and di-anions as super acids through Ï-hole interaction

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C: Energy Conversion and Storage; Energy and Charge Transport

Rational design of mono- and di-anions as super acids through #hole interaction: Implications for Lithium and Magnesium Ion Batteries Mrinal Kanti Si, and Bishwajit Ganguly J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01286 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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The Journal of Physical Chemistry

Rational Design of Mono- and Di-anions as Super Acids Through π-hole Interaction: Implications for Lithium and Magnesium Ion Batteries Mrinal Kanti Si1,2, Bishwajit Ganguly*1,2 1

Computation and Simulation Unit (Analytical Discipline and Centralized Instrument Facility), CSIR-Central Salt & Marine Chemicals Research Institute 2

Academy of Scientific and Innovative Research, CSIR-CSMCRI, Bhavnagar, Gujarat, India364 002. *Corresponding Author. Fax: (+91)-278-2567562, E-mail:[email protected]; [email protected] Abstract: Nature helps to generate multiply charged negative ions in abundance. Such negatively charged ions are stable as crystals with charge compensating cations, but in solution, they are protected with solvent molecules. In the gas phase, such multi-charged anions are not stable due to strong electrostatic repulsion between the extra electrons. Therefore, the generation of multi-charged anions without the influence of environment has been of great interest. The DFT calculations reveal that the π-hole interactions can stabilize the anion of acid using selenoaldehydic group attached to the cyclopentadiene ring to make them super acids. This strategy helped to generate the stability of dianions without any influence of environmental conditions. It is known that most electrolytes used in Li-ion batteries contain halogens, which are however toxic in nature. The designed super-acids can also be exploited as lithium ion batteries. These stable dianions showed the binding energies of lithium ions (5.8 and 63.6 kcal/mol) at M06-2X/6-311+G(2d,p) level of theory and can be considered as an efficient electrolyte in lithium ion battery. The study extended with magnesium ion also showed promise to function as efficient electrolyte in magnesium ion battery using such anions.

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Introduction: The proton is a tiny particle that plays a pivotal role in many chemical and biological phenomena. The principal importance of proton is in acid-base chemistry. According to Arrhenius theory, an acid produces H+ ion in solution. In the dissociation of an acid HA, one important consequence in equilibria (HA

H+ + A-) is that every acid (HA) has a conjugate

base (A-) and vice-versa. The strong acids are molecular compounds that disassociates into H+ ions and anions in aqueous solution. The equilibrium shifts to the right hand side in the (HA

H+ + A-) means the HA can be the strong acid and the A- is the stable conjugate

base. Recently, efforts have been laid in the design of potential neutral organic superacids.1-5 Polycyanated hydrocarbon derivatives have been designed and examined, which behaves as hyperacids.6,7 The parent hydrocarbons of such polycyanated hydrocarbons exhibit tautomerism, which results a large number of tautomers. Some of these tautomers are less stable and possess lower acidity. The substitution of cyano groups in these hydrocarbons generate more number of tautomers where the hydrogen moves around the molecule leads to stable ketene imine species. Such phenomena yields hyper strong superacids in some cases.1 The tautomerism was also seen to play an important role in organic tri- and hexacyclic molecules to generate super- to hyper acids.8 A few strategies in preparing superacids have been reported. In the first strategy, electronic super acceptor substituents were used, however, in second strategy stabilization of the conjugate base through strong anionic resonance triggered by strong electron withdrawing groups.1,6,7,9 Yagupolskii has given the concept of electronic superacceptor substituents that can be showed by =NSO2CF3, where replacing doubly bonded oxygen to S, P and I atoms gives very strong superacids in the gas phase and in acetonitrile. This strategy is based on the use of one or more highly dipolar superacceptor groups and some strongly polarizable


