Synthesis, characterization and catalytic activity of half sandwich ruthenium(II) complexes based on N/N, and N/O and O/O donor ligands

Abstract

This thesis comprises of six chapters, which deals with the synthesis and characterization of various half-sandwich ruthenium(II) complexes and their application in various catalytic reactions. These complexes are revealing its flexible chemistry, supply as excellent precursors and find important applications in many catalytic organic transformations. The first chapter describes the literature survey about the previously reported transition metal based homogeneous catalysts, and motivation to design new nitrogen and oxygen donor ligands based metal complexes for important catalytic reactions. Subsequent chapters describes the detailed study about the synthesis, characterization and catalytic activity of newly synthesized half-sandwich ruthenium(II) complexes based on nitrogen and oxygen donor ligands and systematic investigation on the structure-activity relationship and mechanistic aspects of the studied catalytic reactions. The newly synthesized complexes have been successfully employed for several catalytic reactions such as ring opening, ring hydrogenation and C-H bond activation reactions under ambient reaction conditions. In the last chapter, conclusion and future scope of this research work is briefly discussed.The main objectives of my research work are as following, • To synthesize highly efficient catalytic system based on substituted ethylenediamine donor arene-ruthenium(II) complexes for the ruthenium and formic acid based tandem catalytic transformation of bioderived furans to levulinic acid and diketones in aqueous aerobic conditions. • To study the role of ethylenediamine ligated half-sandwich arene-ruthenium(II) complexes as precursor for in situ generation of active Ru/C catalyst for the catalytic ring hydrogenation of arenes in water. • To design and develop the troponate/aminotroponate arene-ruthenium(II) complexes and study their ligand-tuned mechanistic pathway for direct C-H Bond arylation with aryl chlorides in water. • To synthesize and characterize cyclopentadienyl-ruthenium(II)-pyridylamine complexes and their application in catalytic transformation of bio-derived furans to levulinic acid and diketones in water.Chapter 1. Introduction and background: Design and synthesis of homogeneous catalyst based on transition metal complexes Design, synthesis and characterization of transition metal complexes and their use as active homogeneous catalysts for various organic reactions have received extensive attention in the field of organometallics and catalysis. Most important things about homogeneous catalysts is the easy of studying mechanistic pathway of catalytic reactions and hence on easy fine tuning of catalytic activity and understand structure-activity relationship. Among various metal catalysts explored so far, ruthenium metal gained preference over other metals for homogeneous catalysis, because of its variable oxidation states, its rich coordination properties and ability to accommodate a wide range of ligands in different coordination geometries. Thus, ruthenium based complexes compose a flexible class of active catalysts for synthetic organic chemistry and various other applications. More importantly, cautious selection of metal and ligands plays a very important role to design and synthesize homogeneous catalyst with tuned properties. In this context, we synthesize and characterization of various N/N, N/O and O/O donor ligand based arene-ruthenium and cyclopentadienyl-ruthenium half sandwich complexes has beenundertaken to explore the catalytic activity of these complexes in field of biomass transformation, selective ring hydrogenation and C-H bond activation reactions in aqueous aerobic condition. Moreover, this thesis also comprises a detailed mechanistic investigation to understand the catalytic reactions pathway and their structure-activity relationship in the studied complexes.Chapter 2. Ruthenium and formic acid based tandem catalytic transformation of bioderived furans to levulinic acid and diketones in water This chapter comprises the synthesis and characterization of areneruthenium( II) complexes ([Ru]-1 – [Ru]-8) based on simple and readily available ethylenediamine (or its derivatives) ligands. The studied complexes exhibited high activity for catalytic ring opening of furans and its derivatives under environmentally benign conditions in water.Synthesis of arene-Ru(II) complexes based on simple and easily