Hydrogen-bonded salt cocrystals of xenon difluoride and protonated perfluoroamides

Abstract

The hydrogen-bonding ability of XeF₂ is an important factor influencing its chemical properties and reactivity, yet structurally characterised examples of hydrogen-bonded xenon fluorides remain rare. In this work, three salt cocrystals containing hydrogen-bonded xenon difluoride and hexafluoridoarsenate salts of protonated perfluoroamides—CF₃C(OH)NH₂[AsF₆]·XeF₂, C₂F₅C(OH)NH₂[AsF₆]·XeF₂, and C₃F₇C(OH)NH₂[AsF₆]·XeF₂—were synthesised and structurally characterised. Diverse hydrogen-bonding motifs were observed, and the first crystallographically characterised examples of N–H⋯FXeF hydrogen bonds are presented. In total, eleven new crystal structures are reported, including two perfluoroamides, three protonated and two hemiprotonated perfluoroamides, and one salt cocrystal containing an oxonium ion. The XeF₂-containing cocrystals demonstrate that XeF₂ reliably functions as a hydrogen-bond acceptor and readily forms hydrogen-bonded cocrystals. These findings broaden the scope of noble-gas chemistry and highlight the potential of noble-gas fluorides for cocrystal formation.

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Introduction

Xenon difluoride (XeF₂) is the most common and extensively studied binary noble-gas fluoride and serves as a precursor to a wide range of xenon compounds.^1,2 It is a nonpolar molecular compound with linear geometry. XeF₂ is a good fluoride-ion donor³ and thus forms a variety of Lewis acid–base adducts⁴ and a plethora of coordination compounds with metal cations.^1,2,5

The ability of XeF₂ to act as a hydrogen-bond acceptor strongly influences its physical and chemical properties. As a nonpolar molecule, it is highly soluble in the polar protic solvent anhydrous HF (aHF) (167 g/100 g at 30 °C).⁶ This unusually high solubility arises from the formation of FXe–F⋯HF hydrogen bonds.^7–9 Furthermore, XeF₂ dissolved in aHF is a considerably more potent oxidiser than pure XeF₂, and even trace amounts of HF can catalyse its reactions with organic substrates through the hydrogen-bonding induced polarisation [FXe^δ+–F^δ−⋯HF].¹⁰ HF also facilitates fluorine exchange in XeF₂, enabling the synthesis of ¹⁸F-radiolabelled XeF₂.^11,12 In certain cases, the influence of HF is so pronounced that it can unexpectedly alter reaction outcomes, even when inadvertently generated by reaction with the vessel material.¹³

Despite the ability of XeF₂ to act as a hydrogen-bond acceptor, systematic crystallographic investigations are absent, and reported solid-state examples remain scarce. To date, only a handful of crystallographically characterised examples of hydrogen-bonded XeF₂ have been described. These include O–H⋯FXeF hydrogen bonds observed in H₃O[AsF₆]·2XeF₂ and in HNO₃·XeF₂ cocrystals,^14,15 as well as an F–H⋯FXeF interaction observed in the coordination complex [Cd(HF)₂(XeF₂)(MF₆)₂] (M = Ta, Nb).^5,16

It has also been shown spectroscopically that protonated trifluoroacetamide (CF₃CONH₂) forms a hydrogen-bonded salt cocrystal¹⁷ with XeF₂, CF₃C(OH)NH₂[AsF₆]·XeF₂·xHF.¹⁸ This cocrystal is particularly noteworthy, as it may feature both =OH⁺ and –NH₂ groups as hydrogen-bond donors,¹⁸ potentially offering insight into the hydrogen-bonding preferences of XeF₂.

To investigate the hydrogen-bonding propensity of XeF₂ in the solid state and its tendency to form cocrystals with NH and OH hydrogen-bond donors, the crystal structures of XeF₂ salt cocrystals with protonated CF₃CONH₂, C₂F₅CONH₂, and C₃F₇CONH₂ were studied in this work. The perfluoroamides were selected because of their anticipated resistance to oxidative-fluorination by XeF₂.

