Active centers, bimetallic

The bimetallic mechanism is illustrated in Fig. 7.13b the bimetallic active center is the distinguishing feature of this mechanism. The precise distribution of halides and alkyls is not spelled out because of the exchanges described by reaction (7.Q). An alkyl bridge is assumed based on observations of other organometallic compounds. The pi coordination of the olefin with the titanium is followed by insertion of the monomer into the bridge to propagate the reaction. 

Pt/Ru bimetallic nanoparticles. In the case of dye-sensitized photochemical water splitters, to which much attention has been received recently, noble metal nanoparticles are often used for the active centers to produce hydrogen gas from water. Bimetallic nanoparticles will be easily replaced by these metal nanoparticles for the sake of saving resources. 

It should be added that MS-02 is not necessarily a mono-nuclear complex. It could be shown in a few cases that the catalytic activity of the metal ion is due to the formation of dinuclear metal-substrate complexes. Presumably in these species each oxygen atom of dioxygen coordinates to a different metal center. Such systems were extensively used to model the reactivity patterns of various enzymes containing a bimetallic active center. 

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

This generates a titanium-carbon a bond through ligand exchange. The active center was believed to be the bimetallic species 44 existing in equilibrium with different monometallic species. 

Although bimetallic active centers with activator molecules complexed to the transition metal were suggested and observed, experimental evidence indi-... 

These kinetic and stereochemical results give us a direct evidence for the bimetallic structure of the active center in which alkylaluminum components are involved as important ligands. 

The value of KM decreases with increasing electronwithdrawing capability of the aluminum component, i.e. with decreasing electron density at the vanadium induced by the aluminum component bonded to the vanadium in the bimetallic structure of the active center. This result seems to suggest that electron back-donation from a filled vanadium d orbital to the empty propylene jc obital (it-bonding) is the main factor in determining the vanadium-propylene interaction. 

Divalent organolanthanide complexes can also initiate MMA polymerization. A divalent lanthanide complex, as a single-electron transfer reagent, can readily react with the monomer to generate a radical anion species, which subsequently couples into a bimetallic trivalent lanthanide enolate intermediate, which is the active center. Therefore, divalent organolanthanide complexes serve as bisinitiators for MMA polymerization [160]. 

Recently, Doi152) speculated on the presence of two types of bimetallic active centers, based on 13C NMR analysis of the structure and stereochemistry of polypropylene fractions obtained with different Ziegler-Natta catalyst systems (see Fig. 44). Site A produces highly isotactic polypropylene, site B atactic polypropylene consisting of isotactic and syndiotactic stereoblocks. The formation of the latter fraction would be due to the reversible migration of the aluminum alkyl, made... 

Armstrong was the first to note that the overall energy of the AC is diminished when the octahedral titanium environment is distorted to a trigonal bipyramid due to alkyl migration into a position which is intermediate between two octahedral vacancies This was confirmed by further calculations performed for monometallic 24,128) Qj. bimetallic centers. Thus, the structure of active centers may formally be... 

A CNDO analysis of the insertion step of coordinated ethylene for the bimetallic active center showed that this step proceeds with an activation energy which is lower than the heat of formation of the rt-complex. A low activation energy 1 kcal/mol) for the insertion step was calculated also for the monometallic active center a mutual influence of Ti-ethylene and Ti-alkyl being taken into account. 

The activation energy of the insertion of coordinated ethylene estimated by the ab initio method was found to be 15 kcal/mol Despite the application of a more advanced calculation technique these results are less compatible with the experimental data on solid titanium chloride-based catalysts, when the activation energy of the propagation step is 3-6 kcal/mol (Table 10). Probably, this incompatability is due to the model used in ref. which describes the AC as a bimetallic complex CljTiCHj with A1(CH3)3. However, it is important to note that the calculations performed by means of the nonempirical method confirm the concept implying that in the active center the alkyl group occupies an intermediate position between the octahedral sites and that in olefin coordination the AC structure is reconstructed. 

