252
J. Phys. Chem. 1994,98, 252-258
Novel Redox Behavior of [SIRadialenes Substituted with 1,3-Dithiol Groups Kenji Kano’ Gifu Pharmaceutical University, 5-6-1 Mitahora- higashi, Gifu 502, Japan
Toyonari Sugimoto,’ Yohji Misaki,’ Tajio Enoki, Hitoshi Hatakeyama, Hideaki Oka, Yuka Hosotani, and Zen-ichi Yoshida’ Department of Synthetic Chemistry, Kyoto University. Yoshida, Kyoto 606, Japan Received: July 7 , 1993; In Final Form: October 11, 19930
Only one pair of reversible waves involving a net four-electron transfer has been observed in the redox process of pentakis( 1,3-benzodithiol-2-ylidene)cyclopentane(la), a potent electron-donating molecule. This is the first case of a single-wave four-electron transfer with only one macroscopic redox site in organic redox systems. Normal explicit difference digital simulation analysis assuming Nernstian response has been carried out to elucidate the detailed redox process of la. Our studies have shown that l a mainly follows a two-step twoelectron transfer and is converted to the tetracation (la4+)via the dication (la2+). The estimated redox potentials for the la/la2+ and la2+/la4+couples are 0.394 f 0.003 and 0.386 f 0.003 V vs SCE,respectively. Thus three electrons are transferred at virtually no cost in energy after the first electron is transferred. The 13C N M R study has revealed that la4+ has a unique electronic structure of cyclopentadienide substituted with five 1,3dithiolylium groups. Two factors play important roles in the unusual multiple electron transfer with enhanced interaction: (1) preferential twist of all five 1,3-dithiolylium moieties from the central five-membered ring so as to decrease the intramolecular Coulomb repulsion and steric hindrance; (2) the aromatic stabilization arising from the contribution of the cyclopentadienide structure in la4+.
Introduction Multiple electron transfers have been the subject of several papers.*-’o The multiple electron transfer A neA” is usually considered to involve a sequenceof n discrete one-electron steps with a redox potential corresponding to each pair of the successive states ( E j O , j = 1-n):
+
A+e*A-
A’-
+ e * A3-
El0
E30
A(*l)-+e+A*
En0 Such n-electron-transfer systems can be basically classified into the following threecases. The first caseconcernselectron transfers of molecules containing n identical noninteracting redox centers (case I).Is3 This case might be often encountered in redoxaccessible polymers. The reversible voltammetric wave resulted from such a situation has the same shape as that obtained with the corresponding molecules containing a single center, although the magnitude of the current is enhanced by a factor of n. As Bard and colleagues have pointed out,’ each step is progressively more difficult, and El0 is defined statistically as
El0 = Eo - (RT/F‘)lnb/(n - j + l ) ] (1) where Eo is the standard redox potential of the system (or the average of El0, EzO, ..., and En0). The second case involves repulsive (electrostatic) interaction in a sequenceof one-electron transfers (case II).l*437When this situation governs successive electron transfers, E j 3 are separated by larger potentials than predicted by eq 1 Therefore, their voltammograms often show multiple separated steps, and the successive intermediates are I
Abstract published in Advance ACS Absrracrs, December 1, 1993.
evidently observed. The third case involves enhanced interaction which makes addition (or removal) of subsequent electrons easier (case 111). These EIO’sare closer together compared with that predicted by eq 1, or are inverted (El” < EzO < < En0 for reduction). The Nernstian current-potential response of case I11 has a sharper shape compared with that of case I. For this case to be distinguished from case I, conformational change or reversible chemical reaction must occur. Two examples of case I11 are ( T ~ - C ~ M ~ ~ complex8 ) ~ R U I Iand quinones.9JO The former exhibits a two-electron reduction with inverted one-electron redox potentials due to the arene hapticity change from 76 to q4. In the latter example, the redox reaction of quinones is coupled with reversible proton transfer in aqueous media. In biochemistry this would be known as an allosteric interaction. In this paper we report the first organic compound with only one macroscopicredox site exhibiting one pair of reversible redox waves involving a four-electron transfer. This compound is pentakis( 1,3-benzodithiol-2-ylidene)cyclopentane, also known as 1,3-benzodithiol[Slradialene (la)),11J2 one of a series of electrondonating [n] radialenes ( n = 3-6) synthesized in our laboratory.I3-I6 All radialenes undergo facile multielectron sequentialoxidation by attaching the potent electron-donating1,3-dithiol groupsI7 at the exomethylene positions. For tetrakis( 1,3-benzodithiol-2-ylidene)cyclobutane(2)14and tetrakis( 1,3-benzodithiol2-y1idene)cyclopentanone (3),15 the first redox wave of both radialenes involves a net two-electron transfer, while at more positive potential the second wave of 3 also involves a net twoelectron transfer. In the case of la, however, four electrons are essentially reversibly transferred at effectively the same potential owing to some enhanced interaction (case 111), despite the structural resemblance to 3. Voltammetric study has been also performed on unsymmetric [Slradialenes ( l b and IC)in which one 1,3-benzodithiol-2-ylidenegroup in la is replaced by a 4,sdimethylthio-l,3-dithiol-2-ylidenegroupin lb, or a 4,s-dimethyl1,3-dithiol-2-ylidenegroup in IC. The unusual electrochemistry of la is discussed in terms of the unique electronic and structural properties of la2+ and la4+.
