Syntheses and Structures of Xenon Trioxide Alkylnitrile Adducts

Size Matters: Computational Insights into the Crowning of Noble Gas Trioxides

In pursuit of enhancing the stability of the highly explosive and shock-sensitive compound XeO3, we performed quantum chemical calculations to investigate its possible complexation with electron-rich crown ethers, including 9-crown-3, 12-crown-4, 15-crown-5, 18-crown-6, and 21-crown-7, as well as their thio analogues. Furthermore, we expanded our study to other noble gas trioxides (NgO3), namely, KrO3 and ArO3. The basis set superposition error (BSSE) corrected interaction energies for these adducts range from -13.0 kcal/mol to -48.2 kcal/mol, which is notably high for σ-hole-mediated noncovalent interactions. The formation of these adducts was observed to be more favorable with the increase in the ring size of the crowns and less favorable while going from XeO3 to ArO3. A comprehensive analysis by various computational tools such as the mapping of the electrostatic potential (ESP), Wiberg bond indices (WBIs), Bader's theory of atoms-in-molecules (AIM), natural bond orbital (NBO) analysis, noncovalent interaction (NCI) plots, and energy decomposition analysis (EDA) revealed that the C-H···O interactions, as well as dispersion interactions, play a pivotal role in stabilizing adducts involving larger crowns. A noteworthy outcome of our study is the revelation of a coordination number of 9 for xenon in the complex formed between XeO3 and the thio analogue of 18-crown-6, which is higher than the largest number reported to date.

On the Importance of σ–Hole Interactions in Crystal Structures

Elements from groups 14–18 and periods 3–6 commonly behave as Lewis acids, which are involved in directional noncovalent interactions (NCI) with electron-rich species (lone pair donors), π systems (aromatic rings, triple and double bonds) as well as nonnucleophilic anions (BF4−, PF6−, ClO4−, etc.). Moreover, elements of groups 15 to 17 are also able to act as Lewis bases (from one to three available lone pairs, respectively), thus presenting a dual character. These emerging NCIs where the main group element behaves as Lewis base, belong to the σ–hole family of interactions. Particularly (i) tetrel bonding for elements belonging to group 14, (ii) pnictogen bonding for group 15, (iii) chalcogen bonding for group 16, (iv) halogen bonding for group 17, and (v) noble gas bondings for group 18. In general, σ–hole interactions exhibit different features when moving along the same group (offering larger and more positive σ–holes) or the same row (presenting a different number of available σ–holes and directionality) of the periodic table. This is illustrated in this review by using several examples retrieved from the Cambridge Structural Database (CSD), especially focused on σ–hole interactions, complemented with molecular electrostatic potential surfaces of model systems.

Noble Gases in Solid Compounds Show a Rich Display of Chemistry With Enough Pressure

In this review, we summarize the rapid progress that has been made in the study of noble gas chemistry in solid compounds under high pressure. Thanks to the recent development of first-principles crystal structure search methods, many new noble gas compounds have been predicted and some have been synthesized. Strikingly, almost all types of chemical roles and interactions are found or predicted in these high-pressure noble gas compounds, ranging from cationic and anionic noble gases to covalent bonds between noble gas atoms, and to hydrogen bond-like noble gas bonds. Besides, the recently discovered He insertion reactions reveal a unique chemical force that displays no local chemical bonding, providing evidence that research into noble gas reactions can advance the frontier of chemistry at the very basic level.

