Protonated oxalyl chloride and the ClCO⁺ cation

The reactions of oxalyl chloride were investigated in the binary superacidic systems HF/SbF₅ and DF/SbF₅. O-Monoprotonated oxalyl chloride was isolated and represents the first example of a protonated acyl chloride. Diprotonated oxalyl chloride is only stable in solution. Salts of the ClCO⁺ cation were syn­thesized from the reactions of oxalyl chloride or COClF with SbF₅ in 1,1,1,2-tetra­fluoro­ethane (R-134a, CF₃CFH₂). The colourless salts were characterized by low-temperature vibrational spectroscopy, NMR spectroscopy and single-crystal X-ray diffraction. (1,2-Di­chloro-2-oxo­ethyl­idene)oxidanium hexa­fluor­ido­anti­monate(V), [C₂O(OH)Cl₂][SbF₆], crystallizes in the monoclinic space group P2₁ and carbonyl chloride hexa­deca­fluorido­triarsenate(V) [ClCO][Sb₃F₁₆], in the trigonal space group P3₁, with two and three formula units per unit cell, respectively. Monoprotonated oxalyl chloride and the ClCO⁺ cation both display very short C—Cl bonds with a strong double-bond character.

1. Introduction

Oxalyl chloride was first prepared by Fauconnier in 1892 by the reaction of diethyl oxalate and phospho­rus penta­chloride (Fauconnier, 1892). Nowadays, it is commercially produced by the photochlorination of ethyl­ene carbonate (Pfoertner & Oppenländer, 2012). Due to its high reactivity, oxalyl chloride is one of the most versatile organic reagents in chemical syn­theses. Among others, it is used in chlorinations, oxidations, reductions, dehydrations, deca­rboxylations or formyl­ation reactions (Masaki & Fukui, 1977; Omura & Swern, 1978; Shiri & Kazemi, 2017; Denton et al., 2012; Wasserman & Tremper, 1977; Mendelson & Hayden, 1996). The best-known applications are its use in Friedel–Crafts reactions (Alexandrou, 1969; Ketcha & Gribble, 1985) or the Swern oxidation, i.e. the oxidation of primary and secondary alcohols to aldehydes and ketones (Omura & Swern, 1978).

The reactivity of oxalyl chloride towards strong Lewis acids has been thoroughly investigated. Thus, the chloro­carbonyl cation (ClCO⁺) is observed in the reaction of oxalyl chloride with SbF₅ (Prakash et al., 1991). The ClCO⁺ cation is a representative of the com­pound class of linear triatomic mol­ecules, such as OCS, ONP or ONS⁺, for which numerous theoretical calculations have been performed (Peterson et al., 1991; Pak & Woods, 1997). It can be synthesized by reacting SbF₅ with either oxalyl chloride, phosgene or carbonyl chloride fluoride. Furthermore, the reaction of carbon mon­oxide with chlorine in SO₂ClF/SbF₅ leads to the formation of the ClCO⁺ cation (Prakash et al., 1991; Bernhardt et al., 1999; Christe et al., 1999). The latter was investigated by Olah in 1991 using NMR spectroscopy (Prakash et al., 1991), while Aubke characterized the species by Raman and IR spectroscopy for the first time in 1999 (Bernhardt et al., 1999). However, it has not yet been possible to elucidate the crystal structure of the cation due to its high reactivity. This prompted us to isolate the ClCO⁺ cation and to perform single-crystal X-ray diffraction analysis to structurally characterize the cation and to com­pare its bond lengths with isoelectronic mol­ecules such as OCS, which shows a short C—S bond with a strong C=S double-bond character (Pak & Woods, 1997).

Furthermore, in previous studies by our group, mono- and diprotonated species of oxalic acid, pyruvic acid and parabanic acid were isolated and characterized, whereas the dications constitute vicinal superelectrophiles (Schickinger et al., 2018; Virmani et al., 2022; Beck et al., 2020). In addition, we recently investigated the reactivity of haloacetyl fluorides in superacidic media and observed the protonation of the carbonyl bond, as well as HF addition to the latter, as in the case of di­chloro­acetyl fluoride (Steiner et al., 2022, 2024). This prompt­ed us to perform investigations on the reactivity of oxalyl chloride in the superacidic system HF/SbF₅.

