5129
J. Phys. Chem. 1991, 95, 5129-5133
species are not stable, transforming into 02-inert Ru(I1) species that contain modified ligands. Acknowledgment. This research was supported in Part by Consiglio Nazionale delle Ricerche of Italy (Progetto Finalizzato Chimica Fine), in part by Minister0 della Pubblica Istruzione of and in part by the U S . Department of Energy Italy (Quota a%), (Office of Basic Energy Sciences, Division of Chemical Sciences). and M.Z.H. is part of the The collaboration between Q.G.M.
US.-Italy Cooperative Research Program. We thank Professor D. P. Rillema for samples of some of the complexes, and Dr.G. Neshvad and H. Sun for further synthesis. We also thank G. Gubellini, A. Monti, and L. Ventura for technical assistance. Supplementary Material Available: Observed spectra from pulse radiolysis and corrected spectra of the acid-base forms of the reduced complexes (18 pages). Ordering information is given on any current masthead page.
Oxidation-Reduction Behavior of Bianthrone and Blanthronyi in Basic Dimethyl Sulfoxide Solutions Saba M. Mattar* and Douglas Sutherlandt Department of Chemistry, University of New Brunswick, Bag Service Canada E3B 6E2 (Received: September 28, 1990)
# 45222, Fredericton, New Brunswick,
Simultaneouselectrolysis-electronparamagnetic resonance (SE-EPR) spectroscopy and cyclic voltammetry of bianthrone in dimethyl sulfoxide (DMSO) indicate that the A form of bianthrone is first reduced to generate the dianion of the B form, B". The dianion then diffuses into solution where it encounters another A to give B.' In addition, the OH-ion in DMSO reduces A to B'- and B' to B2- in two distinct consecutive reactions. The first reaction is used to prepare a solution that only contains B*-. The UV-VIS and EPR spectra of this solution unequivocally show that the only stable radical anion in DMSO is B.' The isosbestic points in the UV-VIS spectra prove that A and B are completely depleted before B2- is formed. The formal reduction potentials for the bianthrone species and O2indicate that the oxidation of B* is not thermodynamically favorable. Consequently, the B2- is stable and is easily characterized by UV-VIS spectroscopy. In contrast, the B'-may also be generated via a controlled oxidation of bianthronyl (BH,) solutions. However, if excess O2is present, then two or more equivalents of NaOH cleave the 9,9' C-C bond of BH2 to give the 9,lO-anthrasemiquinoneradical anion.
Introduction Bianthrone' is a sterically hindered ethylene derivative that displays unique photochemical and thermochromic behavior. Because of these unusual properties it was the subject of a large number of investigations. Several review articles summarizing this extensive research have been published? The thermochromic and photochemical properties were eventually found to be independent of one another.' Bianthrone exhibits thermochromic behavior because at 298 K it exists in a yellow form, A. The 9,9' double bond is intact, 0
5
2'
\\
/
-\
0
~
3'
4'
5'
A
6'
0
6
but the steric repulsion between the 1-8' and 1'-8 protons is relieved by the folding of the two anthrone halves away from one another! As the temperature is increased the molecule converts into a second form, B, which is green. This process has an activation energy of 55-63 kJ/mol. In the B form both anthrone halves are planar, but the 9,9' central double bond, connecting the two halves, is twisted. In one account, the twist angle is estimated to be approximately 57O? while theoretical computations, using the M I N D 0 / 3 approximation, estimate it to be 'Present address: Department of Chemistry, University of Western Ontario, London, Ontario, Canada.
