The salts of chloronium ions R–Cl⁺–R (R = CH₃ or CH₂Cl): formation, thermal stability, and interaction with chloromethanes

Abstract

The interaction of CH₃Cl/CD₃Cl or CH₂Cl₂/CD₂Cl₂ with the carborane acid H(CHB₁₁Cl₁₁) (abbreviated as H{Cl₁₁}) generates the salts of CH₃–{Cl₁₁} and CH₂Cl–{Cl₁₁} and their deuterio analogs, respectively, which are analogs of the salts of asymmetric chloronium cations. Next, salts of chloronium cations CH₃–Cl⁺–CH₃, ClCH₂–Cl⁺–CH₂Cl, and ClCH₂–Cl⁺–CH₃ and their deuterio analogs were obtained from the above compounds. The asymmetric ClCH₂–Cl⁺–CH₃ cation was found to be unstable, and at ambient temperature, slowly disproportionated into symmetric cations (CH₃)₂Cl⁺ and (CH₂Cl)₂Cl⁺. At a high temperature (150 °C), disproportionation was completed within 5 minutes, and the resulting cations further decomposed into CH₃–{Cl₁₁} and CH₂Cl–{Cl₁₁}. The molecular fragment ClCH₂–(X) of the compounds (X = {Cl₁₁}, –Cl⁺–CH₂Cl, or –Cl⁺–CH₃) is involved in exchange reactions with CH₂Cl₂ and CHCl₃, converting into CH₃–(X) with the formation of chloroform and CCl₄, respectively.

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Halonium ions (R₂Hal⁺) are well-recognized reactive intermediates in electrophilic chemistry.^1,2 Their stability increases in the order Hal = F, Cl, Br, I. Recently, the evidence of the formation of the symmetrical fluoronium ions in solutions was obtained,^3,4 and the nature of carbon–halogen bonds in the halonium ions was studied.⁵ Mostly stable dimethylbromonium and -iodonium salts are presently commercialized and widely used in chemical ionization mass spectroscopy (gas phase chemistry) as effective methylating^6–12 and protonating agents^13,14 for a variety of compounds. Nevertheless, the chemistry of dialkylhalonium ions in condensed phases is virtually unknown. Recently, the salts of (CH₃)₂Cl⁺ and (C₂H₅)₂Cl⁺ with the exceptionally stable and inert toward reactive cations undecachlorocarborane ions, CHB₁₁Cl₁₁⁻, were obtained and studied using X-ray and infrared (IR) spectroscopy.¹⁵ The solid salt (CH₃)₂Cl⁺(CHB₁₁Cl₁₁⁻) is stable even at elevated temperatures and decomposes at 140 °C releasing CH₃(CHB₁₁Cl₁₁) and CH₃Cl. This is an important method for the isolation of pure CH₃(CHB₁₁Cl₁₁), which can be viewed as a neutral analogue of an asymmetric chloronium ion related to the dimethylchloronium ylide, CH₃ClCH₂.^16,17

In the present work we obtained the salts of symmetric and asymmetric chloronium cations, CH₃–Cl⁺–CH₂Cl and CH₂Cl–Cl⁺–CH₂Cl, both protio and deuterio analogs, with the CHB₁₁Cl₁₁⁻ counterion (hereafter abbreviated as {Cl₁₁⁻}, Fig. S1, ESI†). We also explored their thermal stability and interaction with some simple chloromethanes. The carborane ion {Cl₁₁⁻} was chosen as a counterion for chloronium salts because of its exceptionally low basicity and high thermal stability, which ensure the stability of chloronium salts at room temperature and above.¹⁵

Experimental

Carborane acid H{Cl₁₁} was prepared as previously described.¹⁸ The acid was sublimed at 150–160 °C under a pressure of 10⁻⁵ Torr on cold Si windows of a specially designed IR cell-reactor as a very thin translucent layer.¹⁹ The spectrum of the sublimed acid showed no traces of the H₃O⁺ cation.²⁰ Dry gaseous chloromethanes (CH₃Cl, CH₂Cl₂, and CHCl₃) were injected anaerobically into the IR cell. The values of their partial pressure were calculated as the ratio of absorption intensity to that of the standard spectrum recorded in the same cell filled with vapors at atmospheric pressure.