Results and discussion

Crystal structures of CF₃CF₂CONH₂ and CF₃CF₂CF₂CONH₂

The crystal structures of pentafluoropropionamide (C₂F₅CONH₂) and heptafluorobutyramide (C₃F₇CONH₂) were elucidated by low-temperature single-crystal X-ray diffraction (LT SCXRD) (Tables 1 and S1), whereas the crystal structure of CF₃CONH₂ has been previously reported at 295 K and 110 K.^19,20 For comparison of bond lengths (Table S2), only the structure obtained at 110 K was considered.²⁰

Table 1 Summary of crystal data and refinement results for crystal structures of amides and protonated amides

Compound	C2F5CONH2	C3F7CONH2	CF3C(OH)NH2[AsF6]	C2F5C(OH)NH2[AsF6]	C3F7C(OH)NH2[AsF6]
Space group	C2/c	P1̄	P21/c	Pccn	P21/c
a (Å)	21.7871(5)	5.11713(18)	9.81910(18)	8.12957(13)	6.17592(15)
b (Å)	5.11704(12)	5.27137(14)	7.90095(13)	25.2768(4)	7.94187(19)
c (Å)	10.0754(3)	12.7768(3)	20.5015(4)	9.34322(16)	21.7914(5)
α (°)	90	95.467(2)	90	90	90
β (°)	98.140(2)	91.890(3)	98.6498(18)	90	96.014(2)
γ (°)	90	105.584(3)	90	90	90
V (Å^3)	1111.94(5)	329.847(18)	1572.42(5)	1919.93(5)	1062.95(4)
M	163.06	213.07	302.97	352.98	402.99
Z	8	2	8	8	4
T (K)	100	100	100	100	100
R[F^2 > 2σ(F^2)]	0.048	0.028	0.033	0.027	0.027
wR(F^2)	0.138	0.076	0.087	0.058	0.068

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C₂F₅CONH₂ (Fig. 1a and S1) crystallizes in the monoclinic space group C2/c with Z = 8. The C=O bond length (1.2323(19) Å) is comparable to the distances observed in the crystal structures of other primary amides, and the same applies to the C–N bond (1.317(2) Å).²¹ Two N–H⋯O hydrogen bonds (2.912(2) Å, 171(2)°; 2.8396(17) Å, 149(2)°; Table S3) in the crystal structure form R²₂(8) and R⁴₆(16) hydrogen-bonding motifs,²² which assemble into a corrugated layer parallel to the bc plane (Fig. S2 and S3).

Fig. 1 R²₂(8) hydrogen-bonding motifs in the crystal structures of (a) C₂F₅CONH₂ and (b) C₃F₇CONH₂. Hydrogen bonds are shown as dashed orange lines. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

C₃F₇CONH₂ (Fig. 1b and S4) crystallizes in the triclinic space group P1̄ with Z = 2. The C=O bond distance (1.2293(15) Å) is essentially identical to that in C₂F₅CONH₂, as is the C–N bond (1.3162(16) Å). These bond distances are shorter than the corresponding ones observed in non-fluorinated secondary amides, such as capsaicin.²³ Two N–H⋯O hydrogen bonds (Table S4) are present in the crystal structure (2.9313(14) Å, 174.3(16)°; 2.8495(14) Å, 140.4(15)°), which fall within the typical range for amide molecules.²¹ The R²₂(8) and R²₄(8) hydrogen-bond motifs link the molecules into a ladder along the a-crystallographic axis (Fig. S5).

Protonation of amides in superacidic media HF–AsF₅

All amides are soluble in aHF and readily undergo protonation upon addition of AsF₅. In all cases, protonation occurs at the oxygen atom, consistent with previous observations.^18,24–26 Low-temperature crystallisation from aHF afforded crystals of suitable quality for SCXRD.

