Superelectrophilic Behavior of an Anion Demonstrated by the Spontaneous Binding of Noble Gases to [B12 Cl11 ]<sup/>

Noble Gas Anions: An Overview of Strategies and Bonding Motifs

This review article aims to provide an overview of the strategies employed to prepare noble gas anions under different environments and experimental conditions, and of the bonding motifs typically occurring in these species. Observed systems include anions fixed into synthesized salts, detected in the gas phase or in high-pressure devices. The major role of the theoretical calculations is also highlighted, not only in support of the experiments, but also as effective in predicting still unreported species. The chemistry of noble gas anions overall appears as a varied and rich paint, offering fascinating opportunities for both experimentalists and theoreticians.

Generation and reactivity of the fragment ion [B12I8S(CN)]- in the gas phase and on surfaces

Gaseous fragment ions generated in mass spectrometers may be employed as "building blocks" for the synthesis of novel molecules on surfaces using ion soft-landing. A fundamental understanding of the reactivity of the fragment ions is required to control bond formation of deposited fragments in surface layers. The fragment ion [B12X11]- (X = halogen) is formed by collision-induced dissociation (CID) from the precursor [B12X12]2- dianion. [B12X11]- is highly reactive and ion soft-landing experiments have shown that this ion binds to the alkyl chains of organic molecules on surfaces. In this work we investigate whether specific modifications of the precursor ion affect the chemical properties of the fragment ions to such an extent that attachment to functional groups of organic molecules on surfaces occurs and binding of alkyl chains is prevented. Therefore, a halogen substituent was replaced by a thiocyanate substituent. CID of the precursor [B12I11(SCN)]2- ion preferentially yields the fragment ion [B12I8S(CN)]-, which shows significantly altered reactivity compared to the fragment ions of [B12I12]2-. [B12I8S(CN)]- has a previously unknown structural element, wherein a sulfur atom bridges three boron atoms. Gas-phase reactions with different neutral reactants (cyclohexane, dimethyl sulfide, and dimethyl amine) accompanied by theoretical studies indicate that [B12I8S(CN)]- binds with higher selectivity to functional groups of organic molecules than fragment ions of [B12I12]2- (e.g., [B12I11]- and [B12I9]-). These findings were further confirmed by ion soft-landing experiments, which showed that [B12I8S(CN)]- ions attacked ester groups of adipates and phthalates, whereas [B12I11]- ions only bound to alkyl chains of the same reagents.

Activation of Small Molecules by Modified Dodecaborate Anions

Two of the basic requirements of a good catalyst are that molecules be bound to it with energies intermediate between physisorption and chemisorption and be simultaneously activated in the process. Using density functional theory, we have studied the interaction of small molecules such as H2, O2, N2, CO2, CO, and NH3 with modified dodecaborate anion [B12H12]2-, namely, [B12X11]- and [B12X11]2- (X = H, F, CN). Calculations of the structure, stability, and electronic properties of these species interacting with the above molecules show that they meet the above requirements. In addition, [B12X11]2- (X = F, CN) species are not only more stable than [B12X11]- species but also bind to O2 more strongly than their monoanion counterparts.

Theoretical prediction of donor-acceptor type novel complexes with strong noble gas-boron covalent bond

The experimental identification of NgBeO molecules, followed by the recent theoretical exploration of super-strong NgBO+ (Ng = He-Rn) ions motivated us to investigate the stability of iso-electronic NgBNH+ (Ng = He-Rn) ions using various ab initio-based quantum chemical methods. The hydrogen-like chemical behavior of gold in small clusters and molecules also inspired us to study the nature of the bonding interactions in NgBNAu+ ions compared to that in NgBNH+ ions. The calculated Ng-B bond lengths in the predicted ions have been found to be much lower than the corresponding covalent limits, indicating a covalent Ng-B interaction in both the NgBNH+ and NgBNAu+ ions. In addition, the Ng-B bond dissociation energies are found to be in the range of 136.7-422.8 kJ mol-1 for NgBNH+ and 77.4-319.1 kJ mol-1 for NgBNAu+, implying the stable nature of the predicted ions. Interestingly, the Ng-B bond length (except for Ne) is the lowest reported to date together with the highest He-B and Ne-B binding energies considering all the neutral and cationic complexes containing Ng-B bonding motifs. Moreover, the natural bonding orbital (NBO) and electron density-based atoms-in-molecule (AIM) analysis reveal the covalent nature of the Ng-B bond in the predicted ions. Furthermore, the energy decomposition analysis together with the natural bond orbital in the chemical valence (EDA-NOCV) studies indicate that the orbital interaction energy is the main contributor to the total attraction energy in the Ng-B bonds. All the calculated results indicate the hydrogen-like chemical behavior of gold in the predicted NgBNM+ ions, showing further evidence of the concept of "gold-hydrogen analogy". Also, for comparison, the corresponding Cu and Ag analogs are investigated. All the computed results together with the experimental identification of the NgMX (Ng = Ar-Xe; M = Cu, Ag, Au; X = F, Cl), ArOH+, and NgBeO (Ng = Ar-Xe) systems clearly indicate that it may be possible to prepare and characterize the predicted NgBNM+ ions experimentally using suitable technique(s).

