Chapter 1: Overview of the Catalytic Chemistry of Metal Complexes

Catalysts are widely used as indispensable tools in modern material transformation. A wide variety of catalysts are used in a wide range of applications, from the synthesis of compounds in the laboratory to industrial production. In addition, new catalysts are being developed every day to carry out useful chemical reactions, and methods are being developed to enrich our society.

The history of the Nobel Prize in Chemistry shows that several studies to develop catalytic chemistry have been awarded, starting with W. Ostwald in 1909 (synthesis of nitric acid; discovery of catalysis); Paul Sabatier in 1912 (catalytic hydrogenation of organic compounds); Fritz Haber in 1918 (synthesis of ammonia); K. Ziegler and G. Natta in 1963 (polymer polymerization methods); J. W. Cornforth in 1975 (stereochemistry of enzymatic reactions); W. S. Knowles, R. Noyori, and K. B. Sharpless in 2001 (asymmetric catalysis); R. H. Grubbs, R. R. Schrock, and Y. Chauvin in 2005 (metathesis reactions); and A. Suzuki, E. Negishi, and R. F. Heck in 2010 (cross-coupling). Most of these studies focused on metal complexes as catalysts as well as metal-catalyzed reactions, which is a remarkable indication of the importance and usefulness of metal complexes to conduct desirable chemical conversions.

In nature, many metalloenzymes exist and contribute to the maintenance of life activities through catalyzing various reactions.¹  The active center of a metalloenzyme is still a metal complex, with the surrounding amino acid residues coordinated to metal ions as ligands. Well known metalloenzymes are classified as oxidases, reductases, oxidoreductases, hydrolases, monooxygenases, dioxygenases, dehydrogenases, and dismutases.¹  Among the many redox enzymes involved in photosynthesis, the oxygen-evolving center in photosystem II is the tetranuclear manganese-oxo cluster, whose structure and function have attracted much attention.²  These enzymatic reactions also contain many things to learn, such as the process of active species formation and requirements for the development of reaction selectivity.

There are two types of catalytic activity of metal complexes: one is based on the redox activity of the metal center and the other is due to the Lewis acidity of the metal center. In the former, the oxidation number of the metal center changes, and electrons are transferred between the metal complex and the substrate to oxidize or reduce the substrate. In the latter, the oxidation number of the metal center does not change, but the reaction rate is accelerated by increasing the electrophilicity of the coordinated reacting substrate, by decreasing the LUMO energy of the bound molecule to increase its electron acceptability, or by increasing the acidity of the coordinating water molecules. This book deals with metal-catalyzed reactions involving redox processes of metal centers and does not deal with Lewis-acid catalysis.

Depending on the forms of catalysts, catalysts are classified as homogeneous catalysts or heterogeneous catalysts. Homogeneous catalysts are in the same phase as the reactants, whereas heterogeneous catalysts are in a different phase from the reactants. In other words, homogeneous catalysts are generally dissolved in a solvent and the reaction proceeds in a homogeneous solution. In contrast, heterogeneous catalysts exist as solids and react with substances in the gas or liquid phase to carry out the reaction. The examples awarded to the Nobel Prizes mentioned above can be categorized as follows: homogeneous catalysis includes asymmetric epoxidation reaction by Sharpless et al. using a titanium complex, asymmetric hydrogenation reaction by Noyori et al. using chiral ruthenium complexes, olefin metathesis reaction by Grubbs et al. using ruthenium complexes, and cross-coupling reaction by Suzuki et al. using a palladium complex. The ammonia synthesis by the Haber–Bosch method using solid catalysts based on iron oxide and the stereoselective polymerization reaction of propylene by Ziegler–Natta catalyst, which is a solid with the composition as TiCl₃–AlEt₃, are also classified as heterogeneous catalytic reactions.

Homogeneous and heterogeneous catalysts have their advantages and disadvantages. The advantages of homogeneous catalysts are that the reaction proceeds under relatively mild conditions, and molecular design based on the ingenuity of ligands enables the reaction to proceed with high selectivity and the detailed reaction mechanism to be elucidated toward the improvement. One disadvantage of homogeneous catalysts is that reuse of the catalyst and isolation of the products are difficult because the catalyst and reaction products are in the same solution. However, the advantages of heterogeneous catalysts are that they are generally stable, with high activity expected because high temperature conditions can be applied, and catalyst reuse and separation of the catalyst from the product are easy. The disadvantages of heterogeneous catalysts include difficulty in analyzing and elucidating the reaction mechanism for the improvement and low selectivity. This book deals with homogeneous catalytic reactions using discrete metal complexes and not heterogeneous catalytic reactions.