CF₃C(OH)NH₂[AsF₆] (Tables 1, S1 and S2; Fig. S6) crystallises in the monoclinic space group P2₁/c with Z = 8 and Z′ = 2. Upon protonation, the C=O bonds (1.2795(19), 1.282(2) Å) lengthen and the C–N bonds (1.279(2), 1.281(2) Å) shorten relative to those in CF₃CONH₂ (1.2304(12) and 1.3164(13) Å, respectively).²⁰ These changes in the C=O and C–N bond lengths are consistent with previous crystallographic studies of protonated amides.^25–27 The O–H⋯F hydrogen bonds (2.5860(16) Å, 2.6530(18) Å; Table S5) bracket the value observed in CF₃C(OH)NH₂[SbF₆] (2.600(1) Å), whereas the N–H⋯F hydrogen bonds (2.8236(18)–3.0797(18) Å) are comparable to those in CF₃C(OH)NH₂[SbF₆] (2.884(2), 2.933(2) Å).²⁶ All hydrogen-bond angles (121(2)–179(3)°) fall within the typical range. The [AsF₆]⁻ anions deviate from ideal octahedral geometry, with the longest As–F bonds (1.7524(10), 1.7557(10) Å) participating in hydrogen bonding with =OH⁺ group. In the crystal structure, cations and anions are linked through O–H⋯F and N–H⋯F hydrogen-bonded chains (Fig. 2a and S7).

Fig. 2 Crystal structure of (a) the hydrogen-bonded chain in CF₃C(OH)NH₂[AsF₆] and (b) the discrete hydrogen-bonded cluster in C₂F₅C(OH)NH₂[AsF₆] (only one orientation of the disordered –C₂F₅ unit is shown). Hydrogen bonds are shown as dashed orange lines. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

C₂F₅C(OH)NH₂[AsF₆] (Tables 1, S1 and S2) crystallises in the orthorhombic space group Pccn with Z = 8 and features a disordered –C₂F₅ moiety (Fig. S8). Perfluorinated alkyl chains frequently exhibit disorder in the crystalline state,²⁸ as F⋯F interactions are relatively weak,²⁹ and can therefore adopt various conformations. The C=O (1.2821(15) Å) and C–N (1.2772(16) Å) bonds are longer and shorter, respectively, than those in C₂F₅CONH₂. The [AsF₆]⁻ anion deviates from ideal octahedral geometry, with the mer-As–F bonds involved in hydrogen bonding being longer (1.7253(8)–1.7453(8) Å) than the remaining As–F bonds (1.6976(8)–1.7111(8) Å). The hydrogen bonds (Table S6) between the =OH⁺ and –NH₂ groups and the [AsF₆]⁻ anions (O(H)⋯F, 2.6006(12) Å, 172(2)°; N(H)⋯F, 2.8309(13) Å, 174(2)° and 2.8316(14) Å, 161.3(19)°) lead to the formation of discrete units (Fig. 2b, S8 and S9), exhibiting R²₂(8) and R⁴₄(12) hydrogen-bonding motifs.

C₃F₇C(OH)NH₂[AsF₆] (Tables 1, S1 and S2) crystallises in the monoclinic space group P2₁/c with Z = 4, with the [AsF₆]⁻ anion disordered over two positions (Fig. S10). The C=O bond (1.2797(14) Å) is elongated, and the C–N bond (1.2841(16) Å) is shortened compared to those in C₃F₇CONH₂. A similar C=O(H) bond distance (1.274(2) Å) was observed in the crystal structure of (C₆F₅)₂COH[AsF₆].³⁰ Hydrogen bonds (Table S7) are formed between the =OH⁺ group (2.541(3), 2.557(3) Å; 157(3), 165(3)°) or the –NH₂ group (2.737(4)–3.179(5) Å, 118.0(19)–168.1(19)°) and the [AsF₆]⁻ anions. The O⋯F hydrogen bond is the shortest among the protonated amides in this study, and also shorter than those in CH₃C(OH)NH₂[AsF₆]²⁵ and CF₃C(OH)NH₂[SbF₆].²⁶ The [AsF₆]⁻ anion deviates from ideal octahedral geometry (1.642(3)–1.795(2) Å). The C₃F₇C(OH)NH₂⁺ cations and [AsF₆]⁻ anions are linked into a hydrogen-bonded ribbon (Fig. 3 and S11), exhibiting conjoined R⁴₄(12), R⁴₂(8) and R²₂(8) motifs.

