Vinylidene complex bimetallic

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). 

The geometry of the W2C2 core of 4b is unique for bimetallic vinylidene complexes, and has precedent in only a handful of examples in which bimetallic alkenes bridge S5munetrically to both metal termini in a i-ti2 tj2 manner. The vinylidene ligand is perpendicular to the W(l)-W(2) axis, and the carbide-like vinylidene terminus, C(5), lies almost directly between... 

The authors presume that transient vinylidene intermediates are involved in the observed reactions. This hypothesis is consistent with the observed (Z)-selectivity of dimerization and the unusual facility with which complexes of type 58 form isolable vinylidenes. Intermetallic cooperation in bimetallic complexes has been recognized to facilitate vinylidene-mediated processes [23] however, the operative mechanism in the present case remains unknown. 

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... 

A number of bimetallic complexes which contain bridging alkylidene and vinylidene ligands semi-bonded to palladium are known, and as these also contain bridging carbonyl ligands, they are discussed in Section 8.04.1.8. In these dimers, the carbene ligand is more closely associated with the early transition metal, for which Schrock-type carbene complexes are well known. 

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