J. Phys. Chem. C 2008, 112, 6993-7000
6993
Oxidation of Several p-Phenylenediamines in Room Temperature Ionic Liquids: Estimation of Transport and Electrode Kinetic Parameters Jenny S. Long,† Debbie S. Silvester,† Alexander S. Barnes,† Neil V. Rees,† Leigh Aldous,‡ Christopher Hardacre,‡ and Richard G. Compton*,† Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OX1 3QZ, United Kingdom, and School of Chemistry and Chemical Engineering/QUILL, Queen’s UniVersity Belfast, Belfast, Northern Ireland BT9 5AG, United Kingdom ReceiVed: January 10, 2008; In Final Form: February 19, 2008
The electrochemical oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) has been studied by cyclic voltammetry and potential step chronoamperometry at 303 K in five ionic liquids, namely [C2mim][NTf2], [C4mim][NTf2], [C4mpyrr][NTf2], [C4mim][BF4], and [C4mim][PF6] (where [Cnmim]+ ) 1-alkyl-3methylimidazolium, [C4mpyrr]+ ) N-butyl-N-methylpyrrolidinium, [NTf2]- ) bis(trifluoromethylsulfonyl)imide, [BF4]- ) tetrafluoroborate, and [PF6]- ) hexafluorophosphate). Diffusion coefficients, D, of 4.87, 3.32, 2.05, 1.74, and 1.34 × 10-11 m2 s-1 and heterogeneous electron-transfer rate constants, k0, of 0.0109, 0.0103, 0.0079, 0.0066, and 0.0059 cm s-1 were calculated for TMPD in [C2mim][NTf2], [C4mim][NTf2], [C4mpyrr][NTf2], [C4mim][BF4], and [C4mim][PF6], respectively, at 303 K. The oxidation of TMPD in [C4mim][PF6] was also carried out at increasing temperatures from 303 to 343 K, with an activation energy for diffusion of 32.3 kJ mol-1. k0 was found to increase systematically with increasing temperature, and an activation energy of 31.4 kJ mol-1 was calculated. The study was extended to six other p-phenylenediamines with alkyl/phenyl group substitutions. D and k0 values were calculated for these compounds in [C2mim][NTf2], and it was found that k0 showed no obvious relationship with the hydrodynamic radius, r.
1. Introduction Room-temperature ionic liquids (RTILs) are defined as salts that exist in a liquid state around 298 K.1-3 They are comprised of a bulky asymmetric cation and a weakly coordinating anion and are rapidly becoming established as electrochemical solvents because of their desirable physical and voltammetric properties.1-3 Such properties include a wide electrochemical window, high intrinsic conductivity, and thermal stability. They also have a low volatility, which means RTIL solvents may be considered “greener” alternatives to volatile organic solvents;4,5 however, several issues with regards to their toxicity have been raised.6-8 RTILs are also much more viscous than traditional organic solvents, slowing the diffusion of the electroactive species under study and giving rise to transient behavior even on a microelectrode.1,2 Some theoretical work has investigated the effect of RTILs on the double-layer structure and heterogeneous electron-transfer rates compared to traditional organic solvents.9,10 It has been shown that, for example, the solvent reorganization energies associated with an electron transfer are similar in RTILs and acetonitrile, and the charge-transfer process is “Marcus-like”.10 While the behavior simulated for the RTILs could be interpreted using the continuum model, it was recognized that the mechanisms by which an RTIL responds to an applied field are quite different from a traditional, polar solvent. In the latter, solvent molecules reorient, whereas in an RTIL the molecules must move relative to each other.10 These differences must impact * Corresponding author: e-mail
[email protected]; Tel +44(0) 1865 275 413; Fax +44(0) 1865 275 410. † Oxford University. ‡ Queen’s University Belfast.
