Involving Monometallic Mechanism

It should be noted that where the metal alkyl is known to be dimeric, should be substituted for [A], where [A2] is the concentration of metal alkyl dimer and K is the dissociation constant. 

The overall rate equation, describing the steady-state polymerization period is thus given by 

The above model not only predicts successfully the observed kinetics and molecular weight dependencies of the VCl3/Al(i—Bu)3/4-methyl-l-pentene system but also appears to have fairly general application to many other Ziegler-Natta systems. 

Evaluation of the various kinetic parameters using Eq. (9.39) requires a determination of the concentration of polymerization centers [C j. This quantity is usually determined from experiments in which the C sites are quenched (made inactive) with CHsO H, CO, or C02 [7,25]. Other methods include the use of number-average molecular weight (combined with polymer yield) and C-labeled Group I-III-metal alkyl component. Each of the techniques has limitations that require careful consideration if reliable results are to be obtained. 

Equation (9.39) has been found applicable to the polymerization systems characterized by kinetic rate-time profile of the type (a) in Fig. 9.6. Comparative values for some of the principal kinetic parameters derived in this way are listed in Table 9.6. 

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The monometallic mechanism is illustrated in Fig. 7.13a. It involves the monomer coordinating with an alkylated titanium atom. The insertion of the monomer into the titanium-carbon bond propagates the chain. As shown in... 

Despite the work presented, no definitive mechanism for the homogeneous hydrogenation of olefins with triarylphosphinerhodium(I) halide catalysts is postulated here. However the experimental observations of inhibition by excess hydrogen and possibly excess olefin are inconsistent with the previously postulated mechanisms (2, 3) involving monometallic active species. The analysis presented suggests that a mechanism involving bimetallic intermediates with the metal sites in reasonable proximity can account for all presently available experimental observations. 

The monometallic mechanism of Cossee and Arlman [10] for Ziegler-Natta polymerization has found favor in the literature because it is based upon quantum mechanical considerations rather than on agreement with the kinetic data [5]. According to this mechanism, as described earlier and shown in Figs. 9.3 and 9.4, the initiation process involves interaction of aluminum alkyl with an octahedral ligand vacancy around Ti which results... 

The basic feature of proposals for the monometallic mechanism is that propagation occurs entirely at one metal center. A monometallic mechanism involving titanium in a lower valence state, for example, RTiCl, has been proposed (63) to be an active site for ethylene polymerization with propagation occurring by coordination and insertion into the titanium-carbon bond (Reaction 12). 

In addition, some monometallic mechanisms based on a different mode of monomer insertion were also proposed. An example is a reaction mechanism that was proposed by Ivin and co-woikers. This mechanism is based on an insertion mechanism involving an a-hydrogen reversible shift, carbene, and a metallocyclobutane intermediate ... 

In the following discussion it will be assumed that the polymerization takes place at the transition metal-carbon bond. Both monometallic and bimetallic mechanisms have been proposed for the propagation reaction. A mechanism is defined as monometallic when only the transition metal is concerned in the propagation reaction, whereas in bimetallic mechanisms both transition and nontransition metals are involved. Thus, in the monometallic mechanism, it is irrelevant whether the complex contains one or two metal centers. 

In 1961 Heck proposed what is now generally considered to be the correct monometallic mechanism for [HCo(CO)4]-catalyzed hydroformylation [10]. He also proposed, but did not favor, a bimetallic pathway involving an intermolecular hydride transfer between [HCo(CO)4] and [Co(acyl)(CO)4] to eliminate aldehyde product (Scheme 2). Most proposals concerning polymetallic cooperativity in hydroformylation have, therefore, centered on the use of inter- or intramolecular hydride transfers to accelerate the elimination of aldehyde product. Bergman, Halpem, Norton, and Marko have all performed elegant stoichiometric mechanistic studies demonstrating that intermolecular hydride transfers can indeed take place between metal-hydride and metal-acyl species to eliminate aldehyde products [11-14]. The monometallic [HCo(CO)4] pathway involving reaction of the acyl intermediate with H2, however, has been repeatedly shown to be the dominant catalytic mechanism for 1-aUcenes and cyclohexane [15, 16]. 

Initially, a cyclic monometallic mechanism involving reversible double transmetallation similar to that for the ethylmagnesation of alkenes (Scheme 12) was considered. The mechanism shown in Scheme 15, however, was soon ruled out, when preformed zirconecyclopentene (32) added to a 3 1 mixture of Et3Al and 5-decyne failed to induce the expected catalytic process. Although reversible transmetallation between trialkylalanes and Cl2ZrCp2 is... 

Many mechanisms have been proposed that develop this picture more specifically. These are often so specific that they cannot be generalized beyond the systems for which they are proposed. Two schemes that do allow some generalization are presented here. Although they share certain common features, these mechanisms are distinguished by the fact that one-the monometallic model-does not include any participation by the representative metal in the mechanism. The second—the bimetallic model—does assume the involvement of both metals in the mechanism. 

