Ionic Liquids (ILs) in Organometallic Catalysis

Table of Contents

1. Frontmatter

2. The Nature of Metal Catalysts in Ionic Liquids: Homogeneous vs Heterogeneous Reactions

   Ning Yan

   Abstract

   Despite the extensive investigations on transition-metal-catalyzed reactions in ionic liquids (ILs), the most fundamental issue, i.e., whether the catalysis is homogenous or heterogeneous, is not always clear. This chapter provides a brief description of the methods to distinguish the metal-complex-catalyzed homogenous reactions from the metal-particle-catalyzed heterogeneous reactions in ILs. Following that, a current understanding of the nature of transition metal catalysts in ILs, categorized by the types of reactions, is provided.

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3. Metal Nanoparticle Synthesis in Ionic Liquids

   Christoph Janiak

   Abstract

   The synthesis of metal nanoparticles (M-NPs) in ionic liquids (ILs) can start from metals, metal salts, metal complexes, and in particular metal carbonyls and can be carried out by chemical reduction, thermolysis, photochemical, microwave irradiation, sonochemical/ultrasound-induced decomposition, electroreduction, or gas-phase synthesis, including sputtering, plasma/glow-discharge electrolysis, physical vapor deposition, or electron beam and γ-irradiation. Metal carbonyls, M _x (CO) _y , are commercially available and elegant precursors because the metal atoms are already in the zerovalent oxidation state for M-NPs so that no reduction is necessary. The thermal decomposition of metal complexes, including metal carbonyls in ILs by microwave irradiation, provides a fast and low-energy access to M-NPs. The reason is an excellent absorption efficiency of ILs for microwave energy due to their high ionic charge, high polarity, and high dielectric constant. Ionic liquids allow for the stabilization of M-NPs without the need of additional stabilizers, surfactants, or capping ligands because of the electrostatic and steric properties inherent to ILs. From the IL dispersion, the M-NPs can be deposited on various surfaces, including graphene derivatives and nanotubes. The formation of intermetallic MM′-nanoalloys in ILs has just begun to be explored. Examples for M(M′)-NP/IL dispersions in catalytic reactions (C–C coupling, methanol synthesis, hydrogenation) are noted.

4. Size Control of Monodisperse Metal Nanocrystals in Ionic Liquids

   Pascal Lignier

   Abstract

   During the last decade, a great interest in the preparation of uniform nanocrystals has offered efficient synthetic strategies to precisely engineer metal, metal oxide and alloy at the nanoscale. Due to their physicochemical properties, ionic liquids (ILs) simultaneously demonstrated their potential in different areas such as nanocrystal synthesis, catalysis and energy. As a result, ionic liquids have been employed for the preparation of monodisperse nanocrystals and the control of their size. This chapter highlights the most promising methods for the synthesis of uniform nanocrystals in ionic liquids which act as a solvent, stabiliser, reducing agent and even precursor. As a result, successful preparations of nanoparticles in the presence of ILs are now available for both noble and earth-abundant elements such as gold, platinum, iridium, silver, palladium, ruthenium, rhodium, copper, nickel, cobalt and iron.

   The formation mechanisms of these nanocrystals are discussed as well as our mechanistic understanding in conventional organic and aqueous solvents. In addition, the IL approach is compared to leading methods in conventional solvents to make possible the identification of general principles for most metallic elements. By analogy with conventional solvents, these strategies can be adapted to the preparation of semiconductor nanocrystals. These achievements are going to drive the identification of relationships between the nature of ILs components, the physicochemical properties of ILs, the formation of nanocrystals in ILs and the resulting performances of these nano-objects.