All procedures were performed in a Vacuum Atmospheres Corp. glovebox in the atmosphere of N₂ (O₂ and H₂O < 0.5 ppm). The IR spectra were recorded on a PerkinElmer Spectrum-100 spectrometer inside a dry box in the transmission mode (400–4000 cm⁻¹). The IR data were processed in the GRAMMS/A1 (7.00) software program from Thermo Fisher Scientific.

Results and discussion

To explore the chemical processes involving chloronium ions by means of IR spectroscopy, their detailed IR spectra must be obtained and interpreted.

Chloronium salts and IR spectra

The CH₃–Cl⁺–CH₂Cl cation was obtained by introducing the CH₂Cl₂ vapors at a partial pressure (P_CH2Cl2) of 0.4 atm into an evacuated IR cell containing a film of CH₃–{Cl₁₁} sublimed on their Si-windows. Reaction (1) of CH₃–Cl⁺–CH₂Cl formation proceeds very slowly, without a release of HCl.

CH₃{Cl₁₁} + CH₂Cl₂ → (CH₃–Cl⁺–CH₂Cl){Cl₁₁⁻} (1)

After 40 minutes, the spectrum of the cation was isolated by subtracting the original spectrum of CH₃–{Cl₁₁} multiplied by the scaling factor f = 0.933 (Fig. 1, red). This means that only 6.7% of the CH₃–{Cl₁₁} salt (100–93.3%) has reacted.

Fig. 1 Red: IR spectra of (CH₃–Cl⁺–CH₂Cl){Cl₁₁⁻} formed at low (red) and high (blue) partial pressure of CH₂Cl₂ (0.4 and 1 atm respectively). Both spectra are normalized to intensity of the anion. The red spectrum was isolated after subtracting the spectrum of the unreacted CH₃–{Cl₁₁}. Intense bands (marked by an asterisk) belong to the {Cl₁₁⁻} anion.

If the partial pressure of the injected CH₂Cl₂ vapors was twofold higher (1 atm), then the reaction was accelerated significantly and completed within 6 minutes. The spectrum of the resultant CH₃–Cl⁺–CH₂Cl cation slightly differs from that of the previous sample (Fig. 1), indicating that the cation is sensitive to changes in the environment.

The CH₃–Cl⁺–CD₂Cl cation is formed when the vapors of CD₂Cl₂ are introduced into the IR cell with sublimed CH₃–{Cl₁₁}. The IR spectrum of the salt of this cation is shown in Fig. 2 (red; the gas phase was evacuated).

Fig. 2 IR spectra of the initial CH₃–{Cl₁₁} salt (blue) and the salt of the CH₃–Cl⁺–CD₂Cl cation (red).

The CD₃–Cl⁺–CH₂Cl and CD₃–Cl⁺–CD₂Cl cations were formed when vapors of CH₂Cl₂ or CD₂Cl₂ respectively, were injected into the IR cell with a film of sublimed CD₃–{Cl₁₁} salt on the Si windows. Their IR spectra are presented in Fig. S2 and S3 (ESI†) and in Table 1.

Table 1 IR frequencies of the methyl and bridged C–Cl⁺–C groups of the compounds analyzed, in comparison with the IR spectrum of CH₃Cl (the most characteristic or intense bands, used as markers to identify the cations in their mixtures, are underlined)

Compound	ν asCH3		ν sCH3		δ asCH3	δ sCH3	CH3 rock	ν as(CClC)	ν s(CClC)
CH3Cl (gas)^21	3043	3̲0̲3̲9̲	2968	2879	1452	1355	1017m	732 (νCCl)
CH3–{Cl11}^15	3083	3̲0̲7̲1̲	2962		1409	1335	—	—
CH3–Cl^+–CH3^15		3̲0̲6̲8̲	2957		1417	1324		6̲3̲6̲
H3C–Cl^+–CD2Cl	3064	3̲0̲5̲4̲	2947		1406	1330	934	632	522

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Interpretation of the IR spectra

IR spectra of the salts of the cations CH₃–Cl⁺–CH₂Cl (red), CD₃–Cl⁺–CH₂Cl (blue), and CH₃–Cl⁺–CD₂Cl (green) that are normalized to the intensity of the anion are shown in Fig. 3. The figure shows that in the frequency range of CH stretch vibrations, the sum of the spectra of the fragments ClH₂C–(Cl⁺–) (blue) and CH₃–(Cl⁺–) (green) matches the spectrum of the CH₃–Cl⁺–CH₂Cl cation. The same result was observed in the frequency range of the CH bend vibrations (Fig. 3, right). Interpretations of the IR spectra for all cations follow from the above data (Tables 1 and 2), taking into account that spectra of CH₃{Cl₁₁} and (CH₃–Cl⁺–CH₃){Cl₁₁⁻} were interpreted earlier.¹⁵ It was important to determine the existence of specific absorption bands for each compound under study; we will use these bands (shown in Tables 1 and 2) as markers for the identification of these compounds in the mixtures. Intensity of the marked bands allowed us to estimate relative amounts of the compounds.

