Gas-phase stability of derivatives of the closo-hexaborate dianion B(6)H(6)(2-)

Probing the Electronic Structure of [B10H10]2- Dianion Encapsulated by an Octamethylcalix[4]pyrrole Molecule

Despite being an important closo-borate in condensed phase boron chemistry, isolated [B10H10]2- is electronically unstable and has never been detected in the gas phase. Herein, we report a successful capture of this fleeting species through binding with an octamethylcalix[4]pyrrole (omC4P) molecule to form a stable gaseous omC4P·[B10H10]2- complex and its characterizations utilizing negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum, contributed by both omC4P and [B10H10]2-, is deconvoluted by subtracting the omC4P contribution to yield a [B10H10]2- spectrum. The obtained [B10H10]2- spectrum consists of four major bands spanning the electron binding energy (EBE) range from 1 to 5 eV, with the EBE gaps matching excellently with the energy intervals of computed high-lying occupied molecular orbitals of the [B10H10]2- dianion. This study showcases a generic method to utilize omC4P to capture unstable multiply charged anions in the gas phase for experimental determination of their electronic structures.

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.

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.

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.

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.

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.

Improvement in hydrogen binding ability of closo-dicarboranes via functionalization and designing of extended frameworks

Neutral closo-dicarboboranes are reported to have very low H₂ binding ability. Herein, we report an improvement in H₂ binding energy (E_b) of C₂B₄H₆ by substituting H atoms with different functional groups like X = F, Cl, Br, and XY = BO, CN and NC via quantum-chemical density functional theory based computations. In going from B₆H₆^2- to C₂B₄H₆, the E_b value is reduced from 14.6 kJ mol^-1 to 2.7 kJ mol^-1. C₂B₄X₆ and C₂B₄(XY)₆ systems, which can bind a total of eight H₂ molecules, with one H₂ molecule occupying at each B-B-C face, possess an E_b value per H₂ in the range of 4.5 kJ mol^-1 for X = F, 3.9 kJ mol^-1 for X = Cl, 5.9 kJ mol^-1 for X = Br, 6.8 kJ mol^-1 for XY = BO, 5.8 kJ mol^-1 for XY = CN and 5.2 kJ mol^-1 for XY = NC. The improvement in E_b value is found to be the highest in case of C₂B₄(BO)₆, which has the ability to bind 6.6 gravimetric wt% of H₂. The situation can be made more favorable by applying an external electric field. Energy decomposition analysis reveals that although the dispersion interaction (ca. 55-65%) has significant role in binding H₂ with such types of molecules, contribution from electrostatic and orbital interaction is also considerable. Further, we modeled an extended system by linking C₂B₄(BO)_n through 'C ≡ C' units for H₂ storage purpose. The energy difference between the highest occupied and the lowest unoccupied molecular orbitals gradually lessens with the increase in molecular length. Therefore, it can be tuned gradually by controlling the chain length, which may further open up their potency in the field of electronics. Graphical abstract C₂B₄X₆ (X = F, Cl, Br) and C₂B₄(XY)₆ (XY = BO, CN, NC) show enhanced H₂ binding ability from C₂B₄H₆. Further, 1D, 2D and 3-D frameworks can be built by joining C₂B₄(BO)_n units via 'C ≡ C' linkage.

Rational Design of Stable Dianions by Functionalizing Polycyclic Aromatic Hydrocarbons

Using density functional theory, we have carried out a systematic study of the stability and electronic properties of neutral and multiply charged molecules B_n C_10-n X₈ (n=0, 1, 2; X=H, F, CN). Our main objective is to explore if the replacements of core C atoms and/or H atoms in naphthalene (C₁₀ H₈ ) can enhance the stability of their dianions. Indeed, we find that the dianions of B_n C_10-n (CN)₈ are more stable than their monoanions with energies of 0.61 eV, 0.57 eV, and 1.97 eV for n=0, 1, 2, respectively. In addition, polycyclic aromatic hydrocarbons become stable as dianions only when H atoms are substituted by more electronegative species. Thus, a rational design approach by tailoring composition and ligands can lead to a new class of organic molecules that are capable of carrying multiple charges.