2. Results and discussion

2.1. Syntheses and properties of [C₂O(OX)Cl₂][SbF₆] (X = H or D), [C₂(OH)₂Cl₂][Sb_nF_5n+1]₂, [ClCO][Sb₃F₁₅Cl] and [ClCO][Sb₃F₁₆]

Oxalyl chloride was reacted in the binary superacidic systems HF/SbF₅ and DF/SbF₅. According to reaction (1) in Scheme 1, [C₂O(OH)Cl₂][SbF₆] (1) and [C₂O(OD)Cl₂][SbF₆] (2) were obtained as O-monoprotonated species of oxalyl chloride in qu­anti­tative yields as colourless solids.

To obtain the diprotonated species, oxalyl chloride was reacted in the binary superacidic system HF/SbF₅ with an excess of the strong Lewis acid SbF₅. However, even with a tenfold excess of SbF₅, the isolation of diprotonated oxalyl chloride (3) as a solid was not possible. Instead of the desired species, monoprotonated oxalyl chloride (1) was obtained. To investigate the reaction of oxalyl chloride in anhydrous hy­dro­gen fluoride (aHF) at −60 °C with an excess of SbF₅, NMR spectroscopy was performed employing eight equivalents of SbF₅. Accordingly, the measured ¹H, ¹⁹F and ¹³C NMR spectra indicate the presence of 3 in the solution, as pre­sent­ed in reaction (2) in Scheme 2. Salts 1 and 2 show thermal decom­position at −45 °C.

[ClCO][Sb₃F₁₅Cl] (4) was synthesized by reacting oxalyl chloride with three equivalents of SbF₅ in 1,1,1,2-tetra­fluoro­ethane (R-134a, CF₃CFH₂) at −78 °C. 4 was obtained as a colourless solid according to reaction (3) in Scheme 3. To avoid mixed occupancies of the fluorine positions of the anion with chlorine, carbonyl chloride fluoride was reacted under the same conditions to form [ClCO][Sb₃F₁₆] (5), as pre­sent­ed in reaction (4) in Scheme 3 (Prakash et al., 1991; Bernhardt et al., 1999; Christe et al., 1999).

2.2. Vibrational spectroscopy

The low-temperature Raman (Ra) and infrared (IR) spectra of [C₂O(OH)Cl₂][SbF₆] (1), [C₂O(OD)Cl₂][SbF₆] (2) and C₂O₂Cl₂ are illustrated in Fig. 1. The com­plete vibrational frequencies of 1, 2, 4 and 5, as well as of oxalyl chloride and carbonyl chloride fluoride, are provided in the supporting information (see Figs. S1–S3 and Tables S1–S5) (Bernhardt et al., 1999; Davis et al., 1993; Nielsen et al., 1952).

Figure 1 Low-temperature Raman (bottom) and IR spectra (top) of [C2O(OX)Cl2][SbF6] (1 and 2) (X = H or D) and C2O2Cl2.

For the [C₂O(OH)Cl₂]⁺ cation with C_s symmetry, 15 fun­da­mental vibrational modes are expected, all of which are Raman and IR active. ν_s(O—H) is superposed by con­den­sed water in the IR spectra due to the measuring method. Furthermore, the O—H stretching vibration shows low intensity in the Raman spectra due to the poor polarizability of the O—H group, which does not apply to the O—D group. The O—D stretching vibration of the D-isotopomeric species 2 is observed at 2157 cm⁻¹ in the Raman spec­trum and at 2388 cm⁻¹ in the IR spectrum. The C=O stretching vibration of the protonated COCl moiety is de­tected at 1607 cm⁻¹ (Ra) (1 and 2), as well as at 1605 (1) and 1593 cm⁻¹ (2) (IR), and is significantly red-shifted (by ap­prox­imately 165 cm⁻¹) com­pared to the starting material. The vibrations are red-shifted by around 100–150 cm⁻¹ in com­parison to the corresponding vibrations of protonated acyl fluorides (Steiner et al., 2022, 2024; Bayer et al., 2022). The ν_s(C=O) of the adjacent unprotonated carbonyl group is not affected by the protonation. The C—Cl stretching vibration of the protonated COCl moiety appears at 817 cm⁻¹ (2) in the Raman spectrum and at 833 cm⁻¹ (1), as well as at 818 cm⁻¹ (2), in the IR spectrum, i.e. blue-shifted by approximately 70 cm⁻¹. The ν_s(C—Cl) of the neighboring COCl moiety is not affected by protonation and occurs at 623 cm⁻¹ (2) in the Raman spectrum. Additionally, the C—C stretching vibration is detected at 1118 (Ra) (1) and 1117 cm⁻¹ (1 and 2) (IR), and is thus blue-shifted by approximately 20 cm⁻¹ in com­parison to oxalyl chloride.

