Synthesis and Applications in Chemistry and Materials

The following sections are included:

• Preface
• About the Editors
• Contents

The rational design of transition-metal catalysts is one of the appealing contemporary areas of chemical research, driven by sustainability and waste reduction targets. In this context, many distinct types of ancillary ligands were evaluated as supporting ligands in transition-metal catalysis with heteroatom-stabilized carbenes, i.e., N-heterocyclic (NHCs) and acyclic diaminocarbenes (ADCs), dominating the field. Tuning of the electron-donor capacity and steric properties of aminocarbenes, essential for the success of their catalytic applications, has been mainly achieved via the empirical variation of their structures. In this context, complexes with NHCs are typically assembled through the direct coordination of the carbene species to a metal center, whereas the complexes with ADCs are generally obtained via an alternative route based on metal-mediated nucleophilic addition to isocyanides. The latter approach is atom-efficient and high-yielding, allowing for an easy variation of the periphery of aminocarbenes through the careful selection of a suitable isocyanide–nucleophile pair ensuring high synthetic flexibility. Insofar as the structure of an aminocarbene species is concerned, several key structural points associated with the emergence of catalytic properties are recognized, including the nature of the metal center, steric and electronic properties as well as the binding mode of the aminocarbene ligands, the presence of secondary ancillary ligands and the manifestation of non-covalent interactions in the metal complex structure. In this survey, we reflect on the application of representative metal–ADC complexes in catalysis, focusing on the recognition of the structural features of the carbene complexes leading to the manifestation of their catalytic properties.

The current demand for more sustainable catalytic processes has seen a clear shift from the classical noble-metal-based catalysts to cheaper and abundant metals. The focus on earth-abundant transition-metal-based catalysts has been marred by synthetic and characterization challenges but is ultimately achieving the desired catalytic results. To extract the catalytic potential of earth-abundant transition-metal-based complexes, great care concerning the design of the supporting ligands is required. Under certain conditions, monoanionic bidentate chelates present a good strategy to tackle those goals. The 2-iminopyrrolyl framework is a good example of that: Its huge electronic and steric tuning potential has paved the way for diverse coordination chemistries of first-row transition metals, from manganese to copper. With such metal complexes in hand, several catalytic applications have been studied, namely polymerization, hydrofunctionalization and click chemistry. This chapter features the main catalytic achievements of complexes of earth-abundant metals bearing 2-iminopyrrolyl ligands and their respective mechanistic insights and structure–reactivity relationships.

The catalytic functionalization of C–H, C–OH and C–C bonds belongs to the most important processes in nature and the industry. In nature, this process occurs via involvement of enzymes, effectively and selectively, usually with very high turnover numbers. The pivotal role in enzymatic activity is played by the metal center cofactors, which involve several bioavailable transition metals, such as, iron, copper, manganese and zinc. In the industry, bond functionalization requires the presence of metal catalysts; therefore, a bio-inspired design of metal catalysts is a challenging approach. The recent advances in the catalysis of industrially important reactions, namely the oxidation and hydrocarboxylation of alkanes, the oxidation of alcohols and C–C coupling are reported. Convenient, environmentally friendly methods are presented, and the role and efficacy of the various transition-metal (iron, copper, zinc, manganese, nickel, vanadium, palladium and cobalt) catalysts are explored.

Suzuki–Miyaura (SM) coupling belongs to the catalytic methods most commonly used in the industry for the production of biaryl compounds applied as pharmaceuticals, natural products and agrochemicals. An important advantage of the SM method is its versatility and high tolerance for the presence of functional groups in substrates. Due to the good stability and high activity of the SM catalysts, the reaction can be carried out in air, under mild conditions and in environmentally friendly solvents, including water. In this chapter, representative groups of phosphorus-free Pd catalysts are presented, with special emphasis on their activation and stability under catalytic conditions. Analysis of structural aspects, such as the electronic and steric influence of ligands on the reaction course, is the basis for improvement and the design of new, more active catalysts. Selected examples present the basic SM coupling as well as carbonylative coupling leading to diaryl ketones. In this area, besides CO, esters and amides are activated by Pd catalysts and serve as carbonyl group donors.