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substituents yields an extended conjugated system in their corresponding conjugate bases.10-12 In the second strategy, the anionic resonance can stabilize the conjugate base to increase the gas phase acidity of molecules. The increase in the acidity can relate to the increase in ionization potential of an anionic conjugate base, which can be calculated in Koopmans’ frozen electron density + clamped nuclei approximation.13 The large IPKoop ionization potentials indicate strong anionic resonance or aromatic stabilization upon deprotonation.10,13 These strategies have generated a large number of extremely powerful superacids in the gas phase and in solution.1,6,7,9 Reports on the stability of dianions are known in the literature, though such anions are relatively unstable due to electronic repulsions.14-17 Recently, the dianion of ortho-diethynylbenzene has been synthesized and the proton affinity was calculated using DFT methods.14 A stable and flexible dianion 2-dicyanomethylene1,1,3,4,5,5-hexacyanopentenediide

(DHCP2-)

has

also

been

prepared

as

a

tetraalkylammonium salt from hexacyanobutadiene.15,18-21 Such strong acids can be used for organic synthesis and catalyst for coal liquefaction, hydrocarbon isomerization, cracking, alkylations used in olefin polymerization and also isolation of many short-lived cations.6,22-29 Carboranes and boron clusters have been exploited to make an interesting class of strong acids.6,25,30 Researchers have also shown that tautomeric fulvene derivatives can also act as hyperacids.31 The stable anions have been shown to be important as materials for batteries.16,17,32 The reports suggest that the stability of multicharged anions can be exploited as electrolyte for metal ion batteries.16,17,32 It is reported in the literature that the halogen electrolytes are used in Li-ion battery, however, halogens are toxic in nature.32 Therefore, there are efforts to design the halogen free electrolyte for Li-ion battery.32 The DFT studies reveal that LiCB11H12 has the potential among all halogen-free electrolytes. The metal borohydrides such as LiBH4 can be used as potential candidates for halogen free electrolytes in Li-ion battery.32



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Scheme 1 Structures of substituted selenoaldehydic cyclopentadiene derivatives (1-6) .

H

R1 H

Se

H

H

H H

R2

Se R4

R3 H

R5

H

Se

H R9 H

H

R6

R8

H R7 H

1

2-6

2. R1= H, R2= H, R3= H, R4= H, R5= H, R6= H, R7= H, R8= H, R9= H 3. R1= CN, R2= H, R3= H, R4= H, R5= H, R6= H, R7= H, R8= H, R9= CN 4. R1= CN, R2= CN, R3= H, R4= H, R5= H, R6= H, R7= H, R8= CN, R9= CN 5. R1= CN, R2= CN, R3= CN, R4= H, R5= H, R6= H, R7= CN, R8= CN, R9= CN 6. R1= CN, R2= CN, R3= CN, R4= OCH3, R5= H, R6= H, R7= CN, R8= CN, R9= CN

In this article, we have given a different concept to design superacids using non-covalent interactions using DFT calculations. Further, we have shown that the conjugated system with electron donating group instead of electron acceptor with π-hole bonds can arrive in the range of superacids. Reports reveal that the anion can be stabilized using non-covalent interactions such as anion…π, π…π, σ-hole and π-hole interaction.28 The π-hole bond is another depiction of the anion/lone pair…π interaction.33 The description of π-hole bond is extensive and includes organic and inorganic π-hole systems involving π-bond or conjugated π-bond.33-36 Generally, anion…π interaction conceives of a negative…negative interaction, nevertheless, the intrinsic characteristic of the “π” is described well in the π-hole. Such π-holes describe a region of positive surface electrostatic potential in a molecular entity, and the π-hole bond is also distinct. Generally, the π-molecular orbital or p orbital of one central atom is involved in π-hole bond. It is known that such non covalent interactions i.e., σ- and π-hole interactions are fully directional.33 We have incorporated for the first time the role of π-hole interactions to augment the acidity of cyclopentadiene (Cp) derivatives. Such strategy may lessen the synthetic work to introduce 4 ACS Paragon Plus Environment

