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

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.

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.

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.

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 .

Gas Phase Reactivity of [Mo6X14]2- Dianions (X = Cl - I)

We investigate collision-induced dissociation (CID) of [Mo₆X₁₄]^2- (X = Cl, Br, I) and the reactivity of fragment ions of these precursors with background gases. Ion mobility measurements and theoretical calculations provide structural information for some of the observed ions. Sequential losses of MoX₂ units dominate the dissociation pathways of [Mo₆Cl₁₄]^2-. Meanwhile, loss of X radicals is the main channel for X = Br and I. Ion mobility measurements and computational investigations indicate minor structural changes in the octahedral Mo₆ unit for [Mo₆I_m]^- (m = 6-13) fragments. We observe that mass spectra obtained using CID substantially vary among mass spectrometers: Specifically, ions with molecular formula [Mo₆X_m(O₂)_n]^- (X = Br and I) are observed as dominant species produced through reactions with O₂ in several mass spectrometers, but also adduct free fragment ions were observed in other instruments, depending on the background conditions. Ion-trap fragmentation combined with theoretical investigations indicates that spontaneous losses of X radicals occur upon binding of O₂ to [Mo₆I_m]^- fragments (m ≤ 12). Theoretical investigations indicate that both oxygen atoms are bound to the vacant sites of the Mo₆ units. This study opens up a new vista to generate and study a large variety of hexanuclear Mo₆X_m(O₂)_n anions.

In silico capture of noble gas atoms with a light atom molecule

Noble gas atoms (Ng = He, Ne, Ar, and Kr) can be captured in silico with a light atom molecule containing only C, H, Si, O, and B atoms. Extensive density functional theory (DFT) calculations on series of peri-substituted scaffolds indicate that confined spaces (voids) capable to energy efficiently encapsulate and bind Ng atoms are accessible by design of a tripodal peri-substituted ligand, namely, [(5-Ph₂B-xan-4-)₃Si]H (xan = xanthene) comprising (after hydride abstraction) four Lewis acidic sites within the cationic structure [(5-Ph₂B-xan-4-)₃Si]⁺. The host (ligand system) thereby provides an adoptive environment for the guest (Ng atom) to accommodate for its particular size. Whereas considerable chemical interactions are detectable between the ligand system and the heavier Ng atoms Kr and Ar in the host guest complex [(5-Ph₂B-xan-4-)₃Si·Ng]⁺, the lighter Ng atoms Ne and He are rather tolerated by the ligand system instead of being chemically bound to it, nicely highlighting the gradual onset of (weak) chemical bonding along the series He to Kr. A variety of real-space bonding indicators (RSBIs) derived from the calculated electron and pair densities provides valuable insight to the situation of an "isolated atom in a molecule" in case of He, uncovering its size and shape, whereas minute charge rearrangements caused by polarization of the outer electron shell of the larger Ng atoms results in formation of polarized interactions for Ar and Kr with non-negligible covalent bond contributions for Kr. The present study shows that noble gas atoms can be trapped by small light-atom molecules without the forceful conditions necessary using cage structures such as fullerenes, boranes and related compounds or by using super-electrophilic sites like [B₁₂(CN)₁₁]^- if the chelating effect of several Lewis acidic sites within one molecule is employed.

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...

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.

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.

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.