upon the overall observed rate of electron transfer, even if the activation free energies are similar. That is, while inner-sphere electron-transfer rates might be expected to be very similar in RTILs compared to polar organic solvents, outer-sphere electron transfers will be very different due to the screening response mechanisms. The solvent reorientation in polar solvents is characterized by the longitudinal dielectric relaxation time, τL, which appears in the pre-exponential factor in the Sumi-Marcus model (see eq 1).11,12
k0 ) Q
(ψr )
1/2
ψ r
[(
)]
exp - rB +
(1)
where
Q)
Kpκ0el exp[-B(δ - σ)] 2τLxπ
(2)
and
ψ)
(
)
NAe2 1 1 32π0RT op s
(3)
In this expression, Kp is the equilibrium constant for the formation of the precursor complex, κ0el exp[-B(δ - σ)] is the electronic transmission probability comprised of a maximum probability κ0el, with B a constant, σ the distance of closest approach, and δ an arbitrary distance, op and s are the highand low-frequency limits of the dielectric constant for the solvent, and NA, e, 0, R, and T have their usual meanings.
10.1021/jp800235t CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008
6994 J. Phys. Chem. C, Vol. 112, No. 17, 2008
Long et al. Concentration profiles for species A and B are obtained by solving eqs 5 and 6 subject to appropriate boundary conditions. A Butler-Volmer condition applies at the electrode surface; no flux conditions apply at the insulating boundary, and the electrode symmetry axis and finally bulk concentrations are found at a distance 6xDt in the r and z directions. The equations are discretized over an appropriate simulation grid,18 and the current is calculated from the concentration gradient of species A at the electrode surface using second-order finite difference methods. Data are plotted in the form of currentvoltage curves, allowing peak-to-peak separations in particular to be deduced. 3. Experimental Section
Figure 1. Structures of all the anions and cations employed in the RTILs in this study and the structure of tetraalkylphenylenediamine (TAPD).
However, at present there is no accepted analogous relaxation time for RTILs due to the complexity of the relaxation processes occurring (rotation, translation, and other as yet unidentified concerted molecular motions).13,14 Full theoretical results for outer-sphere electron transfer have yet to be published, with only experimental reports of non-Marcus behavior in RTILs.15,16 In this work, the electrochemical oxidation of N,N,N′,N′tetramethyl-p-phenylenediamine (abbreviated as TMPD) is first reported in several ionic liquids, namely [C2mim][NTf2], [C4mim][NTf2], [C4mpyrr][NTf2], [C4mim][BF4], and [C4mim][PF6] (see Figure 1 for structures). Second, the oxidation of several substituted p-phenylenediamines (TAPDs) is reported in [C2mim][NTf2]. For each TAPD, heterogeneous rate constants are calculated, and these will be used to consider whether the Marcus relation applies for TAPDs in RTIL solvents. 2. Theory: Simulation of Cyclic Voltammetry at Microdisk Electrodes The voltammetry observed throughout this work falls in the intermediate region between linear diffusion and convergent diffusion; hence the need for theoretical simulations. The simulation program used to model cyclic voltammetry in this work has been described in detail in a separate publication17 and will be described briefly here. The process considered in all voltammetric simulations is the one-electron heterogeneous electron transfer at an isolated microdisk electrode as shown in eq 4.
A + e- h B
(4)
Diffusion to the electrode surface is described by Fick’s second law, expressed in cylindrical polar coordinates (z,r) for species A and B respectively in eqs 5 and 6, where DA and DB are the diffusion coefficients of the oxidized and reduced species.
( (
) )
∂2[A] ∂2[A] 1 ∂[A] ∂[A] + + ) DA ∂t r ∂r ∂z2 ∂r2
(5)
∂[B] ∂2[B] ∂2[B] 1 ∂[B] ) DB + + ∂t r ∂r ∂z2 ∂r2
(6)
3.1. Chemical Reagents. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]), N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, ([C4mpyrr][NTf2]), and their bromide salt precursors were prepared by standard literature procedures.19,20 1-Butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4], high purity, halide content
