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Development of Molecular-defined Non-noble Metal Complexes and Nano-structured Materials for Sustainable Redox Reactions Development of Molecular-defined Non-noble Metal Complexes and Nano-structured

Periodic Reporting for period 4 - NoNaCat (Development of Molecular-defined Non-noble Metal Complexes and Nano-structured Materials for Sustainable Redox ReactionsDevelopment of Molecular-defined Non-noble Metal Complexes and Nano-structured)

Reporting period: 2020-06-01 to 2021-11-30

The major goal of the ERC project “NoNaCat” was the development of new catalysts for improved synthetic transformations, specifically related to (de)hydrogenations, which are of interest for organic chemistry in general and which are also of significant practical value for the chemical and life science industries. In this respect, the synthesis and exploration of both molecularly-defined catalysts and nano-structured materials, especially based on non-noble metals such as iron, cobalt and manganese, was proposed. Originally, we projected to synthesize >60 new molecular complexes and >200 supported nanoparticles for the planned target reactions. Indeed, in the past five years within this project more than 1000 new heterogeneous materials were prepared and tested in these model reactions. In addition, around 250 molecular catalysts were prepared and tested in comparison. Several synthesized materials were forwarded to European industries (Evonik, Hofmann LaRoche, Bayer, Symrise) for further testing in their industrial relevant processes. Regarding knowledge transfer students from France (University of Rennes) and Denmark (Aarhus) were trained in the preparation of our catalysts and to perform catalytic experiments including high pressure techniques. Apart from the methodological improvements and specific application examples vide infra, several interesting results have been achieved with respect to material sciences or organometallic chemistry. Some highlights are exemplarily mentioned here:
1. We discovered that unusual nickel silicides can be relatively easy synthesized for the first time following our pyrolysis approach. These materials constitute safe and stable nickel catalysts compared to presently used Raney Nickel and offer new opportunities for material sciences.
2. In comparison to other metals, the organometallic chemistry of group 6 PNP-pincer complexes, in particular of Mo, was poorly developed for a long time. To overcome this limitation, we developed a family of low-valent molybdenum complexes, supported by the pincer ligand (iPr2PCH2CH2)2NH. After suitable activation (NaBHEt3) some coordination compounds were found to be suitable catalysts for the hydrogenation of ketones and olefins.
3. Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N-bound ligands have been prepared and fully characterized for the first time. By replacing CO and H− as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α-alkylation of ketones with alcohols as shown below.
4. Methanol synthesis from syngas (CO/H2 mixtures) is one of the largest manmade chemical processes with annual production reaching 100 million tons. The current industrial method proceeds at high temperatures (200–300 °C) and pressures (50–100 atm) using a copper–zinc-based heterogeneous catalyst. In contrast, we developed a molecularly defined manganese catalyst based on the ERC proposal that allows for low-temperature/low-pressure (120–150 °C, 50 bar) carbon monoxide hydrogenation to methanol.
In the project outline catalyst developments for the following redox transformations vide infra was proposed. All these reactions were investigated in detail and selected obtained results are shown below. For more details, please see also the respective publications (please note that we expect to publish a minimum of 10 more publications this year which are based to a significant part on this ERC project. As described in the proposal most (<90%) of the prepared catalytic materials are based on easily available and less expensive metals such as iron, cobalt, and manganese. Typically, catalysts based on these metals were previously not very efficient for hydrogenation and dehydrogenation processes, at least not under mild conditions. By creating a suitable microenvironment with M-N interactions our systems became active and selective. According to this concept the suitable surrounding was created by using nitrogen-containing pincer ligands or nitrogen-doped graphenes. Most important catalysis results have been achieved for the hydrogenation and transfer hydrogenations of various functional groups. e.g. carboxylic acids, esters, and nitriles as well as carbon monoxide and carbon dioxide. As originally proposed, we paid specific attention to the hydrogenation of amides, which also resulted in some improvements. Unfortunately, the planed work on peptides hydrogenation did not result in a general methodology. Because of the leaching problems in the area of carbonylation reactions this work was stopped. A selection of obtained results are shown above.
An important development of the previous period was the development of a new synthetic methodology for hydrogenation of epoxides to terminal alcohols. The initially developed homogeneous catalysts for this transformation were based on Ru as metal. Afterwards a cobalt-catalyzed hydrogenation of epoxides for the synthesis of anti-Markovnikov alcohols was reported. This method is suitable for internal, as well as terminal, epoxides and works smoothly even with multi-substituted derivatives under mild conditions.
In the past two years, work on the ERC project also allowed to identify new methodologies for specific labelling, especially deuteration of organic compounds. Such methodologies are important in the development of every new pharmaceutical. Another discovered class of catalysts useful for the introduction of deuterium atoms into organic compounds is based on practical and stable heterogeneous copper catalyst, which permits for dehalogenative deuteration via water–gas shift reaction at comparably low temperature.
The testing of our heterogeneous catalyst materials in electrocatalytic reductions of carbon dioxide was continued (originally not proposed in the ERC grant proposal). This outcome led to cooperation with the University of Aarhus in Denmark. In addition, a concept was developed for valorization of electronic waste (E-waste), which is produced from end-of-life electronic equipment.
Notably, E-waste is the fastest growing solid waste stream, and its rapid generation creates significant environmental problems on a global scale. E-waste contains valuable metals in much higher concentrations than their respective primary resources—metal ores. Currently, less than a quarter of all E-waste is being recycled to recover precious metals; thus, most of the waste is being exported to developing countries, where it is often landfilled and stockpiled. Poor treatment of this hazardous waste causes environmental damage and poses serious health risks to inhabitants in these areas. Therefore, the development of new strategies for the recovery and valorization of metals from E-waste is of increasing importance. In our work, we describe a methodology for converting E-waste to useful catalytic materials while producing gold-enriched solids as the byproduct.