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Applications of metal clusters in organic synthesis Metal clusters refer to molecular complexes that contain metal–metal bonds, and through these bonds form triangular closed structures or larger closed structures. Most of the metals in them are in the zero-valence state, and multiple metal atoms form cluster frameworks such as triangular, tetrahedral, square pyramidal, and octahedral shapes. As one of the important interdisciplinary fields in modern chemistry, metal cluster chemistry has developed rapidly. Chemists have not only synthesized a large number of metal clusters but also applied them widely in areas such as catalysis and organic synthesis. It can catalyze various reactions such as hydrogenation, oxidation, isomerization, dimerization, carbonylation, and water-gas shift, which are typically carried out using traditional homogeneous or heterogeneous catalysts; it exhibits special catalytic activity and enables the occurrence of some reactions that would otherwise seem impossible. In mixed metal cluster compounds, different metal sites have varying activities, allowing the same compound to catalyze different reactions. Metal cluster complexes are a type of novel catalyst that has attracted the most attention in the field of catalysis in recent years. 1 Reduction 1.1 Reduction of carbon-carbon double bonds Various metal catalysts can be used to catalyze the hydrogenation of C = C bonds. Metal cluster catalysts exhibit many unique properties in this type of catalytic hydrogenation; for example, Pd7 (Pd1415 Phen60O～1100) / Pd8 (Pd2057 Phen84O～1600) supported on TiO2 shows higher activity in the hydrogenation of olefins compared to commercial Pd/TiO2 catalysts. Mononuclear homogeneous complex catalysts proved to be ineffective for the hydrogenation of aromatics, whereas Rh3(CO)12 clusters could hydrogenate benzene to cyclohexene. Pt, Pd, and Pd2Pt metal cluster catalysts supported on γ2Al2O3 can directly catalyze the conversion of toluene into methylcyclohexane. Johnson further developed a cleaning technology for catalyzing the hydrogenation of aromatic hydrocarbons; using a solvent-free ruthenium cluster catalyst Ru6CSn, naphthalene and polyenes were reduced to monoolefins with a selectivity of 90%, while fully hydrogenated products were obtained in high yield using a Ru6Pd6 catalyst. 1. 2 Reduction of carbon-oxygen double bonds Compared to the hydrogenation of C=C bonds, the hydrogenation of C=O bonds is much more difficult. When both C=C and C=O bonds are present in the same molecule, selectively catalyzing the hydrogenation of the C=O bonds while leaving the C=C bonds untouched represents a challenging task in the field of catalysis. Issue 1 Chemical World ·51· © 1994-2006 China Academic Journal Electronic Publishing House. All rights reserved. http://www.cnki.net Liu Hanfan and others studied the catalytic hydrogenation of cinnamaldehyde, crotonaldehyde, and citral by platinum nanometal clusters to produce unsaturated alcohols. Using the PVP2Pt2FeCl3 system, the selectivity for producing unsaturated alcohols from cinnamaldehyde was as high as 98.5%, while it was 98.8% with the PVP2Pt2CoCl2 system. The selectivity for hydrogenating citral using PVP2Pt2FeCl3 was 97.6%, 98.5% with PVP2Pt2CoCl2, and 99.8% with the PVP2Ru2CoCl2 system. The selective catalytic hydrogenation of crotonaldehyde yielded poorer results. The first step of hydrogenation of asymmetric α2-diynes involves a problem of regioselectivity, resulting in the formation of two products. By introducing metal ions into the polymer-stabilized platinum nanometal cluster (PVP2Pt) catalytic system, regional selectivity in the hydrogenation of 2,3-dimethylpentadiene was achieved, yielding 3,2-dihydroxy-2-methylpentane as the main product. Organic compounds can often yield optically active products through the hydrogenation of C = O groups. Hu Bin and his colleagues investigated the catalytic hydrogenation of ethyl acetoacetate to produce ethyl β2-hydroxybutyrate using various ruthenium catalyst systems; they found that ruthenium-containing tetrahedral clusters possess certain hydrogenation activity, and by using them in asymmetric catalytic reactions, a selectivity of 100% could be achieved. This lays the foundation for using chiral tetrahedral clusters in asymmetric catalytic reactions. 