Protonated Ethylene Carbonate: A Highly Resonance‐Stabilized Cation

Abstract Salts containing the monoprotonated ethylene carbonate species of were obtained by reacting it with the superacidic systems XF/MF5 (X=H, D; M=Sb, As). The salts in terms of [C3H5O3]+[SbF6]−, [C3H5O3]+[AsF6]− and [C3H4DO3]+[AsF6]− were characterized by low‐temperature infrared and Raman spectroscopy. In order to generate the diprotonated species of ethylene carbonate, an excess of Lewis acid was used. However, this only led to the formation of [C3H5O3]+[Sb2F11]−, which was characterized by a single‐crystal X‐ray structure analysis. Quantum chemical calculations on the B3LYP/aug‐cc‐PVTZ level of theory were carried out for the [C3H5O3]+ cation and the results were compared with the experimental data. A Natural Bond Orbital (NBO) analysis revealed sp2 hybridization of each atom belonging to the CO3 moiety, thus containing a remarkably delocalized 6π‐electron system. The delocalization is confirmed by a 13C NMR‐spectroscopic study of [C3H5O3]+[SbF6]−.


Introduction
Lithium ion batteries play an important role in our everyday life. They are applied in mobile phones, notebooks or other batteryoperated tools. [1] A lithium ion battery consists of two electrodes, which are typically separated by a semipermeable membrane, or similar separators, immersed in an ion-conducting electrolyte. Throughout a charge process, or discharge process respectively, lithium ions are transported by an electrolyte from one electrode to the other and are intercalated in the respective layers. [1][2][3][4][5][6] Cathode materials are commonly transition metal oxides in the form of LiMO 2 (M = Co, Ni, Mn, Fe) or LiMn 2 O 4 (spinel type), whereas the anode consists of carbonic materials. The electrolyte is a mixture of organic solvents, such as propylene carbonate, wherein Li salts are dissolved. [7] Graphite was one of the first anode materials, which was not only able to intercalate the Li + ions but also solvent molecules. [4] This problem of co-intercalation made the improvement of anode materials a greater challenge. Replacing graphite by petroleum coke prevented the solvent intercalation. Interestingly, adding ethylene carbonate to the solvent also blocked co-intercalation into the anode material for both graphite and petroleum coke. [4,8] This fact led to the establishment of ethylene carbonate, together with a dialkyl carbonate, as a useful electrolyte. [1] Hence, many investigations on ethylene carbonate were performed, for example on the transportability of binary ethylene carbonate/ propylene carbonate systems [7,9] or cation-solvent interactions. [10] So far, very little is known about the base properties of ethylene carbonate in literature. The only evidence for a protonated species of ethylene carbonate is given by 1 H NMR [11,12] and mass spectroscopic studies. [13] This prompted us to study ethylene carbonate in superacidic solutions.
The superacidic solutions were prepared using an excess of anhydrous hydrogen or deuterium fluoride, which serves as a reagent as well as a solvent. In order to obtain a complete solvation of the Lewis acid, the mixture was homogenized at À 40°C. Afterwards, ethylene carbonate was added to the frozen mixture under nitrogen atmosphere. The reaction mixture was allowed to warm up to À 40°C and salts of monoprotonated ethylene carbonate were formed. The excess of the solvent was removed in a dynamic vacuum at À 78°C overnight. Due to the poor polarizability of the OH stretching vibration, the corresponding Raman lines usually are of low Raman intensity ((b) and (c)). Contrariwise, the OD stretching vibration in (d) is detected at 2339 cm À 1 . The IR spectra (f) and (g) display a broad ν(OH) band at 3406 cm À 1 and 3278 cm À 1 , respectively. In comparison, the corresponding ν(OD) of (e) is observed at 2325 cm À 1 . The red shift is in good agreement with the Teller-Redlich rule for an H/D isotopic effect. [14] These are the most meaningful vibrational modes for providing evidence for a successful protonation. Due to the O-protonation, the CO double bond of ethylene carbonate is weakened, whereas the CO single bonds are strengthened. The stretching vibrations of the CO 3 group display this tendency. Compared to the starting material, the vibration of the former CO double bond is redshifted by about 100 cm À 1 . In contrast, the former CO single bonds are blue-shifted by up to 400 cm À 1 . [15] The COX deformation vibration is observed between 1212 cm À 1 (c) and 1218 cm À 1 (b) for the protonated and at 826 cm À 1 (e) for the deuterated species. The most intense line in the Raman spectra (b-d) occurs at about 900 cm À 1 and is assigned to the skeletal breathing mode. In comparison to ethylene carbonate (a), this mode is nearly unaffected by the protonation. [15,16] This also applies to the rest of the skeletal vibration modes (except for the CO 3 moiety), such as ν(CO), ν(CC), δ(COC), and δ(OCO), respectively. For the anions (M = Sb, As) with an ideal O h symmetry, three Raman lines and two IR bands are expected. In the Raman spectra (b-d), more than three lines are observed and likewise the IR spectra (d-f) show more than two bands for the anions. The increased numbers of vibrations indicate a lowered symmetry of the hexafluoridometalate anions.

