Syntheses and Structures of F6XeNCCH3and F6Xe(NCCH3)2

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.

Covalent and Non-covalent Noble Gas Bonding Interactions in XeF n Derivatives (n = 2-6): A Combined Theoretical and ICSD Analysis

A noble gas bond (also known in the literature as aerogen bond) can be defined as the attractive interaction between any element of group-18 acting as a Lewis acid and any electron rich atom of group of atoms, thus following the IUPAC recommendation available for similar π,σ-hole interactions involving elements of groups 17 (halogens) and 16 (chalcogens). A significant difference between noble gas bonding (NgB) and halogen (HaB) or chalcogen (ChB) bonding is that whilst the former is scarcely found in the literature, HaB and ChB are very common and their applications in important fields like catalysis, biochemistry or crystal engineering have exponentially grown in the last decade. This article combines theory and experiment to highlight the importance of non-covalent NgBs in the solid state of several xenon fluorides [XeF_n]^m+ were the central oxidation state of Xe varies from +2 to +6 and the number of fluorine atoms varies from n = 2 to 6. The compounds with an odd number of fluorine atoms (n = 3 and 5) are cationic (m = 1). The Inorganic Crystal Structural Database (ICSD) strongly evidences the relevance of NgBs in the solid state structures of xenon derivatives. The ability of Xe compounds to participate in π,σ-hole interactions has been studied using different types of electron donors (Lewis bases and anions) using DFT calculations (PBE1PBE-D3/def2-TZVP) and the molecular electrostatic potential (MEP) surfaces.

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.

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.

Synthesis and Characterization of [XeOXe](2+) in the Adduct-Cation Salt, [CH3 CN- - -XeOXe- - -NCCH3 ][AsF6 ]2

Acetonitrile and [FXeOXe- - -FXeF][AsF6 ] react at -60 °C in anhydrous HF (aHF) to form the CH3 CN adduct of the previously unknown [XeOXe](2+) cation. The low-temperature X-ray structure of [CH3 CN- - -XeOXe- - -NCCH3 ][AsF6 ]2 exhibits a well-isolated adduct-cation that has among the shortest Xe-N distances obtained for an sp-hybridized nitrogen base adducted to xenon. The Raman spectrum was fully assigned by comparison with the calculated vibrational frequencies and with the aid of (18) O-enrichment studies. Natural bond orbital (NBO), atoms in molecules (AIM), electron localization function (ELF), and molecular electrostatic potential surface (MEPS) analyses show that the Xe-O bonds are semi-ionic whereas the Xe-N bonds may be described as strong electrostatic (σ-hole) interactions.

3c/4e -type long-bonding competes with ω-bonding in noble-gas hydrides HNgY (Ng = He, Ne, Ar, Kr, Xe, Rn; Y = F, Cl, Br, I): a NBO/NRT perspective

Novel resonance bonding for the HNgY molecule is demonstrated based on natural resonance theory analyses. Ng/Y affects the ω-bonding vs. long-bonding propensity in each of the HNgY molecules.