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1686 A.K. Boudalis et al. / Inorganica Chimica Acta 361 (2008) 1681–1688superimposed in Fig. 2. In good agreement with thosefound by Peover [24], the redox potentials (E 1/2 ) for theparent quinone, MNQ to MNQ (reversible) and MNQ to MNQ 2 (quasi-reversible) are 0.71 and 1.16 V versusSCE, respectively. The redox behaviour of the 3-carboxylsubstituted derivatives, 3 and 4, is very different. The intensityof the two quinone reduction waves has decreased, thequasi-reversible character of the second wave has almostdisappeared, and the potentials for both reductions aremore negative. In addition, two irreversible pre-peaks,E pc = 0.75 and 0.81 and E pa = 0.08 and 0.03 V for 3and 4, respectively, are present. As previously reported[25,26], the shift of the quinone reduction toward negativepotentials with regard to MNQ (Table 4) most probablyoriginates from the presence of an electron-donating 3-substituentin compounds 3 and 4. We notice that the length ofthe aliphatic chain bearing the carboxylic group has no significantinfluence on the first redox potential measuredE 1=2 ¼ 0:87 (3) and 0.86 V versus SCE (4).The occurrence of a prepeak upon addition of acids toquinones [22,25–28] and also in the case of quinones includinga substituent able to act as a proton source or to promotehydrogen bonding [29] has been reported. Itspresence associated with changes in the usual reductionwaves of quinones has been rationalized on the basis ofan ECE (e H + e ) mechanism involving protonation ofthe semiquinone radical prior to the second electron transferand/or stabilization of the electro-generated intermediatespecies by hydrogen bonding [22,25–28]. Since thesubstituent of quinones 3 and 4 bears an acidic proton, theirpeculiar reduction voltammogram may be explained byintramolecular self-protonation and hydrogen bonding:the proton source is the substituent of the quinone, andits hydrogen bonding to the electro-generated base stabilizesthe latter through formation of hetero-conjugateFig. 2. Cyclic voltammograms (scan rate 0.1 V s 1 ) obtained for compoundsMNQ (dashed line), 4 (black line) and 6 (dashed and dotted line)in acetonitrile. The concentration was 10 2 M in all cases (experimentalconditions as described in Table 4).acid–base dimers [29,30]. Indeed, consecutive scans includingthe 0 ! 0.65 ! 1 ! 0.85 ! 1 ! 1.5 ! 1Vranges show that the pre-peaks of 3 and 4 are associatedwith the Q to SQ reduction as illustrated in FigureSM.1. In the transient state, the slope of i/ p v versus log(v)is 0 (0.75–50 V s 1 range, Figure SM.2), indicating that (i)the first electrochemical process ( 0.75 (3), 0.81 V versusSCE (4)) is unique and independent of the sweeping rate,and (ii) taking into account the current measured at the electrode,the process involves most probably one e . On theother hand, in the steady state, while the intensity of the firstelectrochemical process ( 0.75 (3), 0.81 V versus SCE (4))remains constant, the current of the second electrochemicalprocess ( 0.92 (3), 0.91 V versus SCE (4)) increases significantlywith the sweeping rate (Figure SM.3), indicatingpassivation of the electrode: examination of the active electrodesurface with a microscope shows a deposit confirmingthis interpretation. It is likely that the species (probably aradical) formed upon the first reduction process in the0.75 to 0.81 V range converts into a dimer that depositsand passivates the electrode. In addition, because organicacids with medium strength act as proton sources uponboth Q–SQ and SQ to Q 2 electron transfers [26], bothreduction waves of 3 and 4 are negatively shifted. Thepotential of the pre-peak in the presence of H + (originatingfrom the carboxyl substituent) can be written asE 00new ¼ E00 2 þ E00 12þ 2:3RT ðpKQ a2FpK a Þwith pK Q acorresponding to the pK a of the quinonoid partand pK a to the acid substituent [28].The cyclic voltammograms of MNQH 2 and of MNQ inthe presence of an external proton source (0.5 M H + ,ðBF 4Þ, recorded as control experiments, confirm that apre-peak different from the oxidation of MNQH 2 , butrelated to the presence of H + , appears (Figure SM.4).The potential difference between the pre-peak and the firstquinone reduction is of the same order of magnitude in thecase of MNQ (0.19 V) and 3 (0.17 V), supporting ourassumption that the pre-peak in 3 is also related to thepresence of H + .With the goal to check the involvement of the carboxylsubstituent of 3 and 4 in the redox behaviour of these quinonederivatives, esterification