Chloride ligands activation

In Tolman s model ethylene must compete with the strong phosphite ligand for a coordination site, while in the case of the bimetallic complex it only has to compete with a weak chloride ligand. The very high activity of the bimetallic complex (24) as compared to the model complex (21) might be explained in part by this difference. 

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

The nature of the bridging thiolate ligands or the replacement of a terminal chloride ligand by water did not have much effect on the catalytic activity, complexes 105b-d and 106a,b being also operative in these transformations. In contrast, conventional monometallic ruthenium derivatives, as well as diruthenium complexes having no Ru-Ru bond, did not work at all. 

In basic tetraalkylammonium chloride ionic liquids, the active catalyst was suggested to form from the dissociation of the chloride ligand of RuCl2(PPh3)3 in the base. The effect of the cation became evident as the catalyst in tetraalkylammonium chloride was much more active than that in [BMIMJCl. It is known that the bulky tetraalkylammonium cation is weaker in its association with the chloride anion than a planar [BMIM] cation. Therefore, it was concluded that the ionic liquids giving the best catalytic activity appeared to be tetraalkylammonium hydroxide, which melts at approximately room temperature. 

Excision reactions are sometimes accompanied by redox chemistry. For example, dissolution of the 2D solid Na4Zr6BeCli6 in acetonitrile in the presence of an alkylammonium chloride salt results in simultaneous reduction of the cluster cores (144). Here, the oxidation product remains unidentified, but is presumably the solvent itself. As a means of preventing such redox activity, Hughbanks (6) developed the use of some room temperature molten salts as excision media, specifically with application to centered zirconium-halide cluster phases. A number of these solids have been shown to dissolve in l-ethyl-2-methylimidazolium chloride-aluminum chloride ionic liquids, providing an efficient route to molecular clusters with a full compliments of terminal chloride ligands. Such molten salts are also well suited for electrochemical studies. 

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