High-valent Iron Complexes for the Catalytic Oxidative Upcycling of Polymers and Reductive Formation of Ammonia

Chatterjee, Debasmita (2025) High-valent Iron Complexes for the Catalytic Oxidative Upcycling of Polymers and Reductive Formation of Ammonia. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Selective functionalization of unactivated C–H bonds is of immense interest. However, functionalizing these C–H bonds is not easy due to their high inertness. There are reports of late transition metals, such as rhodium, iridium, palladium, etc, that can functionalize C–H bonds under harsh reaction conditions involving a multitude of reagents. Nature has its own way of functionalizing the C–H bonds selectively using different heme or non-heme metalloenzymes that operate under mild conditions. However, understanding their reactivity in vitro by isolating the active site structure is difficult, as they are deeply embedded in a protein core. To understand the structure and reactivity, scientists have been trying to synthesize natural mimic complexes over the years. However, these catalyst frameworks are prone to degradation, and the reactive intermediates formed are unstable and, thus, difficult to characterize. In 2013, our group developed a biuret-modified tetraamido ligand framework that made the catalytic framework robust and stabilized the iron in its high-valent oxidation state. The oxoiron(V) intermediate was characterized by several spectroscopic techniques and catalyzed C–H bond functionalization in a wide range of substrates. Although this catalytic system could perform C–H functionalization reactions selectively, there is a significant drawback: the use of solvents. Solvents are used in more than 1000 equivalents in hydroxylation reactions and thus significantly affect the environment if they are not handled and discarded safely. Another limitation lies in solvent oxidation by this catalyst system, which leads to side product formation and thus restricts the catalyst's reactivity. Again, these C–H bond activation reactions are limited to the substrate solubility while extending the substrate scope to hydrophobic macromolecules that have limited solubility in the acetonitrile/water system, which is the most used solvent in C–H bond functionalization. Thereby, to functionalize the C–H bonds present in hydrophobic molecules, e.g., polymers, we need to introduce solvents like dichloromethane and analogs of benzene, e.g., dichlorobenzene, difluorobenzene, etc. They have significant environmental impacts and effects on human health as they are potent carcinogens. Thus, removing solvents from the reaction system and performing C–H bond activation reactions with elevated yield and selectivity under mild reaction conditions was a challenge. This led us to explore the mechanochemical pathway that essentially operates under solvent-less conditions and depends on the mechanical force imparted in the reaction conditions. Using this methodology, the C–H bond functionalization can be taken to the next level in the post-functionalization of commodity polymers and biopolymers, mostly insoluble in conventional solvents. Like other metalloenzymes, Nitrogenase also has a significant role in reducing nitrogen for the production of ammonia under mild conditions. The nitridoiron(V) species, isoelectronic with oxoiron(V), is speculated to be responsible for carrying out the reactions under mild conditions. However, proper study of this intermediate is still underway. On the other hand, ammonia synthesis in industry by the Haber Bosch process requires harsh reaction conditions, including very high temperatures and pressure. Thus, mimicking the natural system to generate ammonia selectively under mild conditions is necessary. However, very few examples of ammonia synthesis have been reported under mild reaction conditions. Therefore, the generation of such functional mimics for ammonia production is necessary. Thus, the overall goal of my thesis depends on designing catalytic processes involving safer reagents and benign conditions that align with green chemistry principles. Chapter I includes a comprehensive literature survey showing the hazards of solvents, followed by the mechanochemical reaction protocol as a better alternative to solvent-based systems. This chapter also advocates iron as the earth's abundant metal in C–H bond activation reactions by emphasizing biomimetic oxidation reactions that mainly operate under mild reaction conditions. Chapter IIA discusses the successful elimination of solvent, a major component of the reaction system, to generate a green catalytic system. This system involves mechanochemical grinding for oxidizing unactivated C–H bonds using a biomimetic catalyst, Fe-bTAML, and mCPBA as terminal oxidant. Chapter IIB discusses a successful