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

New developments in the field of Lewis acidity are highlighted, with the focus of novel Lewis acids and Lewis superacids of group 2, 13, 14, and 15 elements. Several important basics, illustrated by modern examples (classification of Donor-Acceptor (DA) complexes, amphoteric nature of any compound in terms of DA interactions, reorganization energies of main group Lewis acids and the role of the energies of frontier orbitals) are presented and discussed. It is emphasized that the Lewis acidity phenomena are general and play vital role in different areas of chemistry: from weak "atomophilic" interactions to the complexes of Lewis superacids.

Binding Properties of Small Electrophilic Anions [B6 X5 ]- and [B10 X9 ]- (X=Cl, Br, I): Activation of Small Molecules Based on π-Backbonding

Superelectrophilic 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 [B6 X5 ]- and [B10 X9 ]- (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B12 X11 ]- . The positive charge of vacant boron increases from [B6 X5 ]- via [B10 X9 ]- to [B12 X11 ]- , as evidenced by increasing attachment enthalpies towards typical σ-donor molecules (noble gases, H2 O). However, this behavior is reversed for σ-donor-π-acceptor molecules. [B6 Cl5 ]- binds most strongly to N2 and CO, even more strongly than to H2 O. 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 [B6 X5 ]- >[B10 X9 ]- >[B12 X11 ]- . 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 [B6 Cl5 N2 ]- : 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.

Chemical Synthesis with Gaseous Molecular Ions: Harvesting [B12 Br11 N2 ]- from a Mass Spectrometer

Mass spectrometry frequently reveals the existence of transient gas phase ions that have not been synthesized in solution or in bulk. These elusive ions are, therefore, often considered to be primarily of analytical value in fundamental gas phase studies. Here, we provide proof-of-concept that the products of ion-molecule reactions in mass spectrometers may be collected on surfaces to generate condensed matter and thus serve as building blocks to synthesize new compounds. The highly reactive fragment anion [B12 Br11 ]- was generated in a mass spectrometer and converted to [B12 Br11 N2 ]- in the presence of molecular nitrogen followed by its mass-selection and soft-landing on surfaces. The molecular structure of [B12 Br11 N2 ]- , which has not been synthetically obtained before, was confirmed by conventional methods of molecular analysis, including nuclear magnetic resonance and infrared spectroscopy. The [B12 Br11 N2 ]- ion is stable on surfaces and in solution at room temperature, but thermal annealing induces elimination of N2 and provides access to the highly reactive intermediate [B12 Br11 ]- in the condensed phase, which can be further used as a reagent, for example, for electrophilic aromatic substitutions. Thus, isolation of [B12 Br11 N2 ]- expands the repertoire of the available diazo ions that can be employed as versatile intermediates in various chemical transformations.

Strong carbon - noble gas covalent bond and fluxionality in hypercoordinate compounds

Thermodynamic, kinetic, and chemical bonding analysis at the coupled cluster level has been carried out for a series of hypercoordinated carbon compounds with formula CH4Ng2+ (Ng = He-Rn). Results show that these compounds could be stable at room temperature and Born-Oppenheimer molecular dynamics simulations (BOMD) in conjunction with activation energies indicate high kinetic stability. In addition, all chemical bonding descriptors agree with a strong C-Ng covalent bond and a bonding pattern similar to that of CH5+. Finally, BOMD simulations showed that these compounds are fluxional, with a continuous formation/breaking of H-H and C-H bonds. To the best of the authors' knowledge, these results represent the first series of fluxional compounds possessing a covalent bond between a main group element and a noble gas atom.