Fig. 3 Hydrogen-bonded ribbon in C₃F₇C(OH)NH₂[AsF₆] (only one orientation of the disordered [AsF₆]⁻ anion is shown). Hydrogen bonds are shown as dashed orange lines. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

Two hemiprotonated salts, (CF₃CONH₂)₂H[AsF₆] and (C₃F₇CONH₂)₂H[AsF₆] (Fig. 4 and S12–S17; Tables 2, S1 and S2), were also crystallographically characterised. The former was inadvertently found during the low-temperature crystal selection and mounting of the CF₃C(OH)NH₂[AsF₆]·XeF₂ sample, whereas the latter was identified as an impurity in the sample of C₃F₇C(OH)NH₂[AsF₆]·XeF₂ salt cocrystals. Both compounds crystallise in the triclinic space group P1̄ with Z = 2. In both structures, the C=O bonds are elongated (1.283(6) Å in (CF₃CONH₂)₂H[AsF₆]; 1.2652(9), 1.2459(9) Å in (C₃F₇CONH₂)₂H[AsF₆]), whereas the C–N bonds are shortened (1.274(7) Å in (CF₃CONH₂)₂H[AsF₆]; 1.2942(10), 1.3027(10) Å in (C₃F₇CONH₂)₂H[AsF₆]) compared to the non-protonated amides.²⁰ The values for one of the amide molecules in (CF₃CONH₂)₂H[AsF₆] fall within the range for neutral amide,²⁰ owing to the relatively high standard uncertainties of the bond lengths. The O(H)⋯O hydrogen bond length in (CF₃CONH₂)₂H[AsF₆] (2.426(5) Å, 170(8)°) is essentially identical to that in (C₃F₇CONH₂)₂H[AsF₆] (2.4174(9) Å, 172(2)°) (Tables S8 and S9), and comparable to literature values for such hydrogen-bonded systems.²¹ The nearly equidistant position of the hydrogen atom (O–H, H⋯O: 1.13(9), 1.31(9) Å in (CF₃CONH₂)₂H[AsF₆]; 1.06(2), 1.36(2) Å in (C₃F₇CONH₂)₂H[AsF₆]), together with the relatively short O⋯O distances, indicates strong, positive charge-assisted hydrogen bonding, (+)CAHB.³¹ These structures represent rare examples of proton sharing between two primary amide molecules,^21,32 a motif more commonly observed in secondary and tertiary amides.²¹ The –NH₂ groups are hydrogen-bonded to [AsF₆]⁻ anions (N⋯F, 2.644(5)–3.005(5) Å in (CF₃CONH₂)₂H[AsF₆]; 2.8168(9)–3.1046(9) Å in (C₃F₇CONH₂)₂H[AsF₆]) (Fig. S13–S17), resulting in the formation of ribbons that are further interconnected by the anions into layers parallel to the ab plane.

Fig. 4 Hydrogen-bonded dimers in the crystal structure of (a) (CF₃CONH₂)₂H[AsF₆] and (b) (C₃F₇CONH₂)₂H[AsF₆]. The short O–H⋯O=C hydrogen bonds are shown as dashed orange lines. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

Table 2 Summary of crystal data and refinement results for hemiprotonated amides and H₃O[AsF₆]·2CF₃CONH₂ salt cocrystal

Compound	(CF3CONH2)2H[AsF6]	(C3F7CONH2)2H[AsF6]	H3O[AsF6]·2CF3CONH2
Space group	P1̄	P1̄	Pnma
a (Å)	5.2815(3)	5.32051(4)	11.5349(2)
b (Å)	10.1517(6)	10.45222(9)	14.0649(3)
c (Å)	12.4911(6)	16.10170(12)	7.78994(14)
α (°)	108.936(5)	90.5740(6)	90
β (°)	93.107(5)	90.8760(6)	90
γ (°)	102.904(5)	103.0402(7)	90
V (Å^3)	611.63(6)	872.156(12)	1263.82(4)
M	416.02	616.06	434.04
Z	2	2	4
T (K)	100	100	100
R[F^2 > 2σ(F^2)]	0.047	0.022	0.037
wR(F^2)	0.125	0.055	0.085