Noble Gas Bonding Interactions Involving Xenon Oxides and Fluorides

Noble gas (or aerogen) bond (NgB) can be outlined as the attractive interaction between an electron-rich atom or group of atoms and any element of Group-18 acting as an electron acceptor. The IUPAC already recommended systematic nomenclature for the interactions of groups 17 and 16 (halogen and chalcogen bonds, respectively). Investigations dealing with noncovalent interactions involving main group elements (acting as Lewis acids) have rapidly grown in recent years. They are becoming acting players in essential fields such as crystal engineering, supramolecular chemistry, and catalysis. For obvious reasons, the works devoted to the study of noncovalent Ng-bonding interactions are significantly less abundant than halogen, chalcogen, pnictogen, and tetrel bonding. Nevertheless, in this short review, relevant theoretical and experimental investigations on noncovalent interactions involving Xenon are emphasized. Several theoretical works have described the physical nature of NgB and their interplay with other noncovalent interactions, which are discussed herein. Moreover, exploring the Cambridge Structural Database (CSD) and Inorganic Crystal Structure Database (ICSD), it is demonstrated that NgB interactions are crucial in governing the X-ray packing of xenon derivatives. Concretely, special attention is given to xenon fluorides and xenon oxides, since they exhibit a strong tendency to establish NgBs.

Not Only Hydrogen Bonds: Other Noncovalent Interactions

In this review, we provide a consistent description of noncovalent interactions, covering most groups of the Periodic Table. Different types of bonds are discussed using their trivial names. Moreover, the new name “Spodium bonds” is proposed for group 12 since noncovalent interactions involving this group of elements as electron acceptors have not yet been named. Excluding hydrogen bonds, the following noncovalent interactions will be discussed: alkali, alkaline earth, regium, spodium, triel, tetrel, pnictogen, chalcogen, halogen, and aerogen, which almost covers the Periodic Table entirely. Other interactions, such as orthogonal interactions and π-π stacking, will also be considered. Research and applications of σ-hole and π-hole interactions involving the p-block element is growing exponentially. The important applications include supramolecular chemistry, crystal engineering, catalysis, enzymatic chemistry molecular machines, membrane ion transport, etc. Despite the fact that this review is not intended to be comprehensive, a number of representative works for each type of interaction is provided. The possibility of modeling the dissociation energies of the complexes using different models (HSAB, ECW, Alkorta-Legon) was analyzed. Finally, the extension of Cahn-Ingold-Prelog priority rules to noncovalent is proposed.

Xenon Trioxide Adducts of O-Donor Ligands; [(CH3 )2 CO]3 XeO3 , [(CH3 )2 SO]3 (XeO3 )2 , (C5 H5 NO)3 (XeO3 )2 , and [(C6 H5 )3 PO]2 XeO3

Xenon trioxide (XeO₃ ) forms adducts with triphenylphosphine oxide, dimethylsulfoxide, pyridine-N-oxide, and acetone by coordination of the ligand oxygen atoms to the Xe^VI atom of XeO₃ . The crystalline adducts were characterized by low-temperature, single-crystal X-ray diffraction, and Raman spectroscopy. Unlike solid XeO₃ , which detonates when mechanically or thermally shocked, solid (C₅ H₅ NO)₃ (XeO₃ )₂ , [(C₆ H₅ )₃ PO]₂ XeO₃ , and [(CH₃ )₂ SO]₃ (XeO₃ )₂ are insensitive to mechanical shock. The [(CH₃ )₂ SO]₃ (XeO₃ )₂ adduct slowly decomposes over several days to (CH₃ )₂ SO₂ , Xe, and O₂ . All three complexes undergo rapid deflagration when ignited by a flame. Both [(C₆ H₅ )₃ PO]₂ XeO₃ and (C₅ H₅ NO)₃ (XeO₃ )₂ are room-temperature stable and the [(CH₃ )₂ CO]₃ XeO₃ complex dissociates at room temperature to form a stable solution of XeO₃ in acetone. The xenon coordination sphere of [(C₆ H₅ )₃ PO]₂ XeO₃ , a distorted square-pyramid, provides the first example of a five-coordinate XeO₃ complex with only two Xe- - -O adduct bonds. The xenon coordination spheres of the remaining adducts are distorted octahedra, comprised of three Xe- - -O secondary bonds that are approximately trans to the primary Xe-O bonds of XeO₃ . Quantum-chemical calculations were used to assess the nature of the Xe- - -O adduct bonds, which are described as predominantly electrostatic bonds between the nucleophilic oxygen atoms of the bases and the σ-holes of the electrophilic xenon atoms.