Catalytic Potential of [B12X11]2- (X = F, Cl, Br, I, CN) Dianions

Dodecaborate anions ([B12H12]2-) and their derivatives where hydrogen atoms are replaced by halogen, pseudohalogen, or superhalogen moieties belong to a class of very stable species, even in the gas phase. Their stability is attributed to Wade's electron counting rule that requires n + 1 pairs of skeletal electrons, n being the number of boron atoms. Consequently, [B12X11]2- (X = H, F, Cl, Br, I, CN) dianions that carry one more electron than needed to satisfy Wade's rule should not be stable, assuming that the rule applies to fragments as well. While this is the case for X = H, we show that [B12X11]2- (X = F, Cl, Br, I, CN) dianions are stable with the second electron in [B12(CN)11]2- bound by as much as 3.17 eV. More importantly, the stability of these dianions is found to have a significant effect on the activation of gas molecules such as CO2 and N2, providing a path toward the development of new catalysts.

Prediction of donor-acceptor-type novel noble gas complexes in the triplet electronic state

Closed-shell noble gas (Ng) compounds in the singlet electronic state have been extensively studied in the past two decades after the revolutionary discovery of ¹HArF molecule. Motivated by the experimental identification of very strong donor-acceptor-type singlet-state Ng complex ¹ArOH⁺, in the present article, for the first time, we report new donor-acceptor-type noble gas complexes in the triplet electronic state (³NgBeN⁺ (Ng = He-Rn)), where most of the Ng-Be bond lengths are smaller than the corresponding covalent limits. The newly proposed complexes are predicted to be stable by various computational tools, including coupled-cluster and multireference-based methods, with strong Ng-Be bonding (40.4-196.2 kJ mol^-1). We have also investigated ³NgBeP⁺ (Ng = He-Rn) complexes for the purpose of comparison. Various computational results, including the structural parameters, bonding energies, vibrational frequencies, and atoms-in-molecule properties suggest that it may be possible to prepare and characterize these triplet state complexes through suitable experimental technique(s).

BH4 Ng+ (Ar-Rn): Viable Compounds with a B-Ng Covalent Bond

In this work, we explore, using high-level calculations, the ability of BH₄ ⁺ to interact with noble gases. The He system is energetically unstable, while the Ne system could only be observed at cryogenic temperatures. In the case of the Ar, Kr and Xe systems, all are energetically stable, even at room temperature. The different chemical bond descriptors reveal a covalent character between B and the noble gas from Ar to Rn. However, this interaction gradually weakens the multicentric bond between the boron atom and the H₂ fragment. Thus, although BH₄ Rn⁺ exhibits a strong covalent bond, it tends to dissociate at room temperature into BH₂ Rn⁺ +H₂ .

Superstrong Chemical Bonding of Noble Gases with Oxidoboron (BO+) and Sulfidoboron (BS+)

Inspired by the overwhelming exploration of noble gas-boron (Ng-B) bond containing chemical compounds, the stability of the Ng bound BY⁺ and AlY⁺ (Y = O and S) has been investigated by using various ab initio based quantum chemical methods. Ng atoms are found to form exceptionally strong bonds with BO⁺ species in the predicted NgBO⁺ (Ng = He-Rn) complexes with remarkably high Ng-B dissociation energies ranging from 138.0 to 462.2 kJ mol^-1 for the He-Rn series. It is the highest ever Ng-B binding energy in conjunction with the smallest Ng-B bond length for any of the cationic species involving a Ng-B bond as reported until today. More importantly, the calculated Ng-B bond lengths have been found to be much lower than the respective covalent limits in both NgBO⁺ and NgBS⁺ ions. The electronegativity difference between O and S atoms has been reflected nicely in the Ng-B and Ng-Al binding energies, which are found to be 91.9-346.5, 9.6-169.2, and 6.8-142.1 kJ mol^-1 in NgBS⁺, NgAlO⁺, and NgAlS⁺, respectively. The strong covalent bonding between Ng and B/Al atoms in the predicted chemical systems has also been supported by the natural bonding orbital (NBO) and electron density based atoms-in-molecule (AIM) analysis. In addition, the energy decomposition analysis (EDA) in combination with the natural bond orbital for chemical valence (NOCV) indicates that the orbital interaction term is the prime contributor to the total attraction energy in the Ng-B and Ng-Al bonds. Furthermore, Ng-B and Ng-Al bonding can be assessed using the donor-acceptor model where the σ-electron donation that takes place from Ng (HOMO) → XY⁺ (LUMO) (X = B and Al; Y = O and S) is the major contributor to the orbital interaction energy. All the computational results along with the very recent experimental observation of ArOH⁺ and NgMX (Ng = Ar-Xe; M = Cu, Ag, Au; X = F, Cl) clearly indicate that it might be possible to synthesize and characterize these superstrong complexes, NgXY⁺ (Ng = He-Rn; X = B and Al; Y = O and S), under suitable experimental technique(s).

