Reaction of 1,3-dimethylimidazolium dimethylphosphate with elemental sulfur

Introduction

The combination of high mechanical strength, elasticity, and low cost ensured the growth of interest in the synthesis of sulfur-based polymers and copolymers [1], [2]. The polymer form of elemental sulfur is formed reversibly by a radical mechanism as a result of homolysis of S-S-bonds [3], and at room temperature the equilibrium is shifted towards depolymerization to form S₈ [3], [4], [5], [6]. The equilibrium shift towards polymerization is possible only when the temperature increases to 432 K, thus limiting the possibility of using elemental sulfur polymer [2], [7]. Thermal opening of the S₈ ring made it possible to carry out radical copolymerization of sulfur with unsaturated compounds, and the resulting copolymers are thermodynamically stable at room temperature and do not depolymerize [8], [9], [10], [11]. The processes of synthesis of homopolymer and copolymers of elemental sulfur correspond to the principles of green chemistry, and apparently have significant prospects for upscaling [12].

S₈ ring opening can occur not only by thermal dissociation or as the result of radical reactions, but also via nucleophilic attack. For example, the copolymerization of S₈ and thiiranes in the presence of typical initiators of anionic polymerization is described [13], [14]. It is known that N-methylimidazole opens the S₈ ring to form polysulfide anions [15], [16], and sodium sulfide reacts with elemental sulfur in an aqueous medium [17], [18], [19]. Vulcanization of rubbers associated with S₈ ring opening is also accelerated in the presence of a number of nucleophiles [16]. These facts leave no doubt that the cyclo-octasulfur is able to undergo the ring opening via the anionic mechanism with the formation of a negatively charged sulfur atom at the end of the chain.

Ionic liquids, which are promising green solvents [20], [21] and in some cases have a catalytic effect [22], also showed efficiency in accelerating the vulcanization of rubbers [23], [24], [25]. However, elemental sulfur is insoluble in ionic liquids. In this regard, the recently discovered reaction of S₈ with tri-n-butylmethylphosphonium dimethylphosphate ionic liquid, proceeding with ring opening and accompanied by the formation of a mixture of sulfur oligomers [26], is of great interest. Whether this example of low-temperature initiation of sulfur oligomerization is a common property of ionic liquids with dimethylphosphate anion, remains to be determined. In this paper, it is reported that the interaction of S₈ with 1,3-dimethylimidazolium dimethylphosphate is also accompanied by the opening of the S₈ ring with the formation of short chains containing a terminal dimethylphosphate group.

Experimental part

Results and discussion

In the ¹H NMR spectrum of the initial 1,3-dimethylimidazolium dimethylphosphate ionic liquid, the singlets with the chemical shifts of 9.30 ppm, 7.83 and 4.10 ppm are observed. They correspond to the protons of the 1,3-dimethylimidazolium cation at position 2, equivalent positions 4 and 5, as well as methyl groups at nitrogen atoms. A doublet with a chemical shift of 3.61 ppm corresponds to the protons of the methyl groups of the dimethylphosphate anion in spin-spin interaction with the phosphorus atom. A similar interpretation of signals in the ¹H NMR spectrum of 1,3-dimethylimidazolium dimethylphosphate is given in [28]. ¹³C NMR spectrum of the initial 1,3-dimethylimidazolium dimethylphosphate contains signals with chemical shifts of 139.93 ppm, 125.89 and 37.57 ppm, which correspond to carbon atoms at position two, equivalent positions four and five of the imidazolium ring, and to the carbon atoms of methyl groups at nitrogen atoms. A signal with a chemical shift of 53.90 ppm corresponds to the carbon atoms of the methyl groups of the dimethylphosphate anion. After the reaction of 1,3-dimethylimidazolium dimethylphosphate with elemental sulfur in the benzene medium, the chemical shifts of all hydrogen and carbon atoms in the ¹H and ¹³C NMR spectra of the reaction product practically do not change, and their multiplicities are also preserved. However, in the ¹H and ¹³C NMR spectra, new singlets appear at 7.44 and 130.71 ppm, respectively. These signals may probably correspond to a product of some reaction involving 1,3-dimethylimidazolium cation, or may simply refer to benzene used as a solvent. Registration of ¹H–¹⁵N two-dimensional NMR spectrum (Fig. 1) of the reaction product whose signals were detected by the inverse method (by protons) showed the absence of spin-spin interaction of the proton responsible for the appearance of the new signal with nitrogen atoms of the 1,3-dimethylimidazolium cation, which allows us to confidently attribute it to benzene protons. The presence of benzene in the reaction product is associated with the difficulties of its removal, since benzene removal by distillation may lead to the equilibrium shift towards increasing the chain length of the formed sulfur oligomers, and freezing, on the contrary, will contribute to reducing their length [2]. Because benzene is a toxic compound, it can be further replaced by the natural terpene p-Cymene as a solvent. In the present work we used benzene as a solvent to simplify the analysis of the obtained spectral data.