0022-3654/94/2098-0252So4.50~00 1994 American Chemical Society
...
Novel Redox Behavior of [5]Radialenes
Ir
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 253
lb
1c
Experimental Section Melting points were determined with a Yanaco MP-micromelting-point apparatus without any correction. Microanalyses were performed at the Microanalysis Center, Kyoto University. IH and 13C NMR spectra were measured using a JEOL JNMGX 400 (operating at 400 MHz for 1H and at 100 MHz for l3C) or JEOL FX 9OQ (operating at 90 MHz for 1H) FT NMR spectrometer. Chemicalshifts are reported in 6 units with respect to tetramethylsilane (TMS) at 0.00 ppm. Infrared spectra were recorded on a Jasco A- 102 or Horiba FT-300 spectrometer. Mass spectra were recorded on a JEOL JMX-DX 300 or Shimadzu QP- 1000 instrument. UV-vis absorption spectra were recorded on a Hitachi 340 or U-3410 spectrometer. Materials. Each solvent used in this experiment was purified by distillation over the followingdrying reagents: tetrahydrofuran (THF) from LiAlH4, methylene chloride (CH2C12) and acetonitrile (CH3CN) from CaH2, and benzonitrile (PhCN) from P2O5. The PhCN used for cyclic voltammetry was further treated with basic alumina (ICN active W-200 B; activity super I). Tetraethylammonium perchlorate (TEAP) was used after recrystallization from ethyl acetate several times. The syntheses of 2,3, and ethanediylidene-2,2’-bis(1,3-benzodithiol)(4) were described previously.l4J5J7 Tetrakis( 1,3-benzodithiol-2-ylidene)cyclopentanol(5). To a solution of 3 (610 mg, 0.89 mmol) in THF (60 mL) was added LiAIH4 (117 mg, 3.1 mmol) at 0 OC under argon atmosphere. After the reaction mixture was stirred for 2 h, the excess LiAlH4 was decomposed with cold water. The reaction mixture was extracted with benzene and dried over anhydrous NatS04, and then the solvent was evaporated inuacuo. The residue was purified by column chromatography on deactivated neutral alumina and eluted first with benzene and then with benzene-AcOEt (15:1, v/v) to give 5 (552 mg, 0.80 mmol) as yellow crystals in 93% yield: mp 220 OC; IR (KBr) 3540,3060,2930,1575,1555,1450, 1435, 1290, 1265, 1165, 1125, 1035, 1010 cm-I; ‘H NMR (90 MHz, CDCb) 6 7.50-7.00 (m, 16H). 5.09 (s, lH), 4.97 (s, 1H); ”C NMR (100 MHz, CDCl3) 6 137.8, 137.0 , 136.7, 125.9, 125.69, 125.67, 125.57, 125.54, 124.4, 121.9, 121.73, 121.65, 75.4; MS m / z 686 (M+) 670,486. Anal. Calcd for C33H180S~: C, 57.70; H, 2.64. Found: C, 57.42; H, 2.68. Tetrakis( l,fbenzodithi0l-2-ylidene)cyclopentyliumTetrafluoroborate (6). To a solution of 5 (324 mg, 0.47 mmol) in ether (100 mL) and benzene (10 mL) was added HBF4.0Et2 (217 mg, 1.34 mmol) in ether ( 5 mL) at -70 OC. After stirring for 2 h at -78 OC, the reaction mixturewas warmed up to room temperature. The precipitate was filtered off and washed with ether to give 6 (339 mg, 0.45 mmol) as blue crystals in 95% yield: mp 240-241 OC; IR (KBr) 1565,1450, 1320, 1200,1160,1110, 1085, 1070, 1035,80O~m-~;UV (CH2C12) A,,,(logc) 925 (3.97),610(4.18),
