Active centres monometallic

The monometallic mechanism proposed by Cossee assumes that the active centre is at the Ti-R part of the catalyst, while the aluminium alkyl acts only as an alkylating agent for the TiCl3. When the catalyst... 

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. 

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. 

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... 

The intermolecular hydrocarboxylation of phenylacetylene with a range of aliphatic carboxylic acids has been catalysed by the bimetallic Ru complex 52 (Scheme 19) [97]. The bimetallic stmcture was shown to have a significant impact on the stereoselectivity of the reaction with the anti-Markovnikov -isomer obtained in good preference to the Z-isomer. In comparison, the related monometallic catalyst 53 showed a poor stereoselectivity for the hydrocarboxylation of phenylacetylene with the Z-isomer product slightly predominant with regard to the -isomer product following catalysis. Similar reaction rates were observed with both catalysts. Analysis of the catalysis reaction by ESI-MS and NMR spectroscopy showed that both Ru centres activate a separate molecule of... 

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