Approaches to analytical and synthetic electrochemistry in fluorous solvent-containing media
Introduction
Use of a fluorous solvent phase [1] has emerged as an important complement to chemistry in mixed aqueous/organic phases, finding utility in an array of catalytic applications including radical [2], hydrogenation [3], hydroformylation [4] and Stille cross-coupling [5] reactions. The use of fluorous membranes for the reduction of biofouling of electrochemical sensors has recently been proposed [6]. The key advantage to using fluorous-modified reagents as homogeneous catalysts is the recovery and recycling of expensive or environmentally hazardous materials [7], [8], [9]. Fluorous biphasic catalysis, conceived by Vogt in 1991 [10] and first reported by Horvath and Rabai [1] takes advantage of the thermally regulated miscibility of fluorous and organic solvents. Ideally, a mixed fluorous/organic solvent system is monophasic at elevated temperatures, maximizing reaction efficiency by eliminating mass transfer difficulties across two phases, and reverts to a biphasic system at lower temperature. If the affinity for the fluorous solvent of one of the reagents is increased by its having a fluorous “tag” [11], facile separation of the reagent from the post-reaction mixture is possible.
As fluorous-phase chemistry expands, electrochemistry might prove very useful for purposes of analysis and synthesis in these media. A significant impediment, however, to development of electrochemical methodologies in fluorous media is the lack of suitable supporting electrolytes.
The problem of generally low solubilities of organic and inorganic salts in fluorous solvents is exacerbated by poor dissociation, owing to the fact that most fluorous liquids have extremely low dielectric constants. The sodium salt of tetrakis[3,5-bis(perfluorohexyl)phenyl)]borate was found to be soluble in perfluoroperhydrophenanthrene at millimolar concentrations, and the extremely high ion-pairing constants observed in this medium were used to address potentiometric selectivity concerns in chemical sensing [6]. The present work addresses the conductivity problem in two ways. First, a mixed set of fluorous solvents was employed, giving increased solvent polarity. The mixed fluorous solvent strategy is often employed in fluorous-phase chemistry to address solubility concerns, with benzotrifluoride (BTF) being a favorite component because of its combined organic/fluorous properties [7], [8], [9]. In the present case, BTF was an attractive solvent owing to its moderate dielectric constant (9.2) and the fact that it has been shown to have favorable electrochemical properties [12]. In this study, BTF was used in conjunction with perfluoromethylcyclohexane (PFMCH). Second, a supporting electrolyte salt was prepared which exhibits increased solubility in a fluorous media. The electrolyte cation, a methylimidazolium ion having a fluorous tag, [10.2-mim]+, 1, had been reported earlier as an iodide salt [13], [14]. For our purposes it was linked with the tetrakis(bis-3,5-trifluoromethylphenyl)borate anion, BArF24, which is one of two fluorine-rich tetraarylborate anions recently shown to be effective supporting electrolyte anions in organic solvents [15], [16]. As shown below, 1[BArF24] in 1:1 PFMCH:BTF makes up an electrolyte medium in which routine voltammetry and even bulk electrolyses may be carried out.
Section snippets
General and chemicals
Benzotrifluoride (Aldrich) was dried by refluxing over CaH2 and distilled under nitrogen. Tetrahydrofuran (Aldrich) was dried over potassium/benzophenone and vacuum distilled prior to use. Na[B{C6H3(CF3)2}4] (Boulder Scientific) was used as received. All other chemicals were reagent grade and unless noted otherwise were used without further purification. NMR spectra were acquired using a Bruker ARX-500 NMR with d6-acetone as the solvent. Ferrocene (Strem) was sublimed and 1,1′-bis-1H,1H,2H,2H
Characterization of [10.2-mim][B{C6H3(CF3) 2}4], 1[BArF24]
Table 1 lists the limiting molar conductivities, Λ°, and association constants, KA, of 1[BArF24] in benzotrifluoride (BTF, dielectric constant, ε, = 9.2) and a 1:1 mixture of perfluorocyclohexane (PFMCH):BTF, determined using the Fuoss paired-ion model as previously described [15]. BTF has been shown to be a useful solvent for electrochemical measurements [12], and conductometric data for [NBu4][B(C6F5)4] in this solvent are included for comparison. The slightly lower molar conductivity of 1[BArF
Conclusions
The very low polarity of fluorous solvents is a major impediment to electrochemistry in media containing fluorous components. Solvents such as PFMCH have dielectric constants essentially identical to those of aliphatic hydrocarbons, lower than even electrochemically challenging organic solvents such as benzene (ε = 2.3). The feasibility of electrochemistry in fluorous solvent-containing media can be improved by mixing the fluorous solvent with benzotrifluoride (ε = 9.2), a strategy which has proven
Acknowledgements
The authors thank the National Science Foundation (CHE-0092702 and -0411703) for financial support. We also thank Drs. Hughes and Deelman for the fluorous-tagged samples used in this work.
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Present address: Department of Chemistry and Physics, Chicago State University, Chicago, IL 60628, USA.