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Abstract

Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. By contrast, boron(II) cations are extraordinarily rare and their number is limited to only some few examples with an almost completely unknown reactivity. The compounds are both Lewis acids and electron donors, properties that ensure a rich and diverse chemistry. The use of bridging bicyclic guanidinate substituents in the diborane(4) compound allows the stabilization and investigation of these exceptional boron(II) species. This doctoral thesis deals with the synthesis, characterization and reactivity of base stabilized di-, tri- and tetraborane cations. Starting with the already known diborane(4) compound [HB(hpp)]2 some new potential precursors [XB(hpp)]2 (X = Cl, Me, n-Bu) were obtained. Proceeding from the dichloro-diborane, the chloride abstraction with AlCl3 and GaCl3 was studied. Irrespectively of the salt chosen, the reaction led to formation of a radical tricationic tetraborane [{B(hpp)}4]3+· and permitted some new insights into the reaction mechanism. In the further course of this work a highly electron-deficient sigma-aromatic tetraborane(4) intermediate [{B(hpp)}4]4+ was isolated and fully characterized. This unprecedented compound represents the first cationic structural derivative of [B4H4]4+ and is stabilized by the lattice energy. In addition, the repertoire of electron-deficient cyclic boranes was expanded by a new cationic triborane [{B(hpp)}2(BH2)]+. The second part of this thesis focuses on the double reactivity pattern of boron(II) cations. Starting with the diborane(4) [(TfO)B(hpp)]2 with two easily exchangeable triflate substituents, a facile synthesis of several new boron(II) cations is reported. By systematically increasing the pi-acceptor properties of the sigma-Lewis basic reaction partners, first examples are presented in which the combined Lewis acidity and electron donor properties were used for the reduction of organic substrates. Expanding the organic sigma-donors by redox-active N-heterocyclic linkers resulted in the first members of an unprecedented family of highly charged cationic cyclophanes with integrated diboranyl units. The findings of this thesis provide deeper insight into the structural variety and potential application of borane(II) cations in organic synthesis as well as supramolecular materials.