Bimetallic complex formation

In the stabilization of PVC, the principal mode of action of the various stabilizer systems has been explained in terms of the Frye and Horst mechanism, i.e., substitution of labile chlorines by more stable groups. Evidence for other actions, such as HCl neutralization, addition to polyene sequences, and bimetallic complex formation have also been given. Despite the wide acceptance of the Frye and Horst mechanism, researchers have frequently contended that this could not be the dominant mechanism in the stabilization of PVC. 

Solubility of LiCl in MeOH, EtOH, and"BuOH is 30.4, 19.6, and 13.9%, respectively. That is why after refluxing of the reaction mixture and washing off the precipitate with alcohol, alkoxides free from LiCl are obtained. However, this reaction in many cases is also complicated by formation of bimetallic complexes. Formation of stable intermediate complexes is especially characteristic when LiOR is applied for alkoxylation. Thus Li4Y40(0Bu )12Cl2 was isolated in reaction of YC13 with 2 mols of LiOBu (i.e., on lack of OR-ligands) [553]. 

The presence of N-donors in pendent arms makes Schiff base ligands very effective for the formation of bimetallic complexes with two Ni11 ions. Dinuclear complexes of compartmental Schiff base ligands are covered in Section 6.3.4.12. 

In contrast, Fe-Hg-X complexes show little tendency to form halide bridged species and less is known about complexes containing Zn. We first reported the formation of Fe-Si-O-M four membered ring systems with soft metals M = Ag, Rh, Pd, and Pt, and then prepared bimetallic complexes with more oxophilic metals in order to better understand the conditions for the occurrence of this unusual (t-alkoxy-silyl bridging mode. We have expanded our studies on Cd-containing complexes [3b-d] to Group 13 elements and we report here about the synthesis and reactivity of new, stable heterometallic Fe-M (M =... 

The arene groups in (r/ -arcnc)tricarbonylchromium complexes are typically electron poor and display poor reactivity toward electrophiles. In the case of mercuration reactions, this lack of reactivity can be overcome by attachment of Lewis-basic substituents to the arene ring. For example, in the case of 75a-c, the presence of a pyridyl, oxazolyl, or methyl-A,A-dimcthylami no group promotes ortho-mercuration, leading to the formation of the bimetallic complexes 76a-c (Equation (28)). 07... 

Catalytic asymmetric cyanide addition to imines constitutes an important C—C bondforming reaction, as the product amino nitriles may be converted to non-proteogenic a-amino acids. Kobayashi and co-workers have developed two different versions of the Zr-catalyzed amino nitrile synthesis [73]. The first variant is summarized in Scheme 6.22. The bimetallic complex 65, formed from two molecules of 6-Br-binol and one molecule of 2-Br-binol in the presence of two molecules of Zr(OtBu)4 and N-methylimidazole, was proposed as the active catalytic species. This hypothesis was based on various NMR studies more rigorous kinetic data are not as yet available. Nonetheless, as depicted in Scheme 6.22, reaction of o-hydroxyl imine 66 with 5 mol% 65 and 1—1.5 equiv. Bu3SnCN (CH2C12, —45 °C) leads to the formation of amino nitrile 67 with 91 % ee and in 92 % isolated yield. As is also shown in Scheme 6.22, electron-withdrawing (— 68) and electron-rich (—> 69), as well as more sterically hindered aryl substituents (— 70) readily undergo asymmetric cyanide addition. 

PhI=NTs in MeCN affords a copper species that is indistinguishable by ultraviolet-visible (UV-vis) spectroscopy from an identical solution derived from Cu(OTf)2. Given the strong oxidizing nature of PhI=NTs, it seems likely that both catalysts proceed through a Cu(II) species. Beyond this, little can be said with certainty. If nitrenoid formation proceeds by a two-electron oxidation of the catalyst, one would need to invoke Cu(IV) as an intermediate in this process (77). This issue is resolved if one invokes the intervention of a bimetallic complex in the catalytic cycle. However, attempted observation of a nonlinear effect revealed a linear relationship between ligand enantiopurity and product ee (77, 78). 

The possible mechanism for the formation of C—C coupled (1) and decoupled (2) bimetallic complexes from Cp2M has been investigated [1 lb,c]. We followed a stepwise procedure to arrive at a model that was practical and at the same time realistic. In the first stage, the substituted cyclopentadienyls were replaced by Cp and the substituents on acetylides and butadiynes were replaced by H. The relative energies showed that, the C—C coupled structure 1 for M = Ti when L = Cp and R = H is more stable than 2 by 3 kcal/mol, while 2 is calculated to be 14.8 kcal/mol... 