approach to mitigate the crisis of plastic waste: "chemical upcycling", in which waste plastic is either converted into products with higher economic value. Towards this goal, several metal-catalyzed post-functionalization of polymers have been reported, with variable success, mostly on account of lack of selectivity, use of harsh reaction conditions, and use of environmentally unfriendly solvents. This part of the thesis demonstrates the selective hydroxylation of the backbone 3° C–H bonds in synthetic macromolecules (polyolefin and polystyrene) using in-house developed (Et₄N)₂[FeIII-(Ph,Me-bTAML)] complex and solid Na₂CO₃.1.5 H₂O₂ (SPC; Sodium percarbonate) under solvent-free mechanochemical conditions. The reaction only employs simple mechanochemical grinding or ball-milling at room temperature. The polar functional group -OH was successfully incorporated into the polymer backbone without chain degradation and crosslinking. The same reaction conditions were also employed to selectively hydroxylate small organic molecules, including complex natural products. The rate and selectivity of the reaction towards 3° C–H bonds far exceed that performed under homogeneous conditions. Mechanistic investigation indicates the formation of the well-characterized oxoiron(V) intermediate upon mechanical grinding of 1 and SPC. The high selectivity observed under solvent-free conditions is due to the elimination of solvent-induced side-reaction of this intermediate. This reaction represents an environment-friendly process since it uses environmentally benign reagents (iron complex, "oxygen bleach") and eliminates the use of hazardous solvents. The workup protocol involves simple washing with water, where both the spent catalyst and the oxidant are soluble. Selective mechanochemical oxidation of alkyl and benzylic 3° C–H bonds often found in commercial polymers, such as polyolefin and polystyrene, may offer a potentially useful method to generate oxyfunctionalized material and also provide routes for the deconstruction of macromolecules with strong C–C bonds under mild conditions. Chapter III includes the oxidative degradation of Lignocellulosic biomass, the most abundant natural biopolymer. Although there have been several reports demonstrating the degradation process using multistep harsh treatments involving strong acids and bases, this degradation is easily achieved by solvent-free mechanochemical reaction conditions in one step involving (E₄N)₂lignocellulosic biomass into water-dispersible carboxylate-functionalized cellulose nanospheres (CNS). The catalyst system mimics the dual functions of Lytic Polysaccharide Monooxygenase (LPMO) and Lignin Peroxidase, enabling the efficient breakdown of biomass into cellulose nanospheres without producing harmful byproducts. The reaction mechanism, validated using glucose and cellobiose as model compounds, follows a pathway similar to LPMO, unlike TEMPO-mediated oxidation. The study of the morphological and compositional transformation of corncob microfiber into nanospherical structures reveals that lignin initially oxidizes into smaller, water-soluble fragments, enabling the subsequent transformation of cellulose fibers into dispersible nanospheres. Ecotoxicological assessments confirmed that CNSs and related byproducts are non-toxic and environmentally safe. Furthermore, due to their superior surface activity, CNSs were explored as stabilizers for O/W Pickering emulsions and environment-friendly alternatives to commercial detergents. Chapter IV describes the role of high-valent iron-nitrido(V) species in synthesizing ammonia in water. In this work, we have successfully synthesised FeIII(N₃)-bTAML (1) complex from its chloride[FeIII-(Ph,Me-bTAML)] as a catalyst and sodium percarbonate as the oxidant. This process successfully converted precursor (FeIII(Cl)-bTAML) and characterized it using different spectroscopic techniques, as well as SC-XRD. Upon irradiation with 370 nm light and successive elimination of N₂ gas, FeV(N)-bTAML (2) was formed and was characterized by UV-Vis, EPR, HRMS, etc. Complex 2 is reactive toward the O–H bond as well as C–H bond oxidation, like fluorene, xanthene, indene, and 1,3-cyclohexadiene. Quantitative ammonia formation was observed when complex 1 was irradiated with 370 nm light in water-DMF mixture. Confirmation of the formation of ammonia and its quantification was done by 1H NMR and the indophenol test, and up to 70 TON ammonia formation was noted. To the best of our knowledge, Complex 2 is the first example that can oxidize C–H bonds and form ammonia in the presence of water, mimicking the reactivity of Nitrogenase in aqueous environments. Chapter V discusses the conclusion of the overall work in the thesis and depicts future directions.

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