Rational design of bimetallic coordination polymers by utilizing heteroditopic ligands : synthesis, structural analysis and thermal decomposition

  • Rationales Design von bimetallischen Koordinationspolymeren unter Nutzung heteroditoper Liganden

van Terwingen, Steven; Englert, Ulli (Thesis advisor); Kögerler, Paul (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Coordination polymers are handled as promising candidates for future key technologies such as hydrogen storage or catalytic applications; the latter make up a majority of processes in the chemical industry. Compared to polymers solely constructed of covalent bonds like polyethylene (PE), coordination polymers are often crystalline. This opens up the possibility to use single crystal X-ray diffraction, which utilizes the interaction of X-rays with crystalline material to obtain information of the arrangement of atoms within the crystal. This method is a cornerstone of this thesis, as the information on intra- and intermolecular interactions can directly be analyzed and compared. The main focus of this thesis lies on heterobimetallic coordination polymers, which strive to combine properties of two metals in one polymer. Therefore, ligands are designed which selectively join two different metal cation species into one coordination polymer. This selectivity is achieved by using a ligand with two spatially separated and chemically different donor sites. On one site an acetylacetone will be used, while the other site bears an N donor, such as a nitrile or a pyrazole. The difference in Pearson hardness of the two binding sites accomplishes the selectivity towards different metal species. The most promising ligand of this thesis combines the acetylacetone with a trimethylpyrazole. Additionally, these bimetallic coordination polymers are thermally decomposed to obtain a complex heterogeneous solid. This solid is then tested for catalytic activities. Besides this overall target species, every organic molecule and every intermediate product is structurally analyzed where possible and discussed in relation to the literature. The N donor sites are cocrystallized with halogen bond donors. The single crystal diffraction of these cocrystals provides useful insights about the strength and nature of the N donor. For the trimethylpyrazole functionalized ligand all of the aforementioned steps are discussed in this thesis. Its synthesis is optimized and conducted on a multi-gram scale. The ligand selectively binds hard cations like Fe(III) on the acetylacetone site, whilst soft cations like Au(I) are bound on the pyrazole site. Multiple heterobimetallic species are presented, including the interesting combination of Fe(III) and Ag(I), which could lead to a potential catalyst after thermal decomposition. The potential catalysts exhibit reflections of the base metal oxide as well as reflections of the noble metal in oxidation state 0. First studies on of the decomposed species suggest a high surface area as well as small noble metal nanoparticles, which are smaller than those produced by common catalyst preparation methods. The actual tests for catalytic activity could not be conducted in the timeframe of this thesis; however, they are planned for the near future. The halogen bonding adducts exhibit contacts within the expected range for a pyrazole...I contact. Interestingly, the hydrohalides of the ligand do indeed cocrystallize with halogen bond donors. This partly gives rise to extended structures, in which hydrogen as well as halogen bonds coexist. The direct comparison of these related yet different interactions in the same crystal is conducted. An extended discussion of the theoretical electron density in these systems obtained by DFT methods utilizing Bader's theory of "Atoms in Molecules" is also presented.

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