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a number of substituents in the organic molecules.1,7 We have also shown that the di-anions can also be stabilized through non-covalent interactions. The stability of these dianions can be used as electrolyte in lithium and magnesium ion batteries. Such small molecules with such properties would be promising for various applications in chemistry and materials including batteries as storage devices. There are reports where non-covalent interactions have been exploited to influence the proton affinity of YHXOH (X = 0-3; Y = Li, Be, B, Na, Mg, Al etc.) in presence of Lewis bases. The Lewis bases form the complexes with YHXOH (X = 0-3; Y = Li, Be, B, Na, Mg, Al etc.) through non-covalent interaction which in turn enhances the proton affinity.37 Computational Methods: We have calculated the acidity of the compounds using the enthalpy change (∆Hacid) for the proton dissociation reaction. The proton dissociation reaction is given below AH(g) → A- (g) + H+(g)

(1)

The ∆Hacid are calculated using the following equation 2 ∆Hacid = ∆Eacid + ∆(pV)

(2)

Where ∆Eacid denote the change of energies of the species involved in the equation 1 where the energy includes total electronic energy and thermal enthalpy correction at 298.15 K. The ∆(pV) is pressure-volume work. Gibbs free energy change (∆Gacid) has been calculated using the species involved in eq. (1) at the same level of theory. We have considered the enthalpy of proton in gas phase as 1(5/2)RT or 6.197 kJ/mol at 298.15 K and Gibbs free energy of proton of “(5/2) RT - TS,” which is -26.25 kJ/mol at 298.15 K.31 We have optimized the structures at M06-2X/6-311+G(2d,p) level of theory in gas phase to calculate the electronic energies and Hcorrection.38-40 The optimized structures are minima which are confirmed by the positive harmonic vibrational frequency. The DFT M06-2X is 5 ACS Paragon Plus Environment


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recognized to be well described with dispersion forces.41 Further, we have performed the single point of these molecules in DMSO solvent using the IPCM (Isodensity Polarized continuum model) solvent model.42,43 The acidity of the molecules can be calculated using the equation 3 AH + DMSO → A- + DMSOH+ + ∆rHDMSO

(3)

∆rH is the enthalpy change in the above equation. We have performed the calculations at M06-2X/6-311+G(2d,p)//M06-2X/6-311+G(2d,p) level of theory in solvent using isodensity polarized continuum model (IPCM) by using the thermal correction at M06-2X/6311+G(2d,p) level of theory. The pKa values for compound cyclopentadiene and 6 are calculated by using following equation which is previous reported to calculate the pKa for neutral organic acid.44 pKa(exp) = 0.661. ∆rHDMSO – 7.7

(4)

We have performed the AIM (The Bader’s theory of atom in molecule) analysis taking the wave function generated at M06-2X/6-311+G(2d,p)//M06-2X/6-311+G(2d,p) level of theory in GAUSSIAN09.45-47 The AIM has been performed using multiwfn software.48 The MESP (Molecular electrostatic potential) has been performed using Molekel-4.3 free software. The following equation has been used to calculate the electrostatic potential in space around molecules.49 V(r) = Σ ZA /|RA-r| - ʃρ(ŕ)d ŕ/|ŕ -r|

(5)

The V(r) is the potential at the distance r by the nuclei and electrons of the molecule; and ZA is the charge of nucleus A at the point RA. The ρ(r) is electron density. We have optimized the structure of Li-82-, Li2-82- and Mg-82- (Figure 5) at M06-2X/6311+G(2d,p) level of theory to get a quantitative estimate of the cation binding energy. The


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energies necessary to remove the first and second lithium ion as well as the magnesium ion have been calculated from the following equations.17 ∆ELi1 = E(Li) + E(Li18-) – E(Li28)

(6)

∆ELi2 = E(Li+) + E(82-) – E(Li18-)

(7)

∆EMg = E(Mg2+) + E(82-) – E(Mg8)

(8)

Where ∆ELi1 and ∆ELi2 are the dissociation energies for removal of first and second Li+ ions from the Li2-82- respectively (Figure 5). ∆EMg is the dissociation energy of magnesium of Mg-82-. The binding energies of 1st and 2nd electron of halogen-free electrolytes (8) are calculated in the following equation 9 and 10.12 ∆E1 = E(8) – E(8-)

