Synthesis and characterization of iron-clathrochelates and their applications in materials science

This thesis focuses on the design and the synthesis of mono- and dinuclear iron clathrochelate complexes and their applications in materials science. Dinuclear iron clathrochelate complexes can be synthesized in different oxidation states: i. as negatively charged FeII-FeII complexes or ii. as neutral FeII-FeIII complexes. These compounds display interesting magnetic properties. The negatively FeII-FeII complexes are antiferromagnetic, whereas the neutral FeII-FeIII complexes are paramagnetic. In addition, an interconversion between the two redox states can be achieved, both chemically and electrochemically. The functionalization of clathrochelate complexes with electron-donating groups was found to stabilize the higher oxidation states (FeIII-FeIII), which was evidenced electrochemically. However, the deliberate preparation and isolation of FeIII-FeIII complexes was not achieved. Dinuclear iron clathrochelate complexes were successfully employed as redox-active compounds for redox flow batteries, and they displayed high battery voltages and Coulombic efficiencies. Iron clathrochelate complexes are stable under the harsh conditions of metal-catalyzed coupling reactions (e.g. Suzuki or Sonogashira reactions). The high stability makes them suitable building blocks for the preparation of networks displaying permanent porosity. Both mononuclear and dinuclear iron clathrochelate-based networks displayed apparent BET surface areas between 235 and 593 m2g 1. Moreover, all clathrochelateâbased porous networks displayed high thermal and chemical stabilities. Mononuclear iron clathrochelate building blocks can be decorated with chiral ligands, resulting in homochiral porous networks with apparent BET surface areas up to 548 m2g-1. Such materials were found to selectively adsorb D-tryptophan from water. Porous networks based on iron clathrochelates can also be synthesized via polycondensation reactions. This synthetic strategy involves the combination of an iron salt, a dioxime ligand, and a capping boronic acid. The resulting Fe-templated condensation reactions give networks with apparent BET surface areas up to 927 m2g-1. Similarly, these materials display high chemical and thermal stabilities. Additionally, they were found to efficiently adsorb chromium (VI) from water.

Nickamine and Analogous Nickel Pincer Catalysts for Cross-Coupling of Alkyl Halides and Hydrosilylation of Alkenes

Xile Hu, Renyi Shi, Zhikun Zhang

CONSPECTUS: Ligand development plays an essential role in the advance of homogeneous catalysis. Tridentate, meridionally coordinating ligands, commonly termed pincer ligands, have been established as a privileged class of ligands in catalysis because they confer high stability while maintaining electronic tenability to the resulting metal complexes. Pincer ligands containing "soft" donors such as phosphines are typically used for late transition-metal ions, which are considered "soft" acids. Driven by our interest to develop base-metal catalysis and in view of the "hard" character of base-metal ions, our group explored a pincer ligand containing only "hard" nitrogen donors. A prototypical nickel complex of this ligand, "Nickamine", turned out to be an efficient catalyst in a wide range of organic reactions. Because of its propensity to mediate single-electron redox chemistry, Nickamine is particularly suited to catalyze cross-coupling of nonactivated alkyl halides through radical pathways. These coupling partners have been challenging substrates for traditional, palladium-based catalysts because of difficult oxidative addition and nonproductive beta-H elimination. The high activity of Nickamine for cross-coupling leads to high chemoselectivity and functional group tolerance, even when reactive Grignard reagents are employed as nucleophiles. The scope of the catalysis is broad and encompasses spa(3)-spa(3), sp(3)-sp(2), and sp(3)-sp cross-coupling. The defined nature of Nickamine facilitated the mechanistic study of cross-coupling reactions. Experiments involving radical-probe substrates, presumed intermediates and dormant species, kinetics, and density functional theory computations revealed a bimetallic oxidative addition pathway. In this pathway, two Ni centers each provide one electron to support the two-electron activation of an alkyl halide substrate. The success of Nickamine motivated our systematic structure-activity studies aiming at improved activity in certain reactions through ligand modification. Indeed, better catalysts have been developed for cross-coupling of secondary alkyl halides as well as direct alkynylation of alkyl halides. The improvement is attributed to a more accessible Ni center in the new catalysts than in Nickamine. Surprisingly, the improvement could be obtained simply by replacing a dimethyl amino group in Nickamine with a pyrrolidino group. During the study of the catalytic cycle of Nickamine in cross-coupling reactions, we synthesized the corresponding Ni-H species. Consequently, we explored the catalytic application of Nickamine in Ni-H mediated reactions, such as hydrosilylation. To our delight, Nickamine is a chemoselective catalyst for hydrosilylation of alkenes while tolerating a reactive C=O group. An analogous Ni pincer complex was found to catalyze unusual hydrosilylation reactions using alkoxy hydrosilanes as surrogates of gaseous silanes.

