same substrates, thereupon enabfing overall tandem synthetic process. Several examples have been provided, including Ir/Rh, Ir/Pd, Ir/Pt, and Ru/Pd systems (see also Section 2.3), and several combinations of tandem processes with these metal centers as catalysts. This subject has been thoroughly reviewed in 2014 by... 

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