ATP Synthase: A Moonlighting Enzyme with Unprecedented Functions

ATP synthase’s intrinsic molecular electrostatic potential (MESP) adds constructively to, and hence reinforces, the chemiosmotic voltage. This ATP synthase voltage represents a new free energy term that appears to have...

Binding of Saturated and Unsaturated C6-Hydrocarbons to the Electrophilic Anion [B12Br11]−: A Systematic Mechanistic Study

The highly reactive gaseous ion [B12Br11]– is a metal-free closed-shell anion which spontaneously forms covalent bonds with hydrocarbon molecules, including alkanes. Herein, we systematically investigate the reaction mechanism for binding...

Anion-Anion Chemistry with Mass-Selected Molecular Fragments on Surfaces

While reactions between ions and neutral molecules in the gas phase have been studied extensively, reactions between molecular ions of same polarity remain relatively unexplored. Herein we show that reactions between fragment ions generated in the gas phase and molecular ions of the same polarity are possible by soft-landing of both reagents on surfaces. The reactive [B₁₂ I₁₁ ]^1- anion was deposited on a surface layer built up by landing the generally unreactive [B₁₂ I₁₂ ]^2- . Ex-situ analysis of the generated material shows that [B₂₄ I₂₃ ]^3- was formed. A computational study shows that the product is metastable in the gas phase, but a charge-balanced environment of a grounded surface may stabilize the triply charged product, as suggested by model calculations. This opens new opportunities for the generation of highly charged clusters using unconventional building blocks from the gas phase.

Stable Noble Gas Compounds Based on Superelectrophilic Anions [B12 (BO)11 ]- and [B12 (OBO)11 ]. Chemphyschem 2021;22:2240-2246. [PMID: 34402158 DOI: 10.1002/cphc.202100391]

Superelectrophilic monoanions [B₁₂ (BO)₁₁ ]^- and [B₁₂ (OBO)₁₁ ]^- , generated from stable dianions [B₁₂ (BO)₁₂ ]^2- and [B₁₂ (OBO)₁₂ ]^2- , show great potential for binding with noble gases (Ngs). The binding energies, quantum theory of atoms in molecules (QTAIM), natural population analysis (NPA), energy decomposition analysis (EDA), and electron localization function (ELF) were carried out to understand the B-Ng bond in [B₁₂ (BO)₁₁ Ng]^- and [B₁₂ (OBO)₁₁ Ng]^- . The calculated results reveal that heavier noble gases (Ar, Kr, and Xe) bind covalently with both [B₁₂ (BO)₁₁ ]^- and [B₁₂ (OBO)₁₁ ]^- with large binding energies, making them potentially feasible to be synthesized. Only [B₁₂ (OBO)₁₁ ]^- could form a covalent bond with helium or neon but the small binding energy of [B₁₂ (OBO)₁₁ He]^- may pose a challenge for its experimental detection.

Measuring Electronic Structure of Multiply Charged Anions to Understand Their Chemistry: A Case Study on Gaseous Polyhedral closo-Borate Dianions

Research on multiply charged anions (MCAs) in the gas phase has been intensively performed during the past decades, mainly to understand fundamental molecular physics phenomena, for example, intramolecular Coulomb repulsion and existence of the repulsive Coulomb barrier. However, the relevance of these investigations with respect to understanding MCAs' chemistry appears often vague. Here, we discuss how insights into the electronic structure obtained from negative ion photoelectron spectroscopy (NIPES) combined with theoretical calculations and collision-induced dissociation can provide a fundamental understanding of the intrinsic chemical reactivity of MCAs and their fragments. This is exemplified in our studies on polyhedral closo-borate dianions [B_nX_n]^2- (n = 6, 10, 11, 12; X = H, F-I, CN) and their fragment ions. For example, the rational design of closo-borate dianions with specific electronic properties is described, which leads to generating highly reactive fragments. Depending on the dianionic precursor, these fragments are tuned to either bind noble gases effectively or activate small molecules like CO and N₂. The intrinsic electronic properties of closo-borate dianions are further compared to their electrochemistry in solutions, revealing solvent effects on the redox potentials. Neutral host molecules such as cyclodextrins are found to bind strongly to [B_nX_n]^2-, and gas phase NIPES provides insights into the intrinsic host-guest interactions. Finally, outlooks including the direct NIPES of molecular fragment ions that cannot be generated in the condensed phase and their utilization in preparative mass spectrometry are discussed.