Serendipitous formation of bimetallic complexes 55, 56 with a /7 -coordi-nated phosphonio-benzophospholides was observed in the reaction of the zwitterion 26 with [M2(CO)io] (M=Mn, Re) [43]. Complexes 56a,b were characterised by spectroscopic data and single-crystal X-ray diffraction studies whose results pointed to a low degree of d(M) r (L) charge-transfer and thus low metallacycle character, suggesting that, similar as in the copper r-complex 51, the coordinated double bond acts predominantly as n-donor rather than an as n-acceptor [43]. 

The formation of the bimetallic complexes 118-123 involves combination of di-cationic transition metal fragments with the dianion of compound 14. A further possibility exists, scarcely studied for 12-vertex 0/050-2,l-ReCBjo species but more extensively exploited in the analogous 11-vertex c/o5o-l,2-MCBg system (M = Mn, Re) discussed in Section IV. Treatment of the dianion of 14 with two... 

The bridging caxbenes of the bimetallic complex, which parallel the surface carbenes of the Fischer-Tropsch catalyst, are involved in C—C bond formation. 

The method has been extended to bimetallic complexes (equation 87) and to butadiene type compounds (equation 88).309 These dienes may be used in complex formation producing complexes analogous to butadiene, as shown by equation (89). The structure of (59b) and the diene [Cp(CO)2 Mo=P(R)CH=P(R)] (59c) have been proved by X-ray crystallography. A similar series of dienyl complexes have been synthesized by Huttner et a/.281 (see Section 14.5.1.3.i). 

The process is complicated by the formation of bimetallic complexes NaM OR), or NaM ORJpCl, which may be decomposed by prolonged refluxing or stirring of the reaction mixture after the addition of the reagents is complete. In the case of Ti(OR)4, due to the low stability of alkoxotitanates reaction can be successfully accomplished however, the attempt to synthesize Zr(OR)4 by this reaction failed complexes NaZr2(OR)9 are quite stable and may even be distilled in vacuum without decomposition [115]. Alkoxochlo-... 

The use of NH3 instead of MIOR allows the formation of bimetallic complexes to be overcome thus by this method Zr(OR)4 and Hf(OR)4 were first synthesized [196]. Alcohol derivatives of different elements (such as Si, Ge, V, Nb, Ta, Sb, Mo, W, U, Fe, and Re) ware also prepared using this route. Sometimes instead of NH3 the tertiary amines were used ... 

In conclusion it is necessary to note the considerable change in chemical activity occurring on transformation from the alkoxides into oxocomplexes. An example is the synthesis of a bimetallic Bi-Ti complex. The complex formation of 2 isopropoxides occurs only in the presence of water (h = 0.2-0.7), which leads to the formation of Bi-oxoisopropoxide, which then reacts with Ti(OPr )4 already at room temperature providing BiTi20(0Pr% [447] (see also Chapter 8). Teyssie et al. [760] have proposed a large group of alkoxides of 3d-transition metals, and also those of Zn, Al, and Mo as highly effective selective catalysts for polymerization of lactones, isocyanates, and so on. 

This treatment leads to increased solubility and the formation of bimetallic complexes. 

The interaction of the metal alkoxides with the salts of carboxylic acids or with p-diketonates of other metals is especially attractive for the synthesis of bimetallic molecular precursors in the cases, when the preparation of the alkoxide of the other metal is somehow hindered or it is insoluble or irreactive under the conditions applied. This method has been widely used for the sol-gel preparation of HTSC materials (because of low solubility and reactivity of Cu(OR)2) and lead-containing ferroelectrics (in the view of difficult synthesis and low stability of Pb(OR)2). It should be mentioned that the reaction between a metal alkoxide and a functional derivative does far not always lead to the formation of a mixed-ligand bimetallic complex ... 

As has been mentioned above, of the many works devoted to the studies of the chemical properties of metal alkoxides, we here can consider only the reactions of partial substitution for the alkoxide groups (Chapter 7), decomposition with formation of oxocomplexes (Chapter 5), and hydrolysis with the formation of homo- and heterometallic hydroxides (Chapters 9 and 10). We discuss here the complex formation products of metal alkoxides — the adducts with neutral ligands, M(OR) mL, and the bimetallic alkoxides, precursors of complex oxides in sol-gel technology. 

The synthesis of bimetallic alkoxides can be achieved using the 2 routes first described by Meerwein and Bersin [1101] complex formation of2 alkoxides and the methathesis of a metal halide with an alkali metal alkoxide. 

To determine the composition of bimetallic alkoxides formed via complex formation in solution and in the solid phase and to estimate their stability, we have applied physicochemical analysis — the investigation of the solubility isotherms in the systems M(OR)m- M (OR)n - L (solvent alcohol, ether, hydrocarbon, etc.). This method, common for the studies ofthe interaction of inorganic salts in water solutions, turned out to be rather fruitful in the chemistry of alkoxides. It permitted the study of the interaction ofthe components... 

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