*On the Technology of Heterogenization of Transition Metal Catalysts towards the Synthetic… DOI: http://dx.doi.org/10.5772/intechopen.86846*

loosely bound; therefore, they are liquid under conventional ambient conditions, and they can be tailor-made according to the needs and demands, so they are also known as 'Designer Solvents' and/or 'Solvents of the future'. The design of room temperature ionic liquids has been achieved through appropriate selection of weakly coordinating cations and anions; theoretically, as per the principles of crystallography, the lattice energy has to be brought down as low as possible, and hence, the design lies on the *antithesis* of assembling pure crystal/s, and/or also symmetry in the ions should be destroyed by all means. Chemists around the globe realised the importance of ionic liquids in technology; in synthetic organic chemistry, they designed innumerable classes of ionic liquids, and mathematically through various permutations and combinations, millions of ionic liquids can be synthesized. Unlike other field of research, the field and phenomenon of ionic liquids progressed in stages, and during its progress, more and more technological applications were realized. In the year 1914, the salts of alkylammonium nitrates were identified as liquid in nature , and then chloroaluminates were identified as ionic liquids. In the 1990s, aluminates were replaced with other anions such as tetrafluoroborate [BF4] <sup>−</sup> or hexafluorophosphate [PF6] <sup>−</sup>, and then the field of ionic liquids started gaining familiarity and popularity because of convenient recyclability. After 1990, numerous ionic liquids with variety of unique anions and cations were reported in the literature, and this field received recognition in developing neoteric technologies, very importantly, synthetic organic chemists found that the ionic liquids were useful as recyclable solvent; with this discovery, this field became appealing and convincing to synthetic organic chemists. Almost all name reactions were carried out in the ionic liquids, and the bench chemist has meticulously screened and succeeded through *trial and error* or *hit and miss* methodologies. In relevance to the title of the chapter, the discussion will revolve around the use of transition metal catalysis in ionic liquids towards organic functional group transformations. As the catalytic materials derived from d-block and rare earths remain expensive and yield unwarranted side effects when consumed by human, the chemists and technologists invented a variety of techniques to immobilize the metallic catalytic systems in various ways through high levels of creativity. Among them, ionic liquids proved to be an excellent matrix to retain the charged, neutral or polar transition metal complexes; consequently, the catalyst was recycled and reused without the loss of catalytic activity and selectivity; moreover, the separation of the product seems to be easy from the reaction mixture, the product reaches the extracting solvent, and the catalyst was held through electrostatic forces in the ionic liquid matrix. Consequently, from the last decade, ionic liquids (ILs) have been employed and found to be effective for the immobilization of homogeneous catalytic materials, and new technology has emerged, which is known as supported ionic liquids phase (SILP). In SILP, mostly an IL film is immobilized on a high-surface area porous solid through either physisorption or chemisorption, and the homogeneous catalyst is immersed in an IL matrix. This unique concept has been employed for the immobilization of homogeneous catalytic materials, absorbents, and other functional materials; consequently, this concept is expected to benefit the scientific community to simplify the lives of people at large. With the added advantages, SILP in this era is much exploited in the immobilization of homogenous catalytic systems; ultimately, it will be surely impossible to picturize the complete overview of this area. In this chapter, we decided to limit our discussion to the most important contributions for brevity and conclude with recommendations, future perspectives, and outlook.

*Solvents, Ionic Liquids and Solvent Effects*

all chemical products. Apart from the design and invention of new reactions in this field, process development is also a motivating factor since it renders highly selective organic functional group transformations in attaining structurally complex organic compounds with biological and pharmaceutical importance. Transition metals being situated at the middle part of the periodic table, either in their ionic or neutral form in solution, or *in situ*, are capable of accommodating up to nine valence shell orbitals accounting for the variable oxidation states and forms various transition states to generate hybrid molecular orbitals yielding the desired product with high chemo-, regio- and stereo-selectivity with high turnover number and turnover frequency [3]. With the advancement in the analytical techniques to understand the mechanistic pathway, transition states, and kinetics, chemists and technologists in this epoch were able to tune (through the selection of appropriate ligands and/or external additives) the catalyst according to the needs. However, the major disadvantage of the homogeneous catalytic system is that the catalyst cannot be separated easily and recycled, and also separation of product from the reaction mixture besides, in some cases, poses plethora of potential problems. In the process development of pharmaceutical drugs employing homogenous systems, the separation of the transition metallic species is very important, since if it remains in the drug molecules (product) even in the ppm level, inside our biological system, it can create undesirable side effects owing to its multivalence ability and can interfere in the biological and enzymatic process [4] and attack the immune system. Eventually, separation of the catalytic material, recovering it without losing it in the work up procedure, and subsequent reuse are the major challenges arising from the transition metal catalysis. In most of the cases, the transition metal catalyst loses its original form while quenching the reaction mixture with the treatment of water; eventually, the metal ion should be treated with new chemical procedure to acquire the original catalyst; in practice, the challenges are multifold as far as the catalyst's reuse and recyclability are concerned. On the other hand, though the heterogeneous catalytic system, separation of the catalytic material/s through a simple filtration technique is allowed, thereby the recovery and reuse are solved with ease; heterogeneous system does not offer most of the benefits that arise from that of homogeneous system; thus, the rich chemistry of transition metal catalysis remains at the top rank which is admired by academicians, chemists, technologists and entrepreneurs [5]. In the industrial processes, though the heterogeneous processes get precedence over homogeneous processes for the process of fine chemicals, in many of such processes, the undesirable side reactions give rise to unwanted products and impurities in varying proportions, and process development chemists instruct the graduate chemists to purify the products through various separation processes for instance distillation and recrystallization. Ultimately, an idea emerged in the minds of academicians to invent processes by combining the advantages of homogeneous and heterogeneous catalysis, which is the 'immobilization or heterogenization,' employing tailor-made transition metal catalytic materials. The concept of heterogenization technology has been attempted since the late 1960s [6], and it is continuously growing towards the success; now, there are many commercial processes that are operative with this technology for the production of fine chemicals to pharmaceutically important compounds. Among all the possible ways available, here in this chapter, the discussion will focus its attention on the exploitation of ionic liquids [7] as support for transition metal

Ionic liquids are the modern technological materials made up of molten organic and inorganic salts with a melting point below 100°C, and they are

**126**

catalysts.
