Binding Properties of Small Electrophilic Anions [B ₆X ₅] ⁻ and [B ₁₀X ₉] ⁻ (X=Cl, Br, I): Activation of Small Molecules Based on π‐Backbonding

Abstract

Superelectrophilic anions constitute a special class of molecular anions that show strong binding of weak nucleophiles despite their negative charge. In this study, the binding characteristics of smaller gaseous electrophilic anions of the types [B ₆X ₅] ⁻ and [B ₁₀X ₉] ⁻ (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B ₁₂X ₁₁] ⁻. The positive charge of vacant boron increases from [B ₆X ₅] ⁻ via [B ₁₀X ₉] ⁻ to [B ₁₂X ₁₁] ⁻, as evidenced by increasing attachment enthalpies towards typical σ‐donor molecules (noble gases, H ₂O). However, this behavior is reversed for σ‐donor–π‐acceptor molecules. [B ₆Cl ₅] ⁻ binds most strongly to N ₂ and CO, even more strongly than to H ₂O. Energy decomposition analysis confirms that the orbital interaction is responsible for this opposite trend. The extended transition state natural orbitals for chemical valence method shows that the π‐backdonation order is [B ₆X ₅] ⁻>[B ₁₀X ₉] ⁻>[B ₁₂X ₁₁] ⁻. This predicted order explains the experimentally observed red shifts of the CO and N ₂ stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B ₆Cl ₅N ₂] ⁻: 222 cm ⁻¹. Therefore, strong activation of unreactive σ‐donor–π‐acceptor molecules (commonly observed for cationic transition metal complexes) is achieved with metal‐free molecular anions.