Fig. 3 IR spectra of salts of cations: CH₃–Cl⁺–CH₂Cl (red), CD₃–Cl⁺–CH₂Cl (blue) and CH₃–Cl⁺–CD₂Cl (green).

Table 2 IR frequencies of the methylene chloride and bridged C–Cl⁺–C groups of the compounds analyzed, in comparison with the IR spectrum of CH₂Cl₂ (the most characteristic or intense bands, used as markers to identify the cations in their mixtures, are underlined)

Compound	ν asCH2			ν sCH2	δCH2 scissor		δCH2 waggle			ν asCCl2/νC–Cl		ν as(CClC)
a Not determined.
CH2Cl2 (liquid)^22		3045		2990	1424			1265vs			7̲3̲9̲	—
ClH2C–{Cl11}	3079	3̲0̲6̲3̲		2978	1391	1̲3̲8̲5̲		1̲2̲4̲3̲vs			7̲9̲3̲	—
ClH2C–Cl^+–CH2Cl IIa	3073	3̲0̲6̲4̲	3048	2980	1416	1324		1̲2̲8̲4̲v̲s̲	1248		7̲7̲0̲	635
					1409
ClH2C–Cl^+–CH2Cl IIb		3̲0̲4̲4̲		2964br	1406	1338		1̲2̲8̲3̲v̲s̲	1248	798	7̲8̲3̲	672	593
									1226
ClH2C–Cl^+–CH3		3044		2962	1̲4̲0̲7̲	1386	1261	1̲2̲4̲8̲s̲	1226		7̲8̲6̲
ClH2C–Cl^+–CD3		3044		2962	1̲3̲9̲8̲	1387	1263	1̲2̲5̲1̲s̲	1226		7̲8̲7̲

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Interaction of mono- and dichloromethane with H{Cl₁₁} and chloronium cations

CH₃Cl interacts with the H{Cl₁₁} acid in two stages. At first, it is protonated with the release of HCl and the formation of CH₃{Cl₁₁} (eqn (2)):

CH₃Cl + H{Cl₁₁} → [CH₃Cl–H⁺{Cl₁₁}] → CH₃{Cl₁₁} + HCl (2)

CH₃{Cl₁₁} + CH₃Cl → (CH₃)₂Cl⁺{Cl₁₁⁻} (3)

Then, CH₃Cl binds to CH₃{Cl₁₁}, thus forming (CH₃)₂Cl⁺{Cl₁₁⁻} (eqn (3)). The formation of the resultant CH₃{Cl₁₁} can be observed by monitoring the intensity of the IR absorption of the released HCl (I_HCl). The amount of the resulting ion (CH₃)₂Cl⁺ can be traced from the intensity of its band ν_as(CCl⁺C) at 636 cm⁻¹ (I₆₃₆; Fig. S4, ESI†). Dependence of I₆₃₆ on I_HCl was initially proportional (Fig. 4), which means that both reactions proceed at constant speeds. When the thickness of the layer of the formed surface products increases sufficiently to slow down the diffusion of CH₃Cl molecules to the acid, the rate of CH₃⁺{Cl₁₁⁻} formation decreases and the slope increases (Fig. 4). The cation of the ultimate salt (CH₃)₂Cl⁺{Cl₁₁⁻} does not show any further interaction with CH₃Cl.

Fig. 4 Dependence of the intensity of band ν_as(CClC) at 636 cm⁻¹ of the (CH₃)₂Cl⁺ cation on the intensity of HCl absorption [reflects the dependence of the (CH₃)₂Cl⁺ formation on the CH₃{Cl₁₁} formation].