Fig. 1:

Two-dimensional ¹H–¹⁵N NMR spectrum measured by the inverse method (by protons) of the product of interaction of 1,3-dimethylimidazolium dimethylphosphate with S₈.

Thus, 1,3-dimethylimidazolium cation is inactive in this reaction, and the reaction is associated only with the presence of a dimethylphosphate anion. This conclusion is fully consistent with the fact that S₈ does not interact with ionic liquids not containing dimethylphosphate anion, such as 1-n-butyl-3-methylimidazolium tetrafluoroborate, 1-n-butyl-3-methylimidazolium trifluoromethanesulfonate, 1,3-dimethylimidazolium hexafluorophosphate. It is also noteworthy that in the ³¹P NMR spectra of the reaction product and of the initial 1,3-dimethylimidazolium dimethylphosphate, only one signal with a chemical shift of 1.17 and 1.83 ppm, respectively, is observed. Thus, the only potential site for the interaction of 1,3-dimethylimidazolium dimethylphosphate with S₈ is the negatively charged oxygen atom of the dimethylphosphate anion, since in case of the reaction on other atoms, the changes in chemical shifts in ¹H, ¹³C, ³¹P NMR spectra would be significant. Registration of ¹⁷O NMR spectra confirms this assumption, since two signals of two nonequivalent oxygen atoms are observed for the initial ionic liquid and after its interaction with S₈, the signals of three nonequivalent oxygen atoms were observed [26].

In the MALDI spectrum of the interaction product of 1,3-dimethylimidazolium dimethylphosphate with sulfur, the most intense signal in the negative ion mode is observed at m/z = 153.00, suggesting the formation of C₂H₂PO₄S⁻ anion (Fig. 2, Scheme 1).

Fig. 2:

MALDI spectrum (negative ion mode) of the products of 1,3-dimethylimidazolium dimethylphosphate interaction with S₈.

Scheme 1:

Reactions expected to proceed during laser desorption of the products of 1,3-dimethylimidazolium dimethylphosphate interaction with S₈.

The presence of a signal at m/z = 153.00 and the absence of a signal with m/z ≈ 125 corresponding to the dimethylphosphate anion, is consistent with the reaction between 1,3-dimethylimidazolium dimethylphosphate and S₈. The signals with m/z = 184.98 and 167.02 correspond to the C₂H₂PO₄S₂ anion and its dehydration product, respectively. The signal at m/z = 232.98 appears to correspond to the particle C₆H₆ × C₂H₄PO₄S⁻ (Fig. 2, Scheme 1).

Although the registration of MALDI spectra assumes mild ionization conditions, the cyclodehydrogenation processes in accordance with scheme 1, which explains the appearance of signals at m/z = 153.00, 184.98 and 232.98, is possible. Such cyclodehydrogenation processes have long been described for the interaction of hydrocarbons with sulfur [29].