More vibrations are observed for the [SbF₆]⁻ anion than expected for ideal octa­hedral symmetry due to inter­ionic inter­actions in the solid leading to a symmetry distortion (Weidlein et al., 1988).

2.3. Crystal structure of [C₂O(OH)Cl₂][SbF₆] (1)

The hexa­fluorido­anti­monate of monoprotonated oxalyl chlo­ride (1) crystallizes in the monoclinic space group P2₁ with two formula units per unit cell. The asymmetric unit is illustrated in Fig. 2. Crystal data and structure refinement details are provided in Table 1 and in the supporting information (Tables S6 and S7).

Table 1
Experimental details

Experiments were carried out with Mo Kα radiation using a Rigaku Xcalibur Sapphire3 diffractometer. Absorption was corrected for by multi-scan methods (CrysAlis PRO; Rigaku OD, 2020).

	1	5
Crystal data
Chemical formula	(C2HCl2O2)[SbF6]	(CClO)[Sb3F16]
Mr	363.68	732.71
Crystal system, space group	Monoclinic, P21	Trigonal, P31
Temperature (K)	106	102
a, b, c (Å)	6.1616 (8), 10.8379 (10), 6.8805 (8)	8.0824 (3), 8.0824 (3), 18.3341 (8)
α, β, γ (°)	90, 106.472 (13), 90	90, 90, 120
V (Å^3)	440.61 (9)	1037.22 (9)
Z	2	3
μ (mm^−1)	3.80	6.19
Crystal size (mm)	0.31 × 0.17 × 0.12	0.19 × 0.14 × 0.11
Data collection
Tmin, Tmax	0.737, 1.000	0.882, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	8919, 2930, 2692	6310, 3334, 3029
Rint	0.042	0.044
(sin θ/λ)max (Å^−1)	0.755	0.746
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.030, 0.057, 1.04	0.038, 0.077, 1.03
No. of reflections	2930	3334
No. of parameters	122	199
No. of restraints	2	0
H-atom treatment	Only H-atom coordinates refined	–
Δρmax, Δρmin (e Å^−3)	1.18, −0.66	1.36, −1.09
Absolute structure	Refined as an inversion twin	Twinning involves inversion, so Flack parameter cannot be determined
Absolute structure parameter	0.52 (3)	–
Computer programs: CrysAlis PRO (Rigaku OD, 2020), SHELXT (Sheldrick, 2015a), SHELXL2019 (Sheldrick, 2015b), ORTEP-3 for Windows (Farrugia, 2012) and PLATON (Spek, 2020).

Figure 2 The asymmetric unit of 1, with displacement ellipsoids drawn at the 50% probability level.

Due to the protonation, the C1—O1 bond length [1.225 (8) Å] is significantly elongated com­pared to the starting material [1.180 (2) Å; Danielson et al., 1995] and is longer than a formal C=O bond (1.18 Å; Allen et al., 1987). The C2—O2 bond length [1.184 (8) Å] of the adjacent un­pro­tonated C=O bond is not affected by the protonation [1.180 (2) Å; Danielson et al., 1995]. Furthermore, the C1—Cl1 bond length [1.647 (7) Å] is significantly shortened com­pared to the neutral com­pound [1.747 (3) Å; Danielson et al., 1995] and is in the range between a formal C—Cl single bond (1.76 Å; Allen et al., 1987) and a C=Cl double bond (1.56 Å; Holleman et al., 1987). The same applies for the C2—Cl2 bond length [1.693 (7) Å] [cf. 1.747 (3) Å for oxalyl chloride; Danielson et al., 1995]. The elongation of the C=O bond and the shortening of the C—Cl bonds are consistent with the observed shifts of the ν_s(C=O) and ν_s(C—Cl) in the vibrational spectra. The C1—C2 bond length [1.550 (8) Å] is not affected by protonation [cf. 1.545 (8) Å for oxalyl chloride; Danielson et al., 1995]. This is consistent with the results ob­served for the mono- and diprotonated species of oxalic acid. In both cases, the protonation at the carbonyl groups does not affect the bond lengths of the C—C backbone (Schickinger et al., 2018).

