Electrochemical Investigation of Benzil in Ionic Liquids

The voltammetric behavior of benzil (1,2-ethanedione, 1,2-diphenyl) has been investigated by cyclic voltammetry in the ionic liquids 1-ethyl-3-methylimidazolium tetraﬂuoroborate (EMImBF 4 ), 1-butyl-2,3-dimethylimidazolium tetraﬂuoroborate (BDMIm BF 4 ), and 1-butyl-1-methylpyrrolidinium triﬂuoromethanesulfonate BMPY TfO. At 100 mV/s, protonation of the radical anions of benzil by the EMIm cation is extensive and leads to complicated voltammetry; however, the protonation reaction is slow and is much less signiﬁcant at 1000 mV/s. Use of BDMIm BF 4 , in which the acidic imidazolium 2-position is substituted, precludes protonation of these radical anions. It has been found that addition of BF 3 etherate to solutions of benzil in EMImBF 4 results in the complexation of benzil by BF 3 . Benzil has been found to undergo two successive reversible one-electron redox processes in the ionic liquid 1-butyl-1-methylpyrrolidinium triﬂuoromethanesulfonate (BMPY TfO). This behavior is observed due to the very low proton-donating ability of the BMPY cation. Upon addition of acetonitrile, however, the second redox process becomes irreversible as is observed in acetonitrile alone. The benzil dianion is evidently undergoing protonation by acetonitrile, and the resulting voltammogram is very similar to that in acetonitrile itself.


Experimental
Cyclic voltammograms were obtained using a PAR 283 potentiostat and PowerSuite software, and potentials are reported with respect to a Ag/AgCl (0.10 M EMIm Cl / EMIm BF 4 ) reference electrode (Cypress Systems). Glassy carbon electrodes (GC, 1 mm diameter) were obtained from Cypress Systems or eDAQ Inc. All electrochemical experiments were carried out in a Vacuum Atmospheres glove box at ambient temperature (∼22 • C). BMPY TfO was prepared according to a standard literature method. 30 EMIm Cl was prepared 31 by reaction of chloroethane and 1-methylimidazole (Aldrich), and EMIm BF 4 was prepared by metathesis of EMIm Cl and NaBF 4 (Aldrich) in acetone, using purification method B. 32 1-Butyl-2,3dimethylimidazolium tetrafluoroborate (BDMIm BF 4 ) was prepared by metathesis of BDMIm Cl and NaBF 4 . 33 All ionic liquids were dried on a high-vacuum line at 70 • C to 80 • C for 2-3 days before use. Benzil, acetonitrile (AN, anhydrous grade), and BF 3 etherate (purified, * Electrochemical Society Member. z E-mail: cheek@usna.edu redistilled grade) were obtained from Aldrich Chemical Company. AlCl 3 was obtained from Fluka and was purified by distillation in a closed tube over aluminum wire. 34 Tetraethylammonium tetrafluoroborate (TEA BF 4 , Electrometric grade ) was purchased from Sachem (formerly Southwestern Analytical Chemicals).

Results and Discussion
Electrochemistry of benzil in acetonitrile.-The electrochemistry of benzil has been extensively studied in nonaqueous media, 10-27 and a brief discussion of its electrochemical behavior in such systems serves as a useful comparison to the work in ionic liquids. As is evident in Figure 1, the first redox process in acetonitrile is essentially reversible, whereas the second process shows considerable irreversibility. Similar results have been found in N,N-dimethylfomamide, 14 and the irreversible nature of the second process was attributed to proton donation from the solvent to the benzil dianion. The resulting dianion protonation product is no longer oxidizable at the original potential, accounting for the lack of reversibility. In the present work, an increase in the scan rate to 1.0 V/s resulted in mostly reversible behavior for the second process, showing that the rate of the carbon acid protonation process is relatively slow. A second anodic process also appeared at −0.60 V, as observed in previous work in N,N-dimethylformamide, 14 and this peak was ascribed to oxidation of the cis form of the radical anion. 14,16 In the present case, it seems reasonable that this assignment is correct, considering that protonation of the dianion from acetonitrile no longer occurs and that the new peak is not then due to a protonation product. Figure 2, the electrochemical behavior of benzil in EMIm BF 4 is much more complicated than that in acetonitrile. At 100 mV/s, the voltammetric wave for the first reduction process is relatively much smaller than that of the second process; in addition, the voltammetric profile for the first process is very narrow and does not display diffusional characteristics on the negative side of the peak. Upon increasing the scan rate to 1000 mV/s, however, the current values for the two processes become much more nearly the same, similar to the behavior seen in acetonitrile. In addition, the first reduction process shows partial reversibility at 1.0 V/s. The reduction processes observed between −1.2 V and −1.6 V are possibly due to interactions of the anion radical with the EMIm cation, similar to behavior seen by Evans after addition of alkaline earth salts to acetonitrile/benzil solutions 14 and as reported for 9-fluorenone in this system. 29 As was the case for benzil in acetonitrile above, the scan rate dependence of the benzil voltammetry in EMIm BF 4 suggests the involvement of a slow step in the reduction pathway, the most likely nature of which is a proton abstraction from the EMIm cation. 28,29 The reaction scheme given in Figure  3 is consistent with the voltammetry shown in Figure 2. At low scan rates (100 mV/s), there is time for the one-electron reduction product to be protonated, and the subsequent rearrangement to the benzoin radical species sets the stage for its one-electron reduction, protonation, and following two-electron reduction. Considering that these last reduction processes take place at similar potentials, this scheme accounts for the much larger current response at −1.7 V compared to that at −0.8 V. Increasing the scan rate and thus eliminating the first protonation allows only the second reduction process to occur, giving roughly equal currents for the two processes. Note that the second reduction process is not reversible, probably due to protonation of the benzil dianion by the EMIm ring. For further confirmation of the proposed reduction pathway, additions of EMIm BF 4 (via microsyringe) were made to a solution of benzil in acetonitrile. After an addition of 3 : 1 EMIm BF 4 : benzil, the cyclic voltammogram closely resembled that of benzil in EMIm BF 4 , consistent with the role of the EMIm cation as proton donor for the benzil reduction products.

