J. Chem. Soc., Faraday Trans. I, 1987,83, 51-55 Electron Spin Resonance, ENDOR and TRIPLE Resonance of some 9,lO-Anthraquinone and 9,lO-Anthraquinol Radicals in Solution Mikko Vuolle" and Reijo Makela Department of Chemistry, University of Jyvaskyla, Kyllikinkatu 1-3, SF-40100 Jyvaskyla, Finland E.s.r., ENDOR and TRIPLE resonance spectra have been recorded for 2-methylanthraquinone, anthraquinone-2-sulphonate and anthraquinone- 2,6-disulphonate radical anions, and in a strongly acidic medium for 2-methylanthraquinol, anthraquinol-2-sulphonate and anthraquinol-2,6- disulphonate radical cations. The hyperfine coupling constants (h.f.s.) and g factors are given. The ENDOR spectra show there to be more h.f.s. than were detected earlier by e.s.r. spectroscopy. The spectra of the di-deutero radicals of the anthraquinols and the assignment of h.f.s.are discussed. Quinones are perhaps the most widely studied organic redox system. Semiquinones are relatively stable free radicals in alkaline media, formed by one-electron reduction of quinones or by one-electron oxidation of quinols. In 1961 Vincow and Fraenkell examined the 9,lO-anthrasemiquinone radical anion by e.s.r., and in 1962 Bolton et aZ., examined the 9,lO-anthraquinol radical cation in strong acidic media. Many mono- and di-substituted 9,lO-anthrasemiquinone radical anions have been examined by e.s.r., but not many radical Only a few ENDOR and TRIPLE spectra of anthrasemiquinone radical anions and semiquinol radical cations have been published. Assignment of the splitting constants of aromatic protons requires comparison with data for substituted radicals having the same basic anthraquinone structure.We have measured the e.s.r. and ENDOR spectra of the radical anions and cations of eight compounds, but publish here the spectra of only three compounds. Most of the splitting constants of semiquinone radical anions included in the tables were originally assigned by the authors. However, some of these are incomplete and we are currently revising all assignments in the light of the better-resolved ENDOR and TRIPLE spectra. Quinones play an important role in biological processes and anthracycline anticancer drugs contain both quinone and hydroquinone structures. Experimental Materials 2-Methyl-9,lO-anthraquinone 97 % and 9,l O-anthraquinone-2,6-disodiumsulphonate (m.p.> 325 "C) from Aldrich and 9,1O-anthraquinone-2-sodiumsulphonate (puriss. p.a.) from Fluka were used without further purification. Other chemicals were Na,S,O, (Merck, lab.), D,SO, (Merck, 96-98 % , deuteration degree not less than 99% ), CF,SO,H (Fluka, purum), FS0,H (Fluka, techn.) S0,ClF (Cationics Inc.), KO, (Fluka, pract.), DMSO (Merck, scintillation grade), 18-crown-6 (Fluka, purum), linolic acid (Merck, p.a.), TCNE (Aldrich, 98%) and NH, (Merck, 99.9%). 5152 9,lO-Anthraquinone and 9,l Q-Anthraquinol Radicals Equipment E.s.r. spectra were recorded on a Varian E-9 spectrometer with a field-frequency lock, a Varian variable-temperature control unit, a Takeda Riken Industry Co TR 5211 microwave counter, a Varian E 500 gauss meter and an Apple I1 computer.ENDOR and TRIPLE spectra were measured with a Bruker ER 200 D-SRC spectrometer (laboratory-made ENDOR coil). U.V. illumination was provided by an Airam HQU 300 W mercury lamp. Sample Preparation Samples were prepared in two different