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

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.

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.

Bonding Nature between Noble Gases and Small Gold Clusters

Noble gases are usually seen as utterly inert, likewise gold, which is typically conceived as the noblest of all metals. While one may expect that noble gases bind to gold via dispersion interactions only, strong bonds can be formed between noble gas atoms and small gold clusters. We combine mass spectrometry, infrared spectroscopy, and density functional theory calculations to address the bonding nature between Au_n⁺ (n ≤ 4) clusters and Ar, Kr, and Xe. We unambiguously determine the geometries and quantitatively uncover the bonding nature in Au_nNg_m⁺ (Ng = Ar, Kr, Xe) complexes. Each Au cluster can form covalent bonds with atop bound noble gas atoms, with strengths that increase with the noble gas atomic radius. This is demonstrated by calculated adsorption energies, Bader electron charges, and analysis of the electron density. The covalent bonding character, however, is limited to the atop-coordinated Ng atoms.

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.