Outer Sphere Electroreduction of CCl 4 in 1-Butyl-3-methylimmidazolium Tetrafluoroborate: An Example of Solvent Specific Effect of Ionic Liquid Mohsin Ahmad Bhat, † Pravin P. Ingole, Vijay R. Chaudhari, and Santosh K. Haram* Department of Chemistry, UniVersity of Pune, Ganeshkhind, Pune 411007, India ReceiVed: NoVember 4, 2008; ReVised Manuscript ReceiVed: December 19, 2008 Electrodics of CCl 4 reduction in 1-butyl-3-methylimmidazolium tetrafluoroborate [BMIM][BF 4 ] room temperature ionic liquid (RTIL) is reported. A convolutive analysis of the cyclic voltammograms suggests that CCl 4 electroreduction follows stepwise (outer sphere) dissociative electron transfer pathway, rather than the sticky dissociative (inner sphere) electron transfer, as in the case of conventional organic solvents. This difference in the mechanism of electron transfer initiated bond cleavage is attributed to the solvent specific effects, namely, stabilization of CCl 4 •- intermediate radical anion in RTIL, which in turn decreases the electron transfer rate and thus the carbon-halogen bond cleavage rates. Electroreduction of CCl 4 in RTIL through outer sphere electron transfer would be a promising pathway for its direct conversion to methane. 1. Introduction Owing to desired ionic conductivity and large electrochemical window, room temperature ionic liquids (RTILs) have received considerable attention as solvents/electrolytes for the electro- chemical investigations and applications. 1-4 To date, most of the electrochemical studies in RTILs are related to their suitability as a “green” alternative to the conventional organic solvents. 5-7 It is presumed that the mechanism of electrochemi- cal processes remains the same as that in conventional solventssalbeit with some effect on kinetic parameters, at- tributed to the difference in the physical properties of RTILs. 8-10 Among various ionic liquids, dialkylimmidazolium-based RTILs (DAI-RTILs) are usually preferred in the electrochemical experiments 1,11,12 due to their appreciable conductivity and air and moisture stability. It is understood that DAI-RTILs form three-dimensional network of cations and anions and are viewed as nanostructured fluids. 13,14 The structural order in DAI-RTILs is envisaged to be responsible for the higher solvent reorganiza- tion, slower double layer interface relaxations, and hence slower electron transfer kinetics. 15 In addition, the possible interaction of electroanalyte or electrogenerated species with the constituent ions of RTIL 16 is also expected to influence the thermodynamic and kinetic aspects of electron transfer and electron transfer initiated processes. 1,17 In spite of these interesting possibilities, RTIL specific effects in electron transfer or electron transfer initiated processes have seldom been reported. 18-20 In this regard, electrochemical investigations of the solvent structure sensitive reactions, such as electron transfer initiated carbon- halogen (C-X) bond cleavage reactions, 21,23 seem to be a suitable choice to explore the solvent specific effects of RTILs. This reaction may proceed through any of following four different pathways, viz. (1) stepwise, (2) concerted, (3) sticky dissociative, and (4) almost dissociative electron transfer, as shown in the following scheme: 23 The most plausible mechanism depends on the stability of radical ion intermediate, electron transfer (k et ) and cleavage (k c ) rate constant of C-X bond, which are strongly influenced by the structure and extent of solvation. Among them, sticky dissociative (SDM) and almost dissociative (ADM) mechanisms represent an intermediate path between the stepwise and dissociative electron transfers and are strongly influenced by solvating capability of the solvent. Cyclic voltammetry has been proved to be very useful for the fundamental understanding and analytical investigations of their mechanistic details. 23-26 One of the most studied systems is the electroreduction of CCl 4 , which is demonstrated to follow SDM 25 in the organic solvents and very sensitive to the solvent characteristics. 26 To our knowledge, the mechanistic aspects of CCl 4 reduction in DAI-RTIL has not yet been studied so for. Therefore, it was chosen as a model reaction to understand solvent specific effect of [BMIM][BF 4 ]. It is worth emphasizing that the dechlorination of CCl 4 is a very important process for the groundwater purification, and RTILs have been proposed to be a suitable medium for the said process. 27 Therefore, the knowledge about the CCl 4 electroreduction in RTIL is expected to help in designing an efficient electrochemical dehalogenating setup for the halogenated pollutants. Electrodics of CCl 4 reduction was therefore investigated with an aim to probe the role of [BMIM][BF 4 ] on the mechanistic aspect of its reduction. Chrono methods have been employed for the estimation of transport parameter, while cyclic voltam- metry (CV) was used for the kinetic investigations. CV data were convoluted and analyzed in the light of various theories proposed for the electroreduction of alkyl halides in the conventional solvents. * Corresponding author: e-mail haram@chem.unipune.ernet.in. Ph +91 20 2560 1394, Fax +91 20 2569 1728. † Permanent address: Department of Chemistry, University of Kashmir, Srinagar 190006, India. J. Phys. Chem. B 2009, 113, 2848–2853 2848 10.1021/jp809749k CCC: $40.75  2009 American Chemical Society Published on Web 02/06/2009