J. Phys. Chem. C 2010, 114, 1285–1292
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Association Reactions of the Anion Radicals of Some Hydroxyquinones: Evidence for Formation of π- and σ-Dimers As Well As a Neutral-Anion Radical Complex Norma A. Macı´as-Ruvalcaba† and Dennis H. Evans*,‡ Facultad de Quı´mica, UniVersidad Nacional Auto´noma de Me´xico, Ciudad UniVersitaria, 04510 Me´xico D.F., Me´xico, and Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: October 26, 2009; ReVised Manuscript ReceiVed: NoVember 20, 2009
The electrochemical reduction of nine quinones has been studied in acetonitrile and, in one case, in dimethyl sulfoxide. Included are seven hydroxyquinones plus 1,4-naphthoquinone, 8, and 9,10-anthraquinone, 9. Each quinone is reduced in two steps, first to the anion radical and then, at more negative potentials, to the dianion. However, digital simulations show that the voltammetric data cannot be explained by these two reactions alone. For three of the hydroxyquinones, 2-4, plus 8 and 9, the fast disproportionation/comproportionation reaction connecting the neutral, anion radical, and dianion must be included along with a diffusion coefficient of the anion radical that is smaller than that of the neutral quinone and a still smaller diffusion coefficient for the dianion. Three other hydroxyquinones, 5-7, require, in addition, the formation of a neutral-anion radical complex. Finally, 5-hydroxy-1,4-naphthoquinone, 1, in acetonitrile involves both σ- and π-dimerization of the anion radical with subsequent reduction of the π-complex, whereas in dimethyl sulfoxide the σ-dimer is replaced by the aforementioned formation of the neutral-anion radical complex. The differences in behavior are discussed in terms of distribution of spin density in the anion radicals and intermolecular and intramolecular hydrogen bonding. Introduction Hydroxyquinones are of significant interest owing to their important biological functions. There have been numerous electrochemical studies of the reduction of a variety of hydroxyquinones,1-20 studies which have revealed several effects of hydroxyl substituents including self-protonation, in which the initially formed anion radical is protonated by the neutral quinone, and stabilization of reduced forms of the quinone by intramolecular hydrogen bonding between the hydroxylic proton and a quinoidal oxygen. We have recently shown21 that the anion radicals of certain quinones and cyclic 1,2-diketones undergo dimerization to form either σ- or π-dimers and that these reactions have subtle but easily detectable effects on the detailed shape of the cyclic voltammograms. We have now extended these studies to a series of hydroxyquinones, 1-7, designated as HQ. For the sake of comparison, we have also examined the unsubstituted quinones, 1,4-naphthoquinone (8) and 9,10-anthraquinone (9). We have avoided strongly acidic hydroxyquinones such as 2-hydroxy-1,4-napthoquinone (pKa(aqueous, 298 K) ) 3.9822) so that the results would not be dominated by self-protonation reactions. Rather, in all cases the quinones contain a so-called β-hydroxyl group where the hydroxyl substituent is in the closest possible position in a ring that is adjacent to the quinoidal ring. Here, the principal effect of the intramolecular hydrogen bonding in the anion radical and dianion is to render the reduction potentials considerably less negative than in the unsubstituted quinone.2 * Corresponding author: Telephone: (765) 494-5454. E-mail: evansd@ purdue.edu. † Universidad Nacional Auto´noma de Me´xico. ‡ Purdue University.
We have found for compounds 2-4, 8, and 9 in acetonitrile that the data for a wide range of concentrations and scan rates can be accounted for by reactions 1-3
HQ + e- h HQ•+ e h HQ
•-
h HQ + HQ
HQ
2HQ
-
(1)
•-
22-
(2) (3)
with the provision that the diffusion coefficients of the anionic forms are smaller than for the neutral quinone. For compounds 5-7 it was necessary to add association reaction 4
HQ•- + HQ h (HQ)2•-
(4)
to reactions 1-3 in order to adequately account for the results. Compound 1, 5-hydroxy-1,4-naphthoquinone, represents a special case. In acetonitrile as solvent, reactions 1-3 were joined by reactions 5 and 6
10.1021/jp910225p 2010 American Chemical Society Published on Web 12/15/2009
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J. Phys. Chem. C, Vol. 114, No. 2, 2010
2HQ•- f σ-(HQ)22•-
2HQ
2-
h π-(HQ)2
Macı´as-Ruvalcaba and Evans
(5) (6)
the formation of σ- and π-dimers, respectively, as well as reaction 7
π-(HQ)22- + e- h π-(HQ)2•3-
(7)
a reaction that was found to be necessary to account for the reduction of some quinones and cyclic 1,2-diketones.21 In dimethyl sulfoxide (DMSO), however, reaction 5 must be replaced by reaction 4. Experimental Section Source and Purification of the Hydroxyquinones. 5-Hydroxy-1,4-naphthalenedione, 1 (Aldrich 97%), was used as received. 5,8-Dihydroxy-1,4-naphthalenedione, 2 (Aldrich 85%), was purified by sublimation. 2,3-Dichloro-5,8-dihydroxy-1,4-naphthalenedione, 3 (Aldrich 95%), was recrystallized from DMF-ethanol. 1-Hydroxy-9,10-anthracenedione, 4, was purified by sublimation. 1,8-Dihydroxy-9,10-anthracenedione, 5 (Aldrich), was recrystallized from ethanol. 1,4-Dihydroxy-9,10-anthracenedione, 6 (Merck 97%), was recrystallized from ethanol. 1,5-Dihydroxy-9,10-anthracenedione, 7 (Aldrich 85%), was purified by successive recrystallizations from ethanol. 1,4-Naphthalenedione, 8, and 9,10anthracenedione, 9, were purified by sublimation. In the text, these compounds will be referred to by their older names, 1,4-naphthoquinones and 9,10-anthraquinones. Source of Electrolytes and Solvents. Anhydrous acetonitrile, AN (Sigma-Aldrich 99.8%,
