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Phosphine oxides and arsine oxides feature highly polarized pnictoryl groups (Pn + –O − /Pn = O; Pn = P, As) and react as Brønsted bases through O-centered lone pairs. We recently reported the first example of a monomeric stibine oxide, Dipp 3 SbO (Dipp = diisopropylphenyl), allowing periodic trends in pnictoryl bonding to be extended to antimony for the first time. Computational studies suggest that, as the pnictogen atom becomes heavier, delocalization of electron density from the O-centered lone pairs to the Pn–C σ* orbitals is attenuated, destabilizing the lone pairs and increasing the donor capacity of the pnictine oxide. Herein, we assess the Brønsted basicity of a series of monomeric pnictine oxides (Dipp 3 PnO; Pn = P, As, and Sb). Stoichiometric reactivity between Dipp 3 PnO and a series of acids demonstrates the greatly enhanced ability of Dipp 3 SbO to accept protons relative to the lighter congeners, consistent with theoretical isodesmic reaction enthalpies and proton affinities. 1 H NMR spectrometric titrations allow for the p K aH,MeCN determination of Dipp 3 AsO and Dipp 3 SbO, revealing a 10 6 -fold increase in Brønsted basicity from Dipp 3 AsO to Dipp 3 SbO. The increased basicity can be exploited in catalysis; Dipp 3 SbO exhibits dramatically increased catalytic efficiency in the Brønsted base-catalyzed transesterification between p -nitrophenyl acetate and 2,2,2-trifluoroethanol. Our results unambiguously confirm the drastic increase in Brønsted basicity from Dipp 3 PO < Dipp 3 AsO < Dipp 3 SbO, a direct consequence of the variation in the electronic structure of the pnictoryl bond as the pnictogen atom increases in atomic number. 

In contrast to phosphine oxides and arsine oxides, which are common and exist as stable monomeric species featuring the corresponding pnictoryl functional group (Pn=O/Pn⁺–O⁻; Pn = P, As), stibine oxides are generally polymeric, and the properties of the unperturbed stiboryl group (Sb=O/Sb⁺–O⁻) remain unexplored. We now report the isolation of the monomeric stibine oxide, Dipp₃SbO (where Dipp = 2,6-diisopropylphenyl). Spectroscopic, crystallographic and computational studies provide insight into the nature of the Sb=O/Sb⁺–O⁻bond. Moreover, isolation of Dipp₃SbO allows the chemistry of the stiboryl group to be explored. Here we show that Dipp₃SbO can act as a Brønsted base, a hydrogen-bond acceptor and a transition-metal ligand, in addition engaging in 1,2-addition, O-for-F₂exchange and O-atom transfer. In all cases, the reactivity of Dipp₃SbO differed from that of the lighter congeners Dipp₃AsO and Dipp₃PO.