Borate-Based Ligand Design: Tailored Coordination for Scandium-Element Multiple Bonds

Abstract

Strategic ligand design plays an integral part in controlling the reactivity of transition metal complexes. By optimizing structure using tailored coordination environments, new bonding interactions may be realized that would otherwise be inaccessible. This thesis explores this concept through the development of two new borate based ligand scaffolds, recruiting four-coordinate boron to incorporate negative charge in the second coordination sphere. Expanding on previous work established with the dianionic B2Pz4Py pentadentate ligand, pursuit of a dinucleating analogue is described using the 1,8-naphthyridine linker. Finding this system inaccessible with current methodology, adaptation to a monosubstituted naphthyridine system was made to study boron-carbon bond formation in detail. Investigating synthesis with a variety of different methods, this ligand was obtained in small scale via transmetallation of a tin-functionalized substrate, and preliminary metalation with ruthenium explored. Following this, successful preparation of the monoanionic pentadentate ligand BPz2Py3 is then presented. Synthesis via direct lithiation provides this scaffold on gram scale, and its facile metalation of first row transition metals is described. Demonstrating the optimized structure of this ancillary ligand for stabilizing rare and very reactive scandium-element multiple bonds, alkane elimination pathways have provided access to a terminal scandium imido complex. Through parallel reactivity, the heavier phosphinidene analogue is proposed to form, but its presence is fleeting due to increased reactivity and yields unique C-C bond cleavage of the ligand framework. Lastly, pursuit of a terminal scandium-oxo is described, with the BPz2Py3 ligand providing access to a benzophenone masked species that retains oxo-like reactivity.