Variation in pnictogen–oxygen bonding unlocks greatly enhanced Brønsted basicity for the monomeric stibine oxide

Abstract

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, Dipp₃SbO (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 (Dipp₃PnO; Pn = P, As, and Sb). Stoichiometric reactivity between Dipp₃PnO and a series of acids demonstrates the greatly enhanced ability of Dipp₃SbO to accept protons relative to the lighter congeners, consistent with theoretical isodesmic reaction enthalpies and proton affinities. ¹H NMR spectrometric titrations allow for the pK_aH,MeCN determination of Dipp₃AsO and Dipp₃SbO, revealing a 10⁶-fold increase in Brønsted basicity from Dipp₃AsO to Dipp₃SbO. The increased basicity can be exploited in catalysis; Dipp₃SbO 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 Dipp₃PO < Dipp₃AsO < Dipp₃SbO, a direct consequence of the variation in the electronic structure of the pnictoryl bond as the pnictogen atom increases in atomic number.