Abstract
Both fields, hydrogen storage and Frustrated Lewis Pair chemistry, have evolved tremendously in the past decades to use molecular hydrogen in the best possible ways. Nevertheless, it is yet unclear if Al-N compounds could contribute by activating molecular hydrogen to store hydrogen or catalyze hydrogenation reactions. Therefore, this thesis aims to guide future studies by stepping into this gap and additionally introduce mechanochemical approaches to synthesize precursors. The first step towards hydrogen storage and hydrogenation reactions is the activation of molecular hydrogen. This is reported in Chapter 2, where several intermolecular classical Al-N Lewis pairs and an intramolecular ortho-ala-aminoarene activate molecular hydrogen. Their ability is investigated using the isotope exchange reaction from HD to H2 and D2. The herein studied intermolecular Lewis pairs are based on alkylalanes and N-methyldiphenylamine, while the intramolecular Lewis pair is (o-TMP-C6H4)AlH2 ((2-(2,2,6,6-tetramethyl-piperidine-1-yl)phenyl)-aluminium dihydride). The activation of molecular hydrogen is carried out in toluene under mild conditions and monitored by 1H and 2H NMR spectroscopy. Furthermore, the Al-N interaction is probed by 27Al NMR spectroscopy using an aring sequence and crystallographic studies. Additionally, the crystal structure of pure AliBu3 is determined. These studies may attribute the pronounced reactivities of these Al-N compounds to elongated Al-N bond lengths. For a deeper understanding of Al-N compounds, Chapter 3 discusses the synthesis of a set of Al-N compounds starting from amino-organolithium compounds. The focus of this chapter is a thorough structural study of small, easily accessible amino-organolithium compounds with bridging phenyl and naphthyl moieties. Their crystal structures most likely represent their aggregation as tetramers and dimers in both hydrocarbon and ethereal solvents. These amino-organolithium compounds are further used to generate their corresponding aluminium compounds as a model system. Their crystal structures are reported too. All structures are discussed with a focus on the different sterically demanding and bridging moieties. Additionally, the crystal structures of neophyllithium and decomposition products of some of the compounds mentioned above are reported. Here, also the mechanochemical synthesis of AlNp2Cl is reported. This study provides additional data for the future design and synthesis of amine stabilized organolithium and –aluminium compounds. Furthermore, the mechanochemical synthesis of secondary and tertiary alanes AlR3 (R = Np or Mes; HAlR2 R = Np or Mes) is described in Chapter 4. In this chapter, the above-mentioned tertiary and secondary alanes are reacted with several acenaphthene- and phenanthrene-bridged α-diimines to give a series of aluminium-derived radicals of the form [(diimine)AlR2]•. EPR and several crystallographic studies are reported to confirm their structures with the assistance of DFT calculations. These species are thought to form via hydro- or carboalumination and subsequent elimination reactions. This view is supported by the structural data obtained for the minor hydroalumination product C12H7(NHDipp)(NDipp)AliBu2 and the rearrangement product C13H8(C(iBu)=N(m-Xy)(NH(m-Xy)))AliBu2 from a carboalumination reaction. Additionally, the characterization of (C6F5)2B(OC(C6F5)OC12H8) infers that such a carboboration pathway also provides access to related boron-derived radicals. Finally, Chapter 5 provides more specific information on the employed inert gas techniques and useful knacks in the lab beyond the standard glovebox- and Schlenk-techniques. These comments were not explicitly written in the manuscripts of Chapters 2-4 but could serve as a guide to new users and successors. This includes the design of a Schlenkline, the sealing of NMR tubes with a torch, safe-operation of pressurized NMR tubes, notes on hetero-nuclei NMR spectroscopy, the synthesis of Li-sand from the melt under oil, milling of organometallics, and comments on additional analytical techniques. In future studies, it would be interesting to see if the herein described compounds can be used for catalytic hydrogenation reactions and if geminal or C2-spaced Al-N compounds can be synthesized from enamines by hydro- or carboalumination. This may broaden the scope of compounds that can activate hydrogen and be used to investigate the influence of steric demand and the basicity of the amines.