70-75°.6 The rates governing the interconversion of the A and B forms have been determined.' The bianthrone 9,9' central double bond is responsible for the thermochromic behavior. Consequently, the electronic and structural properties of this molecule once the 9,9' double bond is reduced are of prime interest. Neta and Evanss were the first to study the interconversion process of the corresponding radical anions A*-
(1)
Their pulse radiolysis experiments indicate that the conversion rate of A'- to B' is approximately 7 X lo' s-' . It was also found that once B'-is produced it disporportionates to BH- and B.8*9 Olsen and Evans'O have shown that the two different conformations, A and B, have different thermodynamic, kinetic, spectral, (1) This compound has been given more than one name. The most common is bianthrone. It represents lo-( 1O-oxe9( lOH)-anthracenylidene)-9(lOH)-anthracenone,shown as structures A and B. Another equivalent name is lO,lO'-bianthronylidcne. (2) Kortum, G. Ber. Bunsen-Ges Phys. Chem. 1974,78, 391-403. Bercovici, T.; Korenstein, R.; Muszcat, K. A.; Fischer, E. Pure Appl. Chem. 1970, 24, 531-65. Margerum, J. D.; Miller, L. J. In Techniques of Chemistry; Brown, G. H., Ed.;Wiley Insterscience, New York, 1971; Vol. 3, pp 558432. Fischcr, E. Fortschr. Chem. Forsch. 1967,7, 605-41. (3) (a) Kortum, G.; Zollcr, W. Chem. Ber. 1967,100,280. (b) Kortum, G.; Koch, K. W. Ibid. 1967, 100, 1515. (4)Harnik, E.; Schmidt, G. M. J. J . Chem. Sa. 1954, 3295. (5) Korenstein, R.; Muszliat, K. A.; Sharafy-Ozeri,S.J . Am. Chem. Soc. 1973, 95,6177. (6)Kikuchi, 0.;Kawakami, Y . THEOCHEM 1986, 137,356. (7) (a) Tapuhi, Y.; Kalisky, 0.;Agranat, I. J. Org. Chem. 1979,44, 1949. (b) Dombrowski, L. J.; Groncki, C. L.; Strong, R. L.; Richtol, H. H. J. Phys. Chem. 1969,73, 3481. ( c ) Bcrcovici, T.; Fischer, E. Isr. J . Chem. 1969, 7, 127. (8) Neta, P.; Evans, D. H. J . Am. Chem. Soc. 1981, 103,7041. (9) Neutral bianthronyl (BH2) is the protonated form of B". It has the name [9,9'-bianthracene]-lO,lO'(9H.9'H)dione. On several occasions it has also been named I0,lW-bianthrone.
0022-3654/91/2095-5129$02.50/00 1991 American Chemical Society
5130 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 and electrochemical properties. As a result of this, the direct one-electron reduction of A to B'- is not observed by cyclic voltammetry.I0 The reduction of A is irreversible and involves a conformational change to yield the B2- dianion. After the B2is formed, it can be oxidized in a stepwise manner first to B' and then to B. Only then can B be transformed back to A at room temperature. By varying the voltage scan rates they were able to determine the interconversion rates of A to B and estimate the upper limits of the rate constants for the remaining processes.1° Molecules that have similarly strained ethylene moieties, such as dixanthylene and 10,lO'-dimethyl-9,9'-biacridylidene, were also found to have similar properties."J2 Both the thermolysis and photolysis of bianthrone produce a phenoxy radical due to the abstraction of a H atom from its environment. The study of these neutral radicals by electron paramagnetic resonance (EPR) shows that the unpaired electron resides only on half of the m ~ l e c u l e . ~ ~ - ~ ~ To the best of our knowledge there is only one published account on the EPR spectroscopy of the bianthrone radical anion.I4 The radical was generated by the reduction of bianthrone with potassium terr-butoxide in dimethyl sulfoxide (DMSO) and was proposed to have a twisted configuration." Neta and Evans6have also suggested that the bianthrone radical anion may exist in the B form. However, at present no definitive characterization using EPR has been obtained. In this work, experimental proof is provided that B' is the only stable radical anion in DMSO solutions. In addition, simultaneous electrolysis-EPR (SE-EPR), UV-VIS absorption spectroscopy, and cyclic voltammetry are used to characterize the redox products of bianthrone and bianthronyl ([9,9'-bianthracene]-9,9'-dihydro-lO,lO'-dione, BH2). 0 5
*'3'
I
4
m /
4'
I(
8' 7'
5'
2
Although the ability of the hydroxide ion (OH-) to act as a one-electron reducing agent is very small in aqueous media, it is greatly enhanced in aprotic solvents.15 One of the factors affecting the OH- reactivity is its solvation energy. The ionization energy of OH- in water is 4.4 eV larger than the corresponding gas-phase value.I6 This large solvation energy in aqueous media inhibits the single-electron-transfer reaction
HO' + e-
=I1
-=il7-
i
N2
a ' Figure 1. Sample cell used for the SE-EPR experiments: (a) platinum working electrode; (b) platinum auxiliary electrode; (c) silver reference electrode. Outer diameter (0.d.) of the tube in the vicinity of the working electrode is 4.0 mm, while the 0.d. of the top end is 15.0 mm.
to be a more efficient one-electron donor.Is The OH- ion will easily reduce 9,lO-anthraquinone and its derivatives to their corresponding radical anions.I8 This was found to be true for a variety of other quinones such as benzoquinones, naphthoquinones, and phenanthr~nes.'~J~J~ The first step in the reduction process is a fast and reversible polar-group-coupling reaction. This nucleophilic addition step results in a quinonehydroxy complex followed by a slower electron-transfer step that produces the ~emiquinone.'~J~*'~ In the present study, use is made of one-electron reducing properties of OH- in DMSO as an alternate route (other than electrolysis and cyclic voltammetry) to generate'B and B" from A. Experimental Section
0
OH-
Mattar and Sutherland
(2)
In aprotic solvents, such as acetonitrile and DMSO,electrochemical measurements indicate that the OH- solvation energy is approximately 1.0 eV less than that in water." This causes a decrease in the OH- ionization energy and enhances its ability (IO) Olsen, B. A.; Evans, D. H. J . Am. Chem. Soc. 1981, 103, 839.