In the positive ion mode, a series of signals with a molecular mass difference of 32 is observed in the MALDI spectrum (Fig. 3). A similar pattern of fragmentation is characteristic of elemental sulfur [30]. Since in the MALDI spectrum of the product of interaction of equimolar quantities of 1,3-dimethylimidazolium dimethylphosphate and S₈, the signal with m/z ∼ 125 was not observed in the negative ion mode, we can assume that dimethylphosphate anion nearly quantitatively reacted with S₈. Therefore, the appearance of signals characteristic of elemental sulfur in the MALDI spectrum in the positive ion mode is associated with its release during the decomposition of C₂H₆PO₄S_n ⁻ (Scheme 1). In addition to signals (S_n)⁺ with different n, there is also a signal of 1,3-dimethylimidazolium cation (m/z = 96.99), which is consistent with its stability in the reaction of 1,3-dimethylimidazolium dimethylphosphate with S₈ (Fig. 3).

Fig. 3:

MALDI spectrum (positive ion mode) of the product of 1,3-dimethylimidazolium dimethylphosphate interaction with S₈.

In the ESI mass spectrum obtained in the positive ion mode, the fragmentation pattern characteristic of sulfur was not observed. This can be explained by the thermal decomposition of C₂H₆PO₄S_n ⁻ when the sample is dispersed in a nebulizer at a temperature of 473 K for 30 s before the sample is fed to the ionization chamber. The signals with m/z = 97.0761 and m/z = 319.1533 correspond to the 1,3-dimethylimidazolium cation and to the positively charged ionic associate [(1,3-Me₂Im)₂Me₂PO₄]⁺ (Fig. 4). The low-intensity signal at m/z = 252.9727 probably corresponds to the loss of hydrogen by the addition of one sulfur atom to 1,3-dimethylimidazolium dimethylphosphate. The low intensity of this signal is also indirectly consistent with the thermal decomposition of C₂H₆PO₄S_n ⁻ before entering the ionization chamber.

Fig. 4:

ESI mass spectrum (positive ion mode) of the product of interaction of 1,3-dimethylimidazolium dimethylphosphate with S₈.

In the ESI mass spectrum obtained in the negative ion mode, the most intense signal with m/z = 125.0000 corresponds to dimethylphosphate anion. Since no signals typical for the initial ionic liquid were observed in MALDI spectra, the detection of the C₂H₆PO₄ ⁻ anion is probably associated with the decomposition of C₂H₆PO₄S_n ⁻ when the sample is dispersed in a nebulizer before entering the ionization chamber. Also, the signals at m/z = 255.2321 and 414.9618 with significant intensities were observed in the mass spectrum (Fig. 5). These signals probably correspond to the products of hydride ion addition, as well as one sulfur atom and six sulfur atoms addition to the initial ionic liquid, respectively. Hydride ion transfer processes are described in the literature [31], [32], [33]. Since only monomolecular reactions are possible under the conditions of mass spectra registration, it is possible that hydride ion transfer can be realized in ionic associates, the formation of which was noted above. In this case, the signal at m/z ≈ 223, the presence of which would be expected for the unreacted 1,3-dimethylimidazolium dimethylphosphate, was not observed.

Fig. 5:

ESI mass spectrum (negative ion mode) of the product of interaction of 1,3-dimethylimidazolium dimethylphosphate with S₈.

Summarizing the data obtained by ¹H, ¹³C, ¹⁵N, ³¹P, ¹⁷O NMR spectroscopy, MALDI and ESI mass spectra, we can conclude that the studied reaction can be described as the addition of sulfur to a negatively charged oxygen atom of dimethylphosphate anion of the initial ionic liquid (Scheme 2). Since in the MALDI spectra there are no signals corresponding to high molecular weight compounds, we suppose that only the products with short sulfur chains for in the studied reaction. It should be noted that 1,3-dimethylimidazolium dimethylphosphate ionic liquid was used in equimolar amounts with respect to S₈. This contributes to the formation of low molecular weight products, especially if the initiation stage is rapid. In the reaction of S₈ with tri-n-butylmethylphosphonium dimethylphosphate, the products of higher molecular weight were formed [26], indicating a lower activity of this ionic liquid in reaction with S₈ compared to 1,3-dimethylimidazolium dimethylphosphate.

