The Corannulene Reduction Mechanism in Ionic Liquids is Controlled by Ion Pairing Eden E. L. Tanner, † King Yoong Foong, ‡ Md. Mokarrom Hossain, ‡ Christopher Batchelor-McAuley, † Leigh Aldous,* ,‡ and Richard G. Compton* ,† † Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom ‡ School of Chemistry, UNSW Australia, Sydney NSW 2052, Australia ABSTRACT: The electroreduction of corannulene (C 20 H 10 ) has been investigated in a room temperature ionic liquid (RTIL) for the first time. In the RTIL 1-butyl-1- methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([Bmpyrr][NTf 2 ]) the resultant voltammetry shows a peak-to-peak separation of 100 mV, and this separation does not vary with scan rate (as predicted by a simple E mechanism). We propose a square scheme that is capable of accurately describing this behavior. Specifically, the use of a square scheme takes into account the effect of ion pairing between the ionic liquid cation and the corannulene anion on the overall reaction mechanism. Importantly, investigation in acetonitrile with a range of conventional electrolytes does not display the trends observed in the RTIL. This result likely provides a general insight into all RTILs as a class of electrolyte, because of the high concentration of ions and the proclivity of RTILs to ion-pair. ■ INTRODUCTION Interest in the corannulene (C 20 H 10 ) molecule is high due to its special electronic structure, 1,2 its reactivity, 3 and the possibility of construction of other carbon nanomaterials using this molecule as a starting point. 4 Corannulene can be viewed as being a fragment of a buckministerfullerene, 5 and its most stable neutral geometry is bowl-shaped. 6 Electrochemically, the reduction of corannulene to its anion, dianion, and trianion species has been documented at low temperature in a number of nonaqueous solvents by Bruno et al. 7 The solvent used was found to have a large effect on the appearance of the voltammetry and, therefore, the reaction pathway. Ion pairing effects were shown to influence the formal potential of the C/ C - couple and therefore the ability to electrochemically generate and stabilize the higher anions within the available electrochemical window. Room temperature ionic liquids (RTILs) usually consist of a bulky, asymmetric organic cation and an inorganic anion 8 and are liquid below 100 °C. 9 RTILs, because of their charged components, are well documented in their proclivity to engage in ion-pairing with the reaction substrate. 10-12 RTILs have been known to alter reaction mechanisms, 13-15 particularly through their affinity for ion-pairing. 12,16,17 Currently, there is no experimental voltammetry of the reduction of corannulene in a RTIL or an exploration of what might occur in an ionic solvent. Complementary to the work of Bruno et al., 7 this work investigates how ion pairing can lead to distorted voltammetric wave shapes as opposed to altered thermodynamics of the reduction process. Voltammetry provides an opportunity to probe changed electron transfer kinetics or chemical reactivity. A peak-to-peak separation of greater than n 57 mV (n is the number of electrons transferred) at 25 °C is commonly taken as diagnostic of slow electron transfer kinetics, 18 which RTILs are noted for revealing due to their higher viscosity, slowing mass transport rates relative to electron transfer rates thus promoting a switch from electrochemical reversibility to irreversibility. 19,20 However, the understanding of the role an RTIL plays purely in terms of slower electron kinetics can lead to misinterpretation of the experimental data, and the impact of ion-pairing on the reaction mechanism needs to be more closely considered. In this paper, we use a square scheme, 21 as first proposed by Jacq 22 and Laviron, 23 to exemplify how coupled homogeneous processes can lead to voltammetry in which the peak-to-peak separation exceeds 57 mV, but the rate at which this separation changes with scan rate does not vary as predicted from a simple one electron process. Under certain kinetic regimes, the electrochemical mechanism may follow a different pathway on the forward and reverse scans. Consequently, the peak position reflects the thermodynamics of bound and unbound redox species and not the electron transfer kinetics. We study the one electron reduction of corannulene to the monoanion in both acetonitrile and the RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpyrr][NTf 2 ]) to dem- onstrate the importance of properly considering the role ion pairing plays in the reaction mechanism in ionic solvents, as seen in Figure 1, and thus the use of a square scheme to describe the electron transfer dynamics and nonideal voltammetry observed in the RTIL. Received: March 11, 2016 Revised: April 5, 2016 Published: April 5, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 8405 DOI: 10.1021/acs.jpcc.6b02551 J. Phys. Chem. C 2016, 120, 8405-8410