Geometrically Constrained Organoboron Species as Lewis Superacids and Organic Superbases.

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>SbF ₅ ) and hydride ion affinity (HIA>B(C ₆ F ₅ ) 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 Et ₃ SiH and AgSbF ₆ 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.
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