Silica immobilized pincer-metal complexes : catalysis, recycling, and retrospect on active species

Abstract

Science is continuously striving for a sustainable progress of society. This progress must be made on the economical as well as the environmental front concomitantly. Many industrial processes are being reviewed to make them environmentally more sustainable. Catalysis emerges as an important player to achieve this goal. No wonder, about 85% of the present chemical processes are run using a catalyst. Out of these about 75% are run based on heterogeneous catalysts due to their advantage of easy separation from product streams and of their recycling and reuse. On the other hand, homogeneous catalysts are appreciated in terms of their activity and selectivity, particularly in the case of enantioselective processes. These catalysts generally consist of a transition metal surrounded by an ingeniously designed (chiral) ligand system to fine tune the catalytic properties. This often makes homogeneous catalyst systems expensive and consequently, recycling and reuse of these catalysts become pertinent. One solution to achieve this is by immobilizing the homogeneous catalyst on an insoluble support, i.e. by heterogenizing the homogeneous catalyst. An inexpensive and abundantly available support is silica. In this thesis, efforts have been made to anchor ECEpincer metal complexes on silica surfaces (both amorphous and structured silica) and to use the resulting systems in catalysis. Chapter 1 provides an overview of the various approaches to immobilize homogeneous catalysts on to silica support. Chapter 2 describes the synthesis of various para-OH functionalized ECE-pincer metal complexes and their functionalization with a siloxane tether in order to immobilize them on silica. In Chapters 3 and 4, these complexes are immobilized on various silicas, ordinary as well as mesoporous materials such as MCM-41 and SBA-15. The resulting hybrid materials have been used in an aldol condensation reaction and their recovery and reuse was investigated. Chapters 5 and 6 provide details of the role that the various ECE-pincer metal catalysts play in the aldol condensation reaction and a new catalytic mechanism is proposed. In Chapter 7, the synthesis of a novel PCS-pincer palladium complex is described along with its activity in the aldol reaction. Finally, the involvement of silver-based reagents as catalysts in the aldol reaction is discussed in Chap8.