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Abstract

This thesis deals with the chemistry of double guanidinate-stabilized diborane(4) compounds. The compounds have an electron-precise B−B bond and a pronounced nucleophilic character due to the strong electron donor properties of the guanidinate ligands. The nucleophilicity and reactivity of the compounds can be further influenced by varying the boron substituents. By reacting the already known ditriflato-diborane with ammonium salts, it was possible to synthesize a large number of neutral diboranes(4) with various substituents. Mechanistic studies have showed that the exchange occurs via an SN1 mechanism. With the reaction of dihalogeno-diboranes with trimethylsilyl compounds, an alternative route for the functionalization of neutral diboranes(4) was established. Besides the ditriflato-diborane, the dibromo-diborane in particular has promising leaving groups. The reactivity of the two diboranes was compared on the basis of substitution reactions with the Lewis bases pyridine and DMAP. While with the ditriflato-diborane both triflate groups are rapidly released at room temperature, with the dibromo-diborane only one substituent is exchanged under analogous conditions. The exchange of the remaining substituent can then be initiated by increasing the temperature. In contrast to simple nitrogen bases, no difference in reactivity is observed when the two diboranes are reacted with ortho-quinones. The electrons of the B−B bond are transferred to the quinone. Monocationic adducts with two sp3-hybridized boron atoms are formed. In addition to the hpp ligands, one boron atom is coordinated only with the quinone and the other to the quinone and a substituent. Further functionalization is also possible by reaction with ammonium salts. A mechanism for electron transfer was proposed which, in combination with quantum chemical calculations, explains not only the results of the reaction of the ditriflato-diborane with ortho-quinones, but also those with other 1,2-diketones. The reaction of the ditriflato-diborane with the dihydrido-diborane results in the formation of the dicationic tetraboron compound [H2B4(hpp)4]2+. The tetraborane(6) has already been synthesized by the reaction of dihydrido-diborane with B(C6F5)3 and has an interesting electronic structure with two 3c-2e bonds in the B4 core. The route found in this work allows the synthesis with an alternative counterion and suggests a mechanism involving the insertion of a hydrogen atom into the B−B bond. The reaction of the ditriflato-diborane with the diazido-diborane also leads to the formation of a dicationic compound with four boron atoms. In the synthesized [N2B4(hpp)4]2+, the four boron atoms are bridged via two nitrogen atoms, so that two boron atoms are sp3-hybridized and two are sp2-hybridized. The mechanism proposed for the formation again considers the pronounced nucleophilicity of the B−B bond, triggering an oxidative insertion of the nitrogen atoms. In general the diazido-diborane has an interesting electronic structure and reactivity. Of the diboranes examined it is the only one whose HOMO is not located at the B−B bond but at the substituents. In addition, the diborane shows a remarkable stability for azidoboranes, enabling thermally induced 1,3-dipolar cycloaddition reactions are possible at both azide groups. By varying the terminal alkyne, which acts as a reactant and solvent, a large number of new bis-triazole-diborane compounds with different functional groups can be synthesized. In addition, the cycloaddition is characterized by a high selectivity for the 1,4-adduct. The adduct from the diazido-diborane and the 2-ethinylpyridine has two bidentate coordination sites, so that complexation of the transition metals zinc and cobalt is possible. In this way, cyclic complexes are formed in which two metal centers are bridged by two bis-triazole diboranes and whose size varies with the metal. Overall, the present work enables the synthesis of a variety of neutral diboranes(4) and makes them accessible for applications. In addition, the reactivity of selected diboranes has already been examined in more detail. The results obtained contribute to a deeper understanding of the reactivity of nucleophilic diboranes and stimulate further investigations in this field.