Relevance of π‐Backbonding for the Reactivity of Electrophilic Anions [B₁₂X₁₁]⁻ (X=F, Cl, Br, I, CN)

Abstract

Electrophilic anions of type [B₁₂ X₁₁ ]^- posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B₁₂ X₁₁ ]^- with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N₂ as σ-donor-π-acceptors. Temperature-dependent formation of [B₁₂ X₁₁ NG]^- indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B₁₂ X₁₁ ]^- and steric effects. The binding of CO and N₂ to [B₁₂ X₁₁ ]^- is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B₁₂ X₁₁ CO]^- and [B₁₂ X₁₁ N₂ ]^- were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N₂ stretching frequencies <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ν</mml:mi> <mml:mrow><mml:mi>C</mml:mi> <mml:mi>O</mml:mi></mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ν</mml:mi> <mml:msub><mml:mi>N</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> , respectively. Observed shifts of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ν</mml:mi> <mml:mrow><mml:mi>C</mml:mi> <mml:mi>O</mml:mi></mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>ν</mml:mi> <mml:msub><mml:mi>N</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:msub> </mml:math> are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B₁₂ X₁₁ ]^- reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions.