Noble-Gas Chemistry More Than Half a Century After the First Report of the Noble-Gas Compound

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Review Noble-Gas Chemistry More than Half a Century after the First Report of the Noble-Gas Compound

Zoran Mazej Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova cesta 39, SI–1000 Ljubljana, Slovenia; [email protected]; Tel.: +386-1-477-3301

Academic Editor: Felice Grandinetti Received: 8 June 2020; Accepted: 30 June 2020; Published: 1 July 2020 Abstract: Recent development in the synthesis and characterization of noble-gas compounds is reviewed, i.e., noble-gas chemistry reported in the last five years with emphasis on the publications issued after 2017. XeF2 is commercially available and has a wider practical application both in the laboratory use and in the industry. As a ligand it can coordinate to metal centers resulting n+ in [M(XeF2)x] salts. With strong Lewis acids, XeF2 acts as a fluoride ion donor forming [XeF]+ or [Xe F ]+ salts. Latest examples are [Xe F ][RuF ] XeF , [Xe F ][RuF ] and [Xe F ][IrF ]. 2 3 2 3 6 · 2 2 3 6 2 3 6 Adducts NgF CrOF and NgF 2CrOF (Ng = Xe, Kr) were synthesized and structurally characterized 2· 4 2· 4 at low temperatures. The geometry of XeF6 was studied in solid argon and neon matrices. + + Xenon hexafluoride is a well-known fluoride ion donor forming various [XeF5] and [Xe2F11] salts. + + A large number of crystal structures of previously known or new [XeF5] and [Xe2F11] salts were reported, i.e., [Xe2F11][SbF6], [XeF5][SbF6], [XeF5][Sb2F11], [XeF5][BF4], [XeF5][TiF5], [XeF5]5[Ti10F45], [XeF5][Ti3F13], [XeF5]2[MnF6], [XeF5][MnF5], [XeF5]4[Mn8F36], [Xe2F11]2[SnF6], [Xe2F11]2[PbF6], [XeF ] [Sn F ], [XeF ][Xe F ][CrVOF ] 2CrVIOF , [XeF ] [CrIVF ] 2CrVIOF , [Xe F ] [CrIVF ], 5 4 5 24 5 2 11 5 · 4 5 2 6 · 4 2 11 2 6 [XeF ] [CrV O F ], [XeF ] [CrV O F ] 2HF, [XeF ] [CrV O F ] 2XeOF , A[XeF ][SbF ] (A = Rb, Cs), 5 2 2 2 8 5 2 2 2 8 · 5 2 2 2 8 · 4 5 6 2 Cs[XeF5][BixSb1-xF6]2 (x = ~0.37–0.39), NO2XeF5(SbF6)2, XeF5M(SbF6)3 (M = Ni, Mg, Zn, Co, Cu, Mn and Pd) and (XeF5)3[Hg(HF)]2(SbF6)7. Despite its extreme sensitivity, many new XeO3 adducts were synthesized, i.e., the 15-crown adduct of XeO3, adducts of XeO3 with triphenylphosphine oxide, dimethylsulfoxide and pyridine-N-oxide, and adducts between XeO3 and N-bases (pyridine and 4-dimethylaminopyridine). [Hg(KrF2)8][AsF6]2 2HF is a new example of a compound in which KrF2 · + + serves as a ligand. Numerous new charged species of noble gases were reported (ArCH2 , ArOH , + + + + + [ArB3O4] , [ArB3O5] , [ArB4O6] , [ArB5O7] , [B12(CN)11Ne]−). Molecular ion HeH was finally detected in interstellar space. The discoveries of Na2He and ArNi at high pressure were reported. Bonding motifs in noble-gas compounds are briefly commented on in the last paragraph of this review.

Keywords: helium; neon; argon; krypton; xenon; noble gases; ﬂuorides; oxides

1. Introduction After almost 20 years, when the Christe’s paper “A renaissance in noble gas chemistry” appeared [1], we can say that the renaissance still lasts, although to a lesser extent than in the past. Times are changing, and with that, the fields of research in science. However, new topics are also present in noble-gas (Ng) chemistry. Beside classical Ng chemistry, which includes the syntheses of larger quantities of noble gas compounds (bulk-phase compounds), nowadays there are many other kinds of studies connected with the identification and characterization of molecules of Ng compounds. They include the preparations of Ng compounds in cold matrices, syntheses in liquid and supercritical noble gases, formations of Ng compounds under high pressures, and syntheses of neutral and gas-ion compounds in the gas phase [2]. Lastly, we should not forget to mention a large number of theoretical papers predicting new Ng compounds, which still need to be experimentally confirmed.

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The last extensive review of Ng compounds was published in 2018 [2]. It gives a short overview of bulk-phase compounds, molecules of Ng compounds formed in cold matrices, molecules formed in liquid and supercritical noble gases, and the formation of Ng compounds under high pressures. Chapters that are more extensive are related to theoretical chemistry and to gas-phase chemistry of the noble gases. Before that reviews were given by Haner and Schrobilgen in 2015 (xenon(IV) compounds) [3], Nabiev et al. in 2014 (structures of Ng compounds) [4], Brock et al. in 2013 (general about Ng compounds) [5], and Hope in 2013 (coordination chemistry of Ng compounds) [6]. An extensive list of older references of Ng chemistry reviews can be found in the list of references given in the works of Saha et al. from 2019 [7], Grochala from 2018 [8], and Grandinetti from 2018 [2]. The present review is related to the synthetic Ng chemistry of some noble-gas compounds reported in the last ﬁve years, with emphasis on the examples reported after 2017, and few studies published before that, which were not mentioned in a 2018 review [2]. Since the present contribution is related to experimental synthesis, pure theoretical papers that predict new Ng compounds are not included. Concerning their number, a separate contribution can be written.

2. Xenon Chemistry of xenon represents the field with the largest number of synthesized Ng compounds. Xenon(II) fluoride is the only Ng compound that is commercially available and has a wider practical application both in the laboratory use and in the industry. Its main use is in its ability of fluorination of various organic compounds [9,10] and as an etching reagent for surfaces of various metals, oxides, nitrides, etc. [11–16]. It is also used for the preparation of fluorographenes [17–19]. Its applicability for low-temperature insertion of fluorine into oxides systems has been demonstrated with the purpose of modification of magnetic and electronic properties, in particular superconductivity [20]. The 18F labeled XeF2 is used in nuclear medicine. This includes imaging techniques such as positron emission tomography (PET) as an 18F source [21]. Xenon is shown to bind to other molecules not only under matrix isolation (Lewis acid-base complex of 1,2-azaborine and Xe) [22] but also bulk-phases with metal-xenon bonds have been prepared and their crystal structures determined [23]. In a recent work from surface science, the Xe-Ru interactions with a significant amount of charge transfer were demonstrated at room temperature and low pressures [24]. Using new strategies taking advantage of confinement effects, these new two-dimensional structures allow the study of noble gas metal interactions down to the atomic scale, using the very sensitive toolkit of surface science methodology. The general ideas for using confinement effects for favoring interactions of noble gases with other elements at mild conditions is described in a recent topical review [25].

2.1. Xenon(II)

Since the first reported case of a compound with a XeF2 ligand coordinated to a metal center, i.e., [Ag(XeF2)2](AsF6)[26], a large number of other examples have been synthesized [27]. A variety of compounds exist, where different numbers of XeF2 molecules are bound to various metal centers n+ yielding [M(XeF2)x] cationic part. The oxidation states of the metals are M(I), M(II), or M(III). The most important aspects that influence the formation of XeF2 coordination compounds are charge and the size of the cation, the type of the anion that compensates the positive charge of the cationic part, solubility of the salt, and the concentration of the XeF2 ligand [27]. + Beside acting as a neutral ligand, XeF2 acts as a fluoride ion donor forming [XeF] salts with strong Lewis acid pentafluorides (MF5;M = As, Sb, Bi, etc.) [28]. Although the ionic formulations, i.e., + [XeF] [MF6]−, implies complete F− transfer from XeF2 to the strong Lewis axcid MF5, in reality the [XeF]+ cation and its anion exists as ion pairs in their salts that interact through Xe–F M bridges [28]. + ··· An excess of XeF2 oftentimes results in the formation of [Xe2F3] salts. Oxidation of ruthenium and iridium metal with an excess of XeF2 in anhydrous HF as a solvent and further crystallization resulted in the crystal growth of [Xe F ][RuF ] XeF , [Xe F ][RuF ], and [Xe F ][IrF ] (Figure1)[ 29]. 2 3 6 · 2 2 3 6 2 3 6 Crystal structure determination of [Xe2F3][MF6] (M = Ru, Ir) showed that their structures are isotypic. MoleculesMolecules 2020, 2 520, x20 FOR, 25, PEERx FOR REVIEW PEER REVIEW 3 of 19 3 of 19

in the crystal growth of [Xe2F3][RuF6]∙XeF2, [Xe2F3][RuF6], and [Xe2F3][IrF6] (Figure 1) [29]. Crystal in theMolecules crystal2020 growth, 25, 3014 of [Xe2F3][RuF6]∙XeF2, [Xe2F3][RuF6], and [Xe2F3][IrF6] (Figure 1) [29]. Crystal3 of 18 2 3 6 structurestructure determination determination of [Xe of2F 3[Xe][MFF6][MF] (M =] Ru,(M =Ir) Ru, showed Ir) showed that their that structurestheir structures are isotypic. are isotypic.

(a) (a) (b) (b)

FigureFigure 1. 1.(a ) Packing(a) Packing diagram diagram of [Xe F of][RuF [Xe]2 alongF3][RuFb-axis.;6] along (b) Packing b-axis. diagram; (b) Packing of [Xe F ][RuF diagram] XeF of. Figure 1. (a) Packing diagram of 2[Xe3 2F3][RuF6 6] along b-axis.; (b) Packing diagram2 3 of6 · 2 (reproduced[Xe2F3][RuF6]·XeF from2. Ref. (reproduced [29]; published from Ref. under [29 the]; published terms and under conditions the terms of the and Creative conditions Commons of the [Xe2F3][RuF6]·XeF2. (reproduced from Ref. [29]; published under the terms and conditions of the Attribution 4.0 International License CC BY 4.0.). CreativeCreative Commons Commons Attribution Attribution 4.0 International 4.0 International License License CC BY CC 4.0 BY.). 4.0.).

TheThe adductsadducts XeF XeF2 CrOF2∙CrOF4 4and and XeF XeF2 2CrOF2∙2CrOF4 were4 were synthesized synthesized and and structurally structurally characterized characterized at low at The adducts XeF2∙CrOF· 4 and XeF2∙2CrOF· 4 were synthesized and structurally characterized at temperatureslow temperatures (Figure (Figure2)[ 28 ].2) The [28] fluoride. The fluoride ion affi ionnity affinity of CrOF of4 isCrOF too low4 is too to enable low to fluoride enable fluoride ion transfer ion low temperatures (Figure 2) [28]. The fluoride ion affinity+ of CrOF4 is too low to enable fluoride ion from XeF2 to form2 ion-paired salts of the [XeF] cations having+ either the [CrOF5]− or [Cr2O25F–9]− transfertransfer from from XeF2 XeFto form to form ion-paired ion-paired salts saltsof the of [XeF] the + [XeF] cations cations having having either either the [CrOF the [CrOF5]– or ] or anions2 2 [289 –]. [Cr2O[Cr2F9]–O anions.F ] anions. [28] [28]

Figure 2. The crystal packing of XeF CrOF viewed along the b-axis of the unit cell. Copyright Figure 2. The crystal packing of XeF2∙CrOF2 4 viewed4 along the b-axis of the unit cell. Copyright (2019) Figure 2. The crystal packing of XeF2∙CrOF4 viewed· along the b-axis of the unit cell. Copyright (2019) (2019) Wiley. Used with permission from Ref. [28]. Wiley.Wiley. Used withUsed permission with permission from Ref. from [28 Ref.]. [28]. + + 2.2. Xenon(VI) Fluoride and its [XeF5] and [Xe2F11] Salts + + 2.2. Xenon(VI) Fluoride and its [XeF+ 5] and [Xe+ 2F11] Salts 2.2. Xenon(VI) Fluoride and its [XeF5] and [Xe2F11] Salts 129 XeF6 was prepared and isolated in solid argon and neon matrices [30]. The IR spectra of XeF6 129 136XeF6 was prepared and isolated in solid argon and neon matrices [30]. The IR spectra129 of XeF6 XeFand6 wasXeF prepared6 isotopologues, and isolated recorded in solid in argon the neon and neon matrix, matrices agree [ with30]. T theoreticalhe IR spectra ones of forXeF the6 C3v 136 136and XeF6 isotopologues, recorded in the neon matrix, agree with theoretical ones for the C3v and conformerXeF6 isotopologues, of the XeF6 molecule recorded in in the the neon neon matrix. matrix, As an agree internal with reference theoretical matrix-site ones for and the Xe-isotope C3v conformer of the XeF6 molecule in the neon matrix. As an internal reference matrix-site and Xe- conformersplitting of of the corresponding XeF6 molecule Xe–F in and the Xeneon=O matrix. stretching As modes an internal of XeOF reference4 were analyzed matrix-site in solid and neon Xe- [30]. isotope splitting of corresponding Xe–F and Xe=O stretching modes of XeOF4 were analyzed+ in solid+ isotope splittingXenon of hexaﬂuoride corresponding is a well-knownXe–F and Xe=O ﬂuoride stretching ion donor modes forming of XeOF various4 were analyzed [XeF5] and in solid [Xe2 F11] neon [30]. + neon salts[30]. [31]. In recent years a large number of crystal structures of previously known or new [XeF5] + + Xenon +hexafluoride is a well-known fluoride ion donor forming various [Xe+ F5] and+ [Xe+ 2F11] Xenonand [Xe hexafluoride2F11] salts were is a reported.well-known The fluoride crystal structureion donor of forming [Xe2F11][SbF various6] consists [XeF5] of and [Xe 2[XeF112]F11cations] + salts [31]. In recent years a large number of crystal structures of previously known or new [XeF+ 5] and salts [and31]. [SbFIn recent6]− anions years a (Figure large number3)[ 32]. of The crystal crystal structures structures of previously of [XeF5][SbF known6][ 33or], new [XeF [XeF5][Sb5] 2 andF11][ 33], + + [Xe+ 2F11] salts were reported. The crystal structure+ of [Xe2F11][SbF6] consists of [Xe+ 2F11] cations+ and [Xe2Fand11] salts [XeF 5were][BF4 reported.][32] are allThe built crystal from structure [XeF5] cations of [Xe2 (FigureF11][SbF3).6] Positiveconsists chargeof [Xe2 ofF11 the] cations [XeF 5 ]andcations – [SbF– 6] anions (Figure 3) [32]. The crystal structures of [XeF5][SbF6] [33], [XeF5][Sb2F11] [33], and [SbF6is] compensatedanions (Figure by 3) [SbF [326]].− ,The [Sb 2 crystalF11]− or structures [BF4]−, respectively, of [XeF5][SbF anions6] [33 (Figure], [XeF3).5][Sb2F11] [33], and

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Molecules[XeF5][BF20204] ,[2532, 3014] are all built from [XeF5]+ cations (Figure 3). Positive charge of the [XeF5]+ cations4 of 18is compensated by [SbF6]–, [Sb2F11]– or [BF4]–, respectively, anions (Figure 3).

(a) (b)

Figure 3. (a) Part of the crystal structure of [Xe2F11][SbF6]. Copyright (2017) Wiley. Used with Figure 3. (a) Part of the crystal structure of [Xe2F11][SbF6]. Copyright (2017) Wiley. Used with permission from Ref. [32]; (b) Part of the crystal structure of the XeF5SbF6 showing the interactions permission from Ref. [32]; (b) Part of the crystal structure of the XeF5SbF6 showing the interactions + between the [XeF5] cation and the [SbF6]− anions. Copyright (2015) Elsevier. Used with permission between the [XeF5]+ cation and the [SbF6]– anions. Copyright (2015) Elsevier. Used with permission from Ref. [33]; (c) Part of the crystal structure of XeF5Sb2F11 showing the interactions between the 5 2 11 from Ref. [33]; (c) Part of the crystal+ structure of XeF Sb F showing the interactions between the [Sb2F11]− anion and the three [XeF5] cations. Copyright (2015) Elsevier. Used with permission from [Sb2F11]– anion and the three [XeF5]+ cations. Copyright (2015) Elsevier. Used with permission from Ref. [33]; (d) Part of the crystal structure of [XeF5][BF4]. Copyright (2017) Wiley. Used with permission Ref. [33]; (d) Part of the crystal structure of [XeF5][BF4]. Copyright (2017) Wiley. Used with permission from Ref. [32]. from Ref. [32].

Reactions between various amounts of XeF2 and MF2, MF3, or MF4 (M = Ti, Mn, Sn, Pb) with Reactions between various amounts of XeF2 and MF2, MF3, or MF4 (M = Ti, Mn, Sn, Pb) with UV- UV-photolyzed F2 in liquid anhydrous hydrogen fluoride (aHF) [34] resulted in the crystal growth photolyzed F2 in liquid anhydrous+ hydrogen+ fluoride (aHF) [34] resulted in the crystal growth of a of a large number of [XeF5] and [Xe2F11] salts upon crystallization from corresponding solutions. large number of [XeF5]+ and [Xe2F11]+ salts upon crystallization from corresponding solutions. Investigation of XeF2/TiF4/UV-irradiated system resulted in the crystal structure determinations of Investigation of XeF2/TiF4/UV-irradiated system resulted in the crystal structure determinations of [XeF5][TiF5] (XeF6 TiF4), [XeF5]5[Ti10F45] (XeF6 2TiF4), and [XeF5][Ti3F13] (XeF6 3TiF4)[35]. The main [XeF5][TiF5] (XeF6·∙TiF4), [XeF5]5[Ti10F45] (XeF6∙2TiF· 4), and [XeF5][Ti3F13] (XeF6∙3TiF· 4) [35]. The main structural feature of [XeF5][TiF5] is an infinite chain of distorted TiF6 octahedra joined by cis vertices. structural feature of [XeF5][TiF5] is an infinite chain of distorted TiF6 octahedra joined by cis vertices. Anionic part of [XeF5][Ti3F13] is built from tetrameric Ti4F20 and octameric Ti8F36 units sharing vertexes Anionic part of [XeF5][Ti3F13] is built from tetrameric Ti4F20 and octameric Ti8F36 units sharing vertexes and alternatively linked into ([Ti3F13]−) columns. In both cases the charge balance is maintained by – and alternatively+ linked into ([Ti3F13] )∞ columns. In both cases the charge balance is maintained by [XeF5] cations which form secondary Xe F contacts with fluorine atoms of anionic parts (Figure4). [XeF5]+ cations which form secondary Xe···F··· contacts with fluorine atoms of anionic parts (Figure 4).

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(a) (b) (a) (b)

Figure 4. (a) Part of the ([TiF5]–)∞ chain in [XeF5][TiF5]; (b) Part of the infinite ([Ti3F13]–)∞ column in Figure 4. (a) Part of the ([TiF5]−) – chain in [XeF5][TiF5]; (b) Part of the inﬁnite ([Ti3F13]−) – column in Figure 4. (a) Part of the ([TiF5∞] )∞ chain in [XeF5][TiF5]; (b) Part of the infinite ([Ti3F13]∞)∞ column in [XeF[XeF55][Ti33F13]].. Ref. Ref. [ [3535];]; reproduced reproduced by by permission permission of of The The Royal Royal Society Society of Chemistry (RSC) on behalf [XeF5][Ti3F13]. Ref. [35]; reproduced by permission of The Royal Society of Chemistry (RSC) on behalf ofof thethe CentreCentre NationalNational dede lala RechercheRecherche ScientiﬁqueScientifique (CNRS)(CNRS) andand thethe RSC.RSC. of the Centre National de la Recherche Scientifique (CNRS) and the RSC.

+ + TheThe crystal crystal structure structure of of [XeF [XeF5]55[Ti]5[Ti10F1045]F consists45] consists of [XeF of5 [XeF] cations5] cations and the and largest the known largest discrete known The crystal structure of [XeF5]5[Ti10F45] consists of [XeF5]+ cations and the largest known discrete 5– 5 discrete[Ti10F45] [Ti anion10F45 ]built− anion from built ten fromTiF6 octahedra, ten TiF6 octahedra, sharing vertices sharing in vertices the sha inpe the of shape a double of a-star double-star (Figure [Ti10F45]5– anion built from ten TiF6 octahedra, sharing vertices in the shape of a double-star (Figure (Figure5). It crystallizes5). It crystallizes in two incrystal two crystalmodification modiﬁcation at low at(α- lowphase, ( α -phase,150 K) and 150 K)ambient and ambient (β-phase, (β-phase, 296 K) 5). It crystallizes in two crystal modification at low (α-phase, 150 K) and ambient (β-phase, 296 K) 296temperatures. K) temperatures. temperatures.

(a) (b) (a) (b) + 5 Figure 5. (a) [XeF+ 5] cations and5 [Ti– 10F45] − anions, composed of ten TiF6 octahedra, in Figure 5. (a) [XeF5] cations and [Ti10F45] anions, composed of ten TiF6 octahedra, in β-[XeF5]5[Ti10F45]. β-[XeFFigure] 5.[Ti (a) F[XeF].5] Ref.+ cations [35]; and reproduced [Ti10F45]5– byanions, permission composed of Theof ten Royal TiF6 octahedra, Society of in Chemistry β-[XeF5]5[Ti (RSC)10F45]. Ref. [355];5 reproduced10 45 by permission of The Royal Society of Chemistry (RSC) on behalf of the Centre onRef. behalf [35]; of reproduced the Centre by National permission de la of Recherche The Royal ScientiﬁqueSociety of Chemistry (CNRS) and(RSC) the on RSC; behalf (b )of Geometry the Centre National de la Recherche Scientifique (CNRS) and the RSC; (b) Geometry of the [Ti10F45]5– anion 5 5– ofNational the [Ti10 deF45 ] la− Rechercheanion (Figure Scientifique was made (CNRS) by Z.M. and using the RSC crystallographic; (b) Geometry data of the from [Ti10 CIF-ﬁleF45] anion of (Figure was made by Z.M. using crystallographic data from CIF-file of β-[XeF5]5[Ti10F45] [35]). β-[XeF(Figure5]5 [Tiwas10 Fmade45][35 by]). Z.M. using crystallographic data from CIF-file of β-[XeF5]5[Ti10F45] [35]).

Photochemical synthesis between XeF2, MnF3, and UV-irradiated elemental F2 in aHF yielded PhotochemicalPhotochemical synthesis synthesis between between XeF XeF2,2 MnF, MnF3,3 and, and UV-irradiated UV-irradiated elemental elemental F 2Fin2 in aHF aHF yielded yielded [XeF5]2[MnF6] (2XeF6∙MnF4), [XeF5][MnF5] (XeF6∙MnF4) and [XeF5]4[Mn8F36] (XeF6∙2MnF4) [36]. The [XeF[XeF]5][MnF2[MnF6]] (2XeF (2XeF6∙MnFMnF 4), [XeF5][MnF][MnF5] ] (XeF (XeF6∙MnFMnF4) ) and and [XeF [XeF5]4[Mn] [Mn8F36F] (XeF] (XeF6∙2MnF2MnF4) [36)[].36 The]. 5 2 6 6 4 5 5 6 + 4 5 4 8 36 2– 6 4 crystal structure of [XeF· 5]2[MnF6] consists of [XeF5]· cations+ and octahedral [MnF6] anions· 2 (Figure Thecrystal crystal structure structure of [XeF of [XeF5]2[MnF5]2[MnF6] consists6] consists of [XeF of [XeF5]+ cations5] cations and octahedral and octahedral [MnF [MnF6]2– anions6] − anions (Figure 6). (Figure6). 6).

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(a) (b) 2 + Figure 6. (a) The secondary contacts between the [MnF ] anion2– and [XeF ] cations+ in the crystal Figure 6. (a) The secondary(a) contacts between the6 [MnF− 6] anion and(b) [XeF5 5] cations in the crystal structure ofstructure [XeF5] 2 of[MnF [XeF6];5]2[MnF (b) Part6]; (b of) Part the of([MnF the 5 ([MnF]−) 5inﬁnite]–)∞ infinite chain chain in thein the crystal crystal structure structure of of ∞ [XeF5][MnFFigure[XeF5]. Copyright5 ][MnF 6. (a) The5]. Copyright secondary (2017) Wiley. (2017) contacts UsedWiley. between with Used the permission with [MnF permission6]2– anion from and from Ref. [XeF Ref. [536]+cations]. [36]. in the crystal structure of [XeF5]2[MnF6]; (b) Part of the ([MnF5]–)∞ infinite chain in the crystal structure of The main[XeFThe structural5][MnF main5]. structuralCopyright feature (2017) offeature the Wiley. anionic of Used the anionicwith part permission in part [XeF in from5 [XeF][MnF Ref.5][MnF [365]]. is5 similar] is similar to thatto that in in [XeF [XeF5][TiF5][TiF55]] (Figure4a),(Figure although 4a), thealthough compounds the compounds are not are isotypic not isotypic (Figure (Figure6b). A6b study). A study of magnetic of magnetic properties properties The main structural feature of the anionic part in [XeF5][MnF5] is similar to that in [XeF5][TiF5] showed that [XeF5][MnF5] is paramagnetic in the 296–200 K range (μeff = 3.87 μB, θ = −9.3 K). Below showed that(Figure [XeF 4a5),][MnF although5] the is paramagneticcompounds are not in isotypic the 296–200 (Figure 6 Kb). rangeA study ( µofe ﬀmagnetic= 3.87 propertiesµB, θ = 9.3 K). 100 K, there is a weak antiferomagnetic coupling between the Mn(IV) ions (J = −1.3 cm−1) [36−].1 Below 100showed K, there that is [XeF a weak5][MnF antiferomagnetic5] is paramagnetic in coupling the 296–200 between K range ( theμeff = Mn(IV) 3.87 μB, θ ions = −9.3 (J K).= Below1.3 cm − )[36]. + 4– 100 K,Crystal there is structurea weak antiferomagnetic of [XeF5]4[Mn coupling8F36] consists between from the [XeFMn(IV)+ 5] ionscations (J = −and1.3 cm discrete−1) [36]. −[Mn8F364 ] anions Crystal structure of [XeF5]4[Mn8F36] consists from [XeF5] cations and discrete [Mn8F36] − anions (FigureCrystal 7). Thestructure latter of are [XeF built5]4[Mn from8F36 ]eight consists MnF from6 octahedra, [XeF5]+ cations each and sharing discrete three [Mn vertices,8F36]4– anions in the shape of (Figure7). The latter are built from eight MnF 6 octahedra, each sharing three vertices, in the shape of a (Figurea ring (Figure 7). The latter 7). are built from eight MnF6 octahedra, each sharing three vertices, in the shape of ring (Figurea ring7). (Figure 7).

(a) (b)

(a) 4 (b) Figure 7. (a) Discrete octameric [Mn8F36] − anion in the crystal structure of [XeF5]4[Mn8F36]; 4 (b) Geometry of the [Mn8F36] − anion built from eight MnF6 octahedra. Copyright (2017) Wiley. Used with permission from Ref. [36].