ways, by a high-vacuum technique or using a laboratory-developed flow system. Semiquinone radicals were generated by Na,S,O, reduction in 0.1-0.05 mol dm-, NaOH ethanolic water solution, where the ethanol concentration varied from 10 to 90% .g DMSO was added to a mixture of the appropriate quinone, KO, and 18-crown-6, and the sample was degassed before use with two freeze-pumpthaw cycles on a vacuum system. Ammonia was distilled under a nitrogen atmosphere into an e.s.r. ampoule containing a very small piece of alkali metal and a quinone.The system was maintained over a cold bath at -85 "C and the sample was sealed under high vacuum. Radical cations were prepared by dissolving the corresponding quinone in H,SO,, D,SO,, FS0,H or CF,SO,H and the samples were sealed under high vacuum. Radical cations were also produced by dissolving the parent compound in a mixture of D,SO, and SO,ClF, FS0,H and D,SO, or FS0,H and S0,ClF. U.v.-irradiation of solutions, if necessary, was carried out using a mercury lamp outside the e.s.r. cavity. Some of the samples were irradiated overnight because of the slow formation of semiquinones. Results and Discussion After irradiation of 9,lO-anthraquinone by U.V. light in a mixture of TCNE and ethanolic NaOH aqueous solution (80 : 20), the general TRIPLE spectrum was observed.The spectrum shows the coupling constants of anthrasemiquinone anion radical to be of the same sign, in contrast to the INDO calculation reported in table 1. Analysis of the e.s.r. spectrum recorded from 9,lO-anthraquinone dissolved in a mixture of D,SO, and CF,SO,H shows hydroxy protons, a = 0.1 17 mT (ENDOR), to be exchanged for deuterons a = 0.018 mT. Fig. 1 shows the ENDOR and TRIPLE spectra of' the 2-methylanthrasemiquinone radical anion produced by reduction of 2-methylanthraquinone with sodium dithionite in alkaline ethanol-water solution. A careful comparison of the e.s.r., ENDOR and special TRIPLE spectra gives the ENDOR coupling constants shown in table 1. The methyl protons and one ring proton are magnetically equivalent, as noted earlier by Brumby., Except for the coupling constants of protons 4 and 5, values obtained from the ENDOR spectrum are different from Brumby's e.s.r.values. According to the general TRIPLE experiment, the coupling constants of the methyl protons are of opposite sign to those of the ring protons. 2-Me thylanthraquinol cation radical was generated by dissolving 2-met hylan t hra- quinone in CF,SO,H; the e.s.r., ENDOR and TRIPLE spectra are shown in fig. 2 and coupling constants in table 2. The general TRIPLE spectrum shows that the smallest coupling constant has an opposite sign and may belong therefore to the methyl protons. Attempts were made to generate the deuterated radical cation of 2-methylanthraquinol, but a resolved spectrum has not yet been obtained.The ENDOR coupling constants of the semiquinone anion radical of 2,6-disulphonateM . Vuolle and R. Makela 53 Table 1. Hyperfine splitting constants (mT) of semiquinone anions 2-met hylanthra- anthraquinone- anthraquinone- semiquinone 2,6-disulphonate 2-sulphonate 9,lO-anthraquinone ENDOR e.s.r.a ENDOR e.s.r.b ENDOR e.s.r.b ENDOR INDOC 0.106 0.105 0.094d 0.09P 0.094 0.095 0.052 0.052 0.052 0.052 0.074 0.073 0.086 0.090 0.063 0.061 g = 2.004 04e 0.025 0.039 0.109 0.123 0.033 0.040 0.025 0.039 0.109 0.123 0.033 