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A crystal of H₃O[AsF₆]·2CF₃CONH₂ (Tables 2, S1 and S2; Fig. 5 and S18–S20) was fortuitously found during the low-temperature crystal selection and mounting of the CF₃C(OH)NH₂[AsF₆]·XeF₂ sample. It crystallises in the orthorhombic space group Pnma with Z = 4. The amide molecule is not protonated, resulting in C=O (1.236(4) Å) and C–N (1.304(4) Å) bond lengths that are close to those in CF₃CONH₂.²⁰ The amide molecule acts as both a hydrogen-bond donor and acceptor (Table S10, Fig. S19), forming N–H⋯F(As) and N–H⋯O(C) hydrogen bonds. An R²₂(8) motif is observed between two amide molecules, with the N⋯O hydrogen bond (2.959(4) Å, 171(4)°) comparable to that found in CF₃CONH₂.²⁰ The H₃O⁺ cation forms three hydrogen bonds: two symmetrically equivalent O–H⋯O(C) (2.525(3) Å, 167(4)°) and one O–H⋯F hydrogen bond (2.657(5) Å, 173(7)°) with the [AsF₆]⁻ anion. Together, these hydrogen bonds form a hydrogen-bonded cluster represented by R⁶₆(20), R⁴₆(14) and R²₂(8) graph-set motifs²² (Fig. 5), which further extend into a layer parallel to the bc plane (Fig. S20).

Fig. 5 R⁴₆(14) hydrogen-bonded cluster in the crystal structure of the H₃O[AsF₆]·2CF₃CONH₂ salt cocrystal. Hydrogen bonds are shown by dashed orange lines. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

Hydrogen-bonded salt cocrystals of XeF₂

The reaction of amides with equimolar amounts of [XeF][AsF₆] at temperatures down to −30 °C leads to the formation of RC(OH)NH₂[AsF₆]·XeF₂ salt cocrystals. This indicates that a proton from HF is transferred to the amide, generating a protonated amide, while the resulting fluoride anion reacts with [XeF]⁺ to form XeF₂. This behaviour was also reported in a previous study of the CF₃CONH₂–[XeF][AsF₆] system.¹⁸

The salt cocrystals (Tables 3, S1 and S2) thus feature protonated amides cocrystallised with XeF₂ and exhibit a rare O–H⋯FXeF hydrogen bond, as well as the first crystallographically characterised examples of N–H⋯FXeF hydrogen bonds.

Table 3 Summary of crystal data and refinement results for salt cocrystals of protonated amides with XeF₂

Compound	CF3C(OH)NH2[AsF6]·XeF2	C2F5C(OH)NH2[AsF6]·XeF2	C3F7C(OH)NH2[AsF6]·XeF2
Space group	P21/n	Aea2	Pnna
a (Å)	7.41785(9)	8.67561(10)	8.62011(14)
b (Å)	9.84875(11)	31.0125(4)	35.5418(5)
c (Å)	14.90113(17)	8.65174(9)	8.71910(12)
α (°)	90	90	90
β (°)	99.4517(11)	90	90
γ (°)	90	90	90
V (Å^3)	1073.85(2)	2327.77(4)	2671.31(7)
M	472.27	522.28	572.29
Z	4	8	8
T (K)	100	100	100
R[F^2 > 2σ(F^2)]	0.025	0.018	0.026
wR(F^2)	0.067	0.038	0.066

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CF₃C(OH)NH₂[AsF₆]·XeF₂ (Fig. 6 and S21) crystallises in the monoclinic space group P2₁/n with Z = 4. The XeF₂ molecule exhibits slight asymmetry in Xe–F bond distances (1.9669(10), 2.0237(9) Å) compared to pure XeF₂ (1.999(4) Å),³³ and it remains linear (178.10(5) Å). The asymmetry of XeF₂ is slightly smaller than that observed in XeF₂·HNO₃ (1.9737(8), 2.0506(8) Å).¹⁵

Fig. 6 Hydrogen-bonded ribbon in the crystal structure of the salt cocrystal CF₃C(OH)NH₂[AsF₆]·XeF₂. Only one orientation of the disordered –CF₃ moiety is shown. Hydrogen bonds are shown as dashed orange lines. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

