ARTICLE pubs.acs.org/JPCC
The Role of Intermolecular Hydrogen Bonding and Proton Transfer in Proton-Coupled Electron Transfer Timothy M. Alligrant and Julio C. Alvarez* Department of Chemistry, Virginia Commonwealth University, P.O. Box 842006, Richmond, Virginia 23284, United States
bS Supporting Information ABSTRACT: An example of proton-coupled electron transfer (PCET) comprised by the electrochemical oxidation of 1,4-hydroquinone (1,4-H2Q) in acetonitrile was studied in the presence of Brønsted bases in acetonitrile. Of the two types of bases studied, the negatively charged carboxylates, trifluoroacetate (TFAC), benzoate (BZ), and acetate (AC), showed hydrogen bonding with 1,4-H2Q, whereas the neutral amines, pyridine (PY) and N, N0 -diisopropylethylamine (DIPEA), did not. This difference allowed a unique investigation of the effect of proton transfer on PCET with and without the influence of hydrogen bonding using two bases (TFAC and PY) with approximately the same pKa (∼12). The study revealed that hydrogen bonding of 1,4-H2Q with the base TFAC made the half wave redox potential of 1,4-H2Q more negative (easier to oxidize) by 0.186 V with respect to the oxidation in the presence of the same concentration of added PY, which does not hydrogen bond with 1,4-H2Q. Both types of bases studied, carboxylates and amines, showed a combination of kinetic and thermodynamic effects in the oxidation voltammerty of 1,4-H2Q; however, no evidence of concerted pathways was found at the conditions studied as indicated by H/D kinetic isotope experiments. The mechanism from fitted digital simulations for all the bases supports a stepwise PCET, even in the presence of hydrogen bonding, implying that the latter does not prevail in the transition state nor is rate determining. Hydrogen bonding was verified by 1H NMR spectroscopy, while the electrochemical studies were carried out by cyclic voltammetry. The hydrogen bonding constants and diffusion coefficients determined by 1H NMR were used in digital simulations that were fitted to experimental voltammograms.
’ INTRODUCTION Herein, we describe an example of electrochemical protoncoupled electron transfer (PCET) consisting of the oxidation of 1,4-hydroquinone (1,4-H2Q), in which intermolecular hydrogen bonding and proton transfer (PT) with Brønsted bases facilitate the PCET in acetonitrile. The most significant aspect of this work is that it allows a comparative study of the effect of PT on the electrochemical oxidation of 1,4-H2Q in the presence and in the absence of hydrogen bonding with two bases of almost the same pKa. Previously, we have shown that both acetate and pyridine facilitate the oxidation of 1,4-H2Q in acetonitrile even though only acetate shows hydrogen bonding with 1,4-H2Q.1 The favoring effect occurs because both bases accept the Hþ released by 1,4-H2Q during its two-electron oxidation (Scheme 1); however, the comparison is ambiguous because acetate and pyridine have quite different pKa values in acetonitrile. In this work, we extend this investigatigation to other carboxylates including trifluoroacetate which has aproximately the same pKa as pyridine and allows a unique comparison of the effect of PT on PCET with and without the influence of hydrogen bonding (Scheme 1). Reactions involving PCET are currently receiving a great deal of attention because of their fundamental role in relevant chemical and biological processes.26 Of primary interest are the PCET reactions in which the PT and electron transfer (ET) r 2011 American Chemical Society
involve different molecular centers.2,3,7,8 These reactions which may happen at electrodes or in solution, can be concerted or stepwise depending on whether the PT and ET occur in a single step or in two kinetic steps.2,3,7,8 Previous reports on nonelectrochemical PCET9,10 indicate that hydrogen bonding can lead to concerted pathways with a kinetic effect that can favor the PCET. Hydrogen bonding is also considered a key favoring factor for the rate enhancement observed in enzyme and general acidbase catalysis of PT-coupled reactions.1115 For those cases, the actual consensus indicates that the hydrogen bonding should stabilize the transition state rather than the ground state for catalysis to occur.13,14,16 Herein, the kinetic and thermodynamic effects of intermolecular hydrogen bonding of Brønsted bases in an electrochemical PCET reaction are investigated. This study is motivated by evidence indicating that some electrodes containing surface functionalities, presumably basic or acidic, appear to catalyze certain electrochemical reactions involving PCET;1720 however, mechanistic understanding is far from clear. It is only recently that intramolecular hydrogen bonding has been shown to affect the kinetics of some nonelectrochemical Received: September 1, 2010 Revised: April 25, 2011 Published: May 10, 2011 10797
dx.doi.org/10.1021/jp108339k | J. Phys. Chem. C 2011, 115, 10797–10805
The Journal of Physical Chemistry C Scheme 1. Pathways for the Electrochemical Oxidation of 1,4-H2Q in the Presence of Pyridine (C5H5N) and Trifluoroacetate (F3COO) in Aprotic Solution
PCET, leading to concerted pathways.3,5,9,10,21,22 This work is an extension of our recent effort on facilitation of electrochemical reactions by electrogenerated acid and is also closely related to previous reports describing hydrogen bonding and acidbase effects on PCET reactions.1,3,57,2134 Hydrogen bonding was investigated by 1H NMR spectroscopy, while the electrochemical studies were carried out by cyclic voltammetry. The hydrogen bonding constants and diffusion coefficients determined by 1H NMR were used in digital simulations that were fitted to experimental voltammograms. The proposed mechanisms derived this way were in congruence with previously reported studies.1,3537
’ EXPERIMENTAL SECTION A. Chemicals and Reagents. Anhydrous acetonitrile (MeCN, Aldrich, St. Louis, MO, 99.8% with