available ethylenediamine ligand were carried out and characterized by ESI-MS, NMR, CHN elemental analysis and also structure for some of the complexes were authenticated by single crystal XRD analysis. These synthesized complexes exhibited high efficiency for the catalytic activity, stability and water solubility. Remarkably, these complexes exhibited high efficients for catalytic ring opening of bioderived furans to open ring products such as value added ketoacid (LA) and diketones (1-HHD, 3- HHD, and 2,5-HD) in the presence of formic acid at 80 °C in water and open atmosphere. Mechanistic investigation showed the profound effect of an N-H bond present in the arene-ruthenium complexes on the catalytic transformation of furans to ketoacid (LA) and diketones (1-HHD, 3-HHD, and 2,5-HD). 1H NMR experiments with variable formic acid concentration evidenced a tandem mode of catalytic conversion of furfural to LA via the formation of the hydrogenated intermediate, furfuryl alcohol.Chapter 3. Catalytic ring hydrogenation of arenes in water over in situ generated ruthenium nanoparticles immobilized on carbon This chapter comprises the selective catalytic ring hydrogenation of various (hetero)arene, over the in situ generated Ru/C from an organometallic precursor ([(6-C6H6)Ru(en)Cl]PF6) in the presence of formic acid. The cooperative effect of the arene-ruthenium(II) catalyst and ethylenediamine ligand activate formic acid to H2 which resulted in the in situ generation of active ruthenium nanoparticles and H2 gas to achieved facile tandem catalytic hydrogenation of a wide range of arenes and heteroarenes in water at 150 °C. Further, the reaction temperature has a determining role in the generation of Ru/C nanoparticles, by enhancing the rate of reduction of the precursor ethylenediamine ligand based arene-ruthenium(II) complex to ruthenium nanoparticles (Ru(0)). Consequently, the in situ generated ruthenium Ru/C nanoparticle was characterized with the help of TEM, SEM and Powder XRD observations. The Hg(0) poisoning experiments also suggested the heterogeneous nature of the catalyst. Further, using the in situ generated Ru/C catalyst, a wide range of aromatic and heteroaromatic compounds which analogous to lignin-derived fragments were also effectively hydrogenated to corresponding alicyclic products and thus the present catalytic system may also be used in the field of biomasstransformation reactions.Chapter 4. Troponate-/Aminotroponate Ruthenium-Arene complexes: Synthesis, characterization and ligand tuned mechanistic pathway for direct C-H bond arylation with arylchlorides in water This chapter contains the design, synthesis and characterization of a series of highly water soluble half sandwich arene-ruthenium(II) complexes [Ru]-9 – [Ru]- 16 of general formula [(η6-arene)Ru(troponate-/aminotroponate)Cl]+ (arene = C6H6 and C10H14) ligated with bidentate (O,O and O,N) troponate-/aminotroponate ligands. Molecular structure of the complexes [Ru]-13 and [Ru]-15 were also authenticated by single-crystal X-ray diffraction. These newly synthesized (η6- arene)-ruthenium(II) complexes based on troponate-/aminotroponate ligands are highly efficient for C-H bond arylation reaction of 2-arylpyridine in water. Our finding demonstrated that the structure-activity relationship plays a major role to achieve efficient selectivity in C-H activation of arylpyridine by these complexes. Moreover, the selectivity of mono- and diarylation of aryl pyridine is significantly affected by the steric bulkiness of the carboxylate additives.Mechanistic investigations performed by using mass spectral studies demonstrated the presence of several important cyclometallated intermediates species. Hence a possible catalytic pathway has been proposed as the initial formation of cycloruthenated species directed by deprotonation of 2-phenylpyridineassisted by mild base (carbonate) and the relative strength of the troponate- /aminotroponate ligand and 6-arene. This motivates the formation of cyclometallated species with arene-ruthenium(II) and 2-phenylpyridine ([(η6- arene)Ru(κ2-CN-phenylpyridine)(OH2)]+) by removal of troponate/aminotroponate ligands while another cyclometallated species formed by decoordination of η6-arene ring and retaining troponate-/aminotroponate ligand is 2-phenylpyridine-Rutroponate/ aminotroponate ([(κ2-troponate/aminotroponate)Ru(κ2-CNphenylpyridine)( OH2)2]). DFT calculations also support the experimental evidences for the formation of these intermediate species. Subsequently, with the oxidative addition