5. Structural Features and Properties of Metal Complexes in Ionic Liquids: Application in Alkylation Reactions

   Cinzia Chiappe, Tiziana Ghilardi, Christian Silvio Pomelli

   Abstract

   Metal-containing ionic liquids (ILs) represent a promising sub-class of “charged” liquids which increase the tunability of ILs combining the properties of common organic salts with magnetic, photophysical/optical or catalytic properties of the incorporated metal salts. In ILs lacking of coordinating groups on cation dissolution of metal salts is generally associated with the coordination of the metal cation with IL anion(s). Here we report on the anionic speciation of metals in ILs having either highly or poorly coordinating anions and we discuss some peculiar properties of these systems in the light of their structural features.

6. Ionic Liquids in Transition Metal-Catalyzed Hydroformylation Reactions

   Bernhard Rieger, Andriy Plikhta, Dante A. Castillo-Molina

   Abstract

   The latest state of the art in ionic liquid-based hydroformylation is reviewed in detail in this chapter. This multiphase homogenous catalytic system represents a promising strategy in order to reduce catalyst leaching during product separation and achieve the desired ratio of linear-to-branched aldehyde with a high catalytic activity and yield. A series of different catalytic systems, ionic liquids (ILs), and ligands together with their application in the hydroformylation of a variety of alkenes is presented. The features of those ILs derived from their composition and their interactions with substrates and catalysts are also discussed. In addition, recent studies on the catalyst distribution in the bulk and on the surface of ILs are summarized. Herein, the properties of the ligands show an impact in the activity and selectivity of the reaction. Moreover, not only Co and Rh complexes can be applied in the hydroformylation in ILs but also Pt and Ru complexes. On the other hand, the uses of CO₂ as chemical C¹ feedstock or scCO₂ as carrier for the reagents and products in the hydroformylation reaction are commented. Catalytic processes where supported ionic liquid phases (SILPs) and nanocatalysts intervened complement this work.

7. ILs in Transition Metal-Catalysed Alkoxy- and Aminocarbonylation

   Rita Skoda-Földes

   Abstract

   The use of carbon monoxide as a carbonyl source for the preparation of aldehydes, ketones and carboxylic acid derivatives in homogeneous catalytic reactions is a valuable tool in the synthesis of fine chemicals. Amongst immobilising agents for carbonylation catalysts, the application of ILs drew particular attention. Besides enabling catalyst reuse, the ionic liquid may influence the structure of catalysts or catalyst precursors, may affect the chemoselectivity of the reaction or may be capable of solubilising special substrates. The present chapter is intended to summarise the research carried out in the past 15 years involving carbonylation reactions of alkenes/alkynes and aryl/alkenyl halides in the presence of various nucleophiles, such as alcohols, water, amines and thiols.

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8. Metal-Catalyzed Oxidation of C–X (X = S, O) in Ionic Liquids

   Andreia A. Rosatella, Carlos A. M. Afonso

   Abstract

   Ionic liquids (ILs) have attracted the scientific community due to several advantages compared with traditional solvents such as their low volatility and high ability to dissolve different organic and inorganic compounds, among other advantages. Transition metals can be toxic compounds; however, this issue can be minimized when used in catalytic amounts which also allows reaction efficiency and selectivity increases. The combination of metal catalysis with the use of ILs can be a powerful tool for several organic reactions, taking advantage of the most attractive property of the ILs, which is the possibility of an easier recycling of the catalytic system, increasing the greenness of the process. The application of ILs in metal-catalyzed oxidation reactions of alcohols and sulfides is discussed in this chapter.

9. Epoxidation of Olefins with Molecular Catalysts in Ionic Liquids

   Christian J. Münchmeyer, Lilian R. Graser, Iulius I. E. Markovits, Mirza Cokoja, Fritz E. Kühn

   Abstract

   This review gives a summary of the epoxidation of various olefins catalyzed by different transition metal complexes, as well as by metal-free compounds in ionic liquid media. A comparison of the most active systems for different olefins is presented, and the effect of the ionic liquid solvents on the catalytic activity and catalyst reusability is discussed.