1. 3 Reduction of nitro groups: Converting halogenated anilines into their corresponding phenylamines is of great significance in the pharmaceutical, light industry, and pesticide sectors. YANG reported that PVP2Pt metal clusters can be used as catalysts to reduce o-chloroanilines to o-chlorophenylamines at room temperature and pressure. The addition of group 8 metal ions such as Fe3+, Co2+, and Ni2+ can increase the selectivity for o-chlorophenylamines as well as the activity of the PVP2Pt catalyst in the hydrogenation of o-chloroanilines. Subsequently, the research team discovered that PVP2Ru could also catalyze this reaction. The addition of a diamine complex of Zn(II) significantly enhanced the catalytic activity; the reaction selectivity was close to 100%, and the reaction rate was 30 times faster than that of pure PVP2Ru. Converting 2,42***toluene into 2,42-diaminotoluene is also a highly valuable reaction from an industrial perspective. Barone conducted a detailed study of this reaction system using Pt cluster catalysts as simplified molecular models, examining the catalyst’s activity and selectivity, thereby providing a theoretical basis for industrial production. 2 Carbonylation reactions 2.1 CO insertion reaction In the field of organic synthesis, CO insertion reactions are diverse; they involve the addition of one carbon atom to the reactant substrate, and can be used for the preparation of various carbonyl compounds. Under transition metal catalysis, halohydrocarbons undergo carbonylation with carbon monoxide to form carboxylic acids. Commonly used catalysts include nickel carbonyl, cobalt carbonyl, and palladium carbonyl, among others. Yao Yudong applied quantum chemical methods to study the reaction of chlorobenzyl undergoing catalyzed dicarbonylation to produce phenylpropanoic acid. He examined the reactivity of the chemical bonds between chlorobenzyl atoms, as well as the influence of solvents and neutralizing agents on the reaction, and further investigated the effect of para-substituents on the yield of the product. C6H5CH2X + 2CO + H2O → Co2(CO)8 + PEtOHCaO; at 2–5 MPa and 40–60°C, C6H5CH2COCOOH is obtained. Using ruthenium carbonyl complexes as catalysts, aromatic compounds can be synthesized with high selectivity. 2.2 Alkyne hydroformylation: The hydroformylation of terminal alkynes can be used to produce aldehydes, and many metal cluster complexes can serve as catalysts, such as –, Co4(μ-CO)2(CO)8(μ4-PPh)2, and Fe2Co2(CO)11(μ4-PPh)2, among others. Aldehydes can be further reduced to alcohols. Alcohols can be obtained directly by reacting olefins with CO and H2O in a n-butylpyrrolidinium solution of iron carbonyl. If a small amount of basic reagent, such as pyridine derivatives, is added to the reaction, the yield of straight-chain alcohols increases significantly. The SiO2-supported metal cluster RhCo3 (CO)12 is highly active and stable, making it suitable for the carbonylation of ethylene to produce propionaldehyde and n-propanol. 2. 3 Others: In a CO atmosphere, Ru3(CO)12 can catalyze the efficient synthesis of homocarboxamides from aldehydes and olefins; formamide can also undergo carbonylation reactions. 3 Oxidation reaction: PVP2Ag can facilitate the oxidation of ethylene to produce ethylene oxide. This reaction takes place in an ethanol-water mixture at a pressure of 0.1 MPa and a temperature of 90–95 °C. The addition of alkalis can enhance its catalytic activity, and its catalytic efficiency is better than that of commercial silver catalysts. Solid-supported Os catalysts can catalyze the oxidation of trans-stilbene to trans-stilbene oxide, with benzaldehyde, a product of double bond cleavage, as the main by-product; the selectivity for stilbene oxide ranges from 70.5% to 92.6%. The solvent has a significant impact on the reaction; highly polar aprotic solvents facilitate the formation of stilbene oxide. Heteronuclear CoPMn metal clusters can be used as catalysts to oxidize p-xylene to p-phenylbenzoic acid; when the catalyst is CoMn2, the conversion rate is 100% with a selectivity of over 98%. Converting methane directly into useful chemicals is a major challenge in the field of catalysis. However, the thermodynamic barriers associated with this process hinder such reactions. Knops and Gerrits succeeded in converting methane into methanol by using N2O as an oxidant and dinuclear iron cluster compounds as catalysts. 