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The increase of the carbon dioxide concentration in the atmosphere provides a strong impetus to discover new catalysts that are able to reduce CO2. The reduction processes of this greenhouse gas CO2 have recently received enormous efforts in the research area. The objective of this thesis was the photocatalytic reduction of CO2 that is known as an artificial photosynthesis using visible light, and the objective of the thesis was to study the ability and efficiency of different new molecular catalysts towards CO2 reduction. The goals of the thesis are to design and characterize new catalysts that have high efficiency for the catalytic reduction of CO2. After a brief introduction in Chapter 1 about the photocatalytic reduction of CO2, a different catalyst is presented in each chapter with their characterization and examination for the photocatalytic reduction of CO2. In addition, these presented catalysts were also examined for the electrocatalytic reduction of CO2, but they show a good catalytic behavior in the photocatalytic reduction of CO2. The catalytic mechanisms were also suggested for each catalyst and tried to be confirmed by many different experiments. observed to be highly influenced by CO2 concentration. These newly discovered catalysts are based on transition metal complexes that are able to be good catalysts for the photocatalytic CO2 reduction. These new transition metal complexes have been synthesized, characterized and examined for their catalytic reactivity for CO2 reduction. As presented in Chapter 2, new manganese and rhenium ccomplexes bearing a phosphino-amino-pyridine ligand were synthesized, characterized and showed their photocatalytic ability for CO2 reduction. In addition, Chapter 3 presents new Ru catalysts supported by an unprecedented ligand array and documented their photocatalytic ability towards CO2 reduction. Moreover, Chapter 4 focuses on new Zn(II) complexes that are novel catalysts in the photocatalytic CO2 reduction area. Furthermore, Chapter 5 presents a new environment for Re photocatalyst that has the switch in product to formic acid compare to all other reported Re photocatalysts. On the other hand, Chapter 6 shows new dimers and monomers for a series of earth-abundant transition metal dibromide complexes supported by a neutral SNS ligand framework and reveals their applications in the catalysis. Finally, Chapter 7 presents a brief conclusion and a number of future directions. The attempts to explore and discover the new catalysts for CO2 reduction were exciting, successfully and resulted in the discovery of new catalysts. These catalysts show their good ability to reduce carbon dioxide (CO2) to more valuable products such as carbon monoxide (CO) and formic acid (HCOOH).
The use of fossil fuels to meet our energy requirements has contributed significantly to increase the atmospheric carbon dioxide level causing global warming. This calls for the development of processes to reduce CO2 level and its conversion to other value-added products. In this regard, photocatalytic CO2 reduction using sunlight is a key reaction to achieve artificial photosynthesis to produce solar or renewable fuels. Designing efficient and robust catalysts for this reduction reaction is crucial and remains a challenging task. Recently, N-heterocyclic carbenes (NHCs) have emerged as strong donor ligands that are useful for forming transition metal-based catalysts. The combination of two NHCs with a central pyridyl ring in a symmetric fashion results in a tridentate CNC-pincer framework that can promote catalyst stability. Metal complexes containing such CNC-pincer ligands along with other co-ligands have been synthesized, characterized, and evaluated for photocatalytic CO2 reduction reaction (CO2-RR). We have shown that the use of a bidentate co-ligand (2,2ʣ-bipyiridine (bpy)) along with a CNC-pincer in ruthenium complexes ([(CNC)RuIIL(bpy)]n+, L is chloride or acetonitrile) resulted in the development of self-sensitized catalysts for photocatalytic CO2-RR under photosensitizer (PS) free conditions. The effect of various donor groups at the 4-position of the central pyridyl ring in CNC-pincer ligand has been evaluated via structure-activity relationships for sensitized photocatalytic CO2-RR in the presence of an external PS using [(CNC)RuIICl(MeCN)2]+ complexes. We have also compared the positional effect for one of the donor group. To further explore the structure-activity relationships, 11 novel ruthenium complexes with the general formula [(CNC)RuIIL(NN)]n+ (NN = diimine ligands, L = chloride or bromide or acetonitrile) were synthesized, characterized, and evaluated for sensitized and self-sensitized photocatalytic CO2-RR. The structures include two CNC-pincer ligands based on imidazole and benzimidazole derived NHCs and three diimine ligands including bpy, 4,4ʣ-dimethyl-2,2ʣ-bipyridine (dmb), and 1,10-phenanthroline. The development of heterogeneous catalysts has also been pursued. Two highly active homogeneous photocatalysts containing CNC-pincer and bpy co-ligand on ruthenium were considered for surface immobilization to develop heterogeneous catalysts with well-defined active sites. Syntheses of ruthenium complexes via bpy ligand modifications are described that can be immobilized onto an inert solid support for surface organometallic chemistry (SOMC). In addition, we have attempted to explore the use of first-row transition metals to develop catalysts for photocatalytic CO2-RR. The development of nickel CNC-pincer complexes for this reduction reaction has been described. We also aimed to synthesized iron-based catalysts and the efforts are described. The use of bulkier wing-tip groups on the CNC-pincer ligands and phosphine co-ligand resulted in the formation of miscellaneous ruthenium complexes.
