Preview

PDF, German Download (6MB) | Terms of use

Abstract

Abstract The chemistry of neutral boron compounds is usually dominated by their electrophilic nature. Contrary to this, the doubly guanidinate-bridged diborane [HB(hpp)]2 stands out due to a B–B-bond with a high HOMO and hence a nucleophilic character. Especially the ditriflato- and dibromido-derivative [(OTf)B(hpp)]2 and [BrB(hpp)]2 are associated with an exceptional chemistry. Due to the presence of leaving-groups at the boron atoms they react with pyridine-derivatives as Lewis-acids. This enables the integration of the electron-rich B2(hpp)2-unit in viologen-like cyclophanes by reaction of [(OTf)B(hpp)]2 with di(4-pyridyl)-compounds. The obtained boracyclophanes show interesting redox- and optical properties. The first part of this thesis deals with the synthesis and characterization of such cyclophane-compounds. Various tetracationic boracyclophanes as well as a hexacationic cage-compound containing three B2(hpp)2-units have been synthesized. Moreover, precursors have been synthesized that gave information on the formation of the boracyclophanes, which is partly accompanied by radical intermediates. Investigation of the produced showed that the redox chemistry of the cyclophanes is defined by intramolecular π-π-interactions, which can be modified via selection of the integrated di(4-pyridyl)-compounds. The optical properties of the cyclophanes are marked by a charge-transfer from the B2(hpp)2-unit to the dipyridyl-units. The second part of this thesis deals with the reactivity of [(OTf)B(hpp)]2 and [BrB(hpp)]2 with diphenyl(4-pyridyl)methane-derivatives. Reaction of these diboranes with chlorodiphenyl(4-pyridyl)methane und bromodiphenyl(4-pyridyl)methane showed a new kind of reactivity of the hpp-bridged diborane-compounds, which resembles the oxidative addition known from transition metals. Initially, the ligands are bonded as Lewis-bases to the diborane-unit and are subsequently reduced associated with oxidation of the boron atoms and cleavage of the B–B-bond. The electron transfer is accompanied by migration of a halide from the ligand to the B2(hpp)2-unit. The Investigations on the mechanism showed that the rate of this reaction is determined by the leaving-group of the ligand. Therefore, a dissociation equilibrium is postulated, which takes place prior to the electron transfer. In this thesis, the cooperation of the Lewis-acceptor- and π-electron-donor-character of hpp-bridged diboranes was used for the synthesis of various extraordinary boracyclophanes. Furthermore, these properties enabled a type of reaction with metallomimetic nature.