Theoretical Calculations
Structure optimization of the free [C 3 H 5 O 3 ] + cation was carried out on the B3LYP/aug-cc-pVTZ level of theory. IR and Raman intensities as well as vibrational frequencies were calculated in the harmonic approximation. Figure 4 shows the comparison of the cation of the single-crystal X-ray structure (4) and the calculated structure of [C 3 H 5 O 3 ] + together with bond lengths and angles.
Comparing the values of the experimentally obtained geometric parameters with those obtained from the calculation   (4) with estimated standard deviations in parentheses. For (4), interatomic contacts are listed. Symmetry codes: i = 1 = 2 + x, 1 = 2 À y,À 1 = 2 + z; iii = 1 = 2 À x, 1 = 2 + y, 1 = 2 À z. ethylene carbonate [17] (  = 2 À y, À 1 = 2 + z; ii = 1À x, 1À y, À z; iii = 1 = 2 À x, 1 = 2 + y, 1 = 2 À z. Despite the protonation, the sp 2 hybridization of the central carbon atom is conserved compared to the neutral compound. Even more interesting is the hybridization of the oxygen atoms. In order to investigate the hybridization situation of the CO 3 group, a Natural Bond Orbital (NBO) analysis was performed on the B3LYP/aug-cc-pVTZ level of theory. In Figure 5, the calculated NBOs of the lone pairs on the oxygen atoms together with the calculated electron occupancy are illustrated.
All oxygen lone pairs are located in the molecule plane, which suggests sp 2 hybridization on each oxygen atom in the protonated species. In order to confirm this hybridization, the corresponding p-orbitals were considered. Figure 6 shows the calculated NBOs of the corresponding p-orbitals together with the calculated electron occupancy. The p-orbital of the central carbon atom as well as every p-orbital of the oxygen atoms are oriented perpendicular to the molecule plane. The NBO analysis shows a π-bond between O3 and C1 with approximately two electrons, whereas the residual p-orbitals on O2 and O1 are occupied with 1.74 and 1.76 electrons, respectively. Additionally, the NBO of the antibonding π-bond of O3 and C1 is occupied with 0.48 electrons. In Table S3, selected NBOs (concerning the CO 3 group) are listed, together with calculated values for electron occupancy and s-and p-character given in percentage (see Supporting Information). In summary, the NBO analysis indicates that a strong delocalization of the electrons over the Y-shaped CO 3 group is possible.

NMR Spectroscopy of [C 3 H 5 O 3 ] + [SbF 6 ] À (1)
The NMR spectroscopic study of [C 3 H 5 O 3 ] + [SbF 6 ] À (1) was carried out in anhydrous HF (aHF). For better comparability, a reference of neutral ethylene carbonate was measured in aHF as well. The chemical shifts obtained by 1 H, 13 C and 19 F NMR spectroscopy are listed in Table 3.   Due to the large excess of aHF, the resonance of the proton on the oxygen atom is not observable. Nevertheless, the observation of the anion confirms the formation of the [C 3 H 5 O 3 ] + cation. The 19 F signal at À 123.24 ppm is attributed to the SbF 6 À anion, which is in accordance with literature. [23] In the 1 H NMR spectrum, the signal of the protons of the CH 2 groups, which appears as a multiplet, is slightly shifted to higher frequencies compared to ethylene carbonate. The same trend is observed for the corresponding C signal in the 13 C NMR spectrum. Interestingly, the C atom, belonging to the CO 3 group, is also only slightly deshielded. The signal is shifted by about 2.58 ppm. This leads to the conclusion that protonated ethylene carbonate is a cation where the positive charge is strongly delocalized over the CO 3 moiety.

Conclusion
The protonation of ethylene carbonate succeeded for the first time in the superacidic systems HF/MF 5 11 ] À (4) were isolated. The compounds were characterized by IR and Raman spectroscopy and, in the case of 4, by an X-ray structure analysis. For compound 1, an NMR-spectroscopic study in aHF was carried out. The experimental results were compared with quantum chemical calculations on the B3LYP/aug-cc-pVTZ level of theory. To elucidate the bonding situation of the CO 3 moiety, a NBO analysis was performed. This calculation indicates a sp 2 hybridization on the central carbon atom as well as on the oxygen atoms, thus leading to the conclusion that protonated ethylene carbonate is a compound with a remarkable 6π-electron delocalization, located on the CO 3 group.

Experimental Section General
Caution! Avoid contact with any of these compounds. The hydrolysis of all these salts might form HF, which burns skin and causes irreparable damage. Safety arrangements should be taken while using and handling these materials.
All reactions were performed by standard Schlenk technique using a stainless steel vacuum line. All reactions in superacidic media were carried out in FEP/PFA reactors closed with a stainless steel valves. The vacuum line as well as the reactors were dried with fluorine prior to use. Detailed Information about the used apparatus and materials as well as analytic measurement methods are described in the Supporting Information.
Deposition Number 1978961 (for 4) contains the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.