(9)

∆E2 = E(8-) – E(82-)

(10)

The structure of 8 is given in Figure 4. Results and Discussion The stability of cyclopentadienyl anion with substituted selenoaldehyde was examined using density functional theory (DFT) calculations (Scheme 1). The (-CH2-CH2-) chain length is appropriate to achieve the interaction between the selenoaldehyde and the cyclopentadienyl anion. The smaller or longer chain lengths, however, do not align properly for any interaction (Figure 1). The selenoaldehyde attached with cyclopentadiene unit showed the remarkable stability of the corresponding conjugate base. The acidity of cyclopentadiene measured experimentally is 352.6 kcal/mol.31,44 The calculated ∆Hacid for cyclopentadiene with M062X/6-311+G(2d,p) level of theory is 351.0 kcal/mol which is in very good agreement with the experimental results. Compound 1 with selenoaldehyde showed the deprotonation energy computed at the same level of theory is 344.0 kcal/mol. The selenoaldehyde group can



possess σ- and π-holes for effective interaction with the electron rich systems.33,50 The σ-hole is present along the extension of the C=Se bond and π-holes are present above and below of the F2C=Se plane formed by the carbon and fluorine atoms.33,50 The σ- and the π-holes enable a molecule to interact attractively with a negative site such as the lone pairs, π electrons or anions.33 The strength of σ- and π-hole bonds can be modulated with the substituents attached to it. The electron withdrawing nature of the substituents enhances the ability of σ- and π-hole to interact strongly with the electron rich sites of the system.33,50 The presence of selenoaldehyde group in 1 remarkably eased the deprotonation by ~8.0 kcal/mol compared to parent cyclopentadiene. The substituted selenoaldehyde group aligns to interact with the cyclopentadienyl anion and resides at a distance of 3.64 Å (Figure 1). The Molecular electrostatic potential (MESP) calculations reveal that the π-hole of selenoaldehyde interacts with the negative site of cyclopentadienyl anion (Figure 2). The selenoaldehyde aligns properly to interact with the conjugate base, while in the case of parent cyclopentadiene, it is away from the ring (Figure 2). We have performed the MESP analysis to examine the stability arises by the π-hole interactions in conjugate bases of cyclopentadiene derivatives (Figure 2).49 The negatively charged cyclopentadienyl anion in 1 (indicated by red colour) is partially neutralized by the π-hole generated on selenoaldehyde. The negative region of cyclopentadienyl unit is dissipated in the MESP of 1- (Figure 2) causes the depletion of positive potential around -C=Se unit. This analysis clearly depicts the stabilization of charge within the system. To augment the stability of carbanion, additional selenoaldehyde was substituted in 2. The calculated ∆Hacid for 2 is 340.4 kcal/mol (Table 1). This result suggests that the cyclopentadienyl anion is stabilized by the π-holes of both the selenoaldehydic groups, which is appropriately oriented for suitable interactions. The MESP suggests that two selenoaldehyde groups interact with cyclopentadiene anion (Figure 2). To improve the effect


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of π-hole interactions, compound 3 has been computed with the same level of theory. The cyano group was substituted at the selenoaldehyde carbon centers in 3 (Figure 1). The ∆Hacid calculated for 3 is 317.6 kcal/mol at M06-2X/6-311+G(2d,p) level of theory. This result shows significant enhancement in the acidity compared to parent cyclopentadiene. The influence of the electron withdrawing cyano group in the selenoaldehydic chain has been found to be significant to enhance the acidity of cyclopentadiene unit. The designed compounds 4 and 5 with more cyano groups substituted to the alkyl chains falls in the range of superacid (Table 1). It is known that electron withdrawing group like cyano and nitro groups can stabilize the conjugate base of acid through resonance stabilization and the electron donating group can have an opposing effect on the stabilization of a conjugate base.1, 6,7,9

On the contrary, we have employed electron donating group in the cyclopentadiene unit

to augment further the basicity of selenoaldehyde substituted cyclopentadiene derivative 6.