Scheme 2:

Reaction of 1,3-dimethylimidazolium dimethylphosphate with elemental sulfur.

Higher reactivity of 1,3-dimethylimidazolium dimethylphosphate in the reaction with S₈ can be described on the basis of the Pearson hard and soft acids and bases (HSAB) theory. The oligomeric anion is a soft Lewis base, while the hardness of the tri-n-butylmethylphosphonium cation is higher than that of 1,3-dimethylimidazolium cation in which the positive charge is delocalized. Then, in accordance with the HSAB theory [34], [35], one should expect a higher decrease in the energy of the system resulting from the interaction of oligomeric anion with 1,3-dimethylimidazolium cation than with tri-n-butylmethylphosphonium.

In the theoretical study of the possibility of S₈ ring opening in the interaction with dimethylphosphate anion, the Kohn-Sham method at the B3LYP/STO-3G level of theory predicts spontaneous S₈ ring opening upon approaching the dimethylphosphate anion at a distance of 1.717 Ǻ. It is accompanied by a decrease in the total energy of the system by 242 kJ × mol⁻¹ compared to infinitely distant reagents. Modeling at B3LYP/6-31G* and B3LYP/6-311G** levels of theory also shows a decrease in the total energy of the system upon formation of a supermolecule by 399 kJ × mol⁻¹ and 55 kJ × mol⁻¹, respectively. After geometry optimization by B3LYP/6-31G* and B3LYP/6-311G** methods, the calculated distances in the supermolecule between the oxygen atom of the dimethylphosphate anion carrying a negative charge and the sulfur atom entering the S₈ ring are 2.400 Ǻ and 2.421 Ǻ, respectively, and an imaginary frequency is present in the vibrational spectrum.

Thus, the convergence of the S₈ ring and the dimethylphosphate anion at a distance of 1.717 Ǻ in the B3LYP/STO-3G approximation leads to the opening of the S₈ ring, and the energy minimum was achieved upon formation of the reaction product shown in Scheme 2. Calculations at the B3LYP/6-31G* and B3LYP/6-311G** levels of theory predict the formation of supermolecules with distances between reagents greater than the originally specified (1.717 Ǻ). These structures are transient states realized when the system moves along the reaction coordinate from the initial compounds to the reaction products, as evidenced by the presence of imaginary frequencies. The QTAIMC theory for B3LYP/6-31G* and B3LYP/6-311G** optimized supermolecules indicates a bound state between the oxygen atom of the dimethylphosphate anion and the sulfur atom of the S₈ ring: there is a critical bond point (3,−1) between these atoms. The electron density (ρ _b ) values at the critical coupling point are 0.0425 (B3LYP/6-31G*) and 0.0399 a.u. (B3LYP/6-311G**). The electron density Laplacian (∇²ρ _b ) values are 0.1122 (B3LYP/6-31G*) and 0.1138 (B3LYP/6-311G**) a.u.

Conclusions

It is shown that the interaction of 1,3-dimethylimidazolium dimethylphosphate with elemental sulfur is accompanied by the S₈ ring opening. The reaction proceeds only with the participation of dimethylphosphate anion due to the addition of sulfur to the oxygen atom carrying negative charge. The 1,3-dimethylimidazolium cation does not participate in the reaction. The possibility of the S₈ ring opening as a result of nucleophilic attack by dimethylphosphate anion is confirmed by modeling the interaction of these particles by the Kohn-Sham method at the B3LYP/STO-3G level of theory.