Binding of noble gas atoms by superhalogens

Because of their closed shells, noble gas (Ng) atoms (Ng = Ne, Ar, Kr, and Xe) seldom take part in chemical reactions, yet finding such mechanisms not only is of scientific interest but also has practical significance. Following a recent work by Mayer et al. [Proc. Natl. Acad. Sci. U. S. A. 116, 8167-8172 (2019)] on the room temperature binding of Ar to a superelectrophilic boron site embedded in a negative ion complex, B₁₂(CN)₁₁ ^-, we have systematically studied the effect of cluster size and terminal ligands on the interaction of Ng by focusing on B₁₂X₁₁(Ng) (X = H, CN, and BO) and B₁₂X₁₀(Ng)₂ (X = CN and BO) whose stabilities are governed by the Wade-Mingos rule and on C₅BX₅(Ng) (X = H, F, and CN) and C₄B₂(CN)₄(Ng)₂ whose stabilities are governed by the Huckel's aromaticity rule. Our conclusion, based on density functional theory, is that both the cluster size and the terminal ligands matter-the interaction between the cluster and the Ng atoms becomes stronger with increasing cluster size and the electron affinity of the terminal ligands. Our studies also led to a counter-intuitive finding-removing multiple terminal ligands can enable electrophilic centers to bind multiple Ng atoms simultaneously without compromising their binding strength.

The Reaction of Hydrogen Halides with Tetrahydroborate Anion and Hexahydro-closo-hexaborate Dianion

The mechanism of the consecutive halogenation of the tetrahydroborate anion [BH₄]⁻ by hydrogen halides (HX, X = F, Cl, Br) and hexahydro-closo-hexaborate dianion [B₆H₆]²⁻ by HCl via electrophile-induced nucleophilic substitution (EINS) was established by ab initio DFT calculations [M06/6-311++G(d,p) and wB97XD/6-311++G(d,p)] in acetonitrile (MeCN), taking into account non-specific solvent effects (SMD model). Successive substitution of H⁻ by X⁻ resulted in increased electron deficiency of borohydrides and changes in the character of boron atoms from nucleophilic to highly electrophilic. This, in turn, increased the tendency of the B–H bond to transfer a proton rather than a hydride ion. Thus, the regularities established suggested that it should be possible to carry out halogenation more selectively with the targeted synthesis of halogen derivatives with a low degree of substitution, by stabilization of H₂ complex, or by carrying out a nucleophilic substitution of B–H bonds activated by interaction with Lewis acids (BL₃).

Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12 X11 ]- (X=F, Cl, Br, I, CN)

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 ν C O and ν N 2 , respectively. Observed shifts of ν C O and ν N 2 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.

Strong Binding of Noble Gases to [B12X11]-: A Theoretical Study

We systematically explore the stability and properties of [B₁₂X₁₁NG]^- adducts resulting from the binding of noble gas atoms to anionic [B₁₂X₁₁]^- clusters in the gas phase of mass spectrometers. [B₁₂X₁₁]^- can be obtained by stripping one X^- off the icosahedral closo-dodecaborate dianion [B₁₂X₁₂]^2-. We study the binding of the noble gas atoms He, Ne, Ar, Kr, and Xe to [B₁₂X₁₁]^- with substituents X = F, Cl, Br, I, and CN. While He cannot be captured by these clusters and Ne only binds at low temperatures, the complexes with the heavier noble gas atoms Ar, Kr, and Xe show appreciable complexation energies and exceed 1 eV at room temperature in the case of [B₁₂(CN)₁₁Xe]^-. The predicted B-NG equilibrium distance in the complexes with Ar, Kr, and Xe is only 0.10-0.25 Å longer than the sum of the covalent radii of the two corresponding atoms, and a significant charge transfer from the noble gas atom to the icosahedral B₁₂ cage is observed.