The Sb—F bond lengths are in the range between 1.846 (4) and 1.941 (3) Å, and correspond with values reported in the literature (Minkwitz et al., 1999a,b; Minkwitz & Schneider, 1999). Due to inter­ionic inter­actions, the anion displays dis­torted octa­hedral symmetry. The Sb1—F6 bond [1.941 (3) Å] is significantly longer than the other Sb—F bonds with it being involved in hy­dro­gen bonding.

In the crystal structure of 1, the ions are arranged into chains along the a and b axes by the strong O1(—H1)⋯F6 hy­dro­gen bond [2.421 (7) Å] and the C⋯F inter­actions C1⋯F2ⁱ [2.617 (7) Å; see Fig. 4 for symmetry codes] and C1⋯F4ⁱⁱ [2.565 (7) Å] (see Figs. S4–S5) (Jeffrey, 1997; Bondi, 1964). The chains are linked to each other by the Cl⋯F inter­action Cl1⋯F3ⁱⁱⁱ [2.883 (5) Å] to form layers. All inter­atomic C⋯F and Cl⋯F contacts are below the sum of the van der Waals radii (3.17 and 3.22 Å; Bondi, 1964). Inter­atomic distances are listed in Table S7.

2.4. Crystal structure of [ClCO][Sb₃F₁₆] (5)

The crystal structures of 4 and 5 were both obtained by recrystallizing the salts from R-134a (CF₃CFH₂) at −40 °C. As 4 crystallizes as an inversion twin and shows mixed occupancies of all 16 crystallographic fluorine positions with chlorine, the structural parameters show high standard deviations. Therefore, the crystal structure of 5 is used for the discussion of all experimental parameters.

[ClCO][Sb₃F₁₆] (5) crystallizes in the trigonal space group P3₁ with three formula units per unit cell. The asymmetric unit is depicted in Fig. 3. Crystal data and structure refinement details are provided in Table 1 and in the supporting information (Tables S6 and S8).

Figure 3 The asymmetric unit of 5, with displacement ellipsoids drawn at the 50% probability level.

The C1—O1 bond length [1.105 (10) Å] is significantly shortened com­pared to ClFCO [1.173 (2) Å; Oberhammer, 1980]. It is in the range between a formal C=O double bond (1.19 Å) and a C≡O triple bond (1.07 Å) (Allen et al., 1987). It corresponds with the C=O bond length (1.1562 Å) of the isoelectronic mol­ecule OCS (Pak & Woods, 1997). Furthermore, the C1—Cl1 bond length [1.571 (3) Å] is significantly shortened com­pared to ClFCO [1.725 (2) Å; Oberhammer, 1980]. It is in the range of a formal C=Cl double-bond length (1.56 Å) and thus displays a significant double-bond character (Allen et al., 1987). It corresponds with the C—Cl bond length [1.67 (2) Å] of ClCN (Beach & Turkevich, 1939). The Cl1—C1—O1 angle [176.1 (8)°] indicates the essentially linear structure of the cation.

The Sb—F bond lengths of the terminal F atoms are in the range between 1.831 (8) and 1.857 (7) Å. The bridging Sb—F bonds are longer than the terminal ones, with bond lengths up to 2.088 (8) Å. The bond angles Sb1—F6—Sb2 [146.0 (4)°] and Sb2—F11—Sb3 [156.6 (4)°] are also in good agreement with the literature (Faggiani et al., 1986; Gerken et al., 2002).

In the crystal structure of 5, the ions form a helical structure along the c axis via the C⋯F inter­action C1⋯F5ⁱⁱ (2.75 Å) and the Cl⋯F inter­action Cl1⋯F1 [2.56 (1) Å] (see Fig. 4, and Figs. S6–S7 in the supporting information). The cation displays a tetra­coordinated C1 atom and further forms the inter­ionic contacts C1⋯F4ⁱ (2.968 Å), C1⋯F14ⁱⁱⁱ (3.062 Å), C1⋯F15^iv (2.92 Å) and O1⋯F16^v [2.79 (2) Å] (see Fig. 4). All inter­atomic C⋯F, Cl⋯F and O⋯F contacts are below the sum of the van der Waals radii (3.17, 3.22 and 2.99 Å; Bondi, 1964). Selected inter­atomic distances are listed in Table S8.

Figure 4 Inter­atomic contacts of 5, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) x − 1, y, z; (ii) −x + y, −x + 1, z − ; (iii) x, y + 1, z; (iv) −y, x − y, z + ; (v) x − 1, y + 1, z.

