338
J. Phys. Chem. C 2009, 113, 338–345
Contrasting Behavior in the Reduction of 1,2-Acenaphthylenedione and 1,2-Aceanthrylenedione. Two Types of Reversible Dimerization of Anion Radicals Norma A. Macı´as-Ruvalcaba,‡ Greg A. N. Felton,† and Dennis H. Evans*,† Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721, and Facultad de Quı´mica, UniVersidad Nacional Auto´noma de Me´xico, Ciudad UniVersitaria, 04510 Me´xico D.F., Me´xico ReceiVed: NoVember 1, 2008
Voltammetric and spectroscopic studies of 1,2-acenaphthylenedione, 1, and 1,2-aceanthrylenedione, 2, have shown that the anion radicals of 1 form a π-dimer whereas a σ-dimer is found with 2. The formation of both dimers is seen to be reversible. These conclusions are based on analysis of the cyclic voltammograms and, for the anion radical of 1, the appearance of a long wavelength absorption band at 995 nm as observed by spectroelectrochemical monitoring. Support was also obtained by comparison of the results with those obtained with systems known to form π-dimers (anion radical of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, 3) and σ-dimers (anion radical of 9-acetylanthracene, 4). The site of dimerization in the σ-dimer of 2 has been assigned through DFT calculations and found to be at a position analogous to that which is known for 4. Introduction Dimerization is one of the principal reactions of organic radicals, including free (or neutral) radicals as well as ion radicals.1 In most instances, the dimerization is irreversible in the sense that no radical species can be detected in equilibrium with the dimer. An early exception was Gomberg′s discovery2 of triphenylmethyl, for which we now know an appreciable amount of the free radical exists in equilibrium with the dimer. As with triphenylmethyl, most dimers formed from radical species involve the formation of a σ-bond by combination of the unpaired electron from each of the monomeric species. Our focus will be on anion radicals which can also form σ-bonded dimers in an essentially irreversible process, reduction of enones3 and nitroalkenes4 being two examples. However, in some instances reversibility in the dimerization reaction has been detected. Examples include the anion radicals of 9-cyanoanthracene,5 3,5-dinitropyridine,6 1,3,5-trinitrobenzene,6,7 trinitrotoluene5i and 2,3-dicyanonaphthalene.5i When studied by electrochemistry, a new anodic peak, usually assigned to oxidation of the σ-dimer, is observed at less negative potentials than reduction of the neutral starting material and the reversibility of the dimerization is signaled by a decrease in the size of this anodic peak accompanied by an increase in the peak for the oxidation of the monomer anion radical as the scan rate is decreased. A different type of dimer can be formed with anion radicals (and cation radicals and free radicals as well). This dimer is a π-dimer in which the anion radicals undergo a face-to-face interaction forming a π-bond. Numerous examples are known including tetracyanoethylene,8 tetracyanoquinodimethane,9 2,3dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),9,10 chloranil,9 and cyanil.11 In many cases the π-dimers have been observed in the solid state by X-ray crystallography while in solution their presence is indicated by characteristic long-wavelength absorption bands. The species formed by interaction of a radical * Corresponding author. Tel: 1-520-626-0318; fax: 1-520-621-8407; e-mail:
[email protected]. † University of Arizona. ‡ Universidad Nacional Auto´noma de Me´xico.
Figure 1. Voltammogram of 2.33 mM 1,2-acenaphthylenedione, 1, in CH3CN containing 0.10 M Bu4NPF6 at a glassy carbon working electrode. 0.50 V/s.
Figure 2. Voltammogram of 1.96 mM 1,2-aceanthrylenedione, 2. Other conditions as in Figure 1.
anion with the neutral starting material, called the precursor complex for electron self-exchange or pimer, shows a distinctly different absorption spectrum.12 It has been found that the formation of π-dimers is always a reversible process with relatively small equilibrium constants. Often, it is necessary to use low temperatures in order to see
10.1021/jp809667f CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008
Reversible Dimerization of Anion Radicals
Figure 3. Comparison of simulations to background-corrected experimental voltammograms of 3.65 mM 1 at -20 °C at 1.0, 5.0, and 10.0 V/s. Other conditions as in Figure 1. Open circles: simulation; full curve: experimental. Dashed line represents the simulation with reactions 4 and 5 removed. See Table 1 for simulation parameter values.
significant dimerization. Also, the reaction is very rapid with the dimerization rate constant for the anion radical of DDQ being close to the diffusion-controlled limit.10a By contrast, when reversible σ-dimerization is observed, the equilibrium constants are usually larger and the dimerization is slower. In this paper we report the differing dimerization behavior of the anion radicals of two structurally related diketones, 1,2acenaphthylenedione, 1, and 1,2-aceanthrylenedione, 2. Specifically, the anion radical from 1 has been found to form a π-dimer while that of 2 forms a σ-dimer. Some aspects of the reduction of 1 have been reported in the literature,13 but the reduction of 2 has not been investigated.
Experimental Section Compounds 1 and 2 as well as 2,3-dichloro-5,6-dicyano-1,4benzoquinone (3) and 9-acetylanthracene (4) were obtained from Aldrich. They were purified by recrystallization from dichloromethane (1), toluene/acetonitrile (2), cyclohexane/benzene (3), and ethanol (4). N,N-Dimethylformamide (DMF) was obtained from Aldrich (99.8%;