CF₃C(OH)NH₂[AsF₆]·XeF₂ is the only salt cocrystal in this series that exhibits both O–H⋯F(Xe) and N–H⋯F(Xe) hydrogen bonds. One fluorine atom of XeF₂ is acting as a bifurcated acceptor (Fig. 6 and S21; Table S11). The O–H⋯F(Xe) hydrogen bond (2.5467(14) Å, 171(3)°) is shorter than that in H₃O[AsF₆]·2XeF₂ (2.571(3) Å)¹⁴ and HNO₃·XeF₂ (2.690(1) Å).¹⁵ It is also significantly shorter than the O–H⋯F(As) hydrogen bonds in CF₃C(OH)NH₂[AsF₆] and C₂F₅C(OH)NH₂[AsF₆], but comparable to that in C₃F₇C(OH)NH₂[AsF₆]. The N–H⋯F(Xe) hydrogen bonds (2.7865(15) Å, 151(3)°; 3.0894(16), 124(2)°), which involve a single bifurcated donor, are longer than those observed in the other two salt cocrystals described in this study. The C=O (1.2773(15) Å) and C–N (1.2772(16) Å) bond lengths are essentially identical to those in the protonated salts,²⁶ indicating a negligible influence of hydrogen bonding on the overall geometry of the CF₃C(OH)NH₂⁺ cation. The –CF₃ moiety is disordered, as also observed in the crystal structure of CF₃CONH₂.²⁰ The [AsF₆]⁻ anion participates in hydrogen bonding with the –NH₂ group (2.8270(18) Å, 176(3)°; 3.0594(15) Å, 111(2)°), resulting in a slight deviation from ideal octahedral geometry (As–F, 1.7006(12)–1.7417(11) Å).

Hydrogen bonds between CF₃C(OH)NH₂⁺ and XeF₂ form a zigzag chain parallel to the b-crystallographic axis, with pendant [AsF₆]⁻ anions connected to the chain via N–H⋯F(As) hydrogen bonds, giving rise to a ribbon-like structure (Fig. 6 and S22).

Both C₂F₅C(OH)NH₂[AsF₆]·XeF₂ and C₃F₇C(OH)NH₂[AsF₆]·XeF₂ (Tables 3, S1 and S2; Fig. S23–S26) crystallise in orthorhombic space groups, Aea2 and Pnna, respectively, with Z = 8. The asymmetry of the Xe–F bond lengths in C₂F₅C(OH)NH₂[AsF₆]·XeF₂ (1.9734(14), 2.0061(15) Å) and in C₃F₇C(OH)NH₂[AsF₆]·XeF₂ (1.9674(15), 2.0135(16) Å) is comparable. The shorter Xe–F bonds are similar to that observed in the trifluoroacetamide analogue, whereas the longer Xe–F bonds are significantly shorter. In both cocrystals, the F–Xe–F angle is essentially linear (179.88(9)°; 179.57(7)°).

The N–H⋯F(Xe) hydrogen bonds (Tables S12 and S13) in C₂F₅C(OH)NH₂[AsF₆]·XeF₂ (2.688(3), 2.729(3) Å) and C₃F₇C(OH)NH₂[AsF₆]·XeF₂ (2.692(3), 2.737(3) Å) are comparable and are significantly shorter than the corresponding hydrogen bonds in CF₃C(OH)NH₂[AsF₆]·XeF₂. The C=O (1.289(2), 1.285(3) Å) and C–N (1.279(3), 1.280(3) Å) bond lengths in C₂F₅C(OH)NH₂[AsF₆]·XeF₂ and C₃F₇C(OH)NH₂[AsF₆]·XeF₂ are almost identical to those observed in the corresponding protonated salts.

The protonated oxygen atom acts as a hydrogen-bond donor towards the [AsF₆]⁻ anions, forming bifurcated hydrogen bonds (Fig. 7, S23 and S25), which are longer than the O–H⋯F(As) hydrogen bonds observed in the parent protonated salts.

Fig. 7 Hydrogen bonds (dashed orange lines) in the crystal structures of the salt cocrystals (a) C₂F₅C(OH)NH₂[AsF₆]·XeF₂ and (b) C₃F₇C(OH)NH₂[AsF₆]·XeF₂. The OH groups are bifurcated hydrogen-bond donors; however, two [AsF₆]⁻ anions have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms are represented as spheres of arbitrary radius.

