Metal-Ligand Co-operativity

Table of Contents

1. Frontmatter

2. Recent Advances in the Applications of Metal-Ligand Cooperation via Dearomatization and Aromatization of Pincer Complexes

   Amit Kumar, David Milstein

   Abstract

   Metal-ligand cooperation (MLC) has played an important role in the development of new green homogeneous catalytic reactions. In the past we have discovered a new mode of MLC which is based on the dearomatization and aromatization of pincer complexes and have utilized it to uncover new modes of bond activation and new catalytic reactions that are atom-economic, environmentally benign and sustainable. Building upon our earlier discoveries which have been well-reviewed in the past, this chapter discusses recent progresses on the application of the concept of MLC via dearomatization and aromatization of pincer complexes. Bond activation of greenhouse gases such as CO₂ and N₂O and their applications in the development of catalytic reactions have been discussed in detail. Additionally, the new concept of template catalysis where catalytic transformation occurs mainly on the pincer ligand and the metal centre acts as an “anchor” has also been discussed. Template catalysis utilizes the tool of MLC via dearomatization/aromatization to perform Michael reactions forming new C–C, C–O and C–N bonds catalysed by ruthenium, rhenium and manganese pincer complexes.

3. Metal-Ligand Cooperation at Phosphine-Based Acceptor Pincer Ligands

   Martine R. Tiddens, Marc-Etienne Moret

   Abstract

   Acceptor ligands, which predominantly withdraw electron density from a transition metal center, often engage in weak metal-ligand interactions. These can be stabilized by flanking the acceptor moiety with strongly binding phosphines in a pincer motif, affording more robust complexes in which bond activation and/or bond-forming events can take place while preserving the integrity of the molecule as a whole. This contribution highlights recent developments in this area. Compounds incorporating a borane at the central position are discussed first, followed by compounds incorporating an electrophilic C = E (E = C, O, N) π-bond. In both cases, recent examples highlight the ability of these ligands to (1) respond to electronic changes at the metal by modifying their binding mode and (2) accept a nucleophilic fragment (e.g., hydride) from substrate molecules. Applications of acceptor pincer ligands as cooperative catalysts are discussed.

4. Metal-Ligand Cooperativity of Phosphorus-Containing Pincer Systems

   Seji Kim, Yeong-Eun Kim, Yunho Lee

   Abstract

   A metal-bound phosphorus atom can actively participate in various organometallic reactions as an electron reservoir and/or a group transfer site, because a phosphorus atom can adopt several distinct forms such as cationic phosphenium, anionic phosphide, neutral phosphine, or a phosphinyl radical, thus making it more versatile. Furthermore, the ability of the phosphorus atom to have various oxidation states promotes the phosphorus atom to be cooperatively engaged in the reaction occurring at a metal center. By using such properties of phosphorus, metal-ligand cooperative (MLC) reactions occurring at a P-M moiety embedded in several transition metal pincer systems are discussed in this chapter.

5. Cooperative Reactivity by Pincer-Type Complexes Possessing Secondary Coordination Sphere

   Ajeet Singh, Evamarie Hey-Hawkins, Dmitri Gelman

   Abstract

   Pincer complexes represent a family of potent compounds having a tremendous number of manifold applications in organometallic chemistry, synthesis, catalysis, materials science, and bioinorganic chemistry. This chapter overviews the recent developments in the chemistry and catalytic applications of pincer complexes incorporating secondary coordination sphere or an appended functionality that can interact with the catalytic center or can modulate its reactivity via secondary substrate-catalyst interactions. Combining the concepts of modular and stable pincer ligands and secondary interactions provides excellent opportunities for fine-tuning the properties of a coordinated metal center and, consequently, attracts considerable interest.

6. Redox-Active Pincer Ligands

   Jarl Ivar van der Vlugt

   Abstract

   This review aims to provide a comprehensive overview of the emerging field of redox-active pincer ligands. As such, following a short general introduction on ligand-centred redox activity, the recent literature is discussed in two separate sections. The first deals with ligand platforms that predominantly display reductive chemistry from the ‘parent’ scaffold; the second focusses more on systems that readily undergo oxidative chemistry. Fundamental stoichiometric reactivity as well as, where explored, catalytic applications will be discussed. This combined survey hopefully inspires the exploration and uncovering of many new avenues within the realm of redox-active pincer chemistry.

7. A Pincer Motif Etched into a meta-Benziporphyrin Frame

   Karolina Hurej, Lechosław Latos-Grażyński

   Abstract

   The incorporation of a meta-phenylene moiety into a β-alkylated or meso-tetraarylporphyrin framework resulted in the formation of m-benziporphyrins. Their molecular design preserves all the essential virtues of the original tetrapyrrolic architecture of regular porphyrin, including the perfect match between the ionic radii of the inserted metal cation and the size of the macrocyclic (CNNN) core, and steric protection provided by thoughtfully chosen β-alkyl or meso-aryl substituents. The analogous incorporation of a pyridine moiety yielded a class of pyriporphyrins. The organometallic derivatives of m-benziporphyrins and pyriporphyrins are reminiscent of the large family of pincer ligand complexes, in which the metal–arene bond is supported by two amine or phosphine arms. This chapter relates the chemistry of the pincer ligand and that of the m-benziporphyrin (aza-m-benziporphyrin) emphasizing the fact that both groups have been structurally and functionally based on common dominators, i.e., on m-phenylene (aza-m-phenylene) rings.

