Active centre bimetallic

A number of models of the active centres in Ziegler Natta catalysts have been postulated. The diversity of these models arises from the multitude of products found to be formed or believed to be formed in the reaction of the catalyst precursor with the activator [e.g. schemes (4) to (8) and (12)]. The proposed active centres fall into either of two general categories those containing monometallic species with the central transition metal atom (e.g. Ti), and those containing bimetallic species with the central transition metal atom linked via bridges with the metal atom originating from the activator (e.g. Al). 

It has not yet been unambiguously decided whether the active centre is formed by the transition metal atom with its ligands (monometallic centre), I, or by the bimetallic complexes II or III most authors now favour the idea that the centre is bimetallic. 

Pseudomonas diminuta PTE is a 72 kDa dimeric bimetallic enzyme with Zn involved in the catalytic process (Carletti et al, 2009, in press). Substitution of the native Zn ions in the active site with Mn, Co, Ni, or Cd ions results in the almost full retention of catalytic activity. Following the first determination of the three-dimensional structure of P. diminuta PTE (Benning et al, 1994), a series of crystal structures, kinetic, and spectroscopic experiments were described. Nevertheless, the enzyme mechanism is still debated and the functional roles of divalent metal cations and amino acids in the active centre are not yet fully understood (Aubert et al, 2004 Samples et al, 2007 Chen et al, 2007 Wong and Gao, 2007 Jackson et al, 2008). No natural substrate has yet been identified (Ghanem and... 

In conclusion, electronic density of the transition metal may be influenced, case by case, by the effect of the reaction with aluminum alkyl and, as a result, the carbon-transition metal bond stability, olefin coordination and insertion capacity, stereochemical control of active centre and chain transfer and propagation processes, hence polymer MWD, may also be affected. This is particulary true for soluble catalytic systems for which the existence of active centres as bimetallic complexes is likely. 

Present views concerning the operation mechanism of ZN catalysts are not conclusive. Cossee [288, 289] assumes that, in the first step, donor-acceptor interaction occurs between the transition metal and the monomer. A a bond is formed by the overlap of the monomer n orbital with the orbital of the transition metal. A second n bond is formed by reverse (retrodative) donation of electrons from the orbital of the transition metal into the antibonding 7T orbital of the monomer. In the following phase, a four-centre transition complex is formed with subsequent monomer insertion into the metal-carbon bond. This, in principle, monometallic concept is criticized by the advocates of the necessary presence of a further metal in the active centre. According to them, the centre is bimetallic. Monometallic centres undoubtedly exist on the other hand, technically important ZN catalysts are multicomponent systems in which each component has its specific and non-negligible function in active centre formation. The non-transition metal in these centres is their inherent component, and most probably the centre is bimetallic. Even present ideas concerning the structural difference in centres producing isotactic and atactic polymers are not united. 

In the search for information on the composition of active centres, and for materials of improved catalytic performance, very much use has been made of bimetallic catalysts (see Further Reading at the end of the chapter). The term is preferred to alloy as in many cases the degree of intimacy of the components is uncertain, and in some cases interesting behaviour is found with systems exhibiting... 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

The objective of this chapter is to examine the basic research on diene (butadiene, isoprene, and piperylene) polymerisation with the LnHalj-nL-AlRj (Ln = lanthanide, Hal = halogen, ligand (L) = tributyl phosphate (TBP), AlRj = triisobutylaluminum and diisobutylaluminum hydride) catalytic system. The chapter will analyse the role of such factors as the electronic and geometric structure of bimetallic active centres, anti-syn and 7t-o-transitions of the terminal units of the growing polymer chains and the nature of the lanthanide, diene, and organoaluminum component in the mechanism of stereoregulation. 

The assumption about the bimetallic bridge structure of lanthanide catalytic systems was made in many works [66-69]. Nevertheless, the possibility must not be ruled out that active centres contain both types of bonds (tt-allyl and a-bridge). It is important that these bonds may differ dramatically in reactivities. In order to answer the question concerning the possible coexistence of two types of bonds, the polymerisation of butadiene on catalytic systems NdCl3 3L-AlR3, where R is /-C4H9 Ln is Nd or Tb L is TBP, prepared in the presence of a small amount of butadiene and piperylene [71, 72] was investigated. Table 3.3. 

In many centred molecules the interactions between the electro-active centres in a given molecule modify the spacing of the formal Standard Potentials of the successive processes by an amount that depends in the case of coulombic forces in part on the dielectric properties of the local surrounding electrolyte solution. This feature has been observed in Ae case of bimetallic complexes in mixed solvent systems of relatively low bulk dielectric constants, has been used to ascertain Ae impact of electrolyte concentration in particular ion-pairing on electrolyte dielectric behaviour. 

Activity in relation to the redox states of the bimetallic centre in [NiFe] hydrogenases... 

There are several photocatalysts mimicking hydrogenase activity that are not based on metalloporphyrin systems. Among them there are mixed-valence complexes of rhodium or iridium, [41] as well as complex systems encompassing photosensitizers (eg ruthenium complexes) attached to a catalytic bimetallic centre [43], The design of more sophisticated systems approaches that of photosynthetic processes [44],... 

The most effective palladium catalysts are those in which a PdFe bimetallic compound carries a bridging diphosphine ligand, when the reaction is thought to occur at the palladium centre with the iron playing an important role.25 Optically active methyl-phenylneophyltin hydride, [a] +28.9°, reacts with palladium on charcoal to give the optically active distannane, [a] +13.2°, which is optically stable for several weeks.28... 

The reasons why complexes 8 and 9 are active in ATRP are presently nnclear. These complexes possess indeed two 18-electron rathenium centres and, as snch, shonld be nnable to activate the carbon-halogen bond of the initiator or of the growing polymer chain end. On the other hand, the fact that an indnction period was fonnd for the ATRP of MMA indicates that the mtheninm-vinylidene complexes have to be activated prior to the ATRP process. There are in principle several plansible explanations for the formation of a coordinatively unsatnrated 16-electron rathenium species from the rathenium-vinylidene complexes 8 or 9 either the splitting of the bimetallic scaffold into two different unsaturated rathenium intermediates (Path A, Scheme 5), the opening of a p.-chloro bridge (Path B), or the release of the vinylidene ligand (Path C, Scheme 5). 

A bimetallic oxidative addition mechanism involving radical intermediates has been proposed for allqrl-allq l Kumada cross-coupling catalysed by complex 18 (Figure 14.2). In this catalytic cycle, the oxidative addition of allq l halide involves two nickel centres, and the key intermediate for the activation of allqrl halide is the complex [(N2N)Ni-R](R-MgCl). The formation of this species is the turnover-limiting step of the catalysis. ... 

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