B12X11(H2)−: exploring the limits of isotopologue selectivity of hydrogen adsorption

We study the isotopologue-selective binding of dihydrogen at the undercoordinated boron site of B₁₂X₁₁⁻ (X = H, F, Cl, Br, I, CN) using ab initio quantum chemistry. With a Gibbs free energy of H₂ attachment reaching up to 80 kJ mol⁻¹ (ΔG at 300 K for X = CN), these sites are even more attractive than most undercoordinated metal centers studied so far. We thus believe that they can serve as an edge case close to the upper limit of isotopologue-selective H₂ adsorption sites. Differences of the zero-point energy of attachment average 5.0 kJ mol⁻¹ between D₂ and H₂ and 2.7 kJ mol⁻¹ between HD and H₂, resulting in hypothetical isotopologue selectivities as high as 2.0 and 1.5, respectively, even at 300 K. Interestingly, even though attachment energies vary substantially according to the chemical nature of X, isotopologue selectivities remain very similar. We find that the H–H activation is so strong that it likely results in the instantaneous heterolytic dissociation of H₂ in all cases (except, possibly, for X = H), highlighting the extremely electrophilic nature of B₁₂X₁₁⁻ despite its negative charge. Unfortunately, this high reactivity also makes B₁₂X₁₁⁻ unsuitable for practical application in the field of dihydrogen isotopologue separation. Thus, this example stresses the two-edged nature of strong H₂ affinity, yielding a higher isotopologue selectivity on the one hand but risking dissociation on the other, and helps define a window of adsorption energies into which a material for selective adsorption near room temperature should ideally fall.

The extreme H₂ affinity of B₁₂X₁₁⁻ gives a glimpse of how higher selectivities in adsorptive isotopologue separation may be achieved.

Theoretical Prediction on the New Types of Noble Gas Containing Anions OBONgO- and OCNNgO- (Ng = He, Ar, Kr and Xe)

The fluorine-less noble gas containing anions OBONgO⁻ and OCNNgO⁻ have been studied by correlated electronic structure calculation and density functional theory. The obtained energetics indicates that for Ng=Kr and Xe, these anions should be kinetically stable at low temperature. The molecular structures and electron density distribution suggests that these anions are stabilized by ion-induced dipole interactions with charges concentrated on the electronegative OBO and OCN groups. The current study shows that in addition to the fluoride ion, polyatomic groups with strong electronic affinities can also form stable noble gas containing anions of the type Y⁻…NgO.

New Perspectives in the Noble Gas Chemistry Opened by Electrophilic Anions

Binding of noble gases (NGs) is commonly considered to be the realm of highly reactive electophiles with cationic or at least non-charged character. Herein, we summarize our latest results evidencing that the incorporation of a strongly electrophilic site within a rigid cage-like anionic structure offers several advantages that facilitate the binding of noble gases and stabilize the formed NG adducts. The anionic superelectrophiles investigated by us are based on the closo-dodecaborate dianion scaffold. The record holder [B₁₂(CN)₁₁]⁻ binds spontaneously almost all members of the NG family, including the very inert argon at room temperature and neon at 50 K in the gas phase of mass spectrometers. In this perspective, we summarize the argumentation for the advantages of anionic electrophiles in binding of noble gases and explain them in detail using several examples. Then we discuss the next steps necessary to obtain a comprehensive understanding of the binding properties of electrophilic anions with NGs. Finally, we discuss the perspective to prepare bulk ionic materials containing NG derivatives of the anionic superelectophiles. In particular, we explore the role of counterions using computational methods and discuss the methodology, which may be used for the actual preparation of such salts.

Synthesis, Electronic Properties and Reactivity of [B12 X11 (NO2 )]2- (X=F-I) Dianions

Nitro‐functionalized undecahalogenated closo‐dodecaborates [B₁₂X₁₁(NO₂)]²⁻ were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X‐diffraction, mass spectrometry, and gas‐phase ion vibrational spectroscopy. The NO₂ substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B₁₂X₁₂]²⁻ (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum‐chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO^.). The homolytic NO^. elimination from the dianion under very soft collisional excitation in gas‐phase ion experiments results in the formation of the radical [B₁₂X₁₁O]^2−.. Theoretical investigations suggest that the loss of NO^. proceeds via the rearrangement product [B₁₂X₁₁(ONO)]²⁻. The O‐bonded nitrosooxy structure is thermodynamically more stable than the N‐bonded nitro structure and its formation by radical recombination of [B₁₂X₁₁O]^2−. and NO^. is demonstrated.