Figure 1 The structure of 3-(cyclopenta-1,3-dien-1-yl)propaneselenal (1) and it’s hexacyano (5) and methoxy substituted hexacyano (6) compounds and their conjugate bases (1-, 5-, 6-) 9 ACS Paragon Plus Environment


optimized at M06-2X/6-311+G(2d,p) level of theory in gas phase. (distances are given in Å and distance taken between selenium and nearest carbon of Cp ring, grey = carbon, white = hydrogen, blue = nitrogen, red = oxygen, orange = selenium).

The methoxy group was introduced at the R4 position in the cyclopentadiene ring (Scheme 1). The ∆Hacid calculated for 6 with M06-2X/6-311+G(2d,p) level of theory is 279.0 kcal/mol. The compound 6 could act as a stronger super acid compare to strong mineral Brөnsted acids like HNO3, H2SO4 and HClO4. The experimental gas phase ∆Hacid values of these inorganic acids are 324.5, 306.0 and 288.0 kcal/mol, respectively.1 We have introduced the electron donating group rather than electron withdrawing group in the cyclopentadienyl anion unit to increase the electron density in the cyclopentadienyl unit, which in turn would experience stronger attractive interaction with the π-hole of seleno-aldehyde group. On the other hand, if the electron-withdrawing group is attached to the cyclopentadienyl anion then the electron density would deplete in the cyclopentadienyl moiety, and hence the interaction would be weaker with the π-hole of seleno-aldehyde group. To examine this, we have calculated the deprotonation energy (∆Hacid) with electron withdrawing group (-CN) in cyclopentadiene ring of 6 instead of –OMe (6-CN) (Figure S1, Table TS1). The ∆Hacid calculated 6-CN is (284.3 kcal/mol) suggests that -CN substituted cyclopentadienyl anion would be less stable compared to –OMe substituted cyclopentadienyl anion. The calculated MESP for 6 reveals that there is more positive potential around -C=Se region compared to 1 which is neutralized by the negatively charged cyclopentadienyl anion in 6- (Table TS2, Figure 2). The MESP calculation reveals that the positive potential of π-hole shifted toward the Se atom of C=Se on cyano group substitution which facilities the stabilization of conjugate bases (Figure 2). It is important to note that other chalcogens, i.e., -C=O and -C=S can also produce similar stabilizing effect with cyclopentadienyl anion systems. It appears that the longer bond length of –C=Se can approach much closer to the cyclopentadienyl anion compared to its corresponding -C=O and -C=S systems (Figure S1 and Table TS1). We have examined the 10 ACS Paragon Plus Environment

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acidity of 6 using aldehyde and thioaldehyde group (6-C=O and 6-C=S in Figure S1) at the same level of theory (Figure S1 and Table TS1). The calculated results suggest that –C=Se is more suitable to influence the acidity of these cyclopentadiene derivatives (Table 1 and Table TS1). We have calculated free energy change (∆Gacid) for all molecules studied here. The free energy change (∆Gacid) for 6 is lowest compared to other systems examined, which is in good agreement with the (∆Hacid) (Table 1). Table 1 The calculated ∆Hacid and ∆Gacid for compounds (1-6) at M06-2X/6-311+G(2d,p) level of theory in gas phase. (∆Hacid and ∆Gacid are given in kcal/mol) Molecules