(11) Evans, D. H.; Busch, R. W. J . Am. Chem. Soc. 1982, 104, 5057. (12) Ahlberg, E.;Hammerich, 0.; Parker, V. D. J. Am. Chem. Soc. 1981,
103.844.
(13) Falle, H. R.; Luckhurst,G. R.; Lemaire, H.; Marcchal, A.; Rassat, A.; Rey, P. Mol. Phys. 1966, 11. 49. (14) Agnnat, 1.; Rabinovitz, M.; Falle, H. R.; Luckhurst, G. R.; Ockwell, J. N. J . Chem. Soc. 1970, 294. (15) Sawyer, D. T.;Roberts, J. L., Jr. Acc. Chem. Res. 1988, 21, 469. (16) Pearson, R. G. J . Am. Chem. Soc. 1986. 108,6109. (17) (a) Tsang, P. K. S.; Cofre, P.; Sawyer, D. T. I m g . Chem. 1987.26,
3604. (b) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; Marcel Dekker: New York, 1985. (c) Parsons, R. Handbook of Electrochemical Constants; Butterworth: London, 1959, pp 69-73.
The electronic absorption spectra were recorded on a PerkinElmer Model 330 UV-VIS spectrophotometer. Solution quartz cells of 1-mm path length were used. Storage, recall, and display of the spectra were done with a Perkin-Elmer Model 3500 microcomputer directly interfaced to the spectrometer. The cyclic voltammogramswere scanned on a Princeton Applied Research PAR 170 electrochemistry system. The working electrode was a platinum bead (1.0-mm diameter) with a platinum auxiliary electrode (0.25-mm diameter X 20.0-mm length). Voltages were measured relative to a saturated calomel reference electrode (SCE) at 298 K. All the DMSO solutions used in cyclic voltammetry contained 0.1 M tetraethylammonium perchlorate (TEAP) and were purged with dry nitrogen before and during the experiments. The simultaneous EPR and electrochemistry cell is very simple. It uses platinum wires (0.25-mm diameter X 20.0-mm length) for both the working and auxiliary electrodes. Since an SCE was too big to fit in the EPR cell, a silver wire was used as the reference electrode (Figure 1). Unless otherwise stated, all reported potentials are relative to an aqueous SCE. The cyclic voltammograms were digitized by using a Data Translation DT2805 analog-to-digital board operating in a Cordata microcomputer. The EPR spectra were reocrded with a custom-built spectrometer. It is equipped with a 12-inch Varian electromagnet controlled by a V-FR2503 Fieldial regulating unit and a Hall probe. The magnetic field is calibrated by a Bell 640 incremental gaussmeter. The klystron beam and reflector voltages are powered by two Power Designs high-stability power supplies. The microwave frequency is locked to the cavity by a Micronow Instruments Co. Model 210 automatic frequency control stabilizer. (18) Roberts, J. L., Jr.; Sugimoto, H.; Barrette, W. C., Jr.; Sawyer, D. T. J . Am. Chem. Soc. 1985. 107,4556. (19) Mattar, S. M.; Sutherland, D. G. To be published.
The Journal of Physical Chemistry, Vol. 95. No. 13. 1991 5131
Oxidation-Reduction of Bianthrone and Bianthronyl
r m
Reference arm I
t
Observation a r m
Figure 2. Layout of the EPR Spectrometer microwave bridge: (A) Hewlett-Packard X238A 50-dB microwave power attenuator; (C) Mi-
cronow three-port circulator; (D) microwave diode detector (1N23G); (I) ferriteisolator; (K) Varian V-153C microwave klystron; (Oc)Varian V-4531 rectangular cavity; (P) Hewlett-PackardX885A precision phase shifter; (W)Hewlett-Packard X532A cylindrical wave meter. When extra stability of the klystron frequency is needed it is phase-locked, via a feedback loop, to a quartz oscillator thermostated in the oven of a Frequency Engineering Laboratories Model 137A synchronizer. The microwave bridge layout is shown in Figure 2. It is equipped with a reference arm that, in conjunction with the FEL phase-lock synchronizer, enables the detection of both the absorption or dispersion modes of the EPR spectra. The detection unit is a Stanford Research Systems SRS 530 microprocessor-controlled lock-in amplifier. The frequency of the spectrometer is measured by a calibrated wavemeter attached to the reference arm of the bridge via a directional coupler. In recording the EPR spectra, the field modulation amplitude and microwave power are chosen to give the highest intensity without distorting the spectral line shapes. All spectra are recorded at 298 K. Cyclohexene- 1-one, bianthrone, bianthronyl, and DMSO were purchased from the Aldrich Chemical Company of Milwaukee, WS. DMSO (HPLC grade, glass distilled, water content