Dichloromethane vapors (at a partial pressure of 0.65 atm) interacted with sublimed acid in the same way as chloromethane did. At the first stage, IR spectra registered the emergence of absorbance of the gaseous HCl and the surface CH₂Cl–{Cl₁₁} compound, which are formed in accordance with eqn (4):

CH₂Cl₂ + H{Cl₁₁} → CH₂Cl–{Cl₁₁} + HCl (4)

The IR spectrum of CH₂Cl–{Cl₁₁} showed the characteristic band of the C–Cl stretch at 793 cm⁻¹, and the absence of the bands of C–Cl⁺–C group vibrations in the frequency range 650–500 cm⁻¹ (Fig. 5 and Table 2). Intensity of HCl absorption (I_HCl) was used to quantify the total amount of the formed CH₂Cl–{Cl₁₁}, whereas intensity of the band of the terminal C–Cl stretch (I_CCl) reflects the current amount of this compound. Dependence of I_CCl on I_HCl was linear for the first 5 hours of the reaction (Fig. 6); this result means that only CH₂Cl–{Cl₁₁} was formed. Then, the dependence started to drop (point 22 in Fig. 6), indicating the second stage of the reaction: involvement of CH₂Cl–{Cl₁₁} in the formation of the ClH₂C–Cl⁺–CH₂Cl cation (eqn (5)).

CH₂Cl–{Cl₁₁} + CH₂Cl₂ → ClH₂C–Cl⁺–CH₂Cl. (5)

Fig. 5 IR spectra of the CH₂Cl–{Cl₁₁} compound (red) and the salt of CH₂Cl–Cl⁺–CH₂Cl cation (blue). The spectra of unreacted H{Cl₁₁} acid, gaseous HCl and CH₂Cl₂ were subtracted. The region of strong absorption of gaseous CH₂Cl₂ (C–Cl stretches), where compensation does not work, is shaded.

Fig. 6 Dependence of I_CCl on I_HCl, pointing to ClH₂C–{Cl₁₁} formation and its further consumption on the formation of the ClH₂C–Cl⁺–CH₂Cl cation.

Extrapolation of I_CCl from I_HCl to the moment of the reaction stoppage (at the 1380th minute, when CH₂Cl₂ was pumped out) yields I_CCl = 0.084 (arbitrary units; Fig. 6). This value corresponds to the amount of CH₂Cl–{Cl₁₁} that was formed in accordance with the amount of HCl production (eqn (4)). Nevertheless, because CH₂Cl–{Cl₁₁} is further consumed (reaction (5)), I_CCl decreases to 0.040 arbitrary units (Fig. 6), meaning that 48% (0.040/0.084) of this compound survived, and 52% was converted to the chloronium ion.

An IR spectrum of (CH₂Cl)₂Cl⁺ was obtained by subtracting the spectrum of CH₂Cl–{Cl₁₁} from the spectrum of the mixture of (CH₂Cl)₂Cl⁺ with the CH₂Cl–{Cl₁₁} up to complete compensation of the bands νCCl = 792 cm⁻¹ and δCH₂ = 1243 cm⁻¹, which are specific to CH₂Cl–{Cl₁₁} (Fig. 5, blue; Table 2). The frequencies of the (CH₂Cl)₂Cl⁺ cation are very close to those of CH₂Cl–{Cl₁₁} except for one intense band at 1284 cm⁻¹, which can be used as a marker of this cation (Table 2). Furthermore, we will denote CH₂Cl–{Cl₁₁} as compound I and the (CH₂Cl)₂Cl⁺ salt as compound II.

After stoppage of the reaction at minute 1380, an IR spectrum of the sample retained a strong absorption pattern of the unreacted acid, which constituted 70% of the original spectrum of the acid.

To continue reactions (4) and (5) at a higher speed, the CH₂Cl₂ vapors were reintroduced into our IR cell at higher partial pressure (1 atm). Under these conditions, the formation of compound I was completed after 8 minutes with full utilization of the acid and termination of the HCl release (point 30 in Fig. 7). An IR spectrum of this sample is shown in Fig. 8 (black).

Fig. 7 Dependence of CH₂Cl–{Cl₁₁} formation (determined by means of intensity of its band ν_asCH₂ at 3063 cm⁻¹) on the amount of released HCl (determined by means of intensity of the νHCl band).