10. Ionic Liquids in Palladium-Catalyzed Cross-Coupling Reactions

    Piero Mastrorilli, Antonio Monopoli, Maria Michela Dell’Anna, Mario Latronico, Pietro Cotugno, Angelo Nacci

    Abstract

    This chapter surveys the most significant developments in the field of palladium-catalyzed cross-coupling reactions in ionic liquids. The beneficial effect of the ionic liquids in terms of activity, selectivity, and recyclability is commented for all types of reactions discussed, namely the Heck, Suzuki–Miyaura, Stille, Sonogashira, Ullmann, and Negishi cross-couplings. Insights into the reaction mechanisms reveal that the effect of the ionic liquid on C–C bond forming reactions manifests itself not only in the energy lowering of polar transition states (or intermediates) involved in the catalytic cycles but also, depending on the cases, in the stabilization of palladium nanoparticles, the synthesis of molecular Pd complexes with the IL anions, the enhancement of the chemical reactivity of reactants, and others. The synergistic effect found by using appropriate mixtures of ionic liquids is also discussed.

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11. RTILs in Catalytic Olefin Metathesis Reactions

    Cédric Fischmeister, Christian Bruneau

    Abstract

    The homogeneous catalytic olefin metathesis reaction has found a tremendous interest in the past 20 years and multiple applications have now emerged in fine chemical synthesis and polymer chemistry. Immobilization of olefin metathesis (pre)catalysts in room temperature ionic liquids (RTILs) offers the opportunity to recover and in some cases reuse the catalyst and it is also a practical way to reduce the level of metal contaminants in the desired products. This chapter covers the research in this field from the early days with an emphasis on recent results and with a critical look at the origin of catalyst recycling.

12. Ionic Liquids in Transition Metal-Catalyzed Oligomerization/Polymerization

    Anna M. Trzeciak

    Abstract

    This short review presents selected examples of polymerization and oligomerization reactions catalyzed by transition metal complexes in ionic liquid media. Analysis of these data to some extent supports the popular opinion that ionic liquids are not inert solvents but rather should frequently be considered co-catalysts. In particular, the application of imidazolium salts makes possible the formation of carbene complexes and consequently changes catalytic activity in many cases.

13. Ionic Liquids in Transition Metal-Catalyzed Enantioselective Reactions

    Yong Li, Yan-Mei He, Qing-Hua Fan

    Abstract

    Transition metal-catalyzed asymmetric reactions provide a powerful access to the optically active molecules that serve as precursors to pharmaceutically significant compounds. However, separation and recycling of these often expensive chiral catalysts are rather difficult and thus limit their applications in industry. As one of the most promising solutions to these problems, immobilization of a chiral homogeneous catalyst can, in principle, facilitate its separation and recycling, and thus is of considerable interest to both academia and industry. Although many methods have been developed for the immobilization of chiral catalysts via attachment of the catalyst onto a solid support via covalent attachment or noncovalent interactions, obvious decrease in catalytic activity and/or stereoselectivity is often observed due to mass transfer and accessibility of the active sites. Alternatively, chiral catalysts have also been immobilized by the use of aqueous, fluorous, supercritical CO₂ (scCO₂) and ionic liquid phase systems. In this chapter, we present the recent significant achievements of the transition metal-catalyzed asymmetric reactions in ionic liquids (ILs). Their unique properties render ILs ideal “mobile supports” for the immobilization of chiral transition metal catalysts without laborious catalyst modification, and consequently facilitating catalyst separation and recycling through a biphasic operation. Moreover, chiral transition metal catalysts in ILs often show improved catalytic activities and/or stereoselectivities. Representative examples suggested that asymmetric catalysis in ILs would combine the advantages of both traditional homogeneous and heterogeneous catalysis. The positive “IL effects” together with different strategies and concepts (e.g., catalyst with ionic tag and ionic liquid-supported phase catalysis) of IL use in catalytic applications will also be discussed.

14. Backmatter