Introduction The chemistry of transitional metal complexes bearing σ-bonded H−X (X = H, Si, C) species/fragments, the so called σ-complexes, are key intermediates in catalytic processes such as hydrogenation, hydrosilylation, alkane functionalization etc. Particularly, the σ-H2 complexes form the best-known group of σ-complexes in which H2 is bound to the metal center in η2-fashion. Several well characterized examples of η2-silane and η2-borane complexes have also been reported. Moreover, in recent years, the carbon analogues of these complexes in which alkanes are coordinated through η2-C-H bonds to the metal center have been attracting the attention of organometallic chemists. An approach towards direct functionalization of σ-bonds in simple alkanes is the heterolytic activation of the C−H bond using highly electrophilic complexes. After all, for fine catalyst design and the selective functionalization of H−H, silanes or simple alkanes, it is necessary to understand the bonding nature of these σ-complexes in depth. Objectives The objectives of this work are as follows a) An attempt to stabilize and gain insights into the bonding nature and reactivity behavior of various sigma ligands on ruthenium center [Ru(η2-HX)(Tpms)(PPh3)2][OTf], (X = H, SiR (R = Me3 or Me2Ph) and CH3). b) Synthesis, characterization and reactivity studies of electrophilic ruthenium(II) complexes bearing (C6F5)2PCH2CH2P(C6F5)2 (dfppe) ligand towards heterolysis of H2. c) An approach towards preparation of insoluble molecular clusters from [Ru(P(OH)3)(dppe)2][OTf]2 complex and Zn, Cd and Cu acetates to realize σ-bond activation under heterogeneous conditions. Significant results In our attempts to gain insights into the bonding nature and reactivity behavior of σ-H2, silane and methane complexes, we followed two strategies to generate these complexes in solution. First, we synthesized and well characterized two new Ru(II)-complexes [RuH(Tpms)(PPh3)2] and [Ru(OTf)(Tpms)(PPh3)2], (OTf = trifluoromethane sulfonate) where Ru-H and Ru-OTf are the key reactive centers, followed by their subsequent reactions with electrophilic reagents such as HOTf, Me3SiOTf and CH3OTf and with H2, PhMe2SiH and CH4 at low temperature, respectively. These reactions finally resulted in the characterization of σ-H2 and σ-silane complexes, however, no σ-methane complex was observed even at low temperature (Scheme 1). Scheme 1 In order to realize highly eletrophilic metal complexes, a chelating fluorinated phosphine ligand 1,2-bis-(pentafluorophenylphosphino)ethane, (C6F5)2PCH2CH2P(C6F5)2 (dfppe) was employed and the synthesis and structural characterization of a series of new, Ru(II) hydride complexes [RuH(P(OMe)3)(bpy)(dfppe)][OTf], cis-[RuH2(dfppe)(PPh3)2] and [RuH(CO)Cl(PPh3)(dfppe)] were accomplished. Protonation reaction of the hydride complexes [RuH(P(OMe)3)(bpy)(dfppe)][OTf] (Scheme 2) and [RuH(CO)Cl(PPh3)(dfppe)] (Scheme 3) with HOTf at low temperature gave free H2 and five-coordinate species [Ru(P(OMe)3)(bpy)(dfppe)][OTf]2 and [Ru(CO)Cl(PPh3)(dfppe)][OTf], respectively. Surprisingly, in all these reactions, dihydrogen complexes are formed which were unobservable in which the H2 ligand was found to be highly labile. Reaction of is-[Ru(bpy)(dfppe)(OH2)(P(OMe)3)][OTf]2 with H2 however, resulted in the heterolytic activation of the H–H bond and concomitant protonation of H2O to give the corresponding hydride complex cis-[Ru(H)(bpy)(dfppe)(P(OMe)3)][OTf] and H3O+ (Scheme 2) . Scheme 2 Scheme 3 In an attempt to prepare insoluble molecular clusters in order to realize σ-bond activation under heterogeneous conditions, we studied the reactivity of highly electrophilic [Ru(P(OH)3)(dppe)2]2+ (dppe = (C6H5)2PCH2CH2P(C6H5)2) complex with various metal acetates. Usage of Zn(OAc)2.2H2O afforded a novel [Ru2(dppe)4P2(OH)2O4Zn2(OAc)(DMP)(OTf)][OTf]2 (Ru-Zn ) soluble bimetallic complex (Scheme 4) which was characterized in detail by NMR and single crystal X-ray crystallography. To achieve the expected insoluble molecular cluster further studies are required to tune the electronics and the sterics around the phosphorous acid moiety. Scheme 4