4 Fischer2Tropsch synthesis: The Fischer2Tropsch reaction is an important synthetic method for low-carbon alkanes and olefins; it is used to produce liquid fuels from coal and natural gas as alternatives to crude oil. There are many studies on supported Co catalysts used to catalyze the Fischer-Tropsch reaction. Compared with iron catalysts, although they are more expensive, they exhibit better activity and slower deactivation. Jacobs conducted a detailed study on the effects of different supports (such as Al2O3, TiO2, SiO2, and ZrO2) as well as various promoters, including precious metals and metal cations, on catalysts used in the Fischer-Tropsch reaction. Bedel prepared Co cluster catalysts with a pore size of less than 10 nm for Fischer-Tropsch synthesis; the selectivity for hydrocarbon formation at 250 °C was 70%, of which 40% were C2–C4 products. 5 Elimination reactions: Dehydrohalogenation of straight-chain halohydrocarbons is one of the important methods for synthesizing olefins. Kamiguchi and others studied the dehydrohalogenation reaction of halopentanes catalyzed by clusters of metal chlorides such as Nb, Mo, Ta, W, and Re. When catalysts with a hexagonal metal framework such as Nb, Mo, Ta, and W are preheated to 300°C, the reaction products are 12-pentene, cis- and trans-22-pentene; whereas when Re clusters with a triangular framework are used, in addition to dehydrohalogenation reactions, hydrodehydrohalogenation reactions also occur, with products including pentenes and pentanes. Many halogenated organic compounds that were once widely used pose hazards to humans and the environment, especially polyhalogenated aromatics. Hexachlorobenzene is also an environmental pollutant among these polyhalogenated aromatics, and its dehalogenation reaction is difficult to carry out. Gao, Yan, and others used metal cluster catalysts loaded with PVP to carry out this dechlorination reaction. It was found that under the catalysis of PVP2PdCl2, 100% dechlorination of hexachlorobenzene could be achieved in 270 minutes. PVP2Pd2Co exhibited a dual-metal synergistic effect, which enhanced its catalytic activity; the highest catalytic activity was observed at 55 °C when the molar ratio of Co to Pd was 0.5. 6 Other reactions: In the isomerization of dodecene catalyzed by the metal clusters Ru3(CO)12 and Ru3(CO)9(PPh3)3, catalytic activity exists only as long as the cluster framework remains intact; once the Ru2Ru bond breaks, the catalytic activity disappears and the isomerization reaction stops. The main isomeric product is trans-2,2-dimethylhexene (over 61%). Heteronuclear metal cluster catalysts can be used for the hydrosilylation of diphenylacetylene to produce (E)2(1,2-diphenyl)vinyl-2-triethysilane, and for the hydrosilylation of 1,4-bis(trimethylsilyl)butadiyne to yield (E)2,2,2-triethylenesilane-1,4-bis(trimethylsilyl)but-2-3-ene. Chen Zhi and colleagues studied the catalytic styrene cyclopropanation reaction using chiral heteronuclear metal cluster catalysts, and discussed in detail the effects of the substrate ratio, temperature, solvent, and catalyst amount, thereby opening up a new approach for the application of such clusters in asymmetric catalysis. Sulfur-containing metal cluster compounds can be used in the desulfurization of thiophene. 7 Conclusion Since the emergence of metal clusters, metal cluster chemistry has developed rapidly as one of the important interdisciplinary fields in modern chemistry, achieving many breakthrough results. A large number of metal cluster compounds have been synthesized and used in organic synthesis. As a catalyst, it has advantages such as high selectivity and mild reaction conditions, making it a topic worthy of further research. Research on metal clusters still needs to develop new ideas and explore new approaches in order to achieve greater and more significant progress.