The packing in both C₂F₅C(OH)NH₂[AsF₆]·XeF₂ and C₃F₇C(OH)NH₂[AsF₆]·XeF₂ consists of hydrogen-bonded ribbons composed of alternating protonated amide and XeF₂ molecules, similar to those observed in CF₃C(OH)NH₂[AsF₆]·XeF₂. These ribbons are further connected by O–H⋯F(As) hydrogen bonds, and in the case of C₃F₇C(OH)NH₂[AsF₆]·XeF₂, also by N–H⋯F(As) hydrogen bonds (Fig. S24 and S26).

The relatively small difference in Xe–F bond lengths in the present XeF₂ cocrystals suggests that hydrogen bonding has only a minor influence on XeF₂ ionisation (XeF₂ → XeF⁺ + F⁻).³⁴ In particular, the shorter Xe–F bonds (1.9669(10)–1.9734(14) Å) are considerably longer than those found in [XeF]⁺ tight ion pairs^33,35,36 and in [Xe₂F₃]⁺ salts.^35,37 They are comparable to the shortest Xe–F bond lengths in XeF₂ adduct-salts with [BrOF₂]⁺ (1.956(5), 1.960(4) Å)³⁸ and [BrO₂]⁺ cations (1.970(4)–1.978(3) Å).³⁹ Nevertheless, the distortion of hydrogen-bonded XeF₂ observed in the present salt cocrystals is significant when compared with Xe–F bond distances observed in the crystal structures containing cocrystallised XeF₂, e.g., 3XeF₂·2MnF₄ (1.9933(7) Å),³⁶ and in the molecular cocrystals XeF₂·XeF₄ (1.9940(9) Å)³⁷ and XeF₂·XeOF₄ (2.014(5) Å),⁴⁰ in which XeF₂ is centrosymmetric.

Vibrational spectroscopy

To corroborate the findings from LT SCXRD and to gain further insight into the ionisation of XeF₂, low-temperature Raman spectra were measured (Fig. 8 and S27–S40). Two bands at 457–475 and 528–535 cm⁻¹ are observed in all XeF₂ salt cocrystals in this study, corresponding to the elongated and shortened Xe–F bond, respectively. These bands are significantly shifted from that of pure XeF₂ (497 cm⁻¹)⁴¹ and from values observed when cocrystallised XeF₂ does not participate in significant intermolecular interactions, such as in XeF₂·XeOF₄ (494, 503 cm⁻¹),⁴⁰ XeF₂·XeF₄ (505 cm⁻¹),⁴² 3XeF₂·2MnF₄ (508 cm⁻¹),³⁶ and XeF₂·N₂O₄ (509 cm⁻¹).¹⁵ The value of the higher-frequency band is comparable to the Raman shifts reported for the adduct salts [BrOF₂][AsF₆]·XeF₂ (531, 543, 559 cm⁻¹),³⁸ [BrO₂][AsF₆]·nXeF₂ (n = 1, 2; 516–546 cm⁻¹),³⁹ and for the hydrogen-bonded cocrystals H₃O[AsF₆]·2XeF₂ (552 cm⁻¹)¹⁴ and HNO₃·XeF₂ (529 cm⁻¹).¹⁵ However, these shifts are significantly smaller than those observed in [XeF]⁺ tight-ion pair salts (>600 cm⁻¹) and [Xe₂F₃]⁺ cations (580–600 cm⁻¹).^{1,33,35,36,43,44} The band around 535 cm⁻¹ is particularly noteworthy, as this value coincides with that observed for XeF₂ dissolved in aHF, which has been attributed to the FXe–F⋯HF hydrogen bonds.^7–9

Fig. 8 Raman spectra of XeF₂ salt cocrystals with protonated amides recorded at low temperatures (−90 °C). The green dashed line is placed at the position of free XeF₂ (497 cm⁻¹)⁴¹ which was observed as an impurity in the reactions.

In addition to the bands attributed to XeF₂, those arising from [AsF₆]⁻ anions are observed around 375 and 680 cm⁻¹.^18,25,26,45 Vibrations from the protonated amide molecules are also present, including an intense band around 800 cm⁻¹ corresponding to ν(C–C),^25,26 and peaks near 1100 cm⁻¹ attributed to C–F vibrations.^18,26,46 In all protonated amides, the N–H stretching vibrations were observed in 3150–3400 cm⁻¹ range.^18,26,46

Experimental

Caution! Anhydrous HF, AsF₅, XeF₂, [XeF][AsF₆] and the compounds prepared in this study are highly reactive and hazardous. The amides used may cause skin, eye, and respiratory irritation. Contact with the skin must be avoided, and all compounds should be handled exclusively in a well-ventilated fume hood.