2.5. NMR spectroscopy

To trace the reactivity of oxalyl chloride in aHF and the binary superacidic system HF/SbF₅, the ¹H, ¹⁹F and ¹³C NMR spectra were measured at −60 °C with acetone-d₆ as the external standard. The measured NMR spectra and the com­plete NMR data of oxalyl chloride, 1 and 3 are listed in the supporting information (Figs. S8–S18).

The ¹³C NMR spectrum of oxalyl chloride dissolved in aHF at −60 °C shows a singlet at 161.1 ppm for both COCl moieties. Furthermore, no chlorine–fluorine exchange or HF addition to the carbonyl bond is observed under these con­ditions.

By first dissolving equimolar amounts of SbF₅ com­pared to oxalyl chloride in HF and then adding the acyl chloride, monoprotonated oxalyl chloride (1) is formed. The ¹H NMR spectrum shows a singlet at 10.03 ppm for the protonated carbonyl group. The ¹³C NMR spectrum displays two singlets located at 161.7 and 183.4 ppm. These are assigned to the carbonyl groups, whereas the NMR resonance of the protonated COCl moiety is significantly shifted downfield. The ¹⁹F NMR spectrum indicates chlorine–fluorine exchange initiated by the protonation of oxalyl chloride. Thus, the NMR resonance at 17.74 ppm indicates the formation of oxalyl fluoride. The ¹H, ¹⁹F and ¹³C NMR spectra of (COF)₂ at −60 °C in aHF are illustrated in Figs. S10–S11 (see supporting information). Furthermore, in the ¹⁹F NMR spectrum, the resonance at −124.78 ppm is assigned to the [SbF₆]⁻ anion (Dean & Gil­lespie, 1969).

Since the diprotonated oxalyl chloride (3) could not be isolated as a solid, the reaction of oxalyl chloride in HF/SbF₅ at −60 °C was investigated using NMR spectroscopy, whereas an eightfold amount of SbF₅ was applied. Accordingly, the NMR spectra indicate the presence of 3 in the solution. The ¹H NMR spectrum shows a singlet at 9.61 ppm for the protonated COCl⁺ moieties, whereas the ¹³C NMR spectrum displays a singlet at 183.1 ppm. Compared to the neutral com­pound, the ¹³C NMR resonance is significantly shifted downfield. Furthermore, in the ¹⁹F NMR spectrum, multiple resonances located in the range between −117.46 and −143.05 ppm are assigned to the [Sb_nF_5n+1]⁻ polyanions (Dean & Gillespie, 1969). Thus, di­pro­tonated oxalyl chloride is stable in solution at −60 °C. However, after removal of the excess HF at −78 °C, it decom­poses with the formation of 1. Furthermore, as in the NMR spectra of 1, a chlorine–fluorine exchange is observed, as the signal at 17.46 ppm indicates the formation of oxalyl fluoride. Additional signals in the ¹⁹F NMR spectra of 3 at −21.78 and −59.90 ppm are assigned to COF₂ and CF₃OH (Christe et al., 2007). The ¹H, ¹⁹F and ¹³C NMR spectra of COF₂ in aHF at −60 °C are depicted in the supporting information (Fig. S12). COF₂ is likely formed due to the decom­position of oxalyl fluoride under superacidic conditions.

3. Conclusions

Oxalyl chloride was reacted in the binary superacidic systems HF/SbF₅ and DF/SbF₅ to form the O-monoprotonated and its D-isotopomeric species as hexa­fluorido­anti­monates. Both represent the first examples of protonated acyl chlorides. When the Lewis acid SbF₅ is applied in eightfold excess, di­pro­tonated oxalyl chloride is formed, which is only stable in solution. By the reaction of oxalyl chloride or carbonyl chloride fluoride in the aprotic solvent 1,1,1,2-tetra­fluoro­ethane (R-134a, CF₃CFH₂) with a threefold excess of SbF₅, salts of the chloro­carbonyl cation were isolated. The colourless salts were characterized by low-temperature vibrational spectroscopy and low-temperature NMR spectroscopy. The crystal structures of [C₂O(OH)Cl₂][SbF₆] and [ClCO][Sb₃F₁₆] were determined by single-crystal X-ray diffraction analysis. Monoprotonated oxalyl chloride and the chloro­carbonyl cation both display very short C—Cl bonds with strong dou­ble-bond character.

Footnotes

‡Deceased