The term ‘catalysis’ was proposed in 1835 by Jöns Jakob Berzelius. The word originates from the Greek spelling of the word ‘dissolution’ (Kata: down, Lyein: to loosen), meaning to loosen bonds and release atoms.³ 

A catalyst, by definition, is a substance that does not change before or after a reaction, and in small amounts reduces the activation energy of the reaction and speeds up the reaction rate at a given temperature. In addition, a catalyst is a substance that does not appear in chemical equations, i.e., they do not affect stoichiometry. However, no actual catalyst is completely unchanged before and after a reaction and should be considered as ‘something that is quantitatively far less than the reacting substance (substrate or reactant reagent) in a reaction that proceeds directly involving a change in that substance’. It should be noted here that a catalyst can change the reaction rate but cannot change the composition of the compounds when added to a reaction system in equilibrium. That is, they change the kinetic parameters of the reaction (activation barriers: ΔG^‡ or E_a) but do not affect the thermodynamic parameters (ΔG). The role of the catalyst is to create a new reaction pathway with a low activation barrier, which is different from a conventional reaction pathway with a high activation barrier, as shown in Figure 1.1, to increase the reaction rate and allow the reaction to proceed efficiently.

A typical reaction mechanism for catalytic oxidation reactions with metal complexes is shown in Figure 1.2. In catalytic oxidation reactions, the complex as a catalyst first activates the reactant (oxidant) to be converted to a reactive species such as a high-valent metal–oxo complex. The active species react with a substrate to produce a complex of an oxidation product, which eventually releases the oxidation product and regenerates the starting catalyst. For example, in the case of a C–H bond hydroxylation reaction catalyzed by a metal complex in a coordinating solvent such as acetonitrile with a peroxide as the oxidant, the catalytic cycle is known to proceed as shown in Figure 1.3(A).¹  For more details, see the related sections in following chapters that mention oxidation reactions. In catalytic reduction reactions, a similar reaction mechanism may also be operative, e.g., in hydrogenation reactions, as shown in Figure 1.3(B), activation via oxidative addition of molecular hydrogen to the metal–hydrido complexes are formed as active species, which react with alkenes to form alkyl complexes via insertion and then release the hydrogenation products by reductive elimination.⁴ 

However, in catalytic reactions involving organometallic complexes, the reaction often proceeds by activating the substrate at the metal center. An example is the Hoechst–Wacker process for the synthesis of acetaldehyde by oxidation of ethylene, as shown in Figure 1.4.⁵  In this process, the Pd(0) species produced by reductive elimination of the product is formally oxidized with oxygen molecules. In the activation of the substrate, oxidative addition often proceeds to the low-valent metal center. Reactions via such substrate activation are discussed in the sections related to organometallic chemistry for C–H and C–C bond activation, N₂ and CO₂ reduction.

As shown in Figures 1.3 and 1.4, the catalyst provides a pathway completely different from the direct reaction pathway between the substrate and the reacting reagent, by forming various intermediates, allowing the desired reaction to proceed efficiently. Although hydroxylation of a C–H bond of alkanes does not proceed when alkane and peroxide react at room temperature, as shown in Figure 1.3(A), another reaction pathway is provided to activate the peroxide at room temperature to form a high-valent metal–oxo complex as an oxidatively active intermediate; that active species can abstract a hydrogen atom and allow the hydroxylation reaction to proceed. Also, alkenes do not react with hydrogen, but catalysts provide a completely different reaction pathway that activates hydrogen to form metal–hydride complexes, and the hydrogenation reaction proceeds via coordination and insertion of the alkene into the metal–H bond.

Important requirements for catalysts include activity, durability, selectivity, and cost. The catalytic activity of metal complexes is generally manifested by the nature of the metal center and the electronic and steric effects of the ligands. In other words, we can select a metal center, design and synthesize its ligand(s) for complexation and examine the catalytic activity of the generated complex. Since the required elements vary depending on the reaction of interest, for metal–complex catalysts, it is possible to meet this objective by selecting the metal and designing and synthesizing the ligands at will.

For example, the epoxidation of alkenes is performed by m-chloroperbenzoic acid (mCPBA). However, alkene epoxidation with mCPBA does not provide stereoselectivity or asymmetric selectivity of the resulting epoxide. However, when epoxidation is performed using a metal complex catalyst having an optically active ligand, asymmetric epoxidation proceeds based on the steric demand of the ligand, since the oxidizing active species is a complex with an optically active ligand in the reaction mechanism shown in Figure 1.1. For example, Katsuki et al. have developed a highly selective asymmetric epoxidation reaction using an oxygen molecule as the oxidant, catalyzed by a ruthenium complex with a chiral salen (N,N′-bis(salicylidene)ethylenediamine) derivative as a ligand, as shown in Figure 1.5.⁶  The application of catalytic chemistry based on transition metal complexes to such asymmetric synthesis is essential for the synthesis of modern pharmaceuticals and other fine chemicals.

This book describes oxidation catalysis, reduction catalysis, bond formation and activation, and photocatalysis, including the latest results. We hope that this book will arouse the readers’ interest in catalytic chemistry based on transition metal complexes and help the development of catalytic chemistry based on coordination chemistry.