Direct functionalization of C-H bonds by electrophilic anions

Alkanes and [B₁₂X₁₂]^2- (X = Cl, Br) are both stable compounds which are difficult to functionalize. Here we demonstrate the formation of a boron-carbon bond between these substances in a two-step process. Fragmentation of [B₁₂X₁₂]^2- in the gas phase generates highly reactive [B₁₂X₁₁]^- ions which spontaneously react with alkanes. The reaction mechanism was investigated using tandem mass spectrometry and gas-phase vibrational spectroscopy combined with electronic structure calculations. [B₁₂X₁₁]^- reacts by an electrophilic substitution of a proton in an alkane resulting in a B-C bond formation. The product is a dianionic [B₁₂X₁₁C_nH_2n+1]^2- species, to which H⁺ is electrostatically bound. High-flux ion soft landing was performed to codeposit [B₁₂X₁₁]^- and complex organic molecules (phthalates) in thin layers on surfaces. Molecular structure analysis of the product films revealed that C-H functionalization by [B₁₂X₁₁]^- occurred in the presence of other more reactive functional groups. This observation demonstrates the utility of highly reactive fragment ions for selective bond formation processes and may pave the way for the use of gas-phase ion chemistry for the generation of complex molecular structures in the condensed phase.

Properties of gaseous closo-[B6X6]2- dianions (X = Cl, Br, I)

Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B₆X₆]^2- (X = Cl-I) are investigated in direct comparison with their closo-[B₁₂X₁₂]^2- analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B₆X₆]^2- dianions are electronically significantly less stable than the corresponding [B₁₂X₁₂]^2- species. Although [B₆Cl₆]^2- is slightly electronically unstable, [B₆Br₆]^2- and [B₆I₆]^2- are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e^- loss) is observed in CID of [B₆X₆]^2- (X = Cl, Br) but not for [B₆I₆]^2-. Halogenide loss (X^- loss) is common for [B₆X₆]^2- (X = Br, I) and [B₁₂X₁₂]^2- (X = Cl, Br, I). Meanwhile, X˙ loss is only observed for [B₁₂X₁₂]^2- (X = Br, I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e^-, X^- and X˙ loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e^- and X^- losses. A significantly lower RCB for X^- loss than for e^- loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X^- losses are significantly lower for [B₆X₆]^2- than for [B₁₂X₁₂]^2-, while enthalpies for X˙ losses are higher. These observations are consistent with a difference in bond character of the B-X bonds in [B₆X₆]^2- and [B₁₂X₁₂]^2-. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B-X bonds in [B₁₂X₁₂]^2- have a stronger covalent character than in [B₆X₆]^2-, in which X has a stronger halide character.

Superelectrophiles: Recent Advances

Superelectrophiles are reactive species that often carry multiple positive charges. They have been useful in numerous synthetic methods and they often exhibit highly unusual reactivities. Recent advances in superelectrophile chemistry are discussed in this review.

Manipulation of Diatomic Molecules with Oriented External Electric Fields: Linear Correlations in Atomic Properties Lead to Nonlinear Molecular Responses

Oriented external electric fields (OEEFs) have been shown to have great potential in being able to provide unprecedented control of chemical reactions, catalysis, and selectivity with applications ranging from H₂ storage to molecular machines. We report a theoretical study of the atomic origins of molecular changes because of OEEFs since understanding the characteristics of OEEF-induced couplings between atomic and molecular properties is an important step toward comprehensive understanding of the effects of strong external fields on the molecular structure, stability, and reactivity. We focus on the atomic and molecular (bond) properties of a set of homo- (H₂, N₂, O₂, F₂, and Cl₂) and heterodiatomic (HF, HCl, CO, and NO) molecules under intense external electric fields in the context of quantum theory of atoms in molecules (QTAIM). It is shown that the atomic properties (atomic charges, energies, and localization indices) correlate linearly with the field strengths, but molecular properties (bond length, electron density at the bond critical point, and electron delocalization index) exhibit nonlinear responses to the imposed fields. In particular, the changes in the electron density distribution alter the shapes and locations of the zero-flux surfaces, atomic volumes, atomic electron population, and localization/delocalization indices. The topography and topology of the molecular electrostatic potential undergo dramatic changes. External fields also perturb the covalent-polar-ionic characteristic of the studied chemical bonds, hallmarking the impact of electric fields on the stability and reactivity of chemical compounds. The findings are well-rationalized within the framework of the QTAIM and form a coherent conceptual understanding of these effects in prototypical diatomic molecules.