AMER CHEMICAL SOC2019

Synthetic methodologies for C-C and C-N bond formation involving alkyl radicals

Guido Barzanò

C-C and C-N bonds are some of the most common structures in molecules ranging from drugs to catalysts and to food additives. Many coupling reactions were developed to form these types of bonds with excellent selectivity and good performance. Still, the synthesis of some of those species either depends on traditional methods or requires expensive and rare reagents and catalysts. The use of precious metals in chemistry, and particularly in the industry, can make molecular synthesis expensive and energy-intensive since the extraction of those materials is costly. Moreover, in homogeneous catalysis, the recovery of metal-based catalysts is almost impossible, making these processes unsustainable for the future. Thus, the discovery of chemical reactions that donât involve precious materials is indispensable. This thesis concentrates on the development of two chemical transformations, one performing an sp2-sp3 radical reductive coupling, one performing a photocatalyzed alkyl amination, involving inexpensive reagents and catalysts. The first chapter reviews the existing methodologies which use alkyl radicals to form carbon-carbon and carbon-nitrogen bonds. Alkyl radicals are formed from alkyl halides and pseudo-halides (Acids, esters, tosylates, etc.) via oxidation or reduction performed by metal or photoredox catalyst. The wide variety of available methodologies was illustrated, highlighting the advantages and weaknesses of each reaction. The second chapter of this dissertation is devoted to the development of a new iron-based catalytic method to form stereoselectively Z-alkene boron species from the corresponding boron-alkyne. This novel stereoselective method opens a new way to obtain these types of molecules with cheap reagents, in high yields, with excellent stereoselectivity (Z:E ratio >10:1), and with good tolerance of functional group. The reaction is radical based, avoiding the problem of beta-hydride elimination, which usually occurs in ionic reactions involving metal centers. In order to prove the versatility of this method, the vinyl boron compound formed from this reaction was used for further functionalizations. The formal total synthesis of a 5-HT2c receptor agonist was successfully performed, improving the overall yield of the process, and avoiding the use of expensive and polluting chemicals. The third chapter of this thesis is concentrated on the development of a tandem photoredox/metal base catalyzed functionalization of anilines and imines with alkyl carboxylic esters. In this reaction, no late transition metal is used, and the traditional metal-based photocatalyst is substituted with an inexpensive and more efficient organic dye. The alkylation of amines is a challenging topic in organic chemistry and classical nucleophilic substitution lacks selectivity and scope. Harvesting visible light with a photoredox catalyst opens the possibility to a radical pathway, making this coupling easy and with high tolerance for functional groups. In order to understand the catalytic cycle of the reaction, additional mechanistic studies were conducted, leading to some insight on how this reaction works.

EPFL2020

Synthetic methodologies for C-C and C-N bond formation involving alkyl radicals

Guido Barzanò

EPFL2020

Nickamine and Analogous Nickel Pincer Catalysts for Cross-Coupling of Alkyl Halides and Hydrosilylation of Alkenes

Xile Hu, Renyi Shi, Zhikun Zhang

AMER CHEMICAL SOC2019