Preface

Efficient and selective execution of chemical transformation of substrates is very important to ‘make only what you want well’. Such synthesis is also desirable from the perspective of the Sustainable Development Goals (SDGs) in terms of reducing waste, such as by atom economy, reducing energy consumption and the formation of unwanted substances. The catalytic activity of metal complexes has been actively studied to selectively carry out chemical reactions with less energy injection. This is because metal complexes form intermediates and transition states that are quite different from those expected in thermal reactions and provide new reaction pathways with low activation barriers. From the viewpoint of a coordination chemist, the detection, isolation, and identification of reaction intermediates, kinetic considerations of reactions, and elucidation of reaction mechanisms based on these facts provide great pleasure and fun. At the same time, these findings may lead to the development of better catalytic reactions.

Metalloenzymes are probably the first thing that comes to mind as excellent metal-complex catalysts. Organisms carry out the metabolic reactions to sustain life under mild conditions in vivo. In many cases, metal complexes bearing amino acid residues as ligands are formed inside proteins to form the active centres of metalloenzymes. The metal complexes, as active centres, catalyze the reactions and carry out the sometimes unbelievable biosynthesis (manufacturing) and metabolism (decomposition and modification) as the indispensable chemical transformations for life. However, humankind has also developed various metal complexes as catalysts and utilized them to synthesize various molecules and produce useful and beneficial chemical products, laying the foundation for making our lives more comfortable and affluent. Unlike enzymes in nature, we need to consider durability, selectivity, efficiency, and cost in the development of catalysts. Even under such constraints, we continue to advance catalytic chemistry based on coordination chemistry by improving metal complexes by virtue of gaining mechanistic insights into catalysis, achieving better selectivity and efficiency, and enabling previously impossible and difficult chemical transformations.

This book covers a wide variety of redox reactions and focuses on metal complex catalysts effective for those reactions. As molecular catalysts, oxidase (methane monooxygenase, cytochrome P450, etc.) and reductase (nitrogenase, hydrogenase) with metal centres, and their model systems with synthetic metal complexes, and second- and third-series transition metal complexes are discussed. In addition, metal-complex catalysts supported on solid surfaces and metal nanoparticles in metal–organic complexes (MOFs) are also included as heterogeneous catalysts. Important reactions such as C–H bond activation, bond formation, and hydrogenation reactions based on organometallic chemistry are also covered. In this book, we especially try to describe not only what reactions proceed in those catalytic reactions, but also how the reactions proceed. In addition, the book includes approaches to catalytic reactions from the viewpoint of computational chemistry, so that readers can understand what intermediates the catalytic reactions go through and deepen their understanding of the reaction mechanisms. Furthermore, the usefulness of metal complexes in light-driven catalytic reactions, including catalytic oxidation and reduction reactions for artificial photosynthesis, is also described.

To promote the development of useful catalysts, a strong sense of purpose, persistence, ingenuity, and a deep and accurate understanding of the results will be necessary. It will also be necessary to benefit from serendipity with a keen eye backed up by these factors. We hope that this book will be of some help to those who have such aspirations, such as third- and fourth-year undergraduate students, students at technical colleges, graduate students, and researchers who are working at the forefront of metal complex catalyst development. It is also our sincere hope that this book will encourage readers who are newly interested in metal complex catalysis and wish to use metal complexes as catalysts, and that it will provide an opportunity to look at catalytic chemistry from a completely different field and to develop catalysts from a completely new perspective.

Takahiko Kojima

Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba