Research

• Fine organic synthesis

• Revealing the nature of catalytic processes

• Evolution of catalytic species in solution

• Adaptive catalysis

Cross-coupling reaction in organic synthesis (source: DOI)

Catalysis has fundamentally changed chemistry of the 21 ^st century and became the basis of the modern industrial processes. Nowadays, the most widely applied types of catalysts are homogeneous catalysts based on transition metal complexes (fine chemicals, pharmaceuticals, drugs, etc.) and heterogeneous catalysts by metal nanoparticles (processing of hydrocarbons, natural resources, bulk chemicals, etc.). Each type of the catalyst has its advantages and drawbacks, but, currently, none of the available technologies allow to create universal catalytic systems.

In recent decades, the reactions of formation of carbon-carbon and carbon-heteroatom bonds, catalyzed by transition metals, have made an enormous contribution to the production of fine chemicals, pharmaceuticals, natural products, "smart" materials, and building blocks for pharmaceutical industry. Cross-coupling and Heck reactions allowed to incorporate aryl, alkenyl, dienyl, and alkynyl fragments into the organic molecules. In addition, high tolerance to functional groups in reagents has significantly expanded the boundaries of use of these catalytic methods. One of the key milestones in the development of this branch was the increasing interest in catalytic processes of carbon-heteroatom bond formation using cross-coupling reaction.

Addition reactions in organic synthesis (source: DOI)

Transition-metal catalyzed addition of heteroaromatic functions to alkynes is an alternative method of carbon-heteroatom bond formation to form dienyl and alkenyl moieties. Addition reactions are atom-efficient, since they do not produce waste, and, thereby, correspond to the principles of green chemistry. These reactions have been successfully used to form C-N, C-O, C-P, C-S, C-Se, and C-I bonds.

In order to optimize the cost and efficiency of the transformations (which are essential for industrial scale), it is crucial to develop new methods of synthesis that require a minimum quantity of a catalyst. The continuous search for new catalytic systems during the previous decades led to the discovery of the reactions that require minimal quantity of the catalyst (ppm or ppb level). Moreover, it was revealed that the clusters of transition metals possess the highest activity.

Revealing the nature of catalytic processes

The active species of the catalyst (Source: DOI)

Recent studies revealed an unusual observation of extremely high activity of transition metal clusters in catalytic reactions. Until now, the border area between homogeneous and heterogeneous catalysis has not been thoroughly explored. However, the next round of catalysis is the development of new generations of the high performance catalysts (active, selective, stable, and recyclable catalysts).

Despite considerable efforts in the study of catalytic reactions, mechanistic side of catalytic processes still raises a lot of concern. Numerous publications on cross-coupling reactions often contain controversial observations, thus, complicating the puzzle. To date, the key problems are related to the stability and interconversions of the catalyst particles in the reaction, as well as the role of a catalyst precursor. On one hand, catalytic reaction can be initiated with almost any source of metal (metal salts, metal complexes, nanoparticles, colloidal systems, supported metal particles, etc.). It should be mentioned, all of these various catalytic species often demonstrate the same type of activity and, presumably, operate within similar catalytic cycles.

Molecular, nanoscale and species catalysis (Source: DOI)

On the other hand, there are clearly defined differences between the pre-synthesized catalysts and catalysts generated in situ, which cannot be analyzed with the help of the simplified methods. Another important inquiry is the re-use of the catalyst: if the reaction can be initiated using different metal sources as catalyst precursor, then recycling and re-use of the catalyst should be easily achievable. Unfortunately, the practice shows that recycling and re-use remain a challenging task. A detailed study and understanding of these phenomena is essential for development of new catalysts, which will be suitable for re-use without loss of catalytic activity.

Evolution of catalyst's sites in solution

"Cocktail" of catalysts (Source: DOI)

Reactions that occur in solutions highly depend on the type of catalyst precursor and the nature of the catalytic system. There are two border cases:

Single-species catalysis: definite metal complex that contain ligands with a high affinity to metal, which is used as a precursor and subsequently undergoes a slight changes in its chemical structure. The original structure of the metal-ligand complex remains unchanged during the catalytic cycle, while the formation of other types of metallic particles does not typically occur (unless the decomposition of the metal complex occurs).

Multiple-species catalysis and cocktail of catalysts: system contains a number of metal particles with metal species, clusters, and nanoparticles. The system is usually characterized by dynamic interconversion of metal centers during the catalytic reaction. Commonly, this type of the catalytic systems is realized in the case of the catalysts generated in situ or when nanoparticles are used as a catalyst precursor. Studies have shown that clusters of transition metals are of key importance in these processes.

Clusters and nanoparticles of metals in chemical systems (source: DOI)

Absence of additional stabilization makes it possible to form a variety of metal compounds from a catalyst precursor after its introduction into the reaction. Various mechanisms of catalytic processes, which may operate in such systems, include both homogeneous catalysis, based on the application of molecular complexes or clusters and heterogeneous catalysis that occur on the metal nanoparticles. Mononuclear complexes and nanoparticles are used in a wide range of catalytic processes, while the metal cluster catalysis is a relatively new area.

Adaptive Catalysis

Adaptive catalytic systems (source: DOI)

Depending on the reagents and conditions, each of the catalytically active species may contribute to the preparation of the products of the catalytic reaction. Thus, under certain conditions, a "cocktail" of catalysts can be regarded as a self-adjusting system, which can be tuned for a particular set of reactions. Self-adjustment is an important prerequisite for the development of highly effective and universal catalysts.

The subsequent development of dynamic catalytic systems supports the concept of adaptive catalysis. In adaptive systems, the nature of the active species of the catalyst is controlled to achieve the desired reactivity and selectivity. In order to emphasize the difference we should note that in a simple dynamic system several types of metal particles are formed, all of which catalyze the formation of the same products. While in an adaptive catalytic systems, catalyst's properties can be adjusted in such a way that it becomes possible to obtain different products one after another from one reaction mixture, while maintaining high selectivity at each step.