Isolation of Reactive Main Group Fragments by Frustrated Lewis Pairs
Creators: Omaña Moreno, Alvaro A.
Publisher: name: University of Alberta Library
Year: 2022

<p>The work in this Thesis describes significant progress made in the field of frustrated Lewis pairs (FLPs) at the intersection between modern main group chemistry and materials chemistry. Mainly, this work focuses on the use of an intramolecular phosphine-borane FLP (namely, o-iPr2P(C6H4)BCy2 or PB) for the isolation of “bottleable” single-source molecular precursors to valuable materials.<br>
Initially, new Group 14 element-based dihydrides (E = SiH2, GeH2) were prepared via E(II) dihalide capture by PB to form [PB{SiCl2}] and [PB{GeCl2}], and subsequent X/H group exchange gives their respective <a href="E%20=%20Si%20or%20Ge" rel="noopener noreferrer" target="_blank">PB{EH2}</a> adducts in high yields. Upon gentle heating of <a href="%3E%20100%20%C2%B0C" rel="noopener noreferrer" target="_blank">PB{EH2}</a> in solution, the formation of H2 gas and free PB ligand were confirmed by multinuclear (1H, 31P, 11B) NMR spectroscopy, along with observed Si or Ge deposition. The deposited Si or Ge materials were studied by SEM/EDX, XPS and Raman spectroscopy to confirm that: (1) elemental Si(0) and Ge(0) could be deposited; and (2) broad amorphous Si–Si and Ge–Ge stretching could be observed using Raman spectroscopy at 485 cm-1 and 480 cm-1, respectively. Consistently, PB could be re-collected after Group 14 element deposition with ~70 % recovery and re-used, allowing for a closed loop process. Isolation and thermolysis of a polymer precursor, [PB{SiMe2}], was studied, showing that well-defined polydimethylsilane, [SiMe2]n, could be formed at &lt; 110 °C.</p>

<p>Next, the development of FLP-chelated boron nitride precursors was explored by targeting the BN complex, [PB{BN}]. First, a [PB{H2BNH2}] adduct was accessed in high yield by dehydrogenation of ammonia-borane (H3B•NH3) and binding of {H2BNH2} using PB. Unlike free amine-boranes, <a href="and%20new%20related%20FLP-aminoborane%20complexes" rel="noopener noreferrer" target="_blank">PB{H2BNH2}</a> did not undergo catalytic dehydrogenation with transition metal complexes, likely due to low polarity of the Bδ+–Hδ- linkages that would otherwise serve as initiation sites for catalysis. Instead, a step-wise halogenation/dehalogenation route was used to afford [PB{HBNH}], an FLP-chelate of the parent iminoborane HBNH. Unfortunately, due to low reactivity of the B–H and N–H linkages, [PB{HBNH}] did not undergo catalytic dehydrogenation to form [PB{BN}]. Upon addition of strong chlorinating agents to [PB{HBNH}], towards the useful intermediate [PB{ClBNH}], B–Cy bond activation in the ligand was observed.</p>

<p>Finally, a closer look at the reactivity of <a href="X%20=%20Cl,%20Br" rel="noopener noreferrer" target="_blank">PB{SiX2}</a> adducts was performed in the hopes that subsequent reduction would yield [PB{E=E}PB] species; however, only free PB was formed according to NMR spectroscopy. To see if the related [PB{C=C}PB] dimer could be accessed, [PB{CH2}] was isolated; however, deprotonation attempts resulted in P–C bond cleavage in the ligand. New phosphine-borane FLP chelates were synthesized, although these ligands were met with limited success in the binding of Group 14 element centers (Si, Ge). The work in this Thesis demonstrates that FLPs can be used to access valuable main group element motifs, as well as molecular precursors for the low temperature deposition of industrially relevant materials from the nano-scale to the bulk.</p>