Mechanistic Analysis of the Reductive Cleavage of Carbon–Halogen Bonds in Halopentafluorobenzenes A. Muthukrishnan * and M. V. Sangaranarayanan z Department of Chemistry, Indian Institute of Technology, Madras-Chennai 600036-India The mechanistic analysis pertaining to the reductive cleavage of C–X X = Cl, Br, I bonds in halopentafluorobenzenes has been investigated at glassy carbon electrodes using convolution potential sweep voltammetry. The quadratic activation-driving force relation has been found valid, with potential-dependent transfer coefficients. The charge density analysis using density functional calculations, estimation of intrinsic barriers, and standard reduction potentials indicate that the mechanism cannot be classified unambiguously either as stepwise or concerted in these compounds. From various diagnostic criteria, the C–X bond cleavage mechanism appears to be borderline between stepwise and concerted. © 2008 The Electrochemical Society. DOI: 10.1149/1.3025830 All rights reserved. Manuscript submitted May 12, 2008; revised manuscript received October 13, 2008. Published November 25, 2008. The mechanism of dissociative electron transfer reactions is a fascinating investigation on account of the subtle issues arising from the interplay of thermodynamic and kinetic factors, mediated by the solvent characteristics. While it is anticipated that the cleavage of carbon–halogen bonds would occur via the intermediate radical an- ion in simple aromatic halides, the influence of electron- withdrawing groups in the aromatic ring on the mechanistic analysis is not obvious. It is therefore of interest to systematically study the reduction behavior of compounds wherein the five hydrogens are replaced by the fluorines in the benzene ring while the other hydro- gen is replaced by any halogen other than fluorine, thereby yielding halopentafluorobenzenes. Further, it is essential to employ different diagnostic criteria in order to obtain reliable insights. The cleavage of aromatic halides in the presence of electron withdrawing groups, viz., halonitrobenzenes, 1 halobenzophenones, haloacetophenones, 2 haloacetonitriles, 3 and fluoromethylarenes, 4 has been investigated. The nature of the functional group indeed dictates the mechanism of the reduction as in the case of arymethylhalides. 5 Aryl halides in general exhibit a stepwise mechanism iodobenzene 6 being a notable exception. In C 6 F 5 X X = Cl, Br, I, the benzene ring becomes more electropositive, leading to the formation of a stable intermediate radical anion which would imply a stepwise re- duction of the C–X bond. Further, in the case of halopentafluoroben- zenes, the C–F bond is anticipated to undergo reduction at more negative potentials and hence is unlikely to interfere with the reduc- tion of C–X bonds. It is customary to analyze such C–X bond cleav- ages with the help of either stepwise or concerted mechanism Scheme 1. The objectives of this Communication are i to estimate the electrode kinetic parameters pertaining to the reduction of the C–X bond in C 6 F 5 X X = Cl, Br, I using convolution potential sweep voltammetry CPSV and ii to elucidate the mechanism with the help of electron density calculations, intrinsic barrier estimates, and standard reduction potentials. Experimental and Computational Methods Experimental.— Acetonitrile ACN, HPLC grade, SRL, India was distilled over anhydrous calcium hydride protected by 4 Å mo- lecular sieves in argon atmosphere. The supporting electrolyte tet- rabutylammonium perchlorate TBAPFluka, Puriss electrochemi- cal grade, chloropentafluorobenzene Aldrich, bromopentafluoro- benzene Aldrich, iodopentafluorobenzene Aldrich, and ferrocene Puriss, Spectrochem, India were used as received. The cyclic vol- tammetric experiments were carried out using an electrochemical workstation CH Instruments 660A, in a single-compartment elec- trochemical cell thermostatted at 298 K. The working electrode was a glassy carbon GC electrode of 3 mm diameter CH Instruments, USA polished with the alumina slurry and sonicated before use. While the Ag /Ag +  1 mM electrode was the quasi-reference elec- trode, Pt wire served as the counter electrode. Under identical con- ditions of the solvent ACN and supporting electrolyte TBAP, the potential of the silver/silver ion quasi-reference electrode was cali- brated with reference to the ferrocene/ferrocenium couple. The un- compensated solution resistance was measured before each experi- ment and the ohmic drop was compensated leaving the residual resistance of 35  for scan rates below 1 V s -1 and 50  above the scan rate 1 V s -1 . Quantum chemical calculations.— The quantum chemical cal- culations were performed using Gaussian 03 software 7 employing the Becke three parameter hybrid exchange in conjunction with the correlation functional developed by Lee, Yang, and Parr B3LYP. The 6-311+G d basis set was employed for C 6 F 5 Cl and C 6 F 5 Br. For C 6 F 5 I, the CEP-121G was used as the basis set. a The open-shell model unrestricted and the closed-shell model restricted was used for the anionic radicals and neutral molecules, respectively. The conductor-like polarized continuum model was employed for the solvent. The input parameters of the solvent ACN were i atomic radii from united atom topological model, ii cavity model: GePol, iii dielectric constant   = 36.64, iv dielectric constant at infinite frequency     = 1.806, v radius of ACN = 2.155 Å. To estimate the total charge density, the Gaussian 03 version was used. For this purpose, the cubes were generated for total density and the electrostatic potential ESP was obtained with the help of the Gaussview 3.09. The ESP was mapped on the surface of the total electron density to generate the corresponding contour diagrams. The bond dissociation energy was computed 8 from the optimized structure of the molecule using theromodynamic considerations. Results and Discussion Figure 1 depicts the cyclic voltammogram of the compounds under study wherein an irreversible peak is noticed for each com- * Electrochemical Society Student Member. z E-mail: sangara@iitm.ac.in a Because the applicability of the 6–311G basis set is restricted up to the third row nontransition metals, it is imperative to employ other basis sets to obtain accurate results. Hence, we employ here the CEP-121G basis set in the case of C 6 F 5 I RX R +X RX a a b e - e - Scheme 1. Reduction of aromatic halides RX via a stepwise and b concerted mechanism. Journal of The Electrochemical Society, 156 2 F23-F28 2009 0013-4651/2008/1562/F23/6/$23.00 © The Electrochemical Society F23 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.112.157.19 Downloaded on 2014-09-22 to IP