Catalyst systems bimetallic complexes

The characteristics of C NMR spectra for all copolymers were similar. The triad distributions for all copolymo" from NMR monomer insertion are shown in Table 2. Based on the triad distribution of ethylene/l-hex aae copolymers in Table 2, we found that microstructurc of copolymer obtainrai fiom aluminoxane system was slightly different in monomer incorporation, but found significantly when borated system was applied. We suspected that this difference was arising fiom the diffaences in bimetallic complex active species between [aluminoxane] [catalyst] and [Borate] [catalyst] which had the electronic and gMmetric effects fiom the sterric effect of larger molecule of borate compare to the aluminoxane on the behaviors of comonomer insertion in our systems. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

Hsu et al. [15] applied a bimetallic catalyst comprising rhodium and ruthenium for the hydrogenation to combine the high selectivity of the rhodium complex with the lower cost of the ruthenium complex. When the amount of each metal is identical, the catalytic activity of the bimetallic complex catalyst system was similar to that of the single rhodium-complex catalyst, containing... 

A catalyst system with a zinc complex that also induces the chain-transfer reaction has been developed by Jerome et al When the polymerization of PO is conducted by using a mixture of zinc/aluminum bimetallic complex (Bu0)2A10Zn0Al(0Bu)2 and phenoxyethanol ([PO]/[Zn]/[phenoxyethanol] = 1000/1/20), the conversion reaches 97% to give PPO that contains low and high molecular weight fractions. An additive of lithium chloride or... 

The transfer of a nitrene from PhI=NTos as a commercial source can also be catalyzed by a bimetallic iron complex which exhibits moderate activity, as reported by Avenier and Latour (Scheme 9.12) [24]. The iron complex 10 is better suited for aliphatic substrates where better yields were reported compared with copper catalyst systems applied to the same reactions. 

A ternary catalyst system based on Nd alcoholate provides one of the first examples in which the active species could be isolated and analytically characterized [620]. The preformation of Nd(0 Pr)3/TEA/DEAC (molar ratio = 1/10/1.5) results in a crystalline precipitate. Further characterization of single crystals gives evidence for a polynuclear Nd - A1 bimetallic complex in which Nd atoms are coordinated by Cl, alcoholate and alkyl ligands. 

As mentioned earlier, palladium, rhodium, and platinum catalysts lead to superior regioselectivities because they work under milder reaction conditions (20-80 °C, 0.1-1 MPa CO) [11], e.g., bimetallic catalysts based on tin(II) chloride and either platinum or palladium complexes afford linear esters in up to 98 % selectivity [12]. In addition, catalyst systems with preference for branched isomers are known. A recent example employed palladium acetate immobilized on montmorillonite in the presence of triphenylphosphine and an acid promoter for the hydroesterification of aryl olefins (eq. (3)). The reaction is totally regiospecific for the branched isomer of aromatic olefins, while aliphatic olefins afford branched chain esters only regioselectively with n/i = 1 3 [13]. 

In analogy to the Schrock cycle, Nishibayashi et aL postulated a Chatt-like reaction mechanism, where the bimetallic complex breaks into two monometallic fragments in solution, followed by catalytic reduction at a single metal center. However, the proposed intermediates could not be observed, and the reaction mechanism remained unclear. In order to address this problem, DFT calculations on the mechanism of the ammonia synthesis catalyzed by the Nishibayashi system were performed. Importantly, Batista and coworkers found that the bimetallic complex is the effective catalyst instead of the monometallic species that was originally postulated to play this role. Moreover, the dinitrogen-bridged dinuclear structure remains intact throughout... 

Abstract Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond. 

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