∆Hacid

∆Gacid

Molecules

∆Hacid

∆Gacid

1

344.0

337.5

4

299.4

297.0

1-

-

-

4-

-

-

2

340.3

336.0

5

287.0

284.5

2-

-

-

5-

-

-

3

317.6

315.7

6

279.0

275.8

3-

-

-

6-

-

-



Figure 2 MESP calculation of 1, 5, and 6 and their conjugate bases. (Blue and light blue colors indicate the positive potential, and green, yellow, and red colors indicate the negative potential) The Gibbs free energies (∆Gacid) indicate that 6 is more acidic in this series (Table 1 and Table TS1). The Gibbs free energy for selenoaldehyde (6) is also less than that of aldehyde and thioaldehyde substituted in 6-C=O and 6-C=S, respectively (Table TS1). The pKa calculated for the 6 using M06-2X/6-311+G(2d,p) level of theory shows the value of -9.6, whereas the pKa calculated for the reference cyclopentadiene at the same level of theory is 19.0.48 The higher homologue of Se i.e., Te can have an even more significant impact on the acidity of similar derivatives. The calculations performed using telluro aldehydic group with the 5 and 6 show even lower ∆Hacid values than the corresponding C=Se group (Table TS3). These results suggest that the acidity can be modulated with even chalcogens of choice. The stabilization of a conjugate base of an acid with π-hole bonds can have an additional advantage, where the acidity of the system can be modulated with solvents. The non-covalent interactions can be influenced with the change in the environmental conditions.51,52


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Figure 3: The topological analysis of compounds 1, 5, and 6 and their conjugate bases. (orange coloured point indicates the bond critical points (3, -1) appears between two interacting atoms) We have further examined the AIM (Atoms in Molecules) analysis for these systems (Figure 3).47,53 The AIM analysis for 1 reveals that one critical point appears between selenium atom and the carbon atom of Cp- ring after deprotonation, which suggests an interaction between these atoms. The electron density (ρ), Laplacian electron density (∇2ρ) values corroborate the MESP analysis to achieve the stability of the conjugate base of acids (Table TS4).47-53 The V(r)/G(r) ratio indicates that the non-covalent interaction between the C=Se group and the CP- of 1 is purely electrostatic in nature. The electron density values calculated for 6 reveals that the interaction between the C=Se group and the CP- of 6 is predominantly covalent in nature (Figure 3 and Table TS4).53 The calculated bond strength reveals the Se…CP- is stronger in compound 6- (Table TS4). The attraction between the Se group and cyclopentadiene is so strong that these groups fall in the region of the bond formation. The AIM calculated results qualitatively in agreement with the acidity of the systems studied here. The stronger interaction calculated with AIM shows that the conjugate base of that acid is more stable than the conjugate bases of other acids examined (Table 1 and Table TS4). 13 ACS Paragon Plus Environment


We have examined this strategy to examine the stability of dianions of similar systems. We have computed the di anions of cyclopentadiene derivatives.

Figure 4: Structure of di anions of Cyclopentadiene derivatives with selenoaldehyde, optimized at M06-2X/6-311+G(2d,p) level of theory. (∆Hacid is given in kcal/mol) The calculated acidity reveals that ∆Hacid of the parent compound (2Cp in Figure 4) are 349.8 kcal/mol and 467.0 kcal/mol, respectively, which suggests that the 2nd deprotonation would be unlikely in this case. However, the cyclopentadiene derivatives exhibit as super acids with seleno aldehydic group. The ∆Hacid of 1st and 2nd deprotonation of 7 are 291.5 kcal/mol and 353.9 kcal/mol, whereas, with electron donating substituents 8 are 289.6 kcal/mol and 348.4 kcal/mol, respectively (Figure 4). The generation of cyclopentadienyl anion from the cyclopentadiene has been reported in the literature.54 Therefore, the experimental condition maintained to prepare the cyclopentadienyl anion can as well lead to the formation of dianion.