This thesis focuses on the synthesis, characterization and reactivity of group VII transition metal complexes. It begins with exploring a new pincer geometry of Re(I) compounds and then examining both Re(I) and Mn(I) compound as homogenous catalysts for photocatalytic and electrocatalytic reduction of CO2. In the first chapter, I focus on some recently reported approaches to photocatalytic and electrocatalytic reduction of CO2 using homogenous catalysts of transition metal. The second chapter presents efforts to capture Re(I) in a neutral N,N,N pincer scaffold and the resulting enhanced absorption of visible light. Most of these results have appeared in a publication. In this thesis, I only present my work on rhenium compounds that are supported by the bis(imino)pyridine ligand and an examination of the differences in properties between the bidentate and tridentate ligand geometries. Later I examine both tridentate and bidentate complexes for the photocatalytic and electrocatalytic reduction of CO2 to CO. The failure of tridentate Re1 bis(imino)pyridine compounds to reduce CO2 to CO prompted a change in direction to rhenium compounds that are supported with diimine ligands. Thus, I choose 4,5-diazafluoren-9-one as supporting ligand for rhenium and manganese. This chapter explained the reasons behind choosing these particular ligand and metal combinations. ReI and Mn1 compounds of 4,5-diazafluoren-9-one have shown activity for the photocatalytic and electrocatalytic reduction of CO2 to CO. In the fourth chapter, as rhenium and manganese compounds of 4,5-diazafluoren-9-one have shown the great ability of CO2 reduction to CO, the focus here was to modify the ligand by attaching a photosensitizer to the ligand in order to prepare supramolecular complexes that may increase the efficiency and yield of reduction products. In this chapter, I examined two types of the photosensitizer; tris(bipyridine)ruthenium(II)chloride and osmium dichloro bis(4,​4'-​dimethyl-​2,​2'-​bipyridine).
Homogeneous and Heterogeneous Catalysis
Photocatalysis and related processes occupy a strategic position for the future of photochemistry. This volume provides an introduction to basic concepts and explains how applications work at the molecular level.
Homogeneous hydrogenation is one of the most thoroughly studied fields of homogeneous catalysis. The results of these studies have proved to be most important for an understanding of the underlying principles of the activation of small molecules by transition metal complexes. During the past three decades homogeneous hydrogenation has found widespread application in organic chemistry, including the production of important pharmaceuticals, especially where a sophisticated degree of selectivity is required. This volume presents a general account of the main principles and applications of homogeneous hydrogenation by transition metal complexes. Special attention is devoted to the mechanisms by which these processes occur, and the role of the recently discovered complexes of molecular hydrogen is described. Sources of hydrogen, other than H2, are also considered (transfer hydrogenation). The latest achievements in highly stereoselective hydrogenations have made possible many new applications in organic synthesis. These applications are documented by giving details of the reduction of important unsaturated substrates (alkenes, alkynes, aldehydes and ketones, nitrocompounds, etc.). Hydrogenation in biphasic and phase transfer catalyzed systems is also described. Finally, a discussion of the biochemical routes of H2 activation highlights the similarities and differences in performing hydrogenation in both natural and synthetic systems. For researchers working in the fields of homogeneous catalysis, especially in areas such as pharmaceuticals, plastics and fine chemicals.
Hydrogenation reactions can be used to store energy in chemical bonds, and if these reactions are reversible that energy can be released on demand. A new bidentate chelating ligand was designed and synthesized for this project, using an N-heterocyclic carbene ring bound directly to a pyridinol ring (NHC-pyOR). This new ligand was used to make iridium complexes that were studied as catalysts for the hydrogenation of CO2 and dehydrogenation of formic acid. For comparison, analogous bipy derived iridium and ruthenium complexes were also tested. In general, the NHC-pyOR complexes demonstrated modest activity, where hydroxyl-pyridines found in the bipy derived systems are more active for CO2 hydrogenation under basic conditions. However, the trends were quite different for formic acid dehydrogenation reaction which will be discussed in Chapter 2. Other ruthenium (II) and iridium (III) complexes of the NHC-pyOR ligand with difference counter anions from above complexes were also synthesized. The ruthenium complexes were tested for their ability to accelerate CO2 (de)hydrogenation, but our studies show that these complexes all undergo transformations in solution and thus they are not true catalysts, but rather pre-catalysts. The use of new tridentate pincer ligands derived from NHC and pyridinol is also described. A new ligand containing (NHC-pyOR-NHC) rings binding to a metal with the pyridinol derivative were synthesized. A series of metal complexes of the type LnM were synthesized (n = 1 and 2; M = Fe2+, Co3+, and Ru2+). Preliminary results of photocatalytic reduction of CO2 to CO show that ruthenium complexes are the most active catalysts followed by cobalt and iron, respectively. The activation of carbon dioxide and nitrite utilizing bio-inspired and proton responsive catalysts were also studied with tris(triazolyl)hydroborate (Ttz) complexes of zinc(II) and copper(II). For the biomimetic zinc complexes for CO2 activation, the synthetic result was found to be greatly depend on the steric bulk of Ttz ligand which will be discussed in detail in Chapter 6. Moreover, the electrochemical reduction of Ttz-Cu(II) complexes in the presence and absence of a proton source shows processes that are relevant to enzymatic nitrite reduction which also will be studied in Chapter 7.
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Secondly, molecular catalysts for CO2 reduction are studied with the aim of understanding how catalytic activity is influenced by the nature of the monodentate ligands. Whether efficient catalysis requires the routinely employed but photolabile CO ligand is explored. The electrocatalytic reduction of CO2 by Ru-bipyridyl compounds is investigated and their visible-light photochemistry is also discussed.
Table 1 E factors (tonnes of waste generated per tonne of product manufactured [7] Industry segment Annual product tonnage E factor 6 8 Oil refining 10 –10 Approx. 0. 1 4 6 Bulk chemicals 10 –10