Stable Tetra‐ and Penta‐Anions in the Gas Phase

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).

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).

Synergetic ligand and size effects of boron cage based electrolytes in Li-ion batteries

We explore the potential application of boron-based clusters as high-performance electrolytes in lithium-ion batteries using first-principles density functional theory. We use small and halogen-free ligands (such as CN, BO, NH₂, NO₂, and CH₃) to replace H in closo-boron cages with different sizes to investigate the ligand and size effects. According to their geometric and electronic stability, Li⁺ dissociation energy in the lithium salt form, and electrochemical stabilities, we screen nine candidate electrolyte anions potentially overcoming the currently used electrolytes in lithium-ion batteries. We show that, when CH₃ is used as a boron cage ligand, both the highest occupied molecular orbital and the lowest unoccupied molecular orbital levels are high, ensuring their electrochemical stability against the oxidation (or reduction) reaction at the anode (or cathode). Solvent effects are also evaluated and high electrostatic screening was found to be favorable for practical usage.

Metallo-boranes: a class of unconventional superhalogens defying electron counting rules

Superhalogens are a class of highly electronegative atomic clusters whose electron affinities exceed those of halogens. Due to their potential for promoting unusual reactions and role as weakly coordinating anions as well as building blocks of bulk materials, there has been considerable interest in their design and synthesis. Conventional superhalogens are composed of a metal atom surrounded by halogen atoms. Their large electron affinities are due to the fact that the added electron is distributed over all the halogen atoms, reducing electron-electron repulsion. Here, using density functional theory with a hybrid exchange-correlation functional, we show that a new class of superhalogens can be developed by doping closo-boranes (e.g., B₁₂H₁₂) with selected metal atoms such as Zn and Al as well as by replacing a B atom with Be or C. Strikingly, these clusters defy electron counting rules. For example, according to the Wade-Mingos rule, Zn(B₁₂H₁₂) and Al(BeB₁₁H₁₂) are closed-shell systems that should be chemically inert and, hence, should have very small electron affinities. Similarly, Zn(B₁₂H₁₁), Al(B₁₂H₁₂), and Zn(CB₁₁H₁₂), with one electron more than needed for electronic shell closure, should behave like superalkalis. Yet, all these clusters are superhalogens. This unexpected behavior originates from an entirely different mechanism where the added electron resides on the doped metal atom that is positively charged due to electron transfer.

Isolated [B2(CN)6]2-: Small Yet Exceptionally Stable Nonmetal Dianion

We report the observation of a small, yet remarkably stable, metal-free hexacyanodiborate dianion [B₂(CN)₆]^2- in the gas phase. Negative ion photoelectron spectroscopy (NIPES) was employed to measure its spectra at multiple laser wavelengths, yielding a 1.9 eV electron binding energy (EBE) ─a remarkably high value of electronic stability and a ∼2.60 eV repulsive Coulomb barrier (RCB) for electron detachment. This rationalizes the observation of this dianion, although homolytic charge-separation dissociation into two [B(CN)₃]^•- is energetically favorable. Quantum chemical calculations demonstrate a D_3d staggered conformation for both the dianion and radical monoanion, and the calculated EBE and RCB match the experimental values well. The simulated density of states spectrum reproduces all measured electronic transitions, while the simulated vibrational progressions for the ground state transition cover a much narrower EBE range compared to the experimental band, indicating appreciable auto-photodetachment via electronically excited dianion resonances.

Theoretical investigation of substituent effects on the relative stabilities and electronic structure of [BnXn]2- clusters

In this study, we provide a theoretical evaluation of relative stabilities and electronic structure for [B_nX_n]^2- clusters (n = 10, 12, 13, 14, 15, 16). Structural and electronic characteristics of [B_nX_n]^2- clusters are examined by comparison with the [B₁₂X₁₂]^2- counterparts with a focus on the substituent effects (X = H, F, Cl, Br, CN, BO, OH, NH₂) on the electronic structure, electron detachment energies, formation enthalpies, and charge distributions. For the electronic structure and electron detachment energies, substituent effects on boron clusters are shown to follow a very similar trend to the mesomeric and inductive effects (± M and ± I) of π-conjugated systems, and the most stable derivatives in terms of HOMO/LUMO and electron detachment energies are calculated for CN and BO substituents due to strong -M effects. In the case of formation enthalpies for larger boron clusters (n ≥ 13), the icosahedral barrier is shown to increase with the halogen and CN substitution, whereas it is possible to reduce the icosahedral barrier for the cases of X = OH and NH₂. It is shown that this reduction results from destabilizing the [B₁₂X₁₂]^2- cluster with electronic (+ M) and symmetry effects induced by OH and NH₂ ligands.