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The binding energies of the first and second electron with the ligand system can also provide the stability of the anions formed.17 The calculations performed for the incorporation of first and second electrons to the ligand molecule (8) using Gaussian 09 program is discussed in the computation section. We have calculated the binding energies (in kcal/mol) of the first and second electron of 8 at M062X/6311+G(2d,p) level of theory. The calculated results suggest that the binding energy of the first electron is ∆E1 (64.3 kcal/mol) and for the second electron is ∆E2 (27.9 kcal/mol), respectively as obtained in other multi-charged anions.17 The stability predicted for the di-anions suggests that such ligands can be potentially used for applications in batteries.16,17 It has been reported that the potentially stable di-anions can be used as lithium and magnesium ion batteries. Therefore, we have extended our study with the dianion 82- to examine its potential as an electrolyte in lithium and magnesium ion batteries. Li-ion batteries play a pivotal role in today’s portable electronics, because they weigh light and possess high energy density.55-57 Li-ion battery comprises of three components such as anode, cathode, and electrolyte.58-62 The Li+ ions transfer from anode to cathode while discharging and reverse when charging and this is supplied by the electrolytes. Research efforts are underway to augment the efficiency, stability and safety of Li-ion batteries.63-70 The electrolytes function as ion carrier between the anode and cathode when current flows through an external circuit. There are electrolytes available consist of lithium salts such as LiAsF6, LiBF4, LiPF6, LiFePO4, LiClO4, LiN-(SO2F)2, and LiN(SO2CF3)2, combined with organic solvents like ethylene carbonate and dimethyl carbonate.71-74 However, there are disadvantages with such electrolytes.32 To resolve the problems associated with the electrolytes known attempts have been made by introducing new solvents or additives.32 One of the important characteristics is it to make the electrolytes halogen free for safety purposes.17,32 The designed dianions may be useful to fulfill the characteristics for Li+ ion batteries.



These di-anions were also used to examine for their suitability to function as an electrolyte in magnesium ion batteries. The magnesium ion batteries are used as replacement for lithium ion based battery due to their high-oxidative-stability and higher charge density.75 We have calculated the binding energies of metal ions like lithium (Li+) and magnesium (Mg2+) with the stable di-anions 82- using M06-2X/6-31G(2d,p) levels of theory (Figure 5, Table 2).

Figure 5: Structure of metal (Li+ and Mg2+) complexes with compound 82-. The optimized complex structure of Li-82- reveals that the lithium ion interacts with -CN group and the oxygen atom of the methoxy group (Figure 5). The weaker cation…π interaction of the Li+ ion was also observed with the negatively charged 2Cp2- ring (Figure 5). The binding affinity of Li+ with 82- become less as there is weaker cation…π interaction to partially quench the negative charge of anion. However, such cation…π interactions were not observed in Li2-82- complex structure. In Li2-82- complex each lithium ion interacts with CN group and the methoxy oxygen atoms (Figure 5). The interaction of Mg2+ with 82- revealed an interesting trend while complexing (Mg-82-), the magnesium ion interacts with Cp rings rather than complexing with CN and the methoxy group (Figure 5). The cation…π interaction between Mg2+ and Cp rings is weaker as the seleno aldehyde groups partially quench the negative charge of the anion (Figure 5). The calculated binding energies of Li+ and Mg2+ ions with 82- i.e., Li-82-, Li2-82- and Mg-82- at M06-2X/6-31G(2d,p) level of theory in gas phase are given in (Table 2).


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The high performance electrolytes in metal ion batteries should have lower binding affinity towards the metal ions.75 The reports suggest that the dianions such as B12H122- and its cyano derivatives can function as an excellent ligand for lithium and magnesium ion batteries. These studies also suggest that the binding of the metal ions with the anions depend on the size of the ligand anions. The binding energies of metal ions decrease with the increasing size of anions. Here, we have employed a different strategy for weaker binding of metal ions (Li+ and Mg2+) with anions where the anion can be stabilized through the π-hole interaction of seleno-aldehyde group. Therefore, the developed negative charge of the anion is partially quenched with the positive hole of seleno-aldehyde groups. The calculated binding energies for the 1st and 2nd Li+ ion with 82- and 8- are given (Table 2). The binding energies of lithium complexed with 8 (∆ELi1 and ∆ELi2) are 5.8 and 63.6 kcal/mol, respectively using M06-2X/6311+G(2d,p) level of theory. To compare the calculated dissociation energy values for 1st and 2nd Li ion with 82-/8- and the reported B12H122-, additional calculations were performed using B3LYP/6-31+G(d) level of theory in gas phase (Table TS5). The results show that the dissociation energy values are lower compared to the corresponding the lithiated B12H122-. The study extended with the Mg2+ ion with 82- also showed a similar trend as obtained for Li+ ions (Table 2 and Table TS5). These calculated results suggest that the dianion 82- can be suitable to function as an electrolyte in lithium and magnesium ion batteries.16,17 Table 2: Dissociation energies in lithium salts (∆ELi1, ∆ELi2) and magnesium salts (∆EMg) at M06-2X/6-311+G(2d,p) level of theory