Modern humans are heavily dependent on chemicals and have constantly looked for improved molecular designs with better efficiency and simple ways to get them. This is why catalysis, the method of getting upgraded chemicals through modern processes using a suitable catalyst, is an important area of research. At least one catalytic step is involved in 90% of the manufacturing of chemical products nowadays. Since chemical processes are always associated with a potential environmental hazard, the behavior of the catalyst used is of specific significance concerning environmental resistance. Therefore, the used catalysts should be explored for complete restoration even in the case of 100% efficiency. Zeolites are a group of materials with the characteristics of thermal and chemical stability and can be regenerated by calcination. Because of these crucial features, these types of materials have been widely used in various catalytic reactions and large-scale petrochemical procedures. The gas-phase reactions in zeolite have been intensively studied to develop zeolite as acid–base and redox catalysts to generate fine chemicals. Furthermore, the encapsulation of different metal complexes within the zeolite pores makes them attractive catalytic materials to contribute as a potential component for various industrial organic transformation processes.

The potential applications for heterogeneous catalysts (supported metallic complexes, metal–organic frameworks (MOFs) or polynuclear complexes) in C–H and C–OH activation are tantalizing. In response to the opportunity, novel strategies have been recently developed, which present a critical step toward harnessing the experimental factors, such as yield and selectivity, by enabling new techniques and hence real-world applications. This chapter provides an up-to-date survey (from the past five years) of the most promising novel routes by summarizing the progress made in the use of unconventional activation methods for performing oxidation reactions, highlighting the synergy of these technologies with heterogeneous catalysis. Focus is centered on both usual catalysts activation methods and less usual ones, such as ultrasounds, microwaves, grinding (mechanochemistry), as well as their combined use.

The reactivity of a number of mono- and polycyclic hydrocarbons in the selective ring-opening reaction as an approach in increasing the cetane number of diesel fuel is compared. Possible pathways of reactions in the presence of Pt, Rh, Ir and Ru catalysts deposited on different carriers are considered.

The synthesis and reactivity of uranium complexes supported by a bis(aryloxide) cyclam ligand are surveyed. The hemilability of this ligand proved to be particularly appropriate to stabilize highly reactive uranium(III) and uranium(IV) complexes that were able to activate different unsaturated molecules. Of note are the reductive cleavage of azobenzene with the unprecedented formation of a trans-bis(imido) uranium(VI) complex and its reaction with carbon dioxide to generate a trans-oxido-imido uranium(VI) complex and phenyl isocyanate. The formation of a trans-dioxido uranium(VI) (uranyl) complex was also observed in the reaction of uranium(IV) complexes with sodium nitrite. Quantum chemical computations and solution ¹⁵N NMR spectroscopy were employed to evaluate bond covalency in the three {E=U=E}²⁺ complexes (E = O, NR).

This book chapter is dedicated to the synthesis and application of manganese carbonyl alkyl complexes. At first, an overview of the synthesis of such complexes is given. Furthermore, selected stochiometric reactions of manganese carbonyl alkyl complexes with various ligands, including the renowned carbonylation reaction to give carbonyl acyl species, are discussed. Apart from stochiometric transformations, modern applications in catalysis are elucidated. These applications cover the hydrogenation reactions of (polarized) carbon–carbon and carbon–heteroatom multiple bonds. In addition to that, the potential of manganese carbonyl alkyl complexes in the fields of hydrofunctionalization and cross-coupling chemistry is presented.

This chapter aims to illustrate the research that was conducted in Portugal or in collaboration with Portuguese research groups in the 2011–2022 period on the oxidative conversion of volatile organic compounds (VOCs) to useful building blocks. It concerns the selective oxidation under mild catalytic oxidations of some VOCs (toluene, xylene, ethylbenzene, styrene and n-hexane), which are hazardous to human health and the environment. Both homogeneous and heterogeneous catalysts are discussed.

The following section is included:

• Index