Fig. 8 A change in intensity of the IR spectra of compounds CH₂Cl–{Cl₁₁} and (CH₂Cl–Cl⁺–CH₂Cl){Cl₁₁⁻} as reaction (5) proceeds. The arrows indicate isosbestic points. The spectrum of gaseous CH₂Cl₂ was subtracted.

Further interaction of compound I with CH₂Cl₂ and the formation of II is manifested in IR spectra as a decrease in the intensity of compound I and the upregulation of compound II with the appearance of isosbestic points (Fig. 8 and Fig. S5, ESI†). The spectrum of the resultant cation II differed from that of the same cation formed during a slow reaction of CH₂Cl–{Cl₁₁} with CH₂Cl₂ (Fig. 9 and Table 2). Thus, two isomers of CH₂Cl–Cl⁺–CH₂Cl are formed: compound IIa from the slowly proceeding reaction (5) and compound IIb from the rapid reaction (5). They differ in frequencies of stretches CH and CCl. Nonetheless, their bend CH vibrations are very similar and have one specific band δCH₂ (1284 cm⁻¹), which does not overlap with the bands of other types of cations. Therefore, the intensity of this band (I₁₂₈₄) can be used for the estimation of the amount of the IIa + IIb mixture.

Fig. 9 IR spectra of (CH₂Cl)₂Cl⁺{Cl₁₁}, formed by slow (IIa, some hours, blue) and quick interaction of CH₂Cl–{Cl₁₁} with CH₂Cl₂ (IIb, some minutes, red). The region of strong absorption of gaseous CH₂Cl₂ (C–Cl stretches), where compensation does not work, is shaded.

Fig. 10 shows the time dependence of the intensity of absorption bands of compounds I (I₃₀₆₃) and II (I₁₂₈₄) that reflects the formation of the corresponding cations. One can see that the formation of compound II passes through a maximum at point 34, when compound I disappears, and then decreases.

Fig. 10 Kinetic curves of the formation of compounds I–IV and chloroform; these curves were constructed by means of the intensity values of their bands at 3068 (compound I), 1284 (II), 1248 (III), 1322 (IV), and 1220 cm⁻¹ (chloroform). Curves I–IV do not quantitatively indicate the proportion of a cation in the mixture of compounds I–IV.

Simultaneously, the bands indicative of the cation CH₃–Cl⁺–CH₂Cl at 1261 and 1248 cm⁻¹ made an appearance (Table 2) and increased in intensity. We will denote this cation as compound III. Its narrow band δCH₂ = 1248 cm⁻¹ (I₁₂₄₈) does not overlap with the bands of other cations and can be used for the assessment of the relative amount of compound III (Fig. 10, green). Along with the spectrum of compound III, the absorption bands of the cation (CH₃)₂Cl⁺ also develop (hereafter denoted as compound IV). Intensity of its single band at 1324 cm⁻¹ (I₁₃₂₄) was used to assess the formation of compound IV. Fig. 10 summarizes the sequence of the formation of compounds I, II, III, and IV and the relative amounts of each compound, but does not describe the quantitative relations among them.

Cations III and IV can be formed only if the reaction of I or II with CH₂Cl₂ is accompanied by the formation of gaseous products. The spectra of the gas phase revealed the band of H–C–Cl bend vibration of chloroform at 1219 cm⁻¹. The time dependence of its intensity shows that the formation of chloroform is associated with the formation of compounds I and II (Fig. 10). In the situation when only I is formed (P_CH2Cl2 = 0.65 atm), the dependence of the chloroform formation (I₁₂₁₉) on the formation of I (I₁₂₄₂) increases both with an increase in the amount of compound I and with an increase in the contact time of I with dichloromethane (Fig. 11). Subtraction of the spectrum of compound I from the spectrum of the products (at point 18 in Fig. 11) leads to the manifestation of the spectrum of compound CH₃{Cl₁₁} (Fig. S6, ESI†). Hence, CH₂Cl₂ reacts with CH₂Cl{Cl₁₁} according to eqn (6):

Fig. 11 Dependence of CHCl₃ formation (shown as intensity of the band at 1219 cm⁻¹) on CH₂Cl–{Cl₁₁} formation (indicated by intensity of its band at 1242 cm⁻¹) under the conditions when only compound I is formed. Cation II starts to form after point 18.

