Variation in pnictogen–oxygen bonding unlocks greatly enhanced Br?nsted basicity for the monomeric stibine oxide?

Dalton Transactions Pub Date: 2023-08-02 DOI: 10.1039/D3DT02113K

Abstract

Phosphine oxides and arsine oxides feature highly polarized pnictoryl groups (Pn+–O?/Pn = O; Pn = P, As) and react as Br?nsted 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 Br?nsted 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 pKaH,MeCN determination of Dipp3AsO and Dipp3SbO, revealing a 106-fold increase in Br?nsted basicity from Dipp3AsO to Dipp3SbO. The increased basicity can be exploited in catalysis; Dipp3SbO exhibits dramatically increased catalytic efficiency in the Br?nsted base-catalyzed transesterification between p-nitrophenyl acetate and 2,2,2-trifluoroethanol. Our results unambiguously confirm the drastic increase in Br?nsted 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.

Graphical abstract: Variation in pnictogen–oxygen bonding unlocks greatly enhanced Br?nsted basicity for the monomeric stibine oxide
Recommended Literature