Current fossil fuels (Coal and Petroleum) based economy is not sustainable in the long run because of its dwindling resources, and increasing concerns of climate change due to excessive carbon dioxide (CO2) emission. To mitigate CO2 emission and climate change, scientists across the world have been looking for clean and sustainable energy sources. Among them hydrogen gas (H2) could be more promising because it is the most clean fuel and can be produced from cheap source (water) which is renewable and abundant. Nevertheless, the bottleneck for hydrogen economy is lying in the cost of hydrogen production from water. Still there are no any efficient systems developed which can deliver hydrogen from water in economically viable way. Meanwhile, recent research on old molecule ammonia-borane (H3N•BH3, AB) as hydrogen source has increased the hope towards the hydrogen economy, however, catalytic recycling (or efficient regeneration) of AB from the dehydrogenated product polyborazylene (PB or BNHx) is the biggest hurdle which prevents use of AB as practical hydrogen storage material. Therefore, it is imperative to understand the dehydrogenation pathways of ammonia-borane (or related amine-boranes) which lead to polymeric or oligomeric product(s). On the other hand, methane (CH4) is abundant (mostly untamed) but cleaner fuel than its higher hydrocarbon analogs. To develop highly efficient catalytic systems to transform CH4 into methanol (gas to liquid) is of paramount importance in the field of catalysis and it could revolutionize the petrochemical industry. Therefore, to activate CH4, it is crucial to understand its binding interaction with metal center of a molecular catalyst under homogenous condition. However, these interactions are too weak and hence σ–methane complexes are very elusive. In this context, σ-H2 and σ-borane complexes bear some similarities in σ-bond coordination (and four coordinated boranes are isoelectronic with methane) could be considered as good models to study σ-methane complexes. Studying the H−H and B−H bond activation in H2 and amine-boranes, respectively, would provide fundamental insights into methane activation and its subsequent functionalization. Moreover, the proposed methanol economy by Nobel laureate George Olah seems more promising because methanol can be produced from CH4 (CO2 as well). This in turn will gradually reduce the amount of two powerful greenhouse gases from the earth’s atmosphere. Thus, efficient and economic production of methanol from CH4 and CO2 is one of most challenging problems of today in the field of catalysis and regarded as the holy grails. Furthermore, very recently formic acid (HCOOH) is envisaged as a promising reversible hydrogen storage material because it releases H2 and CO2 in the presence of a suitable and efficient catalyst or vice versa under ambient conditions. Objective of the research work: Taking the account of the above facts, the research work in this thesis is mostly confined to utilize electrophilic Ru(II)-complexes for activation of small molecules such as ammonia-borane (H3N•BH3) [and related amine-borane (Me2HN•BH3)], hydrogen (H2), methane (CH4), formic acid (HCOOH) and carbon dioxide (CO2) and investigation of their mechanistic pathways using NMR spectroscopy under homogeneous conditions. Though these molecules are small, they have huge impacts on chemical industries (energy sector and chemical synthesis: drugs/natural products) and environment [CO2 and CH4 are potent green house gases] as well. However, they are relatively inert molecules, especially CH4 and CO2, and impose very tough challenges to activate and functionalize them into useful products under ambient conditions. The partial oxidation of the strong C−H bond in CH4 for its transformation into methanol under relatively mild condition using an organometallic catalyst is considered as a holy grail in the field of catalysis which is mentioned earlier. More importantly, to develop better and highly efficient homogeneous catalytic systems for the activation of these molecules, it is imperative to understand the mechanistic pathways using well defined homogeneous metal complexes. Thus, an understanding of the interaction of these inert molecules with metal center is obligatory. In this context, discovery of a σ-complex of H2 gave remarkable insights into H−H bond activation pathways and its implications in catalytic hydrogenation reactions. Subsequently, σ-borane complexes of amine-boranes were discovered and found to be relatively more stable because of stronger M−H−B interaction and hence act as good models to study the M−H−C interaction of elusive σ-methane complex. On the other hand, HCOOH, a promising hydrogen storage material and its efficient catalytic dehydrogenation/decarboxylation and CO2 hydrogenation back to HCOOH using well defined homogeneous catalysts could lead to a sustainable