The attempts to grow single crystals in XeF6/MF4 (M = Sn, Pb) system were successful in three cases. Single crystals of [Xe2F11]2[SnF6], [Xe2F11]2[PbF6], and [XeF5]4[Sn5F24] were grown from aHF saturated Molecules 2020, 25, x FOR PEER REVIEW 7 of 19

Figure 7. (a) Discrete octameric [Mn8F36]4– anion in the crystal structure of [XeF5]4[Mn8F36]; (b)

MoleculesGeometry 2020 of, 2 5the, x FOR [Mn PEER8F36] 4REVIEW– anion built from eight MnF6 octahedra. Copyright (2017) Wiley. Used7 ofwith 19 permission from Ref. [36]. Figure 7. (a) Discrete octameric [Mn8F36]4– anion in the crystal structure of [XeF5]4[Mn8F36]; (b)

Geometry of the [Mn8F36]4– anion built from eight MnF6 octahedra. Copyright (2017) Wiley. Used with The attempts to grow single crystals in XeF6/MF4 (M = Sn, Pb) system were successful in three Molecules 2020permission, 25, 3014 from Ref. [36]. 7 of 18 cases. Single crystals of [Xe2F11]2[SnF6], [Xe2F11]2[PbF6], and [XeF5]4[Sn5F24] were grown from aHF saturatedThe solutions attempts to upon grow crystallizationssingle crystals in XeF[376]/MF. The4 (M crystal= Sn, Pb) structuressystem were ofsuccessful [Xe2F11 ]in2[SnF three6] and + 2– [Xesolutions2Fcases.11]2[Pb upon SingleF6] are crystallizations crystals isotypic. of [XeThey2F11 [ 37]consist2[SnF]. The6], of [Xe crystal [Xe2F112]F2[PbF11 structures] cations6], and [XeFand of [Xe5 ][MF4[Sn2F56F11]24]] 2 (M[SnF were = 6 Sn,] grown and Pb) [Xe fromanions2F 11 aHF] 2(Figure[PbF 6] saturated solutions upon crystallizations+ [37]. The crystal2 structures of [Xe2F11]2[SnF6] and 8)are. isotypic. They consist of [Xe2F11] cations and [MF6] − (M = Sn, Pb) anions (Figure8). [Xe2F11]2[PbF6] are isotypic. They consist of [Xe2F11]+ cations and [MF6]2– (M = Sn, Pb) anions (Figure 8).

+ 2 Figure 8. Packing of the [Xe2F11] cations and [MF6] − anions in the crystal structures of [Xe2F11]2[MF6] + 2– Figure 8. Packing of the [Xe2F11] +cations and [MF62]– anions in the crystal structures of [Xe2F11]2[MF6] (M =FigurePb, Sn). 8. Packing Copyright of the (2019) [Xe2F11 Wiley.] cations Used and with [MF6 permission] anions in the from crystal Ref. structures [37]. of [Xe2F11]2[MF6] (M =(M Pb, = Sn).Pb, Sn). Copyright Copyright (2019) (2019) Wiley. Wiley. UsedUsed with permission permission from from Ref. Ref. [37 ].[37 ]. The single crystal structure determination of [XeF5]4[Sn5F24] reveals that it is built of two-dimensional 4 + ([Sn5TheF24] The− single) singlegrids crystal andcrystal the structure structure [XeF5] determinationcations determination located of of between [XeF [XeF5]45[Sn]4 them.[Sn5F245F] 24 reveals The] reveals two-dimensional that that it is it built is built ofgrids two of have- two a- ∞ dimensional ([Sn5F424–]4–)∞ grids and the [XeF5]++ cations4 located between them. The two-dimensional dimensionalwave-like conformation ([Sn5F24] )∞ (Figuregrids and9). Thethe ([Sn[XeF55F]24 cations] −) layer located contains, between both them. six- and The seven-coordinated two-dimensional grids have a wave-like conformation (Figure 9). The∞ ([Sn5F24]4–)4∞– layer contains, both six- and seven- gridsSn(IV) have interconnected a wave-like by conformation bridging fluorine (Figure atoms 9). The (Figure ([Sn95).F24 It] is)∞ a layer unique contains, example both of Sn(IV)-fluoridesix- and seven- coordinated Sn(IV) interconnected by bridging2 fluorine atoms (Figure 9). It is a unique example of coordinatedcompound that Sn(IV) does interconnected not consist just by of bridging [SnF6] − anions.fluorine Theatoms coordination (Figure 9). of ItSn(IV) is a unique by seven example fluorine of Sn(IV)-fluoride compound that does not consist just of [SnF6]2– anions. The coordination of Sn(IV) by Sn(IV)atoms- isfluoride unprecedented. compound that does not consist just of [SnF6]2– anions. The coordination of Sn(IV) by seven fluorine atoms is unprecedented. seven fluorine atoms is unprecedented.

(a) (b)

4 + Figure 9. a 4- + Figure (9. )(a Two-dimensional) Two-dimensional ([Sn ([Sn5F524F]24)]∞ grids−) grids with a withwave a-like wave-like conformation conformation and the [XeF and5] cations the [XeF 5] (a) ∞ (b) + cationslocated located between between them them in the in crystal the crystal structure structure of [XeF of5] [XeF4[Sn55F]244].[Sn For5F 24clarity,]. For the clarity, [XeF the5]+ cations [XeF5 ]arecations 4 are presentedpresented with with sticks sticks only only;; (b) (Theb) The ([Sn ([Sn5F24]45-)F∞24 layer] −) in thelayer crystal in the structure crystal of structure [XeF5]4[Sn of5F24 [XeF] contains5]4[Sn 5F24] 4- ∞ + Figurecontainsboth 9. ( both sixa) Two- six-and-dimensional and seven seven-coordinated-coordinated ([Sn5F24 ] Sn(IV))∞ grids Sn(IV) interconnected with interconnected a wave -like by bridgingconformation by bridging fluorine and ﬂuorine atomsthe [XeF atoms (view5] cations (view located between them in the crystal structure of [XeF5]4[Sn5F24]. For clarity, the [XeF5]+ cations are perpendicular to the layer, along the a-axis). Copyright (2019) Wiley. Used with permission from presentedRef. [37]. with sticks only; (b) The ([Sn5F24]4-)∞ layer in the crystal structure of [XeF5]4[Sn5F24] contains both six- and seven-coordinated Sn(IV) interconnected by bridging fluorine atoms (view Reactions between molten mixtures of XeF and CrVIOF proceeds by F elimination to form 6 4 2 [XeF ][Xe F ][CrVOF ] 2CrVIOF , [XeF ] [CrIVF ] 2CrVIOF , [Xe F ] [CrIVF ], and [XeF ] [CrV O F ][38]. 5 2 11 5 · 4 5 2 6 · 4 2 11 2 6 5 2 2 2 8 On the other side, their reactions in aHF and CFCl /aHF yield [XeF ] [CrV O F ] 2HF and 3 5 2 2 2 8 · [XeF ] [CrV O F ] 2XeOF [38]. Their crystal structures contain [XeF ]+ and/or [Xe F ]+ cations, 5 2 2 2 8 · 4 5 2 11 which interact with their respective anions by secondary Xe F interactions. In the case of ··· Molecules 2020, 25, x FOR PEER REVIEW 8 of 19

perpendicular to the layer, along the a-axis). Copyright (2019) Wiley. Used with permission from Ref. [37]. Molecules 2020, 25, x FOR PEER REVIEW 8 of 19

VI Reactionsperpendicular between to the molten layer, along mixtures the a-axis). of XeF Copyright6 and (2019) Cr OF Wiley.4 proceeds Used with by permission F2 elimination from Ref. to form [XeF5][Xe2F[3711].][Cr VOF5]·2CrVIOF4, [XeF5]2[CrIVF6]·2CrVIOF4, [Xe2F11]2[CrIVF6], and [XeF5]2[CrV2O2F8] [38]. On the other side, their reactions in aHF and CFCl3/aHF yield [XeF5]2[CrV2O2F8]·2HF and Molecules 2020Re, 25actions, 3014 between molten mixtures of XeF6 and CrVIOF4 proceeds by F2 elimination to form8 of 18 [XeF5]2[CrV2O2F8]·2XeOF4 [38]. Their crystal structures contain [XeF5]+ and/or [Xe2F11]+ cations, which [XeF5][Xe2F11][CrVOF5]·2CrVIOF4, [XeF5]2[CrIVF6]·2CrVIOF4, [Xe2F11]2[CrIVF6], and [XeF5]2[CrV2O2F8] [38]. interact with their respective anions by secondary Xe∙∙∙F interactions. In the case of On the other side, their reactions in aHF and CFCl3/aHF yield [XeF5]2[CrV2O2F8]·2HF and [XeF5][Xe2F11][CrVOFV 5]·2CrVIOFVI 4 and [XeF5]2[CrIVFIV6]·2CrVIOFVI4, the CrOF4 is introduced into the [XeF5][Xe[XeF25]F2[Cr11][CrV2O2FOF8]·2XeOF5] 2Cr4 [38OF]. Their4 and crystal [XeF5 structures]2[Cr F6 ]contain2Cr OF[XeF4,5] the+ and/or CrOF [Xe4 is2F11 introduced]+ cations, which into the coordination sphere of [Cr· VVOF5]2– and [CrIVIVF6]2–2 anions, respective· ly, upon crystallizations (Figure 10). coordinationinteract sphere with of their [Cr respectiveOF5] − and anions [Cr F 6 by] − anions, secondary respectively, Xe∙∙∙F interactions. upon crystallizations In the case (Figure of 10). IV + IV 2– The crystal structure Vof [Xe2F11VI]2[Cr IVF6] is comprisedIV of [XeVI 2F11] cations+ and [Cr F6IV] anions.2 The crystal[XeF5][Xe structure2F11][Cr ofOF [Xe5]·2Cr2F11]OF2[Cr4 andF 6] [XeF is comprised5]2[Cr F6]·2Cr of [XeOF24,F 11the] cations CrOF4 is and introduced [Cr F6] − intoanions. the coordination sphere of [CrVOF5]2– and [CrIVF6]2– anions, respectively, upon crystallizations (Figure 10). The crystal structure of [Xe2F11]2[CrIVF6] is comprised of [Xe2F11]+ cations and [CrIVF6]2– anions.

