Heterobimetallics, synthesis from

Synthesis of Heterobimetallic Alkoxides from the Component Metal Alkoxides... 

T) -CgH5)PeCo(C0)3(Nj) ( i-CO). The synthesis of heterobimetallic complexes from metal carbonyl complexes of bis(diisopropylamino)phosphine has been described thus... 

Unsymmetric compartmental ligands that allow for the controlled synthesis of unsymmetric Ni2 or heterobimetallic NiM complexes have received particular attention.1876,1892 A wide range of such ligands derived particularly from 2-hydroxy-3-hydroxymethyl-5-methylbenzaldehyde and 2-hydroxy-3-hydroxymethyl-bromo-benzaldehyde has now been prepared and used for Ni com-plexation. These ligands have monopodal iminic pendent arms and either mono- or dipodal aminic pendent arms and the terminal donors of the pendent arms can be provided by pyridine, imidazole, and tertiary amino groups.1893-1897 Complexes are usually prepared by reaction of the requisite Ni11 salts with the preformed ligand. 

Scheme 12 Synthesis of heterobimetallic Ln/Al carboxylate complexes from Ln(02CCF3)3 (Ln = Y, Eu, Nd) and observed ligand coordination patterns (a-c) [132]...

Scheme 37 Synthesis of a heterobimetallic Eu(II)/Al(III) compound from [Eu(OCH2 CH2OMe)2] [191]...

Scheme 43 Synthesis of heterobimetallic Ln(III)/M species from lanthanide amides Ln(NMe2)3(LiCl)3 and MMe3 (Ln = La, Nd M = Al, Ga) [206]...

An intramolecular diastereoselective Refor-matsky-type aldol approach was demonstrated by Taylor et al. [47] with an Sm(II)-mediated cy-clization of the chiral bromoacetate 60, resulting in lactone 61, also an intermediate in the synthesis of Schinzer s building block 7. The alcohol oxidation state at C5 in 61 avoided retro-reaction and at the same time was used for induction, with the absolute stereochemistry originating from enzymatic resolution (Scheme II). Direct re.solution of racemic C3 alcohol was also tried with an esterase adapted by directed evolution [48]. In other, somewhat more lengthy routes to CI-C6 building blocks, Shibasaki et al. used a catalytic asymmetric aldol reaction with heterobimetallic asymmetric catalysts [49], and Kalesse et al. used a Sharpless asymmetric epoxidation [50]. 

For example, an effective procedure for the synthesis of LLB (where LL = lanthanum and lithium) is treatment of LaCls 7H2O with 2.7 mol equiv. BINOL dilithium salt, and NaO-t-Bu (0.3 mol equiv.) in THF at 50 °C for 50 h. Another efficient procedure for the preparation of LLB starts from La(0-/-Pr)3 [54], the exposure of which to 3 mol equiv. BINOL in THF is followed by addition of butyllithium (3 mol equiv.) at 0 C. It is worthy of note that heterobimetallic asymmetric complexes which include LLB are stable in organic solvents such as THF, CH2CI2 and toluene which contain small amounts of water, and are also insensitive to oxygen. These heterobimetallic complexes can, by choice of suitable rare earth and alkali metals, be used to promote a variety of efficient asymmetric reactions, for example nitroaldol, aldol, Michael, nitro-Mannich-type, hydrophosphonylation, hydrophosphination, protonation and Diels-Alder reactions. A catalytic asymmetric nitroaldol reaction, a direct catalytic asymmetric aldol reaction, and a catalytic asymmetric nitro-Mannich-type reaction are discussed in detail below. 

The aim and purpose of the synthesis of tin(II) heterobimetallic derivatives is to prepare such a material that could deliver both elements of a final material simultaneously, leading to formation of complex ceramic materials in a single step, and was envisaged in the deposition of SrTa206 directly from [SrTa2(OEt)6(/u.-OEt)4(/u.-bis-dmap)2] , where dmap = l,3-bis(dimethylamino)-2-propanoate. 

Until the mechanism of formation of heterobimetallics, such as 15, in these one-step reactions is understood better, it is difficult to devize a reliable general synthesis of this type. Consequently, our attempts to synthesize such systems by this method have b n somewhat hit and miss. Thus, our as yet limited attempts to produce heterobimetallics by co-reduction of Co and metal ions other than Rh have so far been unproductive. Such a system has, however, been produced (37) from a related Ni°/Cu° reaction. In this reaction, an Ni°/dppm mixture was treated in the usual manner with NaBH3CN (reactant ratios 1 3.6 4.8)) under CO with an addition time of 10 minutes. The mixture was stirred for two hours after which CuCl2 (1 molar equivalent) was added. From this mixture was obtained 16 (66% yield, P NMR AA XX pattern, 5Ni-P 23.0, 8Cu-P -17.1 (v. broad signals). IR Wco 2000, 1958, Vcn 21W cm ). As will be seen shordy, complex 10 is a probable intermediate in this reaction. An X-ray crystal structure has shown that in the solid state, the molecule has the cradle-like geometry shown in 16. While this is a heterobimetallic system, it is of less interest than homo- and heterobimetallic systems such as 2, 3, 11 and 15 since the metal-metal bond which is so useful in reactions which mimic those which take place on metal surfaces is absent. 

In 1992, Shibasaki et al. reported for the time an application of chiral heterobimetallic lanthanoid complexes (LnLB) as chiral catalysts in asymmetric catalysis, namely the catalytic asymmetric nitroaldol reaction (Henry reaction), which is one of the most classical C-C bond forming processes [11]. Additionally, this work represents the first enantioselective synthesis of (3-nitroalcohol compounds by the way of enantioselective addition of nitroalkanes to aldehydes in the presence of a chiral catalyst. The chiral BINOL based catalyst was prepared starting from anhydrous LaCl3 and an equimolar amount of the dialkali metal salt of BINOL in the presence of a small amount of water [9]. 

Fig. 1.8 Modes of synthesis of PAMAM-dendrimer-encapsulated heterobimetallic Pd-AuNPs (Reprinted with permission from Ref [18k] Crooks group,/ Am. Chem. Soc. 2004, 126, 15583).

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