A stepwise reaction mechanism which involves adsorption of nitrate at a bimetallic site, reduction to nitrite, desorption in to the aqueous phase and re-adsorption at a monometallic e.g. Pd) site has been proposed and is supported by theoretical prediction. A reaction scheme based on the use of a bimetallic catalyst is illustrated in Fig. 2. 

An alternate bimetallic pathway was also suggested, but not favored, by Heck and Breslow (also shown in Scheme 1). The acyl intermediate could react with HCo(CO)4 to undergo intermolecular hydride transfer, followed by reductive elimination of aldehyde to produce the Co-Co bonded dimer Co2(CO)s. A common starting material for HCo(CO)4-catalyzed hydroformylation, Co2(CO)g is well-known to react with H2 under catalysis reaction conditions to form two equivalents of HCo(CO)4. The bimetallic hydride transfer mechanism is operational for stoichiometric hydroformylation with HCo(CO)4 and has been proposed to be a possibility for slower catalytic hydroformylation reactions with internal alkenes.The monometallic pathway involving reaction of the acyl intermediate with H2, however, has been... 

Oxidative addition see Oxidative Addition) ofH-C, H-Si, H-Sn, Cl-C, and other a-bonded atom pairs have been used to add group 14 donor ligands to metal clusters. Additions of H-C, H-Si, and H-Sn bonds require an unsaturated metal center. A number of studies of the mechanism are consistent with a mechanism involving addition at a single metal atom through a three-center, synchronous process, the same as commonly occurs for monometallic complexes. Some examples of H Si addition to unsaturated (equation 10) and lightly stabihzed (equation 11) OS3 clusters are shown. Oxidative additions of unactivated C-H bonds are rarer. Preferential addition of terminal C H bonds to... 

The electronic perturbations described in section 2 could affect the chemical properties of Pd. Carbon monoxide is an ideal molecule to investigate the chemisorption properties of bimetallic sur ces. There is extensive information about the surface chemistry of this molecule on many monometallic substrates [70], and die bonding mechanism is much better known for CO [14,71,72] than for other simple molecule, hr addition, CO is involved in many catalytic processes of industrial importance [1,4,70]. 

Electron transfer effects in porphyrins are of biological relevance for a number of reasons, not least in the understanding of mechanisms involved in photodynamic therapy. Studies reported include fluorescence quenching of porphyrins by oxidants such as p-benzoquinone , photoinduced electron transfer of porphyrin-acceptor molecules in solid state °, and ps experiments on quinone substituted monometallic porphyrin dimer which show evidence for super-exchange mediated electron transfer in photosynthetic systems . 

The scope of rf -i 2 interaction and resultant chemistries at monometallic sites is summarized in Figure 3. This is the generally accepted mechanism for an H" "/e -coupled H2 utilization process which for Fe, for example, would involve on deprotonation of the ( 7 -H2) Fe complex, formation of an [Fe -H] followed by oxidation to [Fe -H]°. On deprotonation of this species, the original two electrons of the H2 moiety are formally shifted to the Fe i.e., removal of H" " from [Fe -H]° is formally a reductive elimination, generating Fe. From Fe another electron is removed and another H2 binds to Fe. This electrochemical mechanism is of the CECE type (Chemical-Electrochemical-Chemical-Electrochemical). While the last two steps might be switched, yielding a CEEC mechanism, there is little doubt of the order of the first two steps. 

Cycloaddition with Azides, Alkynes, Alkenes and Allenes. The copper-catalysed azide-alkyne cycloadditiOTi reaction is typically catalysed by simple, monometallic Cu(I) salts. However, the mechanism of catalysis was recently determined to involve a bimetalhc process. Similar bimetallic mechanisms have also been discovered in the cycloaddition of alkynes with alkenes, allenes and other alkynes using Au catalysts. This reaction is discussed for its broad application to many areas of chemistry and for the potential of bimetalhc catalyst design to enhance the reaction. 

One of the most well-studied bimetallic catalysts used for the C-C coupling of alkynes are the thiolato-bridged diruthenium complexes 7 (Scheme 5) [22]. In the presence of NH4BF4 these complexes catalyse the head-to-head dimerisation of a number of terminal alkynes to selectively yield Z-enynes [23]. In contrast, related monometallic Ru catalysts typically yield a mixture of E- and Z-isomers, with the E-isomer more commonly favoured [24-26]. Previous work has shown that diruthenium complexes such as 7 are exceptionally robust due to the strong bridging ability of the thiolate ligands, which results in retention of the dinuclear core during reaction [27]. The proposed mechanism for the dimerisation reaction involves a concerted activation process where both Ru centres activate one alkyne each via the catalytic cycle shown in Scheme 5. Initial coordination of the first alkyne yields the vinylidene intermediate 8. The second alkyne then coordinates to... 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Several authors have proposed a mechanism involving... 

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