8. The Role of Metal-Ligand Cooperation in Manganese(I)-Catalyzed Hydrogenation/Dehydrogenation Reactions

   Stefan Weber, Karl Kirchner

   Abstract

   The usage of earth-abundant metals as catalysts for chemical synthesis in order to install more sustainable reactions is a major goal in modern synthesis. Within the last few years, well-defined manganese complexes appeared in academic research and were proven to be a powerful player in the field of benign oxidation and reduction reactions. Hydrogenation of polarized double bonds such as aldehydes, ketones, esters, amides, and nitriles, but also carbon-carbon double bonds, can efficiently be achieved by well-defined manganese complexes. In the case of oxidation reactions, typical condensation reactions such as aldol condensation or Michael addition may be carried out with alcohols as starting material by in situ oxidation to carbonyl moieties, employing finely tuned manganese complexes. In this book chapter, we describe the development of the emerging field of manganese-catalyzed hydrogenation/dehydrogenation reactions in conjunction with metal-ligand cooperation processes.

9. Hydrogenation Reactions Catalyzed by PNP-Type Complexes Featuring a HN(CH2CH2PR2)2 Ligand

   Dewmi A. Ekanayake, Hairong Guan

   Abstract

   This chapter first provides a brief background of how hydrogenation mechanisms have evolved over the years leading to the blossom of catalytic systems with metal-ligand cooperativity. The main body of the chapter focuses specifically on complexes supported by ligands of the type HN(CH₂CH₂PR₂)₂. The discussion of hydrogenation systems is organized based on the central metals including Ru, Fe, Os, Rh, Co, Ir, Ni, Pd, Mo, W, Mn, and Re (in that particular order). Substrates involved in these hydrogenation reactions include olefins, aldehydes, ketones, esters, amides, epoxides, nitriles, imines, N-heterocycles, CO₂ (to formate or methanol), silyl formates, CO (to ethylene glycol or methanol), and cyclic carbonates. When appropriate, the presence or the lack of metal-ligand cooperativity in these catalytic systems is highlighted.

10. Catalytic Conversion of Nitriles by Metal Pincer Complexes

    Beibei Guo, Edwin Otten, Johannes G. de Vries

    Abstract

    The nitrile is an extremely useful functional group in organic synthesis: it can be transformed into amides, carboxylic acids, amines and imines; yet it is relatively stable and can be easily carried through several synthetic steps before being converted. The conversions of nitriles under mild conditions are thus very important transformations. Great progress has been made in the last decade in the use of metal pincer complexes as catalysts for quite a number of reactions of nitriles and nitrile-containing molecules. The selective hydrogenation of nitriles either to the amines or to the imines usually follows a Noyori-type outer-sphere mechanism. Coordination of aliphatic nitriles to the metal centre renders the α-proton rather acidic allowing deprotonation followed by carbon-carbon coupling reactions. The pyridine-based metal pincer complexes introduced by Milstein allow for novel mechanisms based on metal-ligand cooperativity in which the pyridine undergoes dearomatisation induced by deprotonation of one of the side arms. The nitrile can undergo a cycloaddition to the complex in its dearomatised form, creating a new bond between the nitrogen atom and the metal, whereas the nitrile carbon atom forms a C-C bond with the carbon atom of one of the pincer side-arms. The resulting metalimide undergoes nucleophilic addition more easily than the nitrile. It can also easily rearrange to the enamide, which can undergo C-C bond forming reactions. Also, oxo- and aza-Michael reactions are facilitated on the unsaturated nitriles, such as acrylonitriles or pentenitriles. Most reactions proceed under mild conditions in excellent yields.

11. The Application of Pincer Ligand in Catalytic Water Splitting

    Hong-Tao Zhang, Ming-Tian Zhang

    Abstract

    The study of catalytic water splitting is one of the most active areas of research across many sub-disciplines of chemistry. To understand the mechanistic details and design artificial molecular catalysts for both water reduction (Hydrogen Evolution Reaction, HER) and water oxidation (Oxygen Evolution Reaction, OER) continue to be a challenge for the development of artificial photosynthetic system. This chapter will focus on the summarization of recent development in the rapidly growing field of artificial molecular catalysts with pincer ligand for both HER and OER.

12. Correction to: Metal-Ligand Cooperativity of Phosphorus-Containing Pincer Systems

    Seji Kim, Yeong-Eun Kim, Yunho Lee