Realization of the Zn3+ oxidation state

Due to unfilled d-shells, transition metal atoms exhibit multiple oxidation states and rich chemistry. While zinc is often classified as a transition metal, electrons in its filled 3d¹⁰ shell do not participate in chemical reactions; hence, its oxidation state is +2. Using calculations based on density functional theory, we show that the chemistry of zinc can fundamentally change when it is allowed to interact with highly stable super-electrophilic trianions, namely, BeB₁₁(CN)₁₂^3- and BeB₂₃(CN)₂₂^3-, which lie 15.85 eV and 18.49 eV lower in energy than their respective neutral states. The fact that Zn exists in +3 oxidation states while interacting with these moieties is evidenced from its large binding energies of 6.33 and 7.04 eV with BeB₁₁(CN)₁₂^3- and BeB₂₃(CN)₂₂^3-, respectively, and from a comprehensive analysis of its bonding characteristics, charge density distribution, electron localization function, molecular orbitals and energy decomposition, all showing a strong involvement of its 3d electrons in chemical bonding. The replacement of CN with BO is found to increase the zinc binding energy even further.

Super-electrophiles of tri- and tetra-anions stabilized by selected terminal groups and their role in binding noble gas atoms

Stabilization of multiply-charged atomic clusters in the gas phase has been a topic of great interest not only because of their potential applications as weakly-coordinating anions, but also for their ability to promote unusual reactions and serve as building blocks of materials. Recent experiments have shown that, after removing one terminal ligand from the closo-dodecacyano-borate, B12(CN)122-, the cluster can strongly bind an argon atom at room temperature. Bearing this in mind, here, we have developed more than a dozen highly stable tri- and tetra-anions using density functional theory (DFT) calculations with hybrid functional (B3LYP) and semi-empirical dispersion corrections. The interactions between the clusters and noble gas atoms, including Ne, Ar and Kr, are studied. The resulting super-electrophilic sites embedded in these charged clusters can bind noble gas atoms with binding energies up to 0.7 eV. This study enriches the database of highly-charged clusters and provides a viable design rule for super-electrophiles that can strongly bind noble gas atoms.

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.

ThH5 : An Actinide-Containing Superhalogen Molecule

Thorium and its compounds have been widely investigated as important nuclear materials. Previous research focused on the potential use of thorium hydrides, such as ThH₂ , ThH₄ , and Th₄ H₁₅ , as nuclear fuels. Here, we report studies of the anion, ThH₅ ^- , by anion photoelectron spectroscopy and computations. The resulting experimental and theoretical vertical detachment energies (VDE) for ThH₅ ^- are 4.09 eV and 4.11 eV, respectively. These values and the agreement between theory and experiment facilitated the characterization of the structure of the ThH₅ ^- anion and showed its neutral counterpart, ThH₅ to be a superhalogen. ThH₅ ^- , which exhibits a C_4v structure with five Th-H single bonds, possesses the largest known H/M ratio among the actinide elements, M. The adaptive natural density partitioning (AdNDP) method was used to further analyze the chemical bonding of ThH₅ ^- and to confirm the existence of five Th-H single bonds in the ThH₅ ^- molecular anion.

Designing stable closo-B12 dianions in silico for Li- and Mg-ion battery applications

The electronic structures and key factors controlling the stability of [B12(ECX)12]2− were revealed. Their good stability and weak binding property towards Li+ and Mg2+ suggest their potential application in Li- and Mg-ion batteries.

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.

Record-high stability and compactness of multiply-charged clusters aided by selected terminal groups

Multiply-charged clusters with compact sizes that are stable in the gas phase are important due to their potential applications as weakly-coordinating ions and building blocks of bulk materials.

Ligand stabilization of manganocene dianions - in defiance of the 18-electron rule

Manganocene [Mn(C5H5)2], a 17-electron system, is expected to have a high electron affinity, as addition of an extra electron would make it a closed-shell 18-electron system. Surprisingly, it has a very low electron affinity of only 0.28 eV. Combined with its high ionization potential of around 7.0 eV, manganocene, therefore, should not be eager to either donate or accept an electron. We show that this property can be fundamentally altered with the proper choice of ligands, even though the total electron count remains the same. For example, the electron affinities of manganocene-derivatives Mn[C5(CN)5]2 and Mn[C5(BO)5]2, created by replacing H with CN or BO, are found to be 4.78 eV and 4.85 eV, respectively, making these species superhalogens. The power of the ligands is further demonstrated by studying the stability of their di-anions. Note that [Mn(C5X5)2]2- (X = H, CN, BO) di-anions, with 19-electrons, have one electron more than necessary to satisfy the 18-electron rule for stability. This factor, combined with the unavoidable repulsion between the two extra electrons, should destabilize [Mn(C5X5)2]2-. While that is the case for [Mn(C5H5)2]2-, we show that both Mn[C5(CN)5]22- and Mn[C5(BO)5]22- are stable against auto-detachment of the second electron by 0.7 eV and 0.38 eV, respectively. These results, based on first-principles calculations, demonstrate that ligand-manipulation can be used as an effective strategy to design and synthesize new materials with novel and tailored properties.