Li2-8 Mg-8

M06-2X/6-311+G(2d,p) ∆ELi1 ∆ELi2/∆EMg (kcal/mol) 5.8 63.6 137.9

Conclusions



In this work, we have reported that π-hole interactions can be exploited to stabilize the anions that can turn an organic acid to super acid. Cyclopentadiene moiety appropriately substituted with selenoaldehydic group 6 can be turned into super acid (∆Hacid = 279.01 kcal/mol). The positive potential on the selenoaldehydic group can effectively interact with the negative region of anions which is formed after deprotonation of selenoaldehyde derivatives. The super acidic behaviour can be augmented with electron donating methoxy group placed to the cyclopentadiene contrary to the conceived opinion of electron withdrawing groups. This strategy has been successfully employed to stabilize the dianions as reports on the stabilization of such species are rare in the literature. The AIM analyses shed light on the nature of non-covalent interactions occurred to stabilize the conjugate base of the substituted cyclopentadiene derivatives. The stability of such anions was further explored for their use in lithium and magnesium ion batteries as this strategy helped to generate the stability of dianions without any influence of environmental conditions. The calculated results suggest that the di-anions can be potentially used for the preparation of lithium and magnesium ion battery electrolytes. The binding energies of lithium complexed with 8 (∆ELi1 and ∆ELi2) are 5.8 and 63.6 kcal/mol respectively using M06-2X/6-311+G(2d,p) level of theory. The stabilization of the negative charge of anions with π-hole interaction of selenoaldehyde groups facilitate the weaker interaction of lithium and magnesium ions with the ligand molecule, which is prerequisite for preparation of electrolyte in effective lithium and magnesium ion batteries. ASSOCIATED CONTENT Supporting Information The AIM calculation and table for ∆Hacid for tellurium substituted cyclopentadiene derivatives are given in SI.


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AUTHOR INFORMATION Corresponding Author Corresponding Author. Fax: (+91)-278-2567562, E-mail: [email protected]; [email protected] Author Contributions B.G. and M.K.S. designed and performed the study and analyzed data. B.G. and M.K.S. have written the manuscript. ACKNOWLEDGMENT CSIR-CSMCRI registration number– 098/2018. The B.G. thanks DST (New Delhi), DBT, New Delhi, DAE-BRNS, Mumbai) for financial support. M.K.S. acknowledges UGC (New Delhi, India) for an SRF and Academy of Scientific & Innovative Research (AcSIR) for enrolment

in

the

PhD

program.

We

thank

the

reviewers

for

their

valuable

comments/suggestions that have helped us to improve the paper. References: 1. Vianello, R.; Maksic´, Z. B. Strong Acidity of Some Polycyclic Aromatic Compounds Annulated to a Cyclopentadiene Moiety and Their Cyano Derivatives – A Density Functional B3LYP Study. Eur. J. Org. Chem. 2005, 3571-3580. 2. Olah, G. A.; Surya Prakash, G. K.; Molnar, A.; Sommer, J. Superacid Chemistry, 1985, ISBN 978-0-471-59668-4(cloth) 3. Kuck, D. Thermochemical Data of Organic Ions Obtained from Investigations in the More or Less "Diluted" Gas Phase. Angew. Chem. Int. Ed. 2000, 39, 125-130. 4. Alcamı´, M.; Mo´, O.; Ya´nˇez, M. Computational chemistry: a useful (sometimes mandatory) tool in mass spectrometry studies. Mass Spectrom. Rev. 2001, 20, 195-245.



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Graphical Abstract :


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and di-anions as super acids through Ï-hole interaction

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and di-anions as super acids through Ï-hole interaction