Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B₁₂Cl₁₁]⁻

Abstract

It is common and chemically intuitive to assign cations electrophilic and anions nucleophilic reactivity, respectively. Herein, we demonstrate a striking violation of this concept: The anion [B Cl ] spontaneously binds to the noble gases (Ngs) xenon and krypton at room temperature in a reaction that is typical of "superelectrophilic" dications. [B Cl Ng] adducts, with Ng binding energies of 80 to 100 kJ mol , contain B-Ng bonds with a substantial degree of covalent interaction. The electrophilic nature of the [B Cl ] anion is confirmed spectroscopically by the observation of a blue shift of the CO stretching mode in the IR spectrum of [B Cl CO] and theoretically by investigation of its electronic structure. The orientation of the electric field at the reactive site of [B Cl ] results in an energy barrier for the approach of polar molecules and facilitates the formation of Ng adducts that are not detected with reactive cations such as [C H ] . This introduces the new chemical concept of "dipole-discriminating electrophilic anions."

Identification of Active Sites and Structural Characterization of Reactive Ionic Intermediates by Cryogenic Ion Trap Vibrational Spectroscopy

“…Warneke and co‐workers recently probed this concept using ion trap mass spectrometry and cryogenic ion trap vibrational spectroscopy . They formed [B 12 Cl 11 ] − by collision‐induced dissociation of the dianion [B 12 Cl 12 ] 2− and subsequently isolated it in a gas‐filled ion trap, where it was allowed to react for 100 ms with Xe, Kr, and Ar at room temperature.…”

“…The estimated electric field strength that acts on the CO molecule is derived from the experimental and theoretical vibrational frequencies (see Ref. ).…”

“…With respect to a non‐polar nucleophile, [B 12 Cl 11 ] − reacts like a superelectrophile in its binding of a noble gas, while its reactivity towards water (usually considered to be a stronger nucleophile than N 2 ) is reduced. Such anions should therefore be classified as “dipole‐discriminating electrophilic anions” rather than as typical superelectrophiles …”

“…Warneke and co-workersr ecently probed this concept using ion trap mass spectrometry and cryogenic ion trap vibrational spectroscopy. [116] They formed [B 12 Cl 11 ] À by collision-induced dissociation of the dianion [B 12 Cl 12 ] 2À andsubsequently isolated it in ag as-filled ion trap, where it was allowed to reactf or 100 ms with Xe, Kr,a nd Ar at room temperature. While [B 12 Cl 11 ] À was unreactive towards Ar,i ts pontaneously formed the Xe and even Kr adducts [B 12 Cl 11 Xe] À and[ B 12 Cl 11 Kr] À ,r espectively (see Figure 9).…”

“…Adapted from Ref. [116].C opyright Wiley-VCH. [ [115] but no reaction with noble gases occursu ndert he same conditions( see text).Adapted from Ref.…”