0.040 g = 2.004 41" - __ - - 0.083 0.075 0.097 0.08 - - 0.056 -0.02 0.122 0.124 0.028 0.028 0.059 0.053 0.094 0.097 0.083 0.079 0.046 0.053 - - - - - - - - - - _. - g = 2.004 08e g = 2.004 02e a Ref. (4). Ref. (7). Ref. (10). CH, group.Our e.s.r. value. m 12 13 m 14 15 m 16 MHz Fig. 1. (a) ENDOR and (b) general TRIPLE spectra of the semiquinone radical anion produced by reduction of 2-methylanthraquinone with Na,S,O,. produced under the reducing conditions of a sodium-ammonia system are listed in table 1 . The general TRIPLE spectrum shows them to be of the same sign for all protons. When anthraquinone-2,6-disodiumsulphonate was dissolved in CF,SO,H the ENDOR and TRIPLE spectra shown in fig. 3 were observed. According to the general TRIPLE spectrum, all coupling constants have the same sign. The highest temperature for the ENDOR measurement was 40 "C using CF,SO,H as solvent; the e.s.r. spectrum was measured at 50 "C and reveals the necessity of higher temperature for good resolution.54 9,1O-Anthraquinone and 9,1O-Anthraquinol Radicals I .I II” ‘ I I 8 m 8 8 8 8 10 12 14 16 18MHz 8 8 8 8 8 1 0 1 2 3 4 5 MHz Fig. 2. (a) E.s.r., (b) ENDOR, (c) special TRIPLE and (d) TRIPLE spectra of the radical cation produced by dissolving 2-methylanthraquinone in CF,SO,H. Table 2. Hyperfine splitting constants (mT) of anthraquinol cations (ENDOR) anthraquinol- 2-sulphonate ~ anthraquinol-2,6- SO,ClF/ D,SO,/ 9,lO-anthraquinol 2-methylanthraquinol disulphonate e.s.r. FS0,H FS0,H d2504 0.378 0.157 0.158 0.184 0.183 0.158 0.363 0.145 0.150 0.161 0.165 0.099 0.268 0.111 0.113 0.144 0.156 0.018 (D) 0.235 0.115 - 0.131 g = 2.003 13 0.093 0.02 1 0.1 12 0.1 10 g = 2.003 08 0.196 g = 2.003 16 0.089 0.088 g = 2.003 90 (deuterated) 0.009 g = 2.002 83 The coupling constants of anthrasemiquinone-2-sulphonate anion radical, which all have the same sign, are shown in table 1.According to the special TRIPLE spectrum, there are two protons with the same coupling constant, 0.053 G. Table 2 gives the coupling constants of cation radicals generated by dissolving anthraquinone-2-sulphonate in a mixture of S0,ClF and FS0,H or D,SO, and FS0,H. The e.s.r. spectra are shown in fig. 4.M. Vuolle and R . Makela 55 " I = 8 rn 8 1 1 12 13 14 15 16MHZ PF 1 (c ) Fig. 3. (a) E.s.r., (b) ENDOR and (c) general TRIPLE spectra of the radical cation produced by dissolving anthraquinone-2,6-disodiumsulphonate in CF,SO,H. Fig. 4. The e.s.r. spectra of the radical cation produced by dissolving anthraquinone-2- sulphonate (a) in a mixture of S0,ClF and FSO,H, (b) D,SO, and FS0,H. References 1 G. Vincow and G. K. Fraenkel, J. Chem. Phys., 1961,34, 1333. 2 J. R. Bolton, A. Carrington and J. dos Santosveiga, Mol. Phys., 1962, 5, 465. 3 P. J. Baugh, G. 0. Phillips and J. C. Arthur Jr, J . Phys. Chem., 1966,70, 3061. 4 S. Brumby, J. Magn. Reson., 1979, 34, 317. 5 J. A. Pedersen, Handbook of EPR Spectra from Natural and Synrhetic Quinones and Quinols (CRC 6 J. A. Pedersen, J . Magn. Reson., 1984, 60, 136. 7 N . J. F. Dodd and T. Mukherjee, Biochem. Pharm., 1984,33, 379. 8 J. A. Pedersen and R. H. Thomson, J . Magn. Reson., 1981,43, 373. 9 S. I. Bailey, Chem. Austr., 1983, 50, 202. Press, Boca Raton, Florida, 1984). 10 J. A. Pople and D. L. Beveridge, Approximate Molecular Orbital Theory (McGraw-Hill, New York, 1970), p. 138. Paper 61853; Received 1st May, 1986