Exploring Photochemical and Redox Reactivity of Four-Coordinated Organoboron Compounds

Abstract

The works described herein are broadly concerned with exploring the characteristic reactivities and properties of four-coordinated organoboron systems, to fully understand and cultivate their utility and applicability. In particular, two different classes of chelate boron compounds are studied, namely N,C- and N,O-, with emphasis on investigating their response to external stimuli and unveiling the unique photophysical/optical properties inherent to these systems. The first section of work described in this thesis focuses on the synthesis and study of new unsymmetrical N,C-chelate organoboron systems bearing two different aryl groups at the boron centre. These phenylpyrazolyl-chelated compounds have been found to undergo a rare, two-stage regioselective photoisomerization. The initial transformation is a di-π-borate rearrangement that affords yellow azaboratabisnorcaradiene isomers which are subsequently converted to unprecedented 14aH- diazaborepins. Spectroscopic and computational studies provide insight into the formation and properties of these unique systems. A new series of N,O-chelate methylaminoanthraquinone-bridged diboron complexes have also been synthesized and their redox behaviour studied via cyclic voltammetry, EPR, and UV/Vis-NIR spectroelectrochemistry. It was found that coordination of boron to a commercial dye offered a redox- stabilizing effect, as well as tuning of the compounds’ colors and electron-accepting/-donating ability based on the substituents attached to boron. BF2 chelation induces a unique and rare golden lustre property, which was not inherent to the ligand itself and shows potential for future application.