2 IV VI Figure 10. The coordination sphere of the [CrF ] 2- in the crystal structure of [XeF ] [Cr IVF ] 2Cr VIOF . Figure 10. The coordination sphere of the [CrF6 6]− in the crystal structure of [XeF5 52]2[Cr F66]·2Cr· OF4. Figure 10. The coordination sphere of the [CrF6]2- in the crystal structure of [XeF5]2[CrIVF6]·2CrVIOF4. Secondary XeXe∙∙∙-F-F and and Cr Cr∙∙∙FF bonding bonding interactions interactions are are indicated indicated by dashed by dashed lines. lines. Copyright Copyright (2019) (2019) Wiley. Secondary··· Xe∙∙∙-F and··· Cr∙∙∙F bonding interactions are indicated by dashed lines. Copyright (2019) Wiley.Used with Used permission with permission from Ref. from [38 Ref.]. [38]. Wiley. Used with permission from Ref. [38]. V V V The crystal structures of [XeF5]2[Cr 2O2F8], [XeF5]2[Cr 2O2F8] 2HF, and [XeF5]2[Cr 2O2F8] 2XeOF4 5 2 V2 V 2 8 5 2 V2 2 8 5 2 V V2 2 8 4 The crystalTheV crystal structures2 structures of [XeFof [XeF] [Cr5]2[CrO2OF2F],8], [XeF [XeF5]]2[Cr[Cr 2O2FF8]·2HF]·2HF· , ,and and [XeF [XeF5]2[Cr] [Cr2O2FO8]·2XeOFF ]·2XeOF· 4 contain [CrV 2O2F82]– − anions, where the latter represents a new structural motif among the known containcontain [Cr 2 O [Cr2FV82]O2 Fanions,8]2– anions, where where the the latter latter represents represents a new structuralstructural motif motif among among the the known known oxyﬂuoroanions of Group 6 (Figure 11)[38]. oxyfluoroanionsoxyfluoroanions of Group of Group 6 (Figure 6 (Figure 11) 11) [38 [38]. ].

(a) (b)

Figure 11. (a) The coordination sphere of the [Cr O F ]2 anion in the crystal structure of Figure 11. (a) The(a) coordination sphere of the [Cr22O22F8]2- −anion in the ( crystalb) structure of V [XeF5][XeF2[Cr5]22[CrO2VF2O8].2F8 Secondary]. Secondary Xe Xe∙∙∙FF bondingbonding interactions and and Cr∙∙∙F Cr bF bridgeb bridge bonds bonds are areindicated indicated by by ··· V ··· 2 b 2-V 2– Figuredasheddashed lines; 11. ( lines;a ( ) ) The (b) coordinationThe coordination coordination environment sphere environment of around the around [Cr the2 theO2 [CrF [Cr8] 22Oanion2FF88]] −anion inanion the in inthe crystal the crystal crystal structurestructure structure of V of [XeF ] V[Cr O F ] 2XeOF . Secondary Xe F bonding interactions and Cr F bridge bonds are [XeF5]2[Cr5 2 2O2F28]. 2Secondary8 · 4Xe∙∙∙F bonding interactions··· and Cr∙∙∙Fb bridge bonds··· b are indicated by indicated by dashed lines. Copyright (2019) Wiley. Used with permission from Ref. [38]. dashed lines; (b) The coordination environment around the [CrV2O2F8]2– anion in the crystal structure

Reactions between ASbF6 (A = Rb, Cs) and [XeF5][SbF6] in aHF in 1:1 molar ratios yielded A[XeF5][SbF6]2 compounds upon crystallization [32]. A similar reaction between CsBiF6 and [XeF5][SbF6] (1:1) yielded mixed-cation/mixed-anion Cs[XeF5][BixSb1-xF6]2 (x = ~0.37–0.39). The A[XeF5][SbF6]2 (A = Rb, Cs) and Cs[XeF5][BixSb1-xF6]2 salts crystallize in two crystal modifications at low (α-phase, 150 K) and ambient (β-phase, 296 K) temperatures (Figure 12). High temperature modifications of A[XeF5][SbF6]2 (A = Rb, Cs) and Cs[XeF5][BixSb1-xF6]2 are structurally related, while low-temperature modifications crystallize isotopically. Molecules 2020, 25, x FOR PEER REVIEW 9 of 19

of [XeF5]2[CrV2O2F8]·2XeOF4. Secondary Xe∙∙∙F bonding interactions and Cr∙∙∙Fb bridge bonds are indicated by dashed lines. Copyright (2019) Wiley. Used with permission from Ref. [38].

Reactions between ASbF6 (A = Rb, Cs) and [XeF5][SbF6] in aHF in 1:1 molar ratios yielded A[XeF5][SbF6]2 compounds upon crystallization [32]. A similar reaction between CsBiF6 and [XeF5][SbF6] (1:1) yielded mixed-cation/mixed-anion Cs[XeF5][BixSb1-xF6]2 (x = ~0.37–0.39). The A[XeF5][SbF6]2 (A = Rb, Cs) and Cs[XeF5][BixSb1-xF6]2 salts crystallize in two crystal modifications at low (α-phase, 150 K) and ambient (β-phase, 296 K) temperatures (Figure 12). High temperature Moleculesmodifications2020, 25, 3014 of A[XeF5][SbF6]2 (A = Rb, Cs) and Cs[XeF5][BixSb1-xF6]2 are structurally related, while9 of 18 low-temperature modifications crystallize isotopically.

(a) (b)

+ + Figure 12. a + + − Figure (12.) ( Packinga) Packing of of Rb Rb and [XeF [XeF5]5 ]cationscations and and[SbF [SbF6] anions6]− anions in the crystal in the structure crystal structureof high- of + + high-temperaturetemperature (296 (296 K) K) form form of of-Rb[XeFβ-Rb[XeF5][SbF5][SbF6]2; (b6) ] Packing2;(b) Packing of Rb+ and of Rb [XeFand5]+ cations [XeF 5 and] cations [SbF6]− and [SbF6anions]− anions in in the the crystal crystal structure structure of of low low-temperature-temperature (150 (150 K) K) modification modiﬁcation of of-Rb[XeFα-Rb[XeF5][SbF5][SbF6]2. 6]2. CopyrightCopyright (2017) (2017) Wiley. Wiley. Used Used with with permission permission from from Ref.Ref. [32].].

ReactionsReactions between between XeF XeF5SbF5SbF6 6and and NONO2SbFSbF6 6oror Cu(SbF Cu(SbF6)2,6 respectively,)2, respectively, at ambient at ambient temperature temperature in ++ + + in aHFaHF as as a solventa solvent yielded yielded the the ﬁrst first [XeF [XeF5]5] /metal/metal and and [XeF [XeF5]5/non] /non-metal-metal mixed mixed-cation-cation salts, salts, i.e., i.e., NO2XeF5(SbF6)2 and XeF5Cu(SbF6)3 (Figure 13) [39]. Further investigations led to six new examples NO2XeF5(SbF6)2 and XeF5Cu(SbF6)3 (Figure 13)[39]. Further investigations led to six new examples of of such type of compounds, i.e., XeF5M(SbF6)3 (M = Ni, Mg, Zn, Co, Mn and Pd) [40]. Additionally, such type of compounds, i.e., XeF5M(SbF6)3 (M = Ni, Mg, Zn, Co, Mn and Pd) [40]. Additionally, single single crystals of (XeF5)3[Hg(HF)]2(SbF6)7 were grown and crystal structure determined (Figure 14) crystals of (XeF5)3[Hg(HF)]2(SbF6)7 were grown and crystal structure determined (Figure 14)[40]. [40]. For XeF5M(SbF6)3 salts, no phase transition was observed for compounds with M2+ (M = Ni, Mg, For XeF M(SbF ) salts, no phase transition was observed for compounds with M2+ (M = Ni, Mg, Cu,5 Zn, Co)6 that3 are smaller than Mn2+. The XeF5Mn(SbF6)3 and XeF5Pd(SbF6)3 crystallize in the low- 2+ Cu, Zn,temperature Co) that (Mn are smallerat 150 K and than Pd Mn at 260. TheK; α- XeFphase)5Mn(SbF and high6)3-temperatureand XeF5Pd(SbF modifications6)3 crystallize (296 K; β in- the low-temperaturephase). The crystal (Mn at structures 150 K and of PdXeF at5M(SbF 260 K;6)3 α(M-phase) = Ni, Mg, and Cu, high-temperature Zn, Co) and α- XeF modiﬁcations5Mn(SbF6)3 are (296 K; β-phase).isotypic. The The crystal main structuresfeature of their of XeF crystal5M(SbF structures6)3 (M is= theNi, tri Mg,-dimensional Cu, Zn, Co)framework and α- consisting XeF5Mn(SbF of 6)3 are isotypic.the M2+ Thecations main interconnected feature of their by the crystal SbF6 structuresoctahedra, is forming the tri-dimensional cavities within framework which the [XeF consisting5]+ cations2+ are located (Figure 13). + ofMolecules the M 2020cations, 25, x FOR interconnected PEER REVIEW by the SbF6 octahedra, forming cavities within which the [XeF10 of5 ]19 cations are located (Figure 13).

