Synopsis: Metallacycles Appear to Be a Relatively New Generation of Catalysts, Which Have Not Been Thoroughly Investigated for Use in the Industry

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Synopsis: Metallacycles appear to be a relatively new generation of catalysts, which have not been thoroughly investigated for use in the industry. Most known metallacycles are phosphorus, and to a lesser extent sulphur, mono- and bidentate metallacycles, whereas relatively little information is available on the corresponding metallacycles based on nitrogen. This study was aimed at establishing whether nitrogen- as well as sulphur-containing metallacycles are comparable, with regard to catalytic activity, to phosphorus-containing metallacycles and if either of these are comparable to the standard palladium and phosphorus catalysts that are usually prepared in situ.

A few of the difficulties that have been experienced with organometallic literature included the lack of detailed descriptions of experimental procedures for metallacycle synthesis as well as the incompleteness or total lack of characterisation data. Metallacycle formation generally involves insertion of a metal into an activated C-H bond. Organometallic literature view metallacycle formation as oxidative addition followed by reductive elimination. From a mechanistic point of view, however, these terms have little meaning. It is possible to rationalise the reaction in terms of acid/base interactions and electrophilic substitution by a process which does not require a change in the oxidation state of the metal.

The critical dependence of palladacycle formation on the reaction conditions became evident in this study. Formation of metallacycles of palladium is always in competition with other processes such as simple complexation and reduction of palladium(II). Direct metallation using palladium acetate was only successful with phosphorus ligands and palladium chloride was exclusively successful with nitrogen ligands. The acetate anion plays an important role, namely to act as a base. The failure of palladium chloride to affect palladacycle formation with phosphorus ligands could be ascribed to its lack of basic properties. In the light of these conclusions, it became clear why it was so difficult to reproduce the poorly described methods in the literature. Some metallacycles cannot be made ii by the direct metallation route and, in our view, are best prepared by oxidative addition reactions with halogen compounds. Distinctive differences in the NMR spectra of the metallacycle from its precursor makes it possible to distinguish between the formation of simple complexes, on the one hand, and the metallacycle, on the other. FAB-MS characterisation has also been shown to be useful in the monitoring of reactions.

The prepared palladacycles were compared to each other as well as with a

Pd(OAc)2/PPh3 system commonly used as catalyst in the Heck reaction. Using an aryl iodide as substrate, it was found that the nitrogen containing palladacycles are superior to the phosphorus-based palladacycles and comparable in activity to the standard non-palladacycle catalysts. Pincer complexes showed almost no activity under these conditions. Much has been speculated on the mechanism of the palladium-catalysed Heck reaction. Some evidence have been presented for a conversion of the palladacycle into a palladium(0) species, but most authors favour a mechanism involving a Pd(II)/Pd(IV) oxidative change.

Two reaction types of great importance to Sasol are isomerisation and hydroformylation/alkoxy-carbonylation of alkenes. With regard to the reactions with carbon monoxide, we found the palladacycles to be inefficient, since they are rapidly reduced to palladium(0). The possibility that these compounds can catalyse hydrogenation reactions is therefore also excluded. However, palladacycles appear to have potential as catalysts for olefin isomerisation reactions.

Analysis of the known catalytic properties of metallacycles shows that these reactions involve a change in the oxidation state of the metal in the catalytic cycle. The stability of metallacycles dictates that this will be a high energy process, thus explaining the low rate of reactions catalysed by these compounds. This practically ensures that metallacycles will find limited application in industry. However, their ability to catalyse reactions which does not require such a change in oxidation state, have not yet received attention.