A Stable Crown Ether Complex with a Noble-Gas Compound

Crown ethers have been known for over 50 years, but no example of a complex between a noble-gas compound and a crown ether or another polydentate ligand had previously been reported. Xenon trioxide is shown to react with 15-crown-5 to form the kinetically stable (CH₂ CH₂ O)₅ XeO₃ adduct, which, in marked contrast with solid XeO₃ , does not detonate when mechanically shocked. The crystal structure shows that the five oxygen atoms of the crown ether are coordinated to the xenon atom of XeO₃ . The gas-phase Wiberg bond valences and indices and the empirical bond valences indicate that the Xe- - -O_crown bonds are predominantly electrostatic and are consistent with σ-hole bonding. Mappings of the electrostatic potential (EP) onto the Hirshfeld surfaces of XeO₃ and 15-crown-5 in (CH₂ CH₂ O)₅ XeO₃ and a detailed examination of the molecular electrostatic potential surface (MEPS) of XeO₃ and (CH₂ CH₂ O)₅ reveal regions of negative EP on the oxygen atoms of (CH₂ CH₂ O)₅ and regions of high positive EP on the xenon atom, which are also in accordance with σ-hole interactions.

Matrix-Isolation and Quantum-Chemical Analysis of the C3v Conformer of XeF6, XeOF4, and Their Acetonitrile Adducts

A joint experimental-computational study of the molecular structure and vibrational spectra of the XeF₆ molecule is reported. The vibrational frequencies, intensities, and in particular the isotopic frequency shifts of the vibrational spectra for ¹²⁹XeF₆ and ¹³⁶XeF₆ isotopologues recorded in the neon matrix agree very well with those obtained from relativistic coupled-cluster calculations for XeF₆ in the C_3v structure, thereby strongly supporting the observation of the C_3v conformer of the XeF₆ molecule in the neon matrix. A C_3v transition state connecting the C_3v and O_h local minima is located computationally. The calculated barrier of 220 cm^-1 between the C_3v minima and the transition state corroborates the experimental observation of the C_3v conformer and the absence of the O_h conformer in solid noble gas matrices. For comparison matrix-isolation spectra have also been recorded and analyzed for the ¹²⁹XeOF₄ and the ¹³⁶XeOF₄ isotopologues. The matrix-isolation complexation shifts obtained for the XeF₆·NCCH₃ relative to those of free matrix isolated XeF₆ and CH₃CN are in good agreement with those reported for crystalline XeF₆·NCCH₃.

Noble gas bond and the behaviour of XeO3under pressure

The covalent Xe–O bond lengths in XeO₃are elongated upon increasing the pressure, which is similar to the change observed with hydrogen bonds under pressure. Moreover, XeO₃rearranges in a highly-ordered manner by O hopping at about 2 GPa, which is analogous to the proton hopping observed among hydrogen bonds.

How to Twist, Split and Warp a σ-Hole with Hypervalent Halogens

Halogen bonds (XB) are no longer newcomers in the chemistry family. However, XB in hypervalent halogens has not been thoroughly studied. We provide a molecular orbital explanation of the shape and strength of XBs in hypervalent halogens and other species, focusing on the charge transfer and electrostatic aspects of these bonds. Our results show that σ-holes (and subsequently the XBs associated with them) can be easily divided and bent by the influence of equatorial substituents. The inductive effect of both the equatorial and axial groups can affect these distortions, but also the angle between the equatorial ligands has a large influence on the shape of the σ-holes and the molecular orbitals acting as electron acceptor. Although the observation of these warped XB can be hindered by other noncovalent interactions, they may be ubiquitous in crystal structures of hypervalent species, where multiple XB can appear as secondary interactions on each halogen. We propose what can be considered the archetypal hypervalent halogen donor (a pincer type iodosodilactone) and a Lewis dot structure that includes the σ-holes.