Homogeneous catalysis is among the most important areas of contemporary chemistry and chemical technology. Homogeneous catalysis, by definition, refers to a catalytic system in which the substrates for a reaction and the catalyst components are brought together in one phase, most often the liquid phase. More recently a narrower definition has become fashionable according to which most of the catalysts used in homogeneous catalysis are coordination complexes. That is, the "classical" homogeneous catalyst consists of the central atom and the ligand(s). Their interplay governs decisively activity, productivity, and selectivity of the catalytic system.
Figure 1. Homogeneous catalytic process
It is worth noting that by varying the size, shape and electronic properties of the ligands, the site at which the substrate binds can be constrained in such a way that only one of many possible products can be produced. This also means that ligands can be used to tune the selectivity of a particular catalyst for the synthesis of a specific desired product. As an example, Figure 2 shows a range of products that might be produced from a mixture containing an alkene, carbon monoxide, hydrogen, and an alcohol. All the products have their uses, but it is a triumph of homogeneous catalysis that any one of the products can now be made with > 90% selectivity by careful selection of the metal center, ligands, reaction conditions and in some cases substrate. It should be added that compared with heterogeneous catalysis, homogeneous catalysis has many important advantages. For example, all catalytic sites are accessible because the homogeneous catalyst is usually a dissolved metal complex. Furthermore, it is often possible to tune the chemoselectivity, regioselectivity, and/or enantioselectivity of the homogeneous catalyst.
Figure 2. Select different ligands or reaction conditions to achieve selective synthesis of products
Many reactions in industry are carried out using highly selective homogeneous catalysts, which are coordination complexes. Homogeneous catalysis by soluble coordination complexes offers many advantages. These advantages arise from the ability of metals to complex with a wide variety of ligands in a number of geometries and to easily change from one oxidation state to another. Two of the most important advantages are as follows:
- In coordination complex-assisted homogeneous catalysis, every single catalytic entity can act as a single active site. This makes coordination complexes more active and selective in nature as homogeneous catalysts.
- In coordination complex-assisted homogeneous catalysis, the properties of the complexes can be adjusted by manipulating the reaction conditions. This makes homogeneous catalytic reactions more controllable.
Among the most significant developments in the field of homogeneous catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of coordination complexes. Examples of the application of complexes in homogeneous catalytic reactions are listed below:
- Acts as a catalyst for hydrogenation reactions
Hydrogenation technology describes the technology of converting unsaturated compounds into a relatively saturated product, whose core is named as "hydrogenation reaction". An increasing number of coordination complexes are being used in hydrogenation reactions. For example, Milstein and co-workers  developed a defined iron pincer complex based on their elegant work on similar ruthenium complexes for the hydrogenation of ketones under mild conditions. Bagh  synthesized an air-stable ruthenium complex using an easily available triazole ligand with good catalytic activity for the transfer of hydrogen from various aldehydes and ketones.
- Acts as a catalyst for cross-coupling reactions
Cross-coupling reactions represent a class of synthetic transformations that involve the combination of an organometallic reagent with an organic electrophile in the presence of groups 8-10 metal catalysts to achieve a C-C, C-H, C-N, C-O, C-S, C-P, or C-M bond formation. The coordination complexes have been widely used for cross-coupling reactions. For example, Van Koten and co-workers  demonstrated the manganese-catalyzed C-C bond formation reaction. The Mn (II) complex exhibited catalytic activity in coupling reactions of Grignard reagents and organic bromides, as well as 1,4-addition reactions of Grignard reagents to α, β-unsaturated ketones in the presence of catalytic amounts of CuCl. Beller and co-workers  have demonstrated that molecular-defined manganese pincer complexes (Mn-PNP) are efficient catalysts for the benign inter- and intramolecular formation of C-N bonds.
- Acts as a catalyst for hydrosilylation reactions
Hydrosilylation reaction is highly important in the silicon industry as it is used in the synthesis of silicon polymers, oils, and resins, as well as in the production of organosilicon reagents for fine chemicals. The hydrosilylation of unsaturated compounds catalyzed by transition metal complexes has received significant attention in recent years. For example, Trovitch and co-workers  reported the first manganese-based pincer catalyst that displays high activity in the hydrosilylation of ketones and esters. Chirik and co-workers  reported the dehydrogenative silylation of alkenes, catalyzed by the 2,6-iminopyridine-cobalt complex.
- Acts as a catalyst for other reactions
Coordination complexes can also be used to catalyze other homogeneous reactions. Examples of the dimerization of ethylene and polymerization of dienes catalyzed by complexes of rhodium; double-bond migration in olefins catalyzed by complexes of rhodium, palladium, cobalt, platinum, and other metals; the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process); the hydration of acetylenes catalyzed by ruthenium chloride; and many others.
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- Milstein, D.; et al. Efficient hydrogenation of ketones catalyzed by an iron pincer complex. Angew. Chem. 2011, 50: 2120-2124.
- Bagh, B.; et al. Transfer hydrogenation of aldehydes and ketones in air with methanol and ethanol by an air-stable ruthenium-triazole complex. ACS Sustainable Chemistry & Engineering. 2021, 9: 4903-4914.
- van Koten, G.; et al. Novel organomanganese (II) complexes active as homogeneous catalysts in manganese (II)/copper(I) catalyzed carbon‐carbon bond formation reactions. Recl. Trav. Chim. Pays-Bas. 1996, 115: 547-548.
- Beller, M.; et al. Efficient and selective N-alkylation of amines with alcohols catalysed by manganese pincer complexes. Nat. Commun. 2016, 7: 12641.
- Trovitch, R. J.; et al. A highly active manganese precatalyst for the hydrosilylation of ketones and esters. J. Am. Chem. Soc. 2014, 136: 882-885.
- Chirik, P. J.; et al. Bis(imino)pyridine cobalt-catalyzed dehydrogenative silylation of alkenes: scope, mechanism, and origins of selective allylsilane formation. J. Am. Chem. Soc. 2014, 136: 12108-12118.