(a) (b)

+ + Figure 13. (a) Packing of [NO2] and [XeF5] cations, and [SbF6]− anions in the crystal structure Figure 13. (a) Packing of [NO2]+ and [XeF5]+ cations, and [SbF6]− anions in the crystal structure of of NO2XeF5(SbF6)2; Only one of the two orientations of the disordered [SbF6]− anions is depicted. NO2XeF5(SbF6)2; Only one of the two orientations of the disordered [SbF6]- anions is depicted. Copyright (2015) Wiley. Used with permission from Ref. [39]; (b) The tri-dimensional [M(SbF6)3]− Copyright (2015) Wiley. Used with permission from Ref. [39]; (b) The tri-dimensional [M(SbF6)3]− framework with the monoclinic unit cell in the crystal structure of α-XeF5M(SbF6)3 (150 K; M = Mn)  5 6 3 framework with2+ the monoclinic unit cell in the crystal structure of+ -XeF M(SbF ) (150 K; M = Mn) isotypic to M = Mg, Co, Ni, Zn, and Cu compounds. The [XeF5] cations located within the cavities isotypic to M2+ = Mg, Co, Ni, Zn, and Cu compounds. The [XeF5]+ cations located within the cavities are omitted for clarity. Copyright (2016) Wiley. Used with permission from Ref. [40]. are omitted for clarity. Copyright (2016) Wiley. Used with permission from Ref. [40].

Although crystal structures of β-XeF5Mn(SbF6)3 and α- and β-modifications of XeF5Pd(SbF6)3, with larger M2+ cations, differ from the other XeF5M(SbF6)3 compounds the main structure motif is preserved, i.e., a three-dimensional (3D) framework constructed of the M2+ cations and SbF6 units and [XeF5]+ cations located inside the cavities (Figure 14).

(a) (b)

Figure 14. (a) Part of the crystal structure of -XeF5Pd(SbF6)3 (296 K). Disorder of [XeF5]+ cations is not depicted. Only F atoms involved in secondary Xe···F contacts are labeled; (b) Packing of the columns

in the crystal structure of (XeF5)3[Hg(HF)]2(SbF6)7 with a triclinic unit cell. Copyright (2016) Wiley. Used with permission from Ref. [40].

2.3. Xenon(VI) Oxide Compounds

One of the main obstacles in the research of XeO3 is its extreme sensitivity. Solid XeO3 detonates when subjected to mild thermal or mechanical shock. For these reasons, experiments are usually limited to small quantities of XeO3 and its compounds (up to 20 mg).

Molecules 2020, 25, x FOR PEER REVIEW 10 of 19

(a) (b)

Figure 13. (a) Packing of [NO2]+ and [XeF5]+ cations, and [SbF6]− anions in the crystal structure of

NO2XeF5(SbF6)2; Only one of the two orientations of the disordered [SbF6]- anions is depicted. Copyright (2015) Wiley. Used with permission from Ref. [39]; (b) The tri-dimensional [M(SbF6)3]−

framework with the monoclinic unit cell in the crystal structure of -XeF5M(SbF6)3 (150 K; M = Mn) isotypic to M2+ = Mg, Co, Ni, Zn, and Cu compounds. The [XeF5]+ cations located within the cavities are omitted for clarity. Copyright (2016) Wiley. Used with permission from Ref. [40].

(a) (b)

+ Figure 14. (a) Part of the crystal structure of β-XeF5Pd(SbF6)3 (296 K). Disorder of [XeF5] cations is Figure 14. (a) Part of the crystal structure of -XeF5Pd(SbF6)3 (296 K). Disorder of [XeF5]+ cations is not not depicted. Only F atoms involved in secondary Xe F contacts are labeled; (b) Packing of the columns depicted. Only F atoms involved in secondary Xe···F ···contacts are labeled; (b) Packing of the columns in the crystal structure of (XeF5)3[Hg(HF)]2(SbF6)7 with a triclinic unit cell. Copyright (2016) Wiley. in the crystal structure of (XeF5)3[Hg(HF)]2(SbF6)7 with a triclinic unit cell. Copyright (2016) Wiley. Used with permission from Ref. [40]. Used with permission from Ref. [40].

Although crystal structures of β-XeF5Mn(SbF6)3 and α- and β-modiﬁcations of XeF5Pd(SbF6)3, 2.3. Xenon(VI) O2+xide Compounds with larger M cations, diﬀer from the other XeF5M(SbF6)3 compounds the main structure motif 2+ is preserved,One of the i.e., main a three-dimensional obstacles in the research (3D) framework of XeO3 is constructed its extreme ofsensi thetivity M . cationsSolid XeO and3 detonates SbF6 units + whenand [XeF subjected5] cations to mild located thermal inside or the mechanical cavities (Figure shock .14 For). these reasons, experiments are usually limited to small quantities of XeO3 and its compounds (up to 20 mg). 2.3. Xenon(VI) Oxide Compounds

One of the main obstacles in the research of XeO3 is its extreme sensitivity. Solid XeO3 detonates whenMolecules subjected 2020, 25, x to FOR mild PEER thermal REVIEW or mechanical shock. For these reasons, experiments are usually11 of 19 limited to small quantities of XeO3 and its compounds (up to 20 mg).

TheThe 15-crown 15-crownadduct adduct ofof XeOXeO33,, i.e., (CH 2CHCH22O)O)5XeO5XeO3 3waswas synthesized synthesized at atambient ambient temp temperatureerature by byreaction reaction of of 15 15-crown-5-crown-5 with with HF HF-acidiﬁed-acidified aqueous aqueous solution solution of XeO XeO33 [[4141].]. ItIt was was structurally structurally characterizedcharacterized by by Raman Raman spectroscopy spectroscopy and and single-crystal single-crystal X-ray X-ray di diffractionﬀraction (Figure (Figure 15 1).5).

Figure 15. Side-on view of the structural unit in the crystal structure of (CH2CH2O)5XeO3. Copyright (2018)Figure Wiley. 15. Side Used-on with view permission of the structural from Ref.unit [ 41in]. the crystal structure of (CH2CH2O)5XeO3. Copyright (2018) Wiley. Used with permission from Ref. [41].

The crystalline adducts of XeO3 with triphenylphosphine oxide, dimethylsulfoxide (DMSO), pyridine-N-oxide,The crystalline and adducts acetonetriphenyl of XeO3 with were triphenylphosphine prepared and characterized oxide, dimethylsulfoxide by low-temperature, (DMSO), single-crystalpyridine-N-oxide, X-ray and diﬀ raction acetonetriphenyl and Raman were spectroscopy prepared [and42]. characterized In [(CH3)2CO] by3XeO low3-temperature,each of the 3 2 3 3 3 XeOsingle3 molecules-crystal X-ray is coordinated diffraction and to threeRaman acetone spectroscopy molecules [42].(Figure In [(CH 16) ).CO] TheXeO structural each of the unit XeO of [(CHmolecules3)2SO] 3(XeO is coordinated3)2 is comprised to threeof three acetone DMSO ligands molecules that bridge (Figure two 16). XeO The3 molecules structural (Figure unit 16 of), [(CH3)2SO]3(XeO3)2 is comprised of three DMSO ligands that bridge two XeO3 molecules (Figure 16), whereas in (C5H5NO)3(XeO3)2 two molecules of XeO3 are O-bridged by C5H5NO ligand and are each O-coordinated to a terminal C5H5NO (Figure 16). The [(C6H5)3PO]2XeO3 (low- and high-temperature modification) provides the only example of a five-coordinate XeO3 adduct which has only two secondary Xe∙∙∙O bonding interactions (Figure 16).

(a) (b)

Molecules 2020, 25, x FOR PEER REVIEW 11 of 19

The 15-crown adduct of XeO3, i.e., (CH2CH2O)5XeO3 was synthesized at ambient temperature by reaction of 15-crown-5 with HF-acidified aqueous solution of XeO3 [41]. It was structurally characterized by Raman spectroscopy and single-crystal X-ray diffraction (Figure 15).

Figure 15. Side-on view of the structural unit in the crystal structure of (CH2CH2O)5XeO3. Copyright (2018) Wiley. Used with permission from Ref. [41].

The crystalline adducts of XeO3 with triphenylphosphine oxide, dimethylsulfoxide (DMSO), pyridine-N-oxide, and acetonetriphenyl were prepared and characterized by low-temperature, Moleculessingle-crystal2020, 25, 3014 X-ray diffraction and Raman spectroscopy [42]. In [(CH3)2CO]3XeO3 each of the XeO3 11 of 18 molecules is coordinated to three acetone molecules (Figure 16). The structural unit of [(CH3)2SO]3(XeO3)2 is comprised of three DMSO ligands that bridge two XeO3 molecules (Figure 16), whereaswhereas in (Cin 5(CH55HNO)5NO)3(XeO3(XeO33)2 twotwo molecules molecules ofof XeO XeO3 are3 are O-bridged O-bridged by C by5H5 CNO5H ligand5NO ligand and are and each are each O-coordinated to a terminal C5H5NO (Figure 16). The [(C6H5)3PO]2XeO3 (low- and high-temperature O-coordinated to a terminal C5H5NO (Figure 16). The [(C6H5)3PO]2XeO3 (low- and high-temperature modification) provides the only example of a five-coordinate XeO3 adduct which has only two modiﬁcation) provides the only example of a ﬁve-coordinate XeO3 adduct which has only two secondary Xe···O bonding interactions (Figure 16). secondary Xe O bonding interactions (Figure 16). ···

Molecules 2020, 25, x FOR PEER REVIEW 12 of 19

(a) (b)

Molecules 2020, 25, x FOR PEER REVIEW 12 of 19

Figure 16. The structural units in the crystal structures of (a) [(CH3)2CO]3XeO3;(b) [(CH3)2SO]3(XeO3)2; FigureFigure 16. The 16. structural The structural units units in the in the crystal crystal structures structures of of ( (aa)) [(CH[(CH33))22CO]CO]3XeO3XeO3; 3(;b ()b [(CH) [(CH3)2SO]3)2SO]3(XeO3(XeO3)2; 3)2; (c) (C5H(c5)NO) (C5H35(XeONO)3(XeO3)2;(3d)2); [(C(d) [(C6H65H)35PO])3PO]22XeOXeO3.. Copyright Copyright (2019) (2019) Wiley. Wiley. Used Used with withpermission permission from Ref. from (c) (C5H5NO)3(XeO3)2; (d) [(C6H5)3PO]2XeO3. Copyright (2019) Wiley. Used with permission from Ref. Ref. [42].[42]. [42].