Unprecedented stability enhancement of multiply charged anions through decoration with negative electron affinity noble gases

The present communication reports unprecedented stabilization of multiply charged anion, B12F122-, through insertion of noble gas (Ng) atoms possessing negative electron affinity into B-F bonds, resulting in the formation of stable icosahedral B12Ng12F122-, where the HOMO is stabilized significantly and the binding energy of the second excess electron is increased remarkably. Unprecedented stability enhancement with Ng is attributed to a strong covalent B-Ng bond, increased charge delocalization and increased electrostatic interaction between the oppositely charged centers.

Rational design of an argon-binding superelectrophilic anion

Chemically binding to argon (Ar) at room temperature has remained the privilege of the most reactive electrophiles, all of which are cationic (or even dicationic) in nature. Herein, we report a concept for the rational design of anionic superelectrophiles that are composed of a strong electrophilic center firmly embedded in a negatively charged framework of exceptional stability. To validate our concept, we synthesized the percyano-dodecoborate [B₁₂(CN)₁₂]^2-, the electronically most stable dianion ever investigated experimentally. It serves as a precursor for the generation of the monoanion [B₁₂(CN)₁₁]^-, which indeed spontaneously binds Ar at 298 K. Our mass spectrometric and spectroscopic studies are accompanied by high-level computational investigations including a bonding analysis of the exceptional B-Ar bond. The detection and characterization of this highly reactive, structurally stable anionic superelectrophile starts another chapter in the metal-free activation of particularly inert compounds and elements.

Superelectrophilicity of 1,2-Azaborine: Formation of Xenon and Carbon Monoxide Adducts

The BN analogue of ortho-benzyne, 1,2-azaborine, is shown to bind carbon monoxide and a xenon atom under matrix isolation conditions, demonstrating its strongly Lewis acidic superelectrophilic nature. The Lewis acid-base complexes involving CO and Xe can be cleaved photochemically and reformed by mildly annealing the matrices. The interaction energy of 1,2-azaborine with Xe is 3 kcal mol^-1 according to quantum chemical computations, and is similar to that of the superelectrophilic carbene difluorovinylidene.

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

Cryogenic ion trap vibrational spectroscopy paired with quantum chemistry currently represents the most generally applicable approach for the structural investigation of gaseous cluster ions that are not amenable to direct absorption spectroscopy. Here, we give an overview of the most popular variants of infrared action spectroscopy and describe the advantages of using cryogenic ion traps in combination with messenger tagging and vibrational predissociation spectroscopy. We then highlight a few recent studies that apply this technique to identify highly reactive ionic intermediates and to characterize their reactive sites. We conclude by commenting on future challenges and potential developments in the field.

Quantum chemical prediction of a superelectrophilic dianion and its binding with noble gas atoms

A counterintuitive superelectrophilic dianion with a positive charge as well as lowest occupied molecular orbital (LUMO) localized on free-Be1 in Dianion1 embedded in the negatively charged framework, forms stable [NgBeB₁₁(CN)₁₁]²⁻ compounds.

Properties of perhalogenated {closo-B10} and {closo-B11} multiply charged anions and a critical comparison with {closo-B12} in the gas and the condensed phase

Dependence of electronic properties and reactivity of closo-borates with size and halogen substituent was investigated.

All-metal aromatic cationic palladium triangles can mimic aromatic donor ligands with Lewis acidic cations

We present that cationic rings can act as donor ligands thanks to suitably delocalized metal-metal bonds. This could grant parent complexes with the peculiar properties of aromatic rings that are crafted with main group elements. We assembled Pd nuclei into equilateral mono-cationic triangles with unhindered faces. Like their main group element counterparts and despite their positive charge, these noble-metal rings form stable bonding interactions with other cations, such as positively charged silver atoms, to deliver the corresponding tetranuclear dicationic complexes. Through a mix of modeling and experimental techniques we propose that this bonding mode is an original coordination-like one rather than a 4-centre-2-electron bond, which have already been observed in three dimensional aromatics. The present results thus pave the way for the use of suitable metal rings as ligands.