Appropriate safety precautions must be observed at all times, and working with minimal quantities is strongly recommended.

Materials and methods

Reactions were carried out in fluorinated ethylene propylene (FEP) vessels equipped with Kel-F or PTFE valves. All vessels were passivated with fluorine prior to use. Volatile substances were handled using a fluorine-resistant metal vacuum line, whereas solids were manipulated inside an N₂-filled glovebox. Detailed synthetic procedures are provided in the SI. Characterisation was performed by low-temperature single-crystal X-ray diffraction and low-temperature Raman spectroscopy. Single-crystal selection and mounting were carried out using a low-temperature crystal-mounting apparatus, as described previously (SI).^30,36,47 Low-temperature Raman spectra were recorded directly on the aluminium trough used for mounting single crystals for X-ray diffraction measurements.

Conclusions

In this work, the perfluoroamides trifluoroacetamide (CF₃CONH₂), pentafluoropropionamide (C₂F₅CONH₂), and heptafluorobutyramide (C₃F₇CONH₂), were protonated in superacidic medium HF–AsF₅, and the crystal structures of the resulting salts, CF₃C(OH)NH₂[AsF₆], C₂F₅C(OH)NH₂[AsF₆], and C₃F₇C(OH)NH₂[AsF₆] were elucidated. Protonation at the carbonyl oxygen atom is consistently observed. In addition, the crystal structures of the amides C₂F₅CONH₂ and C₃F₇CONH₂, the hemiprotonated salts (CF₃CONH₂)₂H[AsF₆] and (C₃F₇CONH₂)₂H[AsF₆], and the oxonium salt cocrystal H₃O[AsF₆]·2CF₃CONH₂ were determined. Low-temperature reactions of the perfluoroamides with [XeF][AsF₆] in aHF yielded rare XeF₂-containing salt cocrystals: CF₃C(OH)NH₂[AsF₆]·XeF₂, C₂F₅C(OH)NH₂[AsF₆]·XeF₂ and C₃F₇C(OH)NH₂[AsF₆]·XeF₂. Their crystal structures reveal a rare example of O–H⋯FXeF and the first crystallographically characterised cases of N–H⋯FXeF hydrogen bonding. The XeF₂ molecule is slightly polarised, as indicated by the differences observed in Xe–F bond lengths compared with those in free XeF₂; this finding is corroborated by low-temperature Raman spectroscopy. The reported crystal structures display diverse hydrogen-bonding motifs involving O–H⋯F(Xe), N–H⋯F(Xe), O–H⋯F(As) and N–H⋯F(As) interactions. The salt cocrystals prepared and structurally characterised in this study demonstrate that XeF₂ readily forms hydrogen-bonded cocrystals and serves as a reliable hydrogen-bond acceptor. These results open new possibilities for the exploration of cocrystal formation with noble-gas fluorides and the expansion of noble-gas chemistry.

Author contributions

Conceptualization, data curation, formal analysis, investigation, visualization, writing – original draft: EU; funding acquisition, methodology, project administration, resources, supervision: ML; validation, writing – review & editing: EU, ML. Both authors agreed on the final version of the article.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information: crystallographic details, Raman spectra, experimental details. See DOI: https://doi.org/10.1039/d5ce00956a.

Crystallographic data for all reported crystal structures has been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers 2493130–2493140.^48a–k

Data for this article, including SCXRD datasets and Raman spectra are available at Zenodo open repository at https://doi.org/10.5281/zenodo.17432981.

Acknowledgements

Financial support from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (Starting Grant No. 950625), the Slovenian Research and Innovation Agency (ARIS, Project No. J1-60022), and the Jožef Stefan Institute Director's Fund is gratefully acknowledged.

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Footnote

† Dedicated to Professor Resnati, celebrating a career in fluorine and noncovalent chemistry on the occasion of his 70th birthday.

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