Binding Properties of Small Electrophilic Anions [B6X5]− and [B10X9]− (X=Cl, Br, I): Activation of Small Molecules Based on π‐Backbonding

Abstract

AbstractSuperelectrophilic 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 [B6X5]− and [B10X9]− (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B12X11]−. The positive charge of vacant boron increases from [B6X5]− via [B10X9]− to [B12X11]−, as evidenced by increasing attachment enthalpies towards typical σ‐donor molecules (noble gases, H2O). However, this behavior is reversed for σ‐donor–π‐acceptor molecules. [B6Cl5]− binds most strongly to N2 and CO, even more strongly than to H2O. 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 [B6X5]−>[B10X9]−>[B12X11]−. This predicted order explains the experimentally observed red shifts of the CO and N2 stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B6Cl5N2]−: 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.