Photophysical Properties and Reactivity Studies of Phosphine-Borane Donor-Acceptor Compounds

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Li, Yufei
This thesis describes the preparation of several phosphine-borane species containing bulky aryl groups on the heteroatoms for steric protection. Depending on the system, they either exhibit interesting photophysical properties, unusual reactivity, or both, based on the environment around the heteroatoms as well as the linking unit connecting them. A highly congested P-B donor-acceptor compound with a 1,8-naphthalene backbone has been synthesized. Despite the high degree of steric congestion, this molecule was found to possess a P-B dative bond which persistents in solution. The new P-B molecule is thermally and photochemically inert, displaying no reactivity toward some common small molecules with the exception of halogens. The high stability of the molecule is attributed to the crowded environment around P and B, as well as the highly rigid 1,8-naphthyl linker. The reaction of this new P-B molecule with halogens in the presence of water leads to the formation of a P-O-B compound. An unbound U-shaped new phosphine-borane compound has been synthesized and displays distinct through-space CT transition and intense dual emission. The use of this new P-B compound in turn-on/switchable fluorescent sensing of fluoride has been demonstrated. The P-B compound was found to have a ratiometric response toward fluoride ions greater than that of the related N-B compounds. Converting the unbound U-shaped donor-acceptor compound to its phosphonium salt greatly enhanced its fluoride binding affinity at the boron center by 2 orders of magnitude. Furthermore, despite the significant steric congestion present in the phosphine-borane species, it behaved as an effective ligand towards metal ions such as Au(I) and Pt(II) yielding their respective coordination complexes. In the case of the Pt(II) complex, it displayed interesting “turn-on” fluorescent to fluoride addition. Finally, the cationic phosphonium-borane compound was found capable of effectively tearing apart an NHC molecule, yielding a vinyl-amine bridged P-B species. In addition to IMe which plays a key role in its self-destruction, a FLP-like ylide-borane and IMeH+ salt have been identified as key species involved in this transformation. The balance of Lewis acidity/basicity and the cooperativity among the phosphonium/ylide, IMe, and borane appear to all be critical in this unprecedented fragmentation.
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