Coordination Chemistry at Boron: Reactivity, Electronic Structure, and Optical Properties Studies Across the Main Group

Abstract

Boron, owing to its vacant pz orbital, has historically enabled the synthesis of diverse Lewis acid-base adducts. The research herein focuses on the structure, bonding, and reactivity of a range of Lewis base-stabilized neutral, cationic, radical, and anionic boron-containing systems. The utilization of various Lewis basic ligands, such as N- heterocyclic carbenes (NHCs), cyclic alkyl(amino) carbenes (CAACs), and phosphines, has enabled the synthesis and study of electron-rich boron-centered radicals, anions, and frustrated Lewis pairs (FLPs). These systems have been shown to participate in discrete bond activation processes, including single electron transfer and nucleophilic attack. In parallel, carbene and carbone ligands have facilitated the isolation of luminescent borafluorenium ions with tunable optoelectronic and stimuli-responsive properties. As a result, boron-centered molecules can be leveraged as key tools for various applications, including hydrogen storage, bond activation reactions, novel heterocycle formation, and stimuli-responsive materials. In conjunction with experimental results, density functional theory has been used to propose reaction mechanisms, identify bonding patterns, and understand key optoelectronic properties within these systems. These results collectively advance the understanding of coordination chemistry involving redox-flexible boron molecules while highlighting their reactivity and potential as functional molecular materials. Chapter 2 focuses on the synthesis of dimethylamineborane-bound magnesium(II) centers supported by NHC ligands. These species were shown to dehydrocouple dimethylaminebo-rane, yielding stoichiometric amounts of dihydrogen. By using various techniques, including NMR spectroscopy and DFT, the migratory behavior of dimethylamine borane within the magnesium complexes is investigated. Chapter 3 describes the activation of elemental sulfur (S 8 ) and diphenyl dichalcogenides using the CAAC-stabilized borafluorene radical and anion. From these non-selective reactions, sulfur chains of varying length (S 8 , S 7 , S 4 , and S 2 ) were each identified and analyzed using experimental and theoretical methods. The corresponding borafluorene-phenylchalcogenide species were analyzed using spectroscopic techniques and displayed characteristics consistent with an internal heavy atom effect. Chapter 4 highlights the ring expansion reactivity of a borinine-containing FLP towards various chalcogen-containing substrates. Subsequent reduction of these species yielded di- boracyclonone molecules bridged by a µ 2 chalcogenide atom. Halide abstraction from the 3 parent FLP yielded an analogous species with a formal chloronium ion bridging the two boron atoms. DFT was used to provide insight into reaction mechanisms and structure and bonding within each species. Chapter 5 depicts the synthesis and photophysical characterization of borafluorenium ions stabilized by hexaphenylcarbodiphosphorane. By installing 3,3’-dimethoxy substituents on the borafluorene core and changing the counteranion from bromide to tetrakis(3,5- bis(trifluoromethyl)phenyl)borate, pronounced changes in stability and optical properties were observed. Substantial increases in fluorescence quantum yields were invoked from solution state to solid state, indicating aggregation-induced emission behavior.