energy cycle. Therefore, it is quite significant to understand the mechanistic pathways of formic acid dehydrogenation/decarboxylation and carbon dioxide reduction to formic acid for the development of next generation efficient catalysts. Chapter highlights: Keeping all these in view, we carried out thorough studies on the activation of these small molecules by electrophilic Ru(II)-complexes. This thesis provides useful insights and perspective on the detailed investigation of mechanistic pathways for the activation of small molecules such as H3N•BH3 [and Me2HN•BH3], H2, CH4, HCOOH and CO2 using electrophilic Ru(II)-complexes under homogeneous conditions using NMR spectroscopy. In Chapter 1 we provide brief overview of small molecule activation using organometallic complexes. This chapter presents pertinent and latest results from literature on the significance of small molecule activation. Although there are several small molecules which need our attention, however, we have focused mainly on H3N•BH3 [and Me2HN•BH3], H2, CH4, HCOOH and CO2. In Chapter 2, we present detailed investigation of mechanistic pathways of B−H bond activation of H3N•BH3 and Me2HN•BH3 using electrophilic [RuCl(dppe)2][OTf] complex using NMR spectroscopy as a model for methane activation. In these reactions, using variable temperature (VT) 1H, 31P{1H} and 11B NMR spectroscopy we detected several intermediates en route to the final products at room temperature including a σ-borane complex. On the basis of elaborative studies using NMR spectroscopy, we have established the complete mechanistic pathways for dehydrogenation of H3N•BH3/Me2HN•BH3 and formation of B−H bond activated/cleaved products along with several Ru-hydride and Ru-(dihydrogen) complexes. Keeping the B−H bond activation of amine-boranes in view as a model for methane activation, we attempted to activate methane using [RuCl(dppe)2][OTf] complex. In addition, [Ru(OTf)(dppe)2][OTf] complex having better electrophilicity than [RuCl(dppe)2][OTf], was synthesized and characterized. The [Ru(OTf)(dppe)2][OTf] complex has highly labile triflate bound to Ru-metal and therefore its reactivity studies toward H2 and CH4 were carried out where H2 activation was successfully achieved, however, no any spectroscopic evidence was found for C−H bond activation of CH4. The Chapter 3 describes the synthesis and characterization of several Ru-Me complexes such as trans-[Ru(Me)Cl(dppe)2], [Ru(Me)(dppe)2][OTf], trans-[Ru(Me)(L)(dppe)2][OTf] (L = CH3CN, tBuNC, tBuCN, H2) with an aim to trap corresponding σ-methane intermediate at low temperature. However, interestingly, we observed spontaneous but gradual methane elimination and orthometalation of [Ru(Me)(dppe)2][OTf] complex at room temperature. We thoroughly investigated mechanistic details of methane elimination and orthometalation of [Ru(Me)(dppe)2][OTf] using VT NMR spectroscopy, NOESY and DFT calculations. Furthermore, H2 activation was confirmed unambiguously by [Ru(Me)(dppe)2][OTf] and Ru-orthometalated complexes using NMR spectroscopy under ambient conditions. An effort was also made to activate methane using Ruorthometalated complex in pressurized condition of methane in a pressure stable NMR tube. Moreover, preliminary studies on protonation reaction of [Ru(Me)(dppe)2][OTf] using VT NMR spectroscopy to trap σ-methane at low temperature was carried out which provided us some useful information on dynamics between proton and Ru-Me species. The Chapter 4 provides useful insights into the mechanistic pathways of dehydrogenation/decarboxylation of formic acid using [RuCl(dppe)2][OTf]. Catalytic dehydrogenation of HCOOH using [RuCl(dppe)2][OTf] was observed in presence of Hunig base (proton sponge). In addition, a complex [Ru(CF3COO)(dppe)2][OTf] was synthesized and characterized using NMR spectroscopy, and found to readily dehydrogenate HCOOH. Moreover, preliminary results on transfer hydrogenation of CO2 into formamide using [RuCl(dppe)2][OTf] as a precatalyst and tert-butyl amine-borane (tBuH2N•BH3) as secondary hydrogen source was confirmed using 13C NMR spectroscopy. The mechanisms were proposed for HCOOH dehydrogenation and transfer hydrogenation of CO2 based on our NMR spectroscopic studies. Furthermore, a few test reactions of transfer hydrogenation of selected alkenes such as cyclooctene, acrylonitrile, 1-hexene using [RuCl(dppe)2][OTf] as pre-catalyst and tert-butyl amine-borane (tBuH2N•BH3) as secondary hydrogen source showed quantitative conversion to hydrogenated products.