Electrochemical Investigation of Fluorenone Complexation by Lewis Acids in Ionic Liquids

Abstract

The electrochemical characteristics of 9-fluorenone have been investigated in various ionic liquids as a means of determining the extent of interaction of representative Lewis acids with this ketone. The ionic liquids studied, with Lewis acids in a particular medium, are : 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI BF 4 ) [BF 3 etherate], 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate (BMPY TfO) [Hf(TfO) 4 and Sc(TfO) 3 ], and aluminum chloride : 1-ethyl-3-methylimidazolium chloride (AlCl 3 : EMICl) [aluminum chloride]. In EMI BF 4 and BMPY TfO, 9-fluorenone undergoes two successive one-electron transfers; however, addition of a Lewis acid produces a new reduction process shifted to a more positive potential. The extent of this shift is taken as an indication of the strength of the Lewis acid interaction with 9-fluorenone. The strongest Lewis acid : 9-fluorenone interaction, judging from the magnitude of the potential shift, is found in the AlCl 3 : EMIC ionic liquid, followed by that of Hf(TfO) 4 in BMPY TfO. The interaction of Sc(TfO) 3 with 9-fluorenone in BMPY TfO is considerably less than that of Hf(TfO) 4. In EMI BF 4 , however, Sc(TfO) 3 interacts quite strongly with 9-fluorenone, indicating that the triflate anion interacts extensively enough with Sc 3+ in BMPY TfO to restrict its complexation of 9-fluorenone.

Studies of Adiponitrile as an Electrochemical Solvent

“…The present abstract represents work involving the use of adiponitrile as an electrochemical solvent for some common redox systems such as ferrocene and 9-fluorenone. Recent work involving 9-fluorenone has been carried out in acetonitrile (5) and the ionic liquids 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate [BMPY TfO] (6) and 1-ethyl-3-methylimidazolium tetrafluoroborate [EMI BF 4 ] (7), providing useful comparisons (8) to the present work.…”

Electrochemical Investigation of Benzil in Ionic Liquids

“…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.…”

“…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.…”

“…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.…”

“…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.…”