With the rapid formation of CH₂Cl{Cl₁₁} in reaction (4) at P_CH2Cl2 = 1 atm, chloroform is formed symbatically (Fig. 12), confirming reaction (6). In contrast, starting from point 32, chloroform formation stopped, whereas from point 34, it begins to decrease, when the amount of II passes through a maximum, and compound I is exhausted (Fig. 10). The subsequent expenditure of chloroform occurs simultaneously with the consumption of compound II and the increasing amount of III. This finding points to reaction (7):

CH₂Cl–Cl⁺–CH₂Cl + CHCl₃ → CH₂Cl–Cl⁺–CH₃ + CCl₄, (7)

(anions are omitted), which is suggestive of CCl₄ formation.

Fig. 12 The link between formation of CH₂Cl–{Cl₁₁} (indicated by intensity of its band at 3063 cm⁻¹) and CHCl₃ (indicated by intensity of its band at 1219 cm⁻¹).

In the same way, compound IV can form:

CH₂Cl–Cl⁺–CH₃ + CHCl₃ → CH₃–Cl⁺–CH₃ + CCl₄, (8)

The detection of CCl₄ was carried out as follows. After the completion of the reaction, the gaseous phase and all surface-adsorbed molecules were removed by pumping. The difference in IR spectra before and after the evacuation represented the spectrum of removed molecules. It consists of a strong absorption pattern of the original dichloromethane, the characteristic band at 1219 cm⁻¹ of chloroform, and a weak band at 790 cm⁻¹, which may belong to the C–Cl stretch of CCl₄. The latter frequency is lower than that of gaseous CCl₄ (795 cm⁻¹), but equals that of CCl₄ solvated with dichloromethane in its solutions (789 cm⁻¹). Therefore, the traces of the formed CCl₄ are adsorbed by superficial chloronium salts.

To sum up, we can conclude that CH₂Cl₂ and CHCl₃ interact with the molecular fragment CH₂Cl–, whose reactivity is highest in CH₂Cl{Cl₁₁} and is consistently reduced in cations II and III. In general, these interactions can be expressed as:

Decreasing the reactivity of the molecular fragment CH₂Cl– in compounds I, II, and III is correlated with its decreasing polarizability, which in turn is determined by the change in ionicity of the bonds in chloronium bridge C–Cl⁺–C.

Ionicity/covalency of the bonds in the C–Cl⁺–C bridge of chloronium ions

The stretch frequencies of the methyl group, especially ν_asCH₃, are sensitive to CH₃ polarization: the higher these frequencies, the stronger the CH₃ group is polarized and the higher is ionicity of the CH₃–Cl(X) bond. In compounds CH₃–{Cl₁₁} and CH₃–Cl⁺–CH₃, the CH stretches differ insignificantly (Table 1), pointing to almost the same ionicity of CH₃⁺ bonding to {Cl⁻} and Cl–CH₃, respectively. Nonetheless, one would expect weaker ionicity (stronger covalency) of the bonds in CH₃–Cl⁺–CH₃ than in CH₃–{Cl₁₁}. The crystal structure of the (CH₃–Cl⁺–CH₃){Cl₁₁⁻} salt shows (Fig. S7 in ESI†) that the chloronium Cl-atom forms six ionic bonds with Cl-atoms of the four {Cl₁₁⁻} anions of its environment. This situation favors an increase in the ionicity of C–Cl⁺–C bonds and contributes to the convergence of polarizability of CH₃ groups in CH₃–{Cl₁₁} and (CH₃–Cl⁺–CH₃){Cl₁₁⁻} salts. If we now examine the salt (CH₃–Cl⁺–CD₂Cl){Cl₁₁⁻}, one can see that CH stretches significantly decrease (Table 1). It is evident that this salt is amorphous and has a disordered structure that may reduce the number of linkages between the chloronium Cl atom and neighboring {Cl₁₁⁻} anions, thus increasing the covalency of the C–Cl⁺–C group. In any case, the ionicity of the CH₃–Cl(–X) bond decreases in the following order:

