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. The only representatives known so far are the fragment ions of the type derived from the closo-dodecaborate dianions . 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.
Overview
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 () 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. Reactions with other anions are also possible and lead to the formation of highly charged ions.
The only superelectrophilic anions known so far are the fragment ions with the molecular formula , which can be generated from the closo-dodecaborate dianions . 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).
Origin of the concept 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."
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, 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.
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 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 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.
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.
Precursors The superelectrophilic anions can be generated from the very stable closo-dodecaborate dianions by fragmentation in the gas phase.
The closo-dodecaborate dianions, which serve as precursors, consist of twelve boron atoms arranged in a highly symmetrical, cage-like closo structure. All twelve boron atoms carry a substituent (e.g. a halogen atom), which together form the outer shell of the ion (see Fig. 3).
Due to their three-dimensional σ-aromatic structure, closo-dodecaborate dianions are remarkably stable compounds and can be used to stabilize reactive cations (cf. weakly coordinating anions).
ions with the halogens (F, Cl, Br, I) or cyano groups (CN) as substituents (X) have been used as precursors for superelectrophilic anions so far.
Generation To generate the superelectrophilic anions from the closo-dodecaborate dianions , fragmentation is carried out in the mass spectrometer using collision-induced dissociation (CID). In this process, a substituent () is cleaved from the dianion precursor. As a result, only eleven of the twelve boron atoms carry a substituent. One boron atom, on the other hand, is vacant and possesses a free binding site (cf. Figures 1 and 4).
Depending on the nature of the substituents, this fragmentation can either occur directly by CID of the precursor , or by fragmentation of a so-called anionic ion pair , which loses the neutral particle MX using CID. The cation can be, for example, an ion or an alkali cation. Direct fragmentation of the precursor is possible for the chlorine, bromine, and iodine variants (X = Cl, Br, I), while generation of the anionic ion pair is required for the precursor variants with fluorine and cyano groups as substituents (X = F, CN).
Characteristics The vacant bonding site on one of the boron atoms created during fragmentation carries a substantial partial positive charge and is therefore strongly electrophilic, although the overall charge of the ion is negative (cf. Fig. 4). At this vacant boron atom, the lowest unoccupied molecular orbital (LUMO) with negative orbital energy is located and the delocalized σ-electron system is interrupted, as shown by Electron Localization Index (ELI) studies.
The stiff and very stable dodecaborate framework prevents possible intramolecular rearrangement reactions, so that the highly reactive electrophilic center remains intact and chemically accessible.
Reactivity and bond formation
Reactions with noble gases The superelectrophilic anions exhibit special properties that together are very unusual and as a result allow the formation of stable noble gas bonds. These include the strong electrophilicity and structural stability of the anions, which have already been discussed in the Characteristics section. In addition, three further aspects were discussed which may be favourable for bond formation to noble gases:
Favored formation and stabilization of the collision complex The strongly electrophilic (positive) binding site within the overall anionic framework generates an unusual electric field near the reactive boron atom. Polar nucleophiles approaching the anion in the preferred orientation must change their orientation to allow the nucleophilic site to react with the anion. This could result in a significant centrifugal barrier and reduce the number of reactive collisions. This effect has no influence on nonpolar molecules such as noble gases (see Fig. 5). The large molecular framework of these polyatomic cage anions allows the collision energy to be redistributed by the collision of the reactants over the many, especially low-energy, vibrational degrees of freedom, so that the collision complex is long-lived enough to be stabilized by collision cooling.
Substitution protection
After the noble gas has been bound to the boron atom, its substitution by a nucleophile in a typical reaction is prevented by the cage structure of the borate, since the noble gas-boron bond is shielded sterically (cf. Fig. 6).
Favorable electrostatics and dispersion forces In addition to the coordinative noble gas-boron bond, dispersion interactions and electrostatics contribute to the stabilization of the noble gas complex. The electrophilic bonding site is located within a crater defined by five partially negatively charged substituents X, which provide a large interaction region for the formation of attractive London-dispersion forces. The bound noble gas atom, on the other hand, provides electron density to the neighboring boron atom and thus becomes weakly positive, which could play a role in an attractive and stabilizing electrostatic interaction with the surrounding substituents (see Fig. 6).
For the reactions of the individual fragment ion variants with the various noble gases, see also section Variants.
Reactions with diatomic molecules: CO, N2 The binding of the diatomic molecules CO and to superelectrophilic anions was studied using mass spectrometry, infrared photodissociation (IRPD) spectroscopy, and theoretical calculations. CO binds to the vacant boron atom of the electrophilic anion via its nucleophilic carbon. However, in contrast to the reaction with noble gases, the binding does not occur only through a σ-bonding of the nucleophile's CO and . The electrophilic anion pushes electron density into the antibonding π-orbitals of and CO via π-backbonding, which further strengthens the bond to the electrophilic anion.
For the reactions of the individual fragment ion variants with CO and , see also section Variants.
Reactions with saturated hydrocarbons
The high reactivity of the 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 and 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 () and a proton (). The carbanion binds to the electrophilic boron atom of , forming the dianion , while the ion remains electrostatically bound to this dianion in the gas phase. The overall singly-charged ion is a Brønsted acid. Upon contact with water, is formed, which can be detected by the characteristic "umbrella mode" of the coordinated hydronium ion (cf. Figs. 8 and 9). In addition, the fragment ions and 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.