Molecular catalyst

The DPE reduction is used as a test reaction to characterize the materials and optimize the preparation conditions of the catalyst. Since hydroaluminations can also be used for the synthesis of carboxylic acids, deuterated products, or vinyl halides via quenching with CO2, D2O or Br2 [44], the method is also a valuable organic synthesis tool. However, as compared with molecular catalysts like Cp2TiCl2 that are known to catalyze hydroaluminations [44], the titanium nitride materials described here are solid catalysts and can be separated by centrifugation. Moreover, they can be reused several times, which is an advantage as compared to molecular catalysts. 

The goal of precise synthesis of supported mononuclear and polynuclear metal complexes can be traced to the early work of Yermakov [1], Ballard [2], and others. Their work stimulated the hvely field referred to as surface organometalhc chemistry [3-6]. The success and importance of precise synthesis of supported molecular catalysts are illustrated by the apphcation of supported metallocene catalysts for industrial alkene polymerization [7j. 

High-throughput screening of molecular catalysts using automated liquid handling, injection and microdevices, Ghimia 56, 11 (2002) 621-626. 

Hydrogenation of aromatic nitro compounds [8,18,29] and hydrogenation of benzene derivatives [2,9,21] have been generally accepted as model reactions to check the heterogeneous nature of catalyst, because homogeneous species are not believed to be active. But at least two well-studied examples show that molecular catalysts can hydrogenate benzene [36,37]. 

Since the reaction rates were found very different in the two catalytic systems, a direct comparison is not possible because the formation of small amounts of active molecular catalyst from the particles could not be excluded. In order to be able to rule out this possibility and more generally to characterise better the colloidal system, control experiments were carried out, founding all of them reproducible. 

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

TEM analyses of the colloids do not evidence any significant change in the size and shape of the particles after seven days catalytic reactions (Figure 4). No NPs were observed at long times of reaction (up to one week) starting with molecular catalysts. 

Enriched I (more than 75% (5)-I) was used as substrate for Pd-catalysed allylic alkylation, using both colloidal [Pd/l]coii and molecular [Pd/lj oi catalysts. As observed in Scheme 3, the colloidal system reacts more slowly with (S)-I enantiomer only 8% of (R)-l is present in the starting substrate, leading to a substrate conversion of ca. 10% with an ee of the remained substrate higher than 99% (S), in agreement with the relative rate calculated previously, k((/J)-I)/k((S)-I) 12 (see above). This relative rate is actually smaller for the molecular catalyst (see above) and consequently a higher conversion was obtained in this case 67% conversion is achieved after 30 h of reaction from a starting substrate constituted by 88.5 R)-I and 11.5(5)- . 

Scheme 3. Pd-catalysed allylic alkykation using an enriched substrate (a) colloidal catalyst (Pd/l/I = 1/1.05/100) (b) molecular catalyst (Pd/l/I = 1/1.25/10,000).

A bis-Co cofacial porphyrin, reported in the early 1980s, is among the best molecular catalysts ever found for the ORR in acidic media in terms of overpotential (about... 

Collman JP, Wagenknecht PS, Hutchison JE. 1994. Molecular catalysts for multielectron redox reactions of small molecules The Cofacial metaUodiporphyrin approach. Angew Chem IntEd 33 1537. 

The above example outlines a general problem in immobilized molecular catalysts - multiple types of sites are often produced. To this end, we are developing techniques to prepare well-defined immobilized organometallic catalysts on silica supports with isolated catalytic sites (7). Our new strategy is demonstrated by creation of isolated titanium complexes on a mesoporous silica support. These new materials are characterized in detail and their catalytic properties in test reactions (polymerization of ethylene) indicate improved catalytic performance over supported catalysts prepared via conventional means (8). The generality of this catalyst design approach is discussed and additional immobilized metal complex catalysts are considered. 

Renewed interest in this method came recently from its adaptation to the immobilization of water/ organic solvent biphasic catalysts, resulting in the so-called supported aqueous phase catalysts (SAPCs).117 The molecular catalyst is immobilized via water, which is hydrogen bonded to the surface silanol groups reactants and products are in the organic phase (Figure 11)... 

Moreover, the molecular catalysts have provided systematic opportunities to study the mechanisms of the initiation, propagation, and termination steps of coordination polymerization and the mechanisms of stereospecific polymerization. This has significantly contributed to advances in the rational design of catalysts for the controlled (co)polymerization of olefinic monomers. Altogether, the development of high performance molecular catalysts has made a dramatic impact on polymer synthesis and catalysis chemistry. There is thus great interest in the development of new molecular catalysts for olefin polymerization with a view to achieving unique catalysis and distinctive polymer synthesis. 

Fig. 5 Schematic structure of a molecular catalyst for olefin polymerization...

Discovery of Highly Active Molecular Catalysts for Ethylene Polymerization... 

Molecular catalysis. The term molecular catalysis is used for catalytic systems where identical molecular species are the catalytic entity, like the molybdenum complex in Figure 8.1, and also large molecules such as enzymes. Many molecular catalysts are used as homogeneous catalysts (see (5) below), but can also be used in multiphase (heterogeneous) systems, such as those involving attachment of molecular entities to polymers. 

Surface catalysis. As the name implies, surface catalysis takes place on the surface atoms of an extended solid. This often involves different properties for the surface atoms and hence different types of sites (unlike molecular catalysis, in which all the sites are equivalent). Because the catalyst is a solid, surface catalysis is by nature heterogeneous (see (6) below). The extended nature of the surface enables reaction mechanisms different from those with molecular catalysts. 

Central to catalysis is the notion of the catalytic site. It is defined as the catalytic center involved in the reaction steps, and, in Figure 8.1, is the molybdenum atom where the reactions take place. Since all catalytic centers are the same for molecular catalysts, the elementary steps are bimolecular or unimolecular steps with the same rate laws which characterize the homogeneous reactions in Chapter 7. However, if the reaction takes place in solution, the individual rate constants may depend on the nonreactive ligands and the solution composition in addition to temperature. 

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