Just as the methyl group in salts CH₃–{Cl₁₁} and (CH₃–Cl⁺–CH₃){Cl₁₁⁻}, the CH₂Cl group in salts ClH₂C–{Cl₁₁} and IIa shows identical CH stretch frequencies (Table 2). Probably, in the case of the slow formation (hours) of the (ClH₂C–Cl⁺–CH₂Cl){Cl₁₁⁻} salt (IIa), its structure is the densest, close to that of the single crystal with a maximal number of interactions between the chloronium Cl atom and atoms of the nearest {Cl₁₁⁻} anions. In the case of quick formation (minutes), the (ClH₂C–Cl⁺–CH₂Cl){Cl₁₁⁻} salt is amorphous with a loose structure and fewer contacts between the chloronium Cl atom and Cl atoms of the neighboring anions. This situation increases the covalency of the bonds in the C–Cl⁺–C bridge and decreases the polarizability of CH₂Cl groups and their CH stretches. We named this salt “isomer IIb”. Polarizability and CH frequencies of the CH₂Cl group of IIb coincide with those of the salt (ClH₂C–Cl⁺–CH₃){Cl₁₁⁻}. Thus, ionicity of the ClH₂C–Cl(–X) bond decreases in the following order:

in accordance with decreasing frequencies of CH stretches, which indicate the polarization of the ClH₂C group. Isomers IIa and IIb differ in frequencies of stretch vibrations of C–Cl⁺–C bridges (Table 2); this observation confirms that this is the starting point of their differences.

The proposed model seems to be a reasonable explanation for the existence of IIa and IIb isomers when they are formed slowly (within hours) or quickly (in minutes), respectively.

Thermal stability of chloronium cations

Ambient temperature. The salt of the symmetric cation (CH₃)₂Cl⁺ can be stored indefinitely (weeks or months) without any changes in the IR spectrum. The spectra of the salts of asymmetric cations are time dependent at ambient temperature: exposure of freshly prepared salt (CD₃–Cl⁺–CH₂Cl){Cl₁₁} to vacuum for one day results in disappearance of the bands of its cation (Fig. 13, blue). They are replaced with the bands of cation IIa: CH₂Cl–Cl⁺–CH₂Cl [labeled (2) in Fig. 13, red]. Weak bands of cations CD₂Cl–Cl⁺–CD₂Cl and (CH₃)₂Cl⁺ also appeared (labeled 2 and 3 respectively), as did the traces of HCl absorption.

Fig. 13 IR spectra of the freshly prepared salt (CD₃–Cl⁺–CH₂Cl){Cl₁₁} (blue) and after one or three days of storage (red). The bands of the cations (CH₂Cl)₂Cl⁺, CD₂Cl–Cl⁺–CD₂Cl and (CH₃)₂Cl⁺ are marked respectively as (1), (2) and (3).

In the range of CD stretch vibrations, the band ν_asCD₃ = 2297 cm⁻¹ of the cation CD₃–Cl⁺–CH₂Cl was replaced with the intense ν_asCD₃ band at 2307 cm⁻¹ of the cation CD₃–Cl⁺–CD₃. Thus, the CD₃–Cl⁺–CH₂Cl cation in the solid phase at ambient temperature is unstable, and during the day, disproportionates into symmetrical cations, predominantly according to eqn (9) and to a slight extent, according to eqn (10)

2CD₃–Cl⁺–CH₂Cl → CH₂Cl–Cl⁺–CH₂Cl + CD₃–Cl⁺–CD₃ (9)

2CD₃–Cl⁺–CH₂Cl → CD₂Cl–Cl⁺–CD₂Cl + CH₃–Cl⁺–CH₃ (10)

The disproportionation looks as if at the chlorine atom, there is an intermolecular exchange of groups CD₃– and –CH₂Cl that is difficult to imagine for the solid phase. Intermediates with the molecular fragments CHDCl– and CH₂D– were not detected by IR spectroscopy.

Increased temperature. Heating of the salt (CH₃–Cl⁺–CH₂Cl){Cl₁₁⁻} to −100 °C for 5 min in a sealed cell led to disappearance of its spectrum (Fig. 14 and Fig. S8, ESI,† blue) and to the emergence of overlapping spectra of the symmetric cation (CH₃)₂Cl⁺ and isomer (CH₂Cl)₂Cl⁺, IIa (with the marked bands a and b, respectively, Fig. 14 and Fig. S8, ESI,† red). The absorption bands of gaseous HCl did not appear.

Fig. 14 IR spectra in the frequency region of CH bend vibrations of the salt (CH₃–Cl⁺–CH₂Cl){Cl₁₁⁻} before (blue) and after heating for 5 minutes at 100 °C (red) and 150 °C (green). The most characteristic bands of the formed compounds are marked with (a) for (CH₃)₂Cl⁺, (b) for (CH₂Cl)₂Cl⁺ (IIa) and (c) for CH₃–{Cl₁₁}.

