Geometrically Constrained Organoboron Species as Lewis Superacids and Organic Superbases

Abstract

This report unveils an advancement in the formation of a Lewis superacid (LSA) and an organic superbase by the geometrical deformation of an organoboron species towards a T-shaped geometry. The boron dication [2]²⁺ supported by an amido diphosphine pincer ligand features both a large fluoride ion affinity (FIA>SbF₅ ) and hydride ion affinity (HIA>B(C₆ F₅ )₃ ), which qualifies it as both a hard and soft LSA. The unusual Lewis acidic properties of [2]²⁺ are further showcased by its ability to abstract hydride and fluoride from Et₃ SiH and AgSbF₆ respectively, and effectively catalyze the hydrodefluorination, defluorination/arylation, as well as reduction of carbonyl compounds. One and two-electron reduction of [2]²⁺ affords stable boron radical cation [2]⋅⁺ and borylene 2, respectively. The former species has an extremely high spin density of 0.798e at the boron atom, whereas the latter compound has been demonstrated to be a strong organic base (calcd. pK_BH ⁺ (MeCN)=47.4) by both theoretical and experimental assessment. Overall, these results demonstrate the strong ability of geometric constraining to empower the central boron atom.