To clarify the mechanism of propylene adsorption on Ru-Co clusters the quantum-chemical calculation of interaction between it and Ru-Co, Ru-Ru, and Co-Co clusters were carried out. During the calculation it was assumed that carbon atoms of C-C bond are situated parallel to metal-metal bond. The distance at which the cluster and absorbable molecule begin to interact is characterized by the nature of active center. Full optimization of C3H6 molecule geometry confirms that propylene adsorbs associatively on Co-Co cluster and forms Jt-type complex. In other cases the dissociate adsorption of propylene is occurred. The presence of Ru atom provides significant electron density transfer from olefin molecule orbitals to d-orbitals of ruthenium in bimetallic Ru-Co- or monometallic Ru-Ru-clasters (independently on either the tertiary carbon atom is located on ruthenium or cobalt atom.). At the same time the olefin C-C bond loosens substantially down to their break. 

LCB-PE (long chain branched polyethylene) is not accessible through simple ethyl-ene/a-olefin copolymerization. Therefore, a bifunctional bimetallic catalyst was developed, in which one active center oligomerizes ethene to long-chain a-olefins, while the other copolymerizes them with ethene. 

Fatty alcohols and their derivatives are widely used as surfactants. In recent years, besides other bimetallic systems (PdRe, ReSn, RhSn, CoSn) Ru-Sn/Al203 catalysts have gained much attention in studying the hydrogenolysis of fatty esters to fatty alcohols [1]. It has been proposed that the active centers are metallic Ru par-... 

Fr . 1. Mechanisms of Ziegler polymerization (a) Bimetallic mechanism, (b) Monometallic mechanism [after Cossee]. (c) Monometallic Cossec mechanism (5). Crystal structure of a-TiC. la with active center in the surface. A propylene rnolocule is inserted in the Cl v acancy forming a 7r-bond with Ti ion A. Figures lb and Ic wore reprinted with the permission of the Faraday Society. 

Zirconocenes and lanthanocenes active for olefin polymerization do, in fact, carry out well-controlled homopolymerizations of (meth)acrylic monomers, but polymerization takes place by an enolate mechanism in which the conjugated carbonyl group plays a crucial role in stabilizing the active center. Both monometallic and bimetallic mechanisms have been documented. Collins and co-workers developed a zirconocene group-transfer polymerization (GTP) technique for the polymerization of methyl methacrylate (MMA) which utilizes a neutral zirconocene enolate as an initiator and the conjugate zirconocene cation as a catalyst (Scheme 3). ... 

Comparison of the chiral bimetallic catalysts, Cu-Pd-TA and Cu-Ru-TA, showed significant differences. In the case of Cu-Ru-TA catalyst, introducing 0.1-0.5% Ru into Cu-TA leads to almost complete loss of enantioselectivify, while in the cases of Cu-Ru and Cu-Pd catalysts such chiral deactivation proceeds only after introduction of more than 5% Pd. The general catalytic activity of the Cu-Ru-TA catalysts increased with increasing Ru content, while the Cu-Pd catalysts exhibited a synergism of catalytic activity, which was explained by a peculiar structure of the active center and by invoking a ligand effect A similar effect for skeletal Cu-Ru-TA catalysts was... 

The above scheme of propagation might also be pictured for bimetallic active centers. Com-plexations precede monomer insertions at the vacant octahedral sites and are followed by insertion reactions at the metal-carbon bonds. When the transition metals are immobilized in crystal lattices, the active centers and the ligands are expected to interchange at each propagation step. 

Equation (19-20) requires a linear relationship between the polymerization rate Vp and the product [ C oi ] /mon independent of the chemical nature of the metal alkyl. Such behavior has also been observed for the 4-methyl pentene-l/VCh/aluminum trialkyl system (Figure 19-3). Thus, the active centers are actually formed by the transition metal halides. An alkylated vanadium species, but definitely not a bimetallic complex, acts as active center. 

The fourth review by Barbara A. Messerle and co-workers is dedicated to the Alkyne Activation Using Bimetallic Catalysts. Two metal centers can induce activation of the triple bond in a variety of different coordination modes as shown below ... 

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