Superelectrophilic anions are a class of molecular ions that exhibit highly electrophilic reaction behavior despite their overall negative charge. Thus, they are even able to bind the unreactive noble gases or molecular nitrogen at room temperature.[1] [2] The only representatives known so far are the fragment ions of the type [B<sub>12</sub>X<sub>11</sub>]– derived from the closo-dodecaborate dianions [B<sub>12</sub>X<sub>12</sub>]2–. X represents a substituent connected to a boron atom (cf. Fig. 1). For this reason, the following article deals exclusively with superelectrophilic anions of this type.
Anions are negatively charged ions and therefore usually exhibit nucleophilic reaction behavior. However, it has been shown that there are anions which behave in a strongly electrophilic manner despite their negative charge. This means that they form bonds with reaction partners in chemical reactions by accepting electron density from them. Their affinity for electrons is so great that they are even able to bind very unreactive small molecules such as nitrogen (N2) or noble gas atoms at room temperature. For this reason, they are called "superelectrophilic anions". Furthermore, these superelectrophilic anions allow the direct reaction with small alkanes such as methane.[3] Reactions with other anions are also possible and lead to the formation of highly charged ions.[4]
The only superelectrophilic anions known so far are the fragment ions with the molecular formula [B<sub>12</sub>X<sub>11</sub>]–, which can be generated from the closo-dodecaborate dianions [B<sub>12</sub>X<sub>12</sub>]2–. The X represents a substituent connected to a boron atom, e.g. Cl or Br.
Due to their high reactivity, these fragment ions can so far only be generated in the evacuated gas phase of a mass spectrometer. Therefore, reactions of this class of compounds have been studied mainly in the gas phase. In the condensed phase, reaction products of the superelectrophilic anions were synthesized in small amounts using the ion soft-landing method.
Potential applications of this research include the preparation of exotic compounds (e.g., noble gas compounds) of interest for fundamental chemical research. If syntheses with superelectrophilic anions would become possible on a larger scale, they might be used for applications, for example the development of cancer drugs for the Boron Neutron Capture Therapy (BNCT).[5]
Originally, the term superelectrophilic was used exclusively for dications.
It was introduced by George A. Olah when he discovered that certain electrophilic monocations can be activated in the condensed phase by the presence of particularly strong acids - the so-called superacids. This makes these cations significantly more reactive than they are under normal conditions. He attributed this increased reactivity to the formation of doubly positively charged dications, which he termed "superelectrophilic."[6]
Later, the term superelectrophilic was also frequently applied in gas phase studies to highly reactive dications that can bind noble gases at room temperature. Noble gases are generally considered to be particularly inert - in order to form a stable bond with a noble gas atom, a pair of electrons must be abstracted from it. Only the strongest electrophiles are able to do this, and these are usually dications or, more rarely, monocations,[7] [8] since high electrophilicity is accompanied by a substantial lack of electrons. In contrast, it is impossible for nucleophiles, which provide electrons for bond formation, to bind a noble gas strongly, because noble gases have negative electron affinities. Only weak interactions (ion- induced dipole and dispersion) are usually present, and do not result in a stable bond at room temperature.[9]
Since anions are negatively charged and formally have an electron excess, they generally exhibit nucleophilic reaction behavior and should therefore not be able to form stable bonds to noble gas atoms. However, in direct contrast to this intuitive concept, it was shown in 2017 that the negatively charged gas-phase fragment anion [B<sub>12</sub>Cl<sub>11</sub>]– can bind the noble gases krypton and xenon at room temperature and thus, must be strongly electrophilic. Furthermore, the electrophilicity of this fragment anion could even be increased by exchanging the chlorine atoms (Cl) with cyano groups (CN). The resulting [B<sub>12</sub>(CN)<sub>11</sub>]– anion also spontaneously binds the particularly unreactive noble gas argon at room temperature and neon up to a maximum temperature of 50 Kelvin. Thus, it is the most electrophilic anion known to date.[10]
Even though anions cannot formally fulfill the concept of superelectrophilic published by Olah (which only refers to cations), these particular anions exhibit reactivity that strongly resembles that of superelectrophilic dications in the gas phase. Thus, the term superelectrophilic anions was used.The high reactivity of the [B<sub>12</sub>X<sub>11</sub>]– anions allows not only reactions with noble gases, but also direct functionalization of saturated hydrocarbons.
To elucidate the reaction mechanism, detailed investigations were carried out on the reactivity of [B<sub>12</sub>Br<sub>11</sub>]– and [B<sub>12</sub>Cl<sub>11</sub>]– with methane in the gas phase. The molecular structures were investigated with the aid of IR spectroscopy. It was shown that the electrophilic boron atom reacts with the less polar C–H bond. The carbon represents the partially negative (nucleophilic) part of the bond. A heterolytic bond cleavage of the C–H bond occurs, which formally leads to a carbanion [CH<sub>3</sub>]– and a proton H+. The carbanion binds to the electrophilic boron atom of [B<sub>12</sub>X<sub>11</sub>]–, forming the dianion [B<sub>12</sub>X<sub>11</sub>CH<sub>3</sub>]2–, while the H+ ion remains electrostatically bound to this dianion in the gas phase. The overall singly charged ion [B<sub>12</sub>X<sub>11</sub>CH<sub>3</sub>]2– [H]+ is a Brønsted acid. Upon contact with water, [B<sub>12</sub>X<sub>11</sub>CH<sub>3</sub>]2–[H<sub>3</sub>O] + is formed, which can be detected by the characteristic "umbrella mode" of the coordinated hydronium ion (H3O) + (cf. Figs. 8 and 9). In addition, the fragment ions [B<sub>12</sub>Br<sub>11</sub>]– and [B<sub>12</sub>Cl<sub>11</sub>]– were deposited on surfaces by the ion soft-landing method, where they reacted with the alkyl chains of organic compounds. The attachment to the nonpolar alkyl chains occurred selectively in the presence of much more reactive functional groups such as aromatic and ester groups. This surprising preference for C–H groups as reactants may be related to the fact that hydrophobic alkyl groups are oriented towards the vacuum in such surface layers. The superelectrophilic anions react directly at the vacuum interface with C–H groups of the phthalate alkyl groups according to the mechanism for methane binding shown in Figure 8.