Therefore, at 100 °C, the asymmetric cation CH₃–Cl⁺–CH₂Cl quickly disproportionates into the more stable symmetric cations:

2CH₃–Cl⁺–CH₂Cl → ClH₂C–Cl⁺–CH₂Cl + CH₃–Cl⁺–CH₃ (11)

Further heating of the sample for 5 minutes at 150 °C led to an equal (threefold) reduction in intensity of the bands at 1324 and 1284 cm⁻¹ of the cations (CH₃)₂Cl⁺ and (CH₂Cl)₂Cl⁺, respectively, and appearance and an increase in intensity of the bands of CH₃–{Cl₁₁} (1335 cm⁻¹: Fig. 14, green) and ClH₂C–{Cl₁₁} (νCCl_term = 793 cm⁻¹). Additionally, bands with a rotational structure at 1270 and 761 cm⁻¹ of gaseous CH₂Cl₂ were observed. Consequently, an increase in temperature facilitates the decomposition of chloronium ions according to eqn (12) and (13); this change should lead to increasing intensity of the IR spectrum of released dichloromethane.

(CH₃–Cl⁺–CH₂Cl){Cl₁₁⁻} → CH₃–{Cl₁₁} + CH₂Cl₂ (12)

(ClH₂C–Cl⁺–CH₂Cl){Cl₁₁⁻} → ClH₂C–{Cl₁₁} + CH₂Cl₂ (13)

Longer heating of the sample at 150 °C did not enhance the spectrum of CH₂Cl₂. It remained virtually unchanged. In contrast, the bands of CHCl₃ appeared (1219 and 772 cm⁻¹) and were enhanced with heating and a very weak absorption pattern of gaseous HCl developed. The solid-phase spectrum shows increased intensity of the CH₃{Cl₁₁} band at 1335 cm⁻¹ and disappearance of the adsorption of ClH₂C–{Cl₁₁}. Taken together, our data indicate that the CHCl₃ formation is caused by the interaction of ClH₂C–{Cl₁₁} with dichloromethane:

ClH₂C–{Cl₁₁} + CH₂Cl₂ → CH₃–{Cl₁₁} + CHCl₃.

To determine the reason for the formation of trace amounts of HCl, additional studies are needed.

Conclusions

The salts of symmetric methylchloronium cations, (CH₃)₂Cl⁺{Cl₁₁⁻} and (CH₂Cl)₂Cl⁺{Cl₁₁⁻}, and their neutral analogs CH₃–{Cl₁₁} and CH₂Cl–{Cl₁₁}, are stable at ambient and increased temperatures. Nevertheless, the asymmetric cation ClCH₂–Cl⁺–CH₃ even at ambient temperature disproportionates into symmetrical (CH₃)₂Cl⁺ and (CH₂Cl)₂Cl⁺. The molecular fragment CH₂Cl– of the chloronium ions enters exchange reactions with CH₂Cl₂ and CHCl₃ with increasing reactivity in the order ClCH₂–Cl⁺–CH₃, ClCH₂–Cl⁺–CH₂Cl, and CH₂Cl–{Cl₁₁}, yielding more stable and less reactive (CH₃)₂Cl⁺{Cl₁₁⁻} and CH₃{Cl₁₁} compounds.

If we take into account the reactivity of H{Cl₁₁} acid or its chloronium salts with CH₂Cl₂, and the simultaneous disproportionation of the CH₃–Cl⁺–CH₂Cl cation (eqn (11); initiated by the elevated temperature), then the end products of interactions at ambient, or slightly evaluated temperature, are the chemically inert CH₃–Cl⁺–CH₃ and CCl₄:

Reactions (9)–(13) (as determined in this study) proceed with the formation of a trace amount of HCl, thus indicating the negligible presence of parallel reactions. Under the other conditions, they may appear to be significant and interesting for future research.

Acknowledgements

This work was supported by grant # 16-13-10151 from the Russian Science Foundation and by the Ministry of Education and Science of the Russian Federation within the Project of the joint Laboratories of the Siberian Branch of the Russian Academy of Sciences and National Research Universities. The author thanks Irina S. Stoyanova for providing the carborane acids and technical support.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6cp00946h

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