Transition metal clusters acetylenes

It has been found in the meantime that reaction (1) is generalizable (752), and that oxidative additions of this type occur for such widely differing substrates H2Y as ethylene, benzene 130), cyclic olefins, alkyl and aryl phosphines, aniline 337, 406), and H2S 130), ail of which give the same product structure with a triply-bridging Y ligand. The stability of these third-row transition metal clusters has stiU prevented catalytic reactions of these species, but it is likely that similar ones are involved in olefin and acetylene reactions catalyzed by other metal complexes. 

Judai K, Abbet S, Worz AS, Ferrari AM, Giordano L, Pacchioni G, Heiz U (2003) Acetylene polymerization on supported transition metal clusters. J Mol Catal A 199 103... 

Most of the catalytic systems derived from transition metal clusters catalyze the hydrogenation of acetylenes to give predominantly olefins... 

A few reactions in which four components are reacted together in the presence of a transition metal cluster as the catalyst are known. These reactions involve an acetylene, an olefin, carbon monoxide and hydrogen, or a hydrogen donor and are catalyzed by the rhodium cluster Rh4(CO)i2. In a typical experiment, an acetone solution of diphenylacetylene is pressurized with ethylene (25 bar), CO (30 bar), and H2 (5 bar) at 150°C for 6 hr the reaction... 

The mechanism of trimerisation of acetylene on extended surfaces is thought to be similar to that observed in homogeneous catalysis using transition-metal cluster compounds.A stepwise mechanism, shown below is thought to occur ... 

Similar reactions applied to transition metal-acetylene complexes appear capable of separating the 2 carbon atoms originally linked by the acetylenic triple bond 18). Thermal isomerization of metal-acetylene complexes may achieve the same result, showing how metal clusters can catalyze scrambling reactions of acetylenes, e.g.. 

The density functional theory and the cluster model approach enable the quantitative computational analysis of the adsorption of small chemical species on metal surfaces. Two studies are presented, one concerning the adsorption of acetylene on copper (100) surfaces, the other concerning the adsorption of ethylene on the (1(X)) surfaces of nickel, palladium and platinum. These studies support the usefulness of the cluster model approach in studies of heterogeneous catalysis involving transition metal catalysts. 

As well as formation of metallacyclobutenones, reactions of cyclopropenones with transition-metal compounds can lead to acetylene-derived products by decarbonylation, to quinones by a nickel-catalyzed cyclodimerization, and to cluster compounds involving a cleavage of the double bond of the cyclopropenone. ... 

It is our intent in this chapter to point out the potential utility of transition metal-alkyne complexes and cluster compounds which may be derived from them as reagents in organic synthesis. In the interests of brevity, we have chosen to limit the discussion to the organic chemistry of isolable acetylene complexes. Those reactions of particular synthetic promise for which alkyne complexes have not been demonstrated to be intermediates, but whose course very likely involves such complexes, will be mentioned only briefly. 

The Cobalt Triad.— The first example of a paramagnetic octahedral cluster, [COeC(CO)i4], shows uneven lengthening of cobalt-cobalt bonds indicative of the anti-bonding character of the extra electron. Transition-metal carbonyls, particularly ICo2(CO)8l catalyse the conversion of CO and H2 at moderate temperatures and pressures into ethanol and other oxygenated products. Parameters obtained by a complete Wolfsberg-Helmholtz (Extended Hiickel) MO analysis of [CoMe(CO)4] have been fitted to the measured u.v. absorption bands. Products from the co-condensation of cobalt vapour with cyclo-pentadiene and various acetylenes include (8—11). ... 

In another reaction of [Mo3S4] +, its aqua complex [Mo3S4(H20)9] + reacts with acetylene at room temperature in I m HCl to yield complex 61, according to Scheme 20. Here a bond is formed between a carbon atom within a hydrocarbon molecule and a sulfur atom of a metal-sulfide cluster. The adduct can be viewed as representing the transition state or intermediate presumed to be formed when an organosulfur molecule reacts with a sulfide vacancy of a heterogeneous catalyst (cf. Section 2.6.4) or a cluster. However, whereas the reaction depicted in Scheme 20 is the first step of a sulfurization process (C-S bond formation), the direction of the conversion would have to be reversed to make it the second step of a desulfurization process (C-S bond cleavage). 

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