Electrochemistry of benzil in EMImBF 4 .-As shown in
In support of the proposed mechanism, the voltammetry of benzil in BDMIm BF 4 , which does not have an acidic proton in the 2 position, has been briefly studied. 25 Figure 4 shows that the first reduction process is reversible at 100 mV/s, whereas in EMIm BF 4 the effect of proton donation to the radical anion greatly distorted this process at 100 mV/s. The remaining two reduction processes in Figure 4 have not been fully explored; however, they have counterparts in the voltammogram of benzil in EMIm BF 4 at 1000 mV/s ( Figure 2).
As in a previously reported study of 9-fluorenone, 29 the complexation of benzil by BF 3 was studied by adding BF 3 etherate to a solution of benzil in EMIm BF 4 . As shown in Figure 5, a new reduction peak, evidently corresponding to a benzil : BF 3 complex, appears at −0.3 V as BF 3 etherate is added. The complexation is virtually complete upon equimolar addition of BF 3 etherate to benzil. By contrast, 9-fluorenone and benzophenone are not fully complexed until more than twice the molar amount of BF 3 etherate has been added. 25,29 This finding indicates that benzil forms a more stable complex with BF 3 than do 9-fluorenone and benzophenone, probably due to the involvement of both benzil carbonyl groups in the complexation. 4 . As can be observed from Figure 6a, benzil undergoes electrochemical reduction in BMPY TfO in two successive, reversible electron-transfer steps which form the radical anion and then the dianion. Similar behavior is observed for 9-fluorenone in BMPY TfO, 29 indicating that the radical anions and dianions for each system are stable in this ionic liquid. In contrast, reduction of benzil in other nonaqueous solvents such as acetonitrile (Figure 1), N,N-dimethylformamide, 14,16 and EMIm BF 4 ( Figure 2) involves considerable irreversibility for the second process. In these cases, the solvents possess activated hydrogen atoms which act as proton donors for the dianions, resulting in protonated species which are no longer oxidizable at the original potentials. The BMPY cation, however, has an aliphatic quaternary ammonium ring system and no activated hydrogen atoms in the system. The low proton donating ability of BMPY TfO, then, accounts for the reversible behavior seen for benzil and 9fluorenone in BMPY TfO. The ionic liquid BMPY TfO serves as a suitable solvent for observing relatively uncomplicated voltammetric behavior of benzil and probably other similar systems. These results also present the possibility of studying the effect of added proton donors in this solvent.  The effectiveness of acetonitrile as a proton donor toward benzil reduction products can be seen in Figure 7, in which increasing amounts of acetonitrile have been added to benzil in BMPY TfO. It is obvious from Figure 7 that a relatively large amount of acetonitrile is necessary to cause voltammetric changes due to proton donation from acetonitrile. The rather large increases in current are due to the dramatic decrease in the viscosity of the ionic liquid upon addition of acetonitrile, which may also affect the availability of acetonitrile as a proton donor. Eventually, the additions of acetonitrile are sufficient to cause the voltammetry to be essentially the same as that in acetonitrile/TEA BF 4 as seen in Figure 1. It should be noted that reversal of the potential scan past the first reduction process gives a reversible response in all cases, signifying that the radical anion formed in the first reduction process is not protonated by the added acetonitrile. The mechanistic pathway at this point is evidently the same in BMPY TfO/AN as it is in AN/TEA BF 4 . An approximate lower value of the pK a value for the protonated dianion can be found from the calculated pK a = 31.3 for acetonitrile in DMSO. 14 A rather high concentration of acetonitrile is required to observe protonation effects, so the pK a for the protonated dianion is probably also approximately 31 in the BMPY TfO/AN system. As referenced to water (vs in DMSO), acetonitrile has a pK a = 25, 35 suggesting a similar value for the protonated dianion on the water reference scale.

Conclusions
The electrochemical characteristics of benzil have been investigated in the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm BF 4 ) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate (BDMIm BF 4 ). From previous studies, 29 it has been found that the EMIm cation can interact with reduction products of these ketones by complex formation, and the present study has shown that the EMIm    cation can also protonate some of these reduction products. Although the fluorenone radical anion is not protonated by EMIm cation, 29 the radical anions of both benzophenone 25 and benzil undergo extensive protonation at 100 mV/s. The protonation process is rather slow, however, and the effects can be largely avoided at 1000 mV/s . In the BDMIm BF 4 ionic liquid, the acidic 2 position of the imidazolium ring is blocked by a methyl group, and protonation of the radical anions is not observed. In contrast to investigations in commonly employed nonaqueous solvents, benzil undergoes two successive reversible oneelectron redox processes in the room-temperature ionic liquid BMPY TfO. This behavior is due to the lack of proton donation from the BMPY cation to the reduced species formed in the reduction process. In the BMPY TfO solvent system, the only possible proton source is the BMPY cation, which is actually a quaternary ammonium cation and therefore has very limited proton donation ability. Addition of acetonitrile to a solution of benzil in BMPY TfO caused the second reduction process, which involves the formation of the benzil dianion, to become irreversible due to its protonation by acetonitrile.