of the carboxyl substituentsof 3 and 4 was carried out, yielding the 3-ester substitutedderivatives, 5 and 6, respectively. The cyclic voltammogramsof 5 and 6 were measured in aprotic and water-free(acetonitrile) media identical to that for MNQ and the carboxylsubstituted derivatives, 3 and 4. As shown in Fig. 2and Table 4, two one-electron redox waves, attributed tothe formation of Q (reversible) and Q 2 (quasi-reversible),respectively, clearly indicate that the redox behaviourof 5 and 6 is similar to that observed for the parent quinoneMNQ. The only difference is a shift in the redox potentialstoward more negative values with regard to MNQ (Table4) which, as previously reported [25,26], most probablyð3Þ
A.K. Boudalis et al. / Inorganica Chimica Acta 361 (2008) 1681–1688 1687originates from the presence of an electron-donating 3-substituentin compounds 5 and 6.5. ConclusionsThe aim of this study was to further extend our investigationson iron-quinone complexes [6], and more specificallyto prepare new ligands allowing us to locate analkylated 1,4-naphthoquinone moiety in the second coordinationsphere of iron(II) through binding of a carboxylsubstituent [31] borne by its alkyl chain. To this purpose,we have prepared, spectroscopically (IR,1 H and13 C NMR) and structurally characterized, and extensivelystudied the electrochemical properties of two quinonederivatives of organic acids, 3-3 0 -methyl-1 0 ,4 0 -dioxo-1 0 ,4 0 -dihydronaphthalen-2 0 -yl propanoic acid (3) and 4-3 0 -methyl-1 0 ,4 0 -dioxo-1 0 ,4 0 -dihydronaphthalen-2 0 -yl butanoicacid (4), derived from MNQ. An improved purificationmethod allowed us to obtain pure 3 and 4 in high yield,and subsequently to grow single crystals allowing to determinetheir molecular structure. The exhaustive electrochemicalstudy of 3 and 4 led us to prepare and fullycharacterize their ethyl esters, 5 and 6, respectively. Thiselectrochemical study has evidenced that the redox behaviourof the 3-carboxyl substituted derivatives 3 and 4 isvery different from that of MNQ, while their esters 5 and6 have the typical redox behaviour of 1,4-naphthoquinoneslike MNQ. The origin of this difference has been elucidated:the presence of a substituent able to act as a protonsource, and to promote hydrogen bonding, associated withchanges in the usual reduction waves of quinones has beenrationalized on the basis of an ECE (e H + e ) mechanisminvolving intramolecular self-protonation of the semiquinoneradical prior to the second electron transfer and stabilizationof the electro-generated intermediate species,probably an hetero-conjugate acid–base dimer, by hydrogenbonding [22,25–30]. It is then expected that coordinationof the (deprotonated) carboxo substituent of 3 and 4to a metal centre may yield an alkylated 1,4-naphthoquinonemoiety in the second coordination sphere of iron(II)showing the typical redox behaviour of 1,4-naphthoquinones:the preparation and study of such metal complexesis presently underway in our laboratories.AcknowledgementsA.K.B. thanks the European Community for partialsupport through a doctoral grant within the frameworkof the TMR contract FMRX-CT980174. We thank Dr.Yannick Coppel, Dr. Penelope Liatsi and Dr. MontserratRodriguez for helpful discussions.Appendix A. Supplementary materialCCDC 258106 and 258107 contain the supplementarycrystallographic data for 3 and 4. These data can beobtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge CrystallographicData Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with thisarticle can be found, in the online version, atdoi:10.1016/j.ica.2007.02.027.References[1] (a) R.H. Thomson, Naturally Occurring Quinones, Academic Press,New York, 1971;(b) R.A. Morton, Biochemistry of Quinones, Academic Press, NewYork, 1965.[2] P.J. O’Brien, Chem. Biol. Interact. 80 (1991) 1.[3] (a) P. Zuman, Substituent Effects in Organic Polarography, PlenumPress, New York, 1967;(b) S. Patai, The Chemistry of the Quinonoid Compounds, Wiley,London, 1974.[4] (a) T. Iyanagi, I. Yamazaki, Biochim. Biophys. Acta 216 (1970) 282;(b) M. Nakamura, I. Yamazaki, Biochim. Biophys. Acta 267 (1972)249.[5] R.M. Buchanan, C.G. Pierpont, Coord. Chem. Rev. 38 (1981) 45.[6] (a) P. Garge, S. Padhye, J.-P. Tuchagues, Inorg. Chim. Acta 157(1989) 239;(b) P. Garge, R. Chikate, S. Padhye, J.-M. Savariault, P. de Loth, J.-P. Tuchagues, Inorg. Chem. 29 (1990) 3315.[7] (a) J. Deisenhofer, O. Epp, K. Miki, R. Huber, H. Michel, J. Mol.Biol. 180 (1984) 385;(b) J. 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