AdductsAdducts between between XeO3 andXeO N-bases3 and N-bases (pyridine (pyridine and 4-dimethylaminopyridine) and 4-dimethylaminopyridine) were alsowere reported also Adducts between XeO3 and N-bases (pyridine and 4-dimethylaminopyridine) were also (Figure reported17)[43]. (Figure Additionally, 17) [43]. the Additionally crystal structures, the crystal of the structures XeO3 adducts of the XeO with3 adducts the ﬂuoride with andthe fluoride biﬂuoride reported (Figure 17) [43]. Additionally, the crystal structures of the XeO3 adducts with the fluoride salts ofand their bifluoride pyridinium salts cationsof their pyridinium were determined cations were (Figure determined 18)[43]. (Figure 18) [43]. and bifluoride salts of their pyridinium cations were determined (Figure 18) [43].

Figure 17. Top-down view of (C5H5N)3XeO3 in the X-ray crystal structure of (C5H5N)3XeO3. Copyright (2018) Elsevier. Used with permission from Ref. [43]. Figure 17. Top-down view of (C H N) XeO in the X-ray crystal structure of (C H N) XeO . Copyright Figure 17. Top-down view of (C5 5H5 5N)3 3XeO3 3 in the X-ray crystal structure of (C5 55H5N)3 3XeO3 3. Copyright (2018)(2018) Elsevier. Elsevier. Used Used with with permission permission from from Ref. Ref. [43 [43].].

(a) (b)

Figure 18. (a) The X-ray crystal structure of [C5H5NH]4[HF2]2[F]2(XeO3)2; (b) The X-ray crystal

structure of [4-(CH3)2NC5H4NH][HF2]XeO3. Copyright (2018) Elsevier. Used with permission from Ref. [43].

3. Krypton Beside xenon, krypton is the only noble gas where isolable compounds in macroscopic amounts can be prepared [44]. Its chemistry is limited to the +2 oxidation state. Known krypton(II) compounds are less stable than corresponding xenon(II) salts. Compounds in which KrF2 serves as a ligand towards Lewis centers have been only recently prepared [45]. The latest example is the synthesis and

Molecules 2020, 25, x FOR PEER REVIEW 12 of 19

Figure 16. The structural units in the crystal structures of (a) [(CH3)2CO]3XeO3; (b) [(CH3)2SO]3(XeO3)2; (c) (C5H5NO)3(XeO3)2; (d) [(C6H5)3PO]2XeO3. Copyright (2019) Wiley. Used with permission from Ref. [42].

Adducts between XeO3 and N-bases (pyridine and 4-dimethylaminopyridine) were also reported (Figure 17) [43]. Additionally, the crystal structures of the XeO3 adducts with the fluoride and bifluoride salts of their pyridinium cations were determined (Figure 18) [43].

Figure 17. Top-down view of (C5H5N)3XeO3 in the X-ray crystal structure of (C5H5N)3XeO3. Copyright Molecules 2020, 25, 3014 12 of 18 (2018) Elsevier. Used with permission from Ref. [43].

(a) (b)

FigureFigure 18. 1(8a.) ( Thea) The X-ray X-ray crystal crystal structure structure of of [C [C5H5H55NH]44[HF[HF2]2[F]]2[F]2(XeO2(XeO3)2; 3(b)2);( Theb) The X-ray X-ray crystal crystal structurestructure of [4-(CH of [4-(CH3)2NC3)2NC5H5H4NH][HF4NH][HF2]]XeOXeO3.3 Copyright. Copyright (2018) (2018) Elsevier. Elsevier. Used Used with with permission permission from from Ref. [Ref.43]. [43].

3. Krypton3. Krypton BesideBeside xenon, xenon, krypton krypton is is the the only only noblenoble gas where where isolable isolable compounds compounds in macroscopic in macroscopic amounts amounts can becan prepared be prepared [44 ].[44 Its]. Its chemistry chemistry is is limited limited to the the +2+2 oxidation oxidation state. state. Known Known krypton(II) krypton(II) compounds compounds are less stable than corresponding xenon(II) salts. Compounds in which KrF2 serves as a ligand are less stable than corresponding xenon(II) salts. Compounds in which KrF2 serves as a ligand towards LewisMoleculestowards centers 2020, Lewis25 have, x FOR centers been PEER onlyhave REVIEW been recently only preparedrecently prepared [45]. The [45 latest]. The latest example example is the is synthesisthe synthesis and and crystal13 of 19 structure determination of [Hg(KrF2)8][AsF6]2 2HF [45]. It is the ﬁrst homoleptic KrF2 coordination crystal structure determination of [Hg(KrF2·)8][AsF6]2∙2HF [45]. It is the first homoleptic KrF2 complex of a metal cation (Figure 19). coordination complex of a metal cation (Figure 19).

2+ Figure 19. The [Hg(KrF2)8] cation in the single-crystal X-ray structure of [Hg(KrF2)8][AsF6]2 2HF 2+ · viewedFigure down19. The the [Hg(KrFC2 axis.2) Copyright8] cation (2018)in the Wiley. single- Usedcrystal with X-ray permission structure from of [Hg(KrF Ref. [45].2)8][AsF6]2·2HF viewed down the C2 axis. Copyright (2018) Wiley. Used with permission from Ref. [45]. The KrF CrOF and KrF 2CrOF adducts were synthesized and their crystal structures 2· 4 2· 4 determinedThe KrF [282∙CrOF]. The4 crystaland KrF structure2∙2CrOF of4 theadducts former were comprises synthesized of F–Kr–F and Cr(O)F their crystalspecies structures in which ··· 4 KrFdetermined2 is weakly [28 coordinated]. The crystal to structure CrOF4 (Figure of the 20former). In the comprises latter both of F ﬂuorine–Kr–F∙∙∙Cr(O) atomsF of4 species KrF2 coordinate in which toKrF CrOF2 is weaklyto form coordinated F (O)Cr F–Kr–F to CrOFCr(O)F4 (Figure. 20). In the latter both fluorine atoms of KrF2 coordinate 4 4 ··· ··· 4 to CrOF4 to form F4(O)Cr∙∙∙F–Kr–F∙∙∙Cr(O)F4.

(a) (b)

Figure 20. (a) The X-ray crystal structures of the near-staggered conformation of KrF2·CrOF4; (b) The

structural unit in the X-ray crystal structure of syn-KrF2·2CrOF4. Copyright (2019) Wiley. Used with permission from Ref. [28].

4. Argon, Neon, Helium In the gas phase all Ng elements including lighter representatives form an exceptionally large family of molecular species, ranging from fragile van der Waals adducts to strongly bound covalent species [2]. Some selected examples from the past include [Ar–N2]+ [46], HCCRg2+ (Rg = Ar and Kr) [47], ArCF22+ [48] and NeH+ [49]. ArH+ was detected in the Crab Nebula, a supernova remnant. It was the first Ng molecule ever found in nature [50]. Charged Ng species, as for example (XeHXe)⁺, (KrHKr)⁺, and (KrHXe)⁺ [51], were synthesized in cold matrix.

4.1. Chemistry of Argon Experimental chemistry of argon is limited to studies in the low-temperature matrices and molecules observed in the gas phase. The HArF is presently the only experimental known neutral molecule containing a chemically bound argon atom that is stable in a low temperature argon matrix

Molecules 2020, 25, x FOR PEER REVIEW 13 of 19 crystal structure determination of [Hg(KrF2)8][AsF6]2∙2HF [45]. It is the first homoleptic KrF2 coordination complex of a metal cation (Figure 19).

Figure 19. The [Hg(KrF2)8]2+ cation in the single-crystal X-ray structure of [Hg(KrF2)8][AsF6]2·2HF

viewed down the C2 axis. Copyright (2018) Wiley. Used with permission from Ref. [45].

The KrF2∙CrOF4 and KrF2∙2CrOF4 adducts were synthesized and their crystal structures determined [28]. The crystal structure of the former comprises of F–Kr–F∙∙∙Cr(O)F4 species in which KrFMolecules2 is weakly2020, 25 ,coordinated 3014 to CrOF4 (Figure 20). In the latter both fluorine atoms of KrF2 coordinate13 of 18 to CrOF4 to form F4(O)Cr∙∙∙F–Kr–F∙∙∙Cr(O)F4.