Deciphering chemical bonding in BnHn2−(n = 2–17): flexible multicenter bonding

Deciphering flexible multicenter bonding incloso-borane dianions B_nH_n²⁻.

Role of ligands in the stability of BnXn and CBn−1Xn (n = 5–10; X = H, F, CN) and their potential as building blocks of electrolytes in lithium ion batteries

We predict a series of boron-cage-based stable (di-)anions, and demonstrate them to be high-performance electrolytes in Li-ion batteries.

Open Shell Dianions Likely To Be Stable in the Gas Phase with Respect to Autoionization

We address the challenge set by Dreuw and Cederbaum [Dreuw, A.; Cederbaum, L. S. Chem. Rev. 2002, 102, 181-200] to develop a set of "small" open shell stable dianions. We offer two families of such species, based on a central diradical center with attached anionic sites. Both families achieve dianion stabilization through alternating zones of positive and negative charge. First, quasi-linear systems such as NB(C2)n-Q-(C2)nBN become diradical dianions stable to autoionization in two cases:  (a) for Q a divalent (carbene) carbon when n is two or greater and (b) for Q a C4 ring diradical when n is one or greater. Second, carbenes with certain anionic aromatic substituents can be stable with respect to autoionization. π-Acid substituted carbenes (A2Q) have triplet ground states generally. If A is cyclopentadienyl anion stabilized by cyano substitution, the resulting triplet dianion is stable with respect to autoionization. In bisphenyl carbenes the triplet is relatively stabilized when ortho substituents force the system toward D2d symmetry. The dianion of bis(orthochlorophenyl) carbene produced by para-substitution with BN retains the triplet preference and is stable with respect to autoionization. These results obtained first by density functional calculations in a small basis, B3LYP/6-31G(d), are confirmed and extended by ROMP2 and CCSD calculations in a more flexible basis, 6-31+G(d). DFT has a systematic bias against systems with excess electrons, which is paradoxically a virtue in the screening of candidate dianions since the DFT calculation does not make false predictions of stability.

Influence of delocalization on the stability of dianions: study of a systematic series of dianions with growing electronic localization

The electronic stability of a dianion is influenced by the degree of delocalization of its electrons, but it is generally not possible to separate this influence from other effects. Here, we investigate by theoretical means the sequence of dianions consisting of phen-1,4-ylenbis(ethynide) and seven of its derivatives obtained by hydrogenating the benzene core in several steps. These dianions are structurally similar and mainly differ by the degree of delocalization of their electrons. We present geometries and electron detachment energies computed at a correlated level of theory. The results point to a classification of the eight dianions in three distinct groups of electronic stability. We are able to explain this grouping by a simple resonance structure picture, which demonstrates why the dianions with more delocalized electrons are less stable.

Dianionic tetraborates do exist as stable entities

To date, B(6)H(6)(2-) and some of its derivatives are the smallest members of the closo-borates that have been synthesized and analyzed in condensed phases. In contrast, no stable dianionic tetraborate has yet been observed, either in solution or solids or in the gas phase. In this work, the gas-phase stability of dianionic tetraborates B(4)X(4)(2-) (X = H, CN, NC, or BO) is investigated with ab initio methods. For this objective, the geometries of the dianions are optimized, the electronic stability is tested, and various fragmentation channels are studied. In agreement with previous examinations, tetrahedral isomers of all examined tetraborates have been found to represent geometrically stable isomers and to exhibit a triplet electronic ground state. However, these isomers are electronically unstable, i.e., their additional electrons are not bound. Furthermore, new D(2)(d)()-symmetric isomers of B(4)X(4)(2-) (X = H, CN, NC, or BO) have been identified that have a closed-shell singlet ground state and are lower in energy than their tetrahedral counterparts. Moreover, B(4)(CN)(4)(2-) and B(4)(BO)(4)(2-) represent stable gas-phase dianions and are predicted to be observable in suitable experiments. The electronic properties and geometries of these dianions are discussed in detail and explained in terms of the electrostatic repulsion of the excess electrons and the aromaticity of the dianions.