UC Santa Cruz

The work presented herein describes the isolation and investigation of the first unsupported monomeric stibine oxides. In Chapter 1, I introduce the topic by discussing synthetic strategies for the isolation of novel, highly reactive main-group motifs, applications of pnictogen-containing species, advances in the isolation and investigation of monomeric pnictinidene chalcogenides and pnictine chalcogenides, and the currently accepted model for pnictoryl bonding (Pn=O/Pn+–O–). In this discussion, we rationalize why monomeric stibine oxides with the unperturbed stiboryl group had remained undiscovered. Chapter 2 focuses on a reinvestigation of previously reported monomeric stibine oxides. I collected new data on these species, including crystallographic, spectral, and computational experiments, which provide unambiguous evidence that these species are in fact hydroxystibonium salts and not monomeric stibine oxides. This chapter is heavily centered on the diffraction and refinement methods we employed, including neutron diffraction, multipole modeling, and Hirschfeld atom refinement with NoSpherA2. In Chapter 3, I present the isolation and investigation of the first monomeric stibine oxide Dipp3SbO (Dipp = 2,6-diisopropylphenyl), as well as its directly comparable lighter congeners, Dipp3PO and Dipp3AsO. Dipp3SbO exists as a bench-stable monomer because the reactive stiboryl group is sterically shielded (i.e., kinetically stabilized) by the bulky Dipp groups. Theoretical analyses of Dipp3PnO (Pn = P, As, Sb) are consistent with our expectation that Dipp3SbO features the most highly polarized pnictoryl bond of the series, arising from inefficient back-bonding between O-centered p-orbitals and Sb–C σ* orbitals. Dipp3SbO was found to engage in several distinct classes of reactivity, including H-bonding, Brønsted base chemistry, coordination chemistry with first, second, and third row transition metals, 1,2-addition chemistry, and oxo transfer. In each case, Dipp3SbO was dramatically more reactive than Dipp3PO or Dipp3AsO. In Chapter 4, I quantify the enhanced Brønsted basicity of Dipp3SbO relative to Dipp3PO or Dipp3AsO that arises from modulations of the electronic structure of the pnictoryl bond. We performed a series of stoichiometric reactions that involved the isolation of hydroxypnictonium salts to provide limiting values for the pKaH of each pnictine oxide. We then conducted a 1H NMR spectrometric titration experiment and determined that Dipp3SbO boasts an approximately one-million fold increase in basicity relative to Dipp3AsO, with pKaH,MeCN values of 19.81(5) and 13.89(13), respectively. The enhanced basicity of Dipp3SbO enables it to catalyze a selected transesterification reaction efficiently, in contrast to the lighter congeners. In Chapter 5, I describe the isolation of Dipp3Bi. Dipp3Bi exhibited very distinct oxidative halogenation reactivity relative to Mes3Bi, as well as Dipp3Sb and Mes3Sb (Mes = mesityl). Our work highlighted the drastic impact that bulky substituents can impose on the reactivity of heavy pnictines. The isolation of Dipp3SbCl2 serves as a curious example of this, as it exhibits a highly unusual square pyramidal geometry in solid state. Finally, Chapter 6 contains an investigation into the chemistry of Mes3SbO, a less sterically encumbered monomeric stibine oxide relative to Dipp3SbO. Mes3SbO is only stable under an inert atmosphere; atmospheric water will readily add across the stiboryl group to form a dihydroxystiborane. Mes3SbO engaged with PbMe3Cl and SnMe3Cl to form classical Lewis adducts. In contrast, GeMe3Cl SiMe3Cl and CPh3Cl added across the unsaturated stiboryl group to form five-coordinate stiboranes. This biphilic reactivity encouraged us to pursue more challenging substrate activations. Mes3SbO was found to similarly activate the C–F and Si–F bonds of C(p-MeOPh)3F and SiEt3F, respectively. Thus, by tuning the steric environment, the reactivity of the genuine stiboryl group could be accessed in a new way to activate among the strongest polar covalent single bonds in chemistry. In conclusion, the use of sterically demanding organic groups has enabled us to access a highly reactive and previously undiscovered class of molecule, monomeric stibine oxides. I hope that this work will inspire other researchers in this area to elucidate novel functional groups that advance periodic trends and expand the frontier of modern chemistry. Gaining insights into fundamental structure and bonding and unlocking new reactivity at Earth-abundant main-group elements holds the promise of practical advances in sustainable catalysis.