(a) (b)

Figure 20. (a) The X-ray crystal structures of the near-staggered conformation of KrF CrOF ;(b) The Figure 20. (a) The X-ray crystal structures of the near-staggered conformation of KrF2·CrOF· 44; (b) The structural unit in the X-ray crystal structure of syn-KrF 2CrOF . Copyright (2019) Wiley. Used with structural unit in the X-ray crystal structure of syn-KrF22·2CrOF· 44. Copyright (2019) Wiley. Used with permissionpermission from from Ref. Ref. [ [2828].]. 4. Argon, Neon, Helium 4. Argon, Neon, Helium In the gas phase all Ng elements including lighter representatives form an exceptionally large In the gas phase all Ng elements including lighter representatives form an exceptionally large family of molecular species, ranging from fragile van der Waals adducts to strongly bound covalent family of molecular species, ranging from fragile van der Waals adducts+ to strongly 2bound+ covalent species [2]. Some selected examples from the past include [Ar–N2] [46], HCCRg (Rg = Ar and species [2]. Some2+ selected examples+ from the+ past include [Ar–N2]+ [46], HCCRg2+ (Rg = Ar and Kr) Kr) [47], ArCF2 [48] and NeH [49]. ArH was detected in the Crab Nebula, a supernova remnant. [47], ArCF22+ [48] and NeH+ [49]. ArH+ was detected in the Crab Nebula, a supernova remnant. It was It was the ﬁrst Ng molecule ever found in nature [50]. Charged Ng species, as for example (XeHXe)+, the first Ng molecule ever found in nature [50]. Charged Ng species, as for example (XeHXe)⁺, (KrHKr)+, and (KrHXe)+ [51], were synthesized in cold matrix. (KrHKr)⁺, and (KrHXe)⁺ [51], were synthesized in cold matrix. 4.1. Chemistry of Argon 4.1. Chemistry of Argon Experimental chemistry of argon is limited to studies in the low-temperature matrices and moleculesExperim observedental chemistry in the gas of phase. argon The is limited HArF is to presently studies in the the only low experimental-temperatureknown matrices neutral and moleculesmolecule containingobserved ain chemically the gas phase. bound The argon HArF atom is thatpresently is stable the in aonly low experimental temperature argon known matrix neutral [52]. moleculeAnother containing story is the a numberchemically of reportedbound argon charged atom species that is stable detected in a inlow the temp gaserature phase. argon Some matrix recent + examples include observation of ArCH2 in mass spectrometry experiments [53] and the production of ArOH+ molecular ion in a pulsed discharge/supersonic expansion of argon seeded with water + + + + vapor [54]. The cation complexes [ArB3O4] , [ArB3O5] , [ArB4O6] , and [ArB5O7] were prepared via laser vaporization supersonic ion source in the gas phase [55]. Superelectophilic fragment anion 2 [B12(CN)11]−, generated from the most stable gas-phase dianion [B12(CN)12] −, binds argon covalently at room temperature [56].

4.2. Chemistry of Neon and Helium Astrophysical detection of HeH+ in nearby interstellar space [57] is one of the greatest discoveries in molecular astrophysics. It was the first molecule to form after the big bang [58]. The synthesis of the isolable compounds containing neon and helium still remains an open challenge [8]. In 1992 it was reported that high pressure stabilizes the formation of a solid van der Waals compound of composition He(N2)11, obtained by compression of helium–nitrogen mixtures [59]. A few years ago, the discovery of non-inclusion compound Na2He at pressures higher than 113 GPa was reported [60]. It was described as an electride, i.e., a crystal made of positively charged ionic cores and with strongly localized valence electrons playing the role of anions. Peculiar Na2He calls for making our definitions of “compound” more precise [8]. Defect perovskites (He2-xx)(CaZr)F6 can be prepared by inserting helium into CaZrF6 at high pressure [61]. Despite these, helium and also neon have not been forced to form genuine chemical compounds in neutral entities to this day [8]. One of the first steps towards a stable neon compound is claimed to be the experimental observation of the molecular anion [B12(CN)11Ne]− [62]. Molecules 2020, 25, 3014 14 of 18

5. Bonding Motifs in Noble-Gas Compounds Despite the inertness of noble gases, Ng chemistry is rich in a variety of species with different bonding motifs [2,4,63]. The bonding motifs are a consequence of the polarization of the spherical electronic cloud of the Ng atoms by binding partners and they range from very weak ‘dispersion’ to stronger ‘induced-dipole’ interactions resulting in neutral and ionic ‘complexes’ of the noble gases, whose character ranges from fragile van der Waals adducts to definitely stable compounds [2]. Clusters of Ng atoms are held together by dispersion forces [2]. In various monocoordinated Ng species [X(Ng)n (n 1)], the character of the occurring interactions may span from weakly bound van ≥ der Waals adducts, held together by dispersion forces, to strongly bound covalent species [2]. Dicoordinated (‘inserted’) compounds have a general formula XNgY (X different or the same as Y), the Ng atom being involved in definitely recognizable interactions with both X and Y [2]. They can be neutral or ionic. The bonding of the noble-gas hydrides with the common formula HNgY compounds + + can be described as (HNg) Y− where (HNg) is mainly covalently bonded and the interaction of the + (HNg) and Y− is strongly ionic [63]. The most frequently drawn picture of the chemical bonding in XeF2 is the molecular orbital approach involving three-center, four-electron bonds 3-center 4-electron bonding (3c–4e) [28,64,65]. A single bond is thus spread over the F-Xe-F system. + XeF2 is a fluoride ion donor and it can form everything from weakly bond complexes to XeF salts with ionic formulation. In the weakly bond complexes [66], the XeF2 geometry stays intact. In XeF adducts as XeF CrOF [28] there is a slight elongation of Xe–F (F = bridging F atom 2 2· 4 b b interacting with Cr) bond, while the Xe–Ft (Ft = terminal F atom) bond is slightly shortened [2]. The elongation (weakening) of Xe–Fb and shortening (strengthening) of Xe–Ft bond is observed also in n+ n [M(XeF2)x] [A] − compounds where XeF2 is coordinated only by one of its fluorine atoms [27,67]. 1 The distortion of XeF2 is visible in its Raman spectrum. Its vibrational band at 497 cm− is replaced 1 by two vibrational bands. One is higher and one lower than 497 cm− [67]. Numerous products of + reactions between XeF2 and various fluorides are formulated as [XeF] salts [28]. Although their formulations imply a simple ionic nature, there is no complete F− transfer between XeF2 and fluoride ion acceptor [28]. The [XeF]+ cation and its anion interact through Xe–F M bridges and the difference ··· between Xe–Fb and Xe–Ft bond lengths is much more pronounced. Structural and vibrational evidences for the ionization pathway XeF XeF+ + F were obtained studying the XeF /XeF AsF system [68] 2 → − 2 5 6 where at least five distinct well-characterized phases exist in the phase diagram of the XeF2/XeF5AsF6 mixture, and each of them exhibit a more or less pronounced dissociation of XeF2 into the ionic [XeF+] [A–F ] form (A = Lewis acid) [63]. ··· − The bonding in XeF6 has caused considerable controversy that is not completely resolved [69–71]. The XeF6 is the strongest fluoride ion donor among XeF2, XeF4, and XeF6. It forms a large number of + + + [XeF5] and [Xe2F11] salts with a variety of Lewis acid fluorides and oxyfluorides [4,5]. The [XeF5] cation geometry may be described in terms of pseudo-octahedral AX5E VSEPR arrangements of five bond pairs (X) and the lone electron pair (E) around Xe (A) which give rise to a square-pyramidal + geometry of Xe and five F atoms [38,72]. Although these compounds are ionic in their nature, the [XeF5] and [Xe F ]+ cations are extensively associated with their anions through Xe F (F belongs to the 2 11 ··· anion) secondary bonding interactions [73]. In XeO3 adducts, the xenon-ligand bonds may be described as predominantly electrostatic, (weakly covalent) interactions between the highly electrophilic σ-holes of the xenon atom and the electronegative ligand atom [42].

6. Conclusions This review shows that noble-gas chemistry is still interesting not just to chemists (at least to some of us) but also to astronomers, geologists, etc. For decades, astronomers have pursued for helium hydride HeH+, made of the two most common elements in the universe. In the laboratory the ion was discovered in 1925 [74]; however, we had to wait almost 100 years for the confirmation of the existence of HeH+ in nearby interstellar space [57]. High-pressure and high-temperature (by laser-heating) Molecules 2020, 25, 3014 15 of 18 study of the possible formation of stable compounds between Ar and Ni at thermodynamic conditions representative of the Earth’s core resulted in the formation of ArNi compound (at 140 GPa and 1500 K) [75]. This result implies that the presence of argon in the Earth’s core is highly probable [75]. Recent review about the noble-gas/noble-metal chemistry suggests an inflation of such complexes, not only in theory or in microscopic amount in cryogenic situation, but also in large-scale syntheses [76]. Additional stimulation in the noble-gas research represents the discovery of noble gas (or aerogen) bonding [77]. It is a novel type of weak attractive noncovalent interaction [78]. According to IUPAC is defined as the attractive interaction between an electron rich atom or group of atoms and any element of Group-18 acting as electron acceptor [79,80].

Funding: The author acknowledges the financial support from the Slovenian Research Agency (research core funding No. P1–0045; Inorganic Chemistry and Technology). Acknowledgments: We thank the anonymous reviewers for their constructive comments. Conflicts of Interest: The author declares no conflict of interest.

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