of arylchloride over these C-H activated cyclometallated intermediate and then the reductive elimination led to the formation of C-H arylated product.Chapter 5. Cyclopentadienyl-Ru(II)-pyridylamine complexes: Synthesis, X-ray structure and application in catalytic transformation of bio-derived furans to levulinic acid and diketones in water This chapter comprises design and synthesis of a series of highly stable and efficient half-sandwich cyclopentadienyl-Ru(II) complexes [Ru]-17 – [Ru]-23 bearing pyridylamine/pyridylimine ligands. Molecular structures of all the complexes were authenticated by single crystal X-ray diffraction.These synthesized complexes were employed for the catalytic ring opening of furans (furfural, 5-HMF and 5-MF) to levulinic acid and various diketones (3-hydroxyhexane-2,5-dione (3-HHD), 1-hydroxyhexane-2,5-dione (1-HHD) and hexane-2,5-dione (HD)) in the presence of formic acid and water at 120 °C. Our experimental finding demonstrated the significant role of the N-substitution (electronic and steric effect), where the complex [Ru]-18 (5-Cp-Ru-N-propylpyridylamine complex) exhibited higher catalytic activity (TON of 16.8 and TTN 58.4) than other 5-Cp-Ru-pyridylamine and 5-Cp-Ru-pyridylimine complexes. Moreover, the flexible pyridylimine ligands may also show a remarkable κ2-κ1-κ2 convertible behavior and hence facilitated the efficient transfer hydrogenation reaction by creating a new vacant site. Thus the observed high catalytic activity of synthesized complexes attributed to the combined effect of bulkiness, flexibility, and basicity of the pyridylamine ligands.Chapter 6. Conclusions and future scope of work 6.1. Conclusions: A series of arene/Cp-Ru(II) complexes containing substituted ethylenediamine, troponate/aminotroponate, and pyridylamine/imine ligands have been synthesized and molecular structure of most of these complexes were confirmed by single crystal X-ray diffraction. Ethylenediamine based arene-Ru(II) complexes and pyridylamine/imine ligands based Cp-Ru(II) complexes were found to be active for the tandem catalytic transformations of bioderived furans, such as furfural, 5-hydroxymethylfurfural (5-HMF), and 5-methylfurfural (5-MF), to levulinic acid (LA) and diketones, 1-hydroxyhexane-2,5-dione (1-HHD), 3-hydroxyhexane-2,5-dione (3-HHD), and hexane-2,5-dione (2,5-HD). Moreover, ethylenediamine based arene-Ru(II) complexes were also employed as precursor for the in situ synthesis of Ru nanoparticles immoblized on carbon (Ru/C) and hence catalyzed the ring hydrogenation of homo/heteroarene in the presence of formic acid and these studies revealed the crucial role of −NH group to facilitate transfer hydrogenation. Moreover, the flexible hemilabile nature of pyridylamine ligands also advantageously contributed in the observed high catalytic activity of 5-Cp-Ru(II) complexes. Machanistic investigations inferred the formation offurfuryl alcohol an important reaction intermediate via transfer hydrogenationwhich further undergoes an acid catalyzed hydrilytic ring opening to LA, hence demonstrate the role of the ruthenium catalyst and the formic acid in reaction. Troponate/Aminotroponate based arene-Ru(II) complexes catalyzed efficient C-H bond arylation of arylpyridine in water, where the selectivity toward mono- vs diarylation of arylpyridines was found to be directed by the steric bulkiness of the carboxylate additives. Mechanistic studies performed using mass-spectral studies revealed the formation of several key cyclometalated intermediates species and therefore provided strong evidence that cloruthenation step was driven by carbonate-assisted deprotonation of 2-phenylpyridine. Moreover, formation of cyclometalated 2-phenylpyridine Ru-arene species, [(η6-arene)Ru(κ2-C,Nphenylpyridine)(OH2)]+ and cyclometalated 2-phenylpyridine Ru-troponate/aminotroponate species [(κ2-troponate/aminotroponate)Ru(κ2-C,N-phenylpyridine)(OH2)2] was depended on the relative strength of the η6-arene and the troponate/aminotroponate ligand by decoordination of troponate/aminotroponate ligands. DFT studied also well complemented over the experimental findings and support the formation of such species for thesecomplexes. Moreover, synthesized complexes were found to be highly air stable even at higher temperature ~120 °C, and most of them are highly water soluble, which help them to easy recovery and reuse for successive catalytic cycles and thus these complexes may be find application for catalytic reaction in aqueous and open atmospheric conditions even at higher temperature.