Variation in pnictogen-oxygen bonding unlocks greatly enhanced Bronsted basicity for the monomeric stibine oxide

Phosphine oxides and arsine oxides feature highly polarized pnictoryl groups (Pn(+)-O-/Pn = O; Pn = P, As) and react as Bronsted 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 & sigma;* orbitals is attenuated, destabilizing the lone pairs and increasing the donor capacity of the pnictine oxide. Herein, we assess the Bronsted 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. H-1 NMR spectrometric titrations allow for the pK(aH,MeCN) determination of Dipp(3)AsO and Dipp(3)SbO, revealing a 10(6)-fold increase in Bronsted 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 Bronsted base-catalyzed transesterification between p-nitrophenyl acetate and 2,2,2-trifluoroethanol. Our results unambiguously confirm the drastic increase in Bronsted 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.