492 (3.74), 383 (4.03), 325 (4.07) nm; I3C NMR (100 MHz, concentrated H2S04) 6 199.6, 146.3, 145.4, 141.9,140.7, 139.8, 134.7, 133.0, 132.7, 128.5, 128.3, 127.1, 126.8, 122.7. Pentalcis( 1,3-benzoditbiol-2-yUdene)cyclopen~ (la). To a suspension of 1,3-benzodithiol-2-yltriphenylphosphoniumtetrafluoroborate (1 16 mg, 0.20 mmol) in THF (10 mL) was added a 1.6 M hexane solution of n-BuLi (0.13 mL, 0.20 mmol) at -78 OC, and the reaction mixture was stirred for 5 min. A solution of 6 (154 mg, 0.20 mmol) in CH&N (10 mL) was added to the resultant yellow solution of phosphorane at -78 OC. After the reaction mixture was stirred for 2.5 h, triethylamine (0.28 mmol, 3.9 mmol) was added and warmed up to room temperature. The reaction mixture was extracted with benzene, and the extract was dried over anhydrous Na2S04. Then the solvent was evaporated in uacuo, and the residue was purified by column chromatography on neutral alumina eluted with benzene. The crude product was reprecipitated from CH2Clz-hexane to give l a (77 mg, 0.094 mmol) as orange crystals in 64% yield: mp 270 OC; IR (KBr) 3040,1565,1480, 1440, 1430,1285, 1260,1115, 1030,750cm-I; UV (benzene) ,A, (log c) 463 (3.92), 356 (4.76) nm; ’H NMR (90 MHz, CSsD2C12) 6 7.56-6.98 (m, 20H); ”CNMR(lOOMHz,CDC13)6 137,4,127.0,125.5,123.2,121.6; MS m / z 820 (M+). Anal. Calcd for CaHZOS10: C, 5830; H, 2.46; S,39.04. Found: C, 58.24; H, 2.35; S, 38.86. The above procedure was also applied to the preparation of l b and ICby using 4,5-dimethylthio-and 4,5-dimethyl-1,3-dithiol2-yltriphenylphosphonium tetrafluoroborates, respectively, in place of 1,3-benzodithiol-2-yltriphenylphosphoniumtetrafluoroborate. 1-(4’,5’-Bi~(m e t a y l ~ 0 ) - 1 ’ , 3 ’ - a 2 ’ - y U ) - ~ ~ ~ (1”,3”-benzodithiol-2”-yUdene)cyclopentane (lb). 55% yield; an orangesolid;mp240 OC (dec);IR(KBr) 3053,2914,1568,1479, 1446,1432,1120 cm-l; IH NMR (90 MHz, CDCl3) 6 7.38-7.04 (m, 16H), 2.42 (s, 6H); 13C NMR (100 MHz, CDCl,) 6 137.4, 137.3, 137.2, 137.1, 129.0, 128.3, 127.2, 126.6, 125.5, 123.4, 122.0, 121.8,121.7, 19.3. MS m / z 862 (M+). Anal. Calcd for C38H22SI~:C, 52.86; H, 2.57; S,44.57. Found: C, 53.02; H, 2.50; S,44.48. 1-(4’,5’-Dlmethyl- 1’,3’-cUthiol-2’-yH~)-~~tetrokls( 1”,3”benzodithiol-2”-ylidene)cyclopentane(IC). 50% yield; an orange solid; mp 240 OC (dec); IR (KBr) 3059,2914,2856,1566,1483, 1446, 1119 cm-1; 1H NMR (90 MHz, CDCl,) 6 7.66-6.94 (m, ~ ~ H ) , ~ . ~ ~ ( s , ~ H ) ; ~ ~ C N M R (137.5,137.2, ~OOMH~,CDCI~ 137.1, 128.3, 127.5, 125.41, 125.36, 123.1, 121.7, 121.6, 13.5; MS m / z 798 (M+). Anal. Calcd for C38H22S10: C, 57.1 1; H, 2.77; S,40.12. Found: C, 57.38; H, 2.64; S,39.98. Pentakis( 1,3-benzodithiol-2-ylidene)cyclopentaniumylTetrakis(tetra8uoroborate) (la4+.4BF,-). A solution of l a (20 mg, 0.024 mmol) in CH2C12 ( 5 mL) was added to a solution of NOBF, (60mg,0.51 mmol)inCH3CN(5mL)atO°C. Afterthereaction mixture was stirred for 2 h, CH2Cl2 (60 mL) was added. The precipitate was collected by filtration and washed with CHzCl2 to give la4+.4BF4- (26 mg, 0.022 mmol) as a red solid in 91% yield: mp 240 OC (dec); IR (KBr) 1470,1450,1355,1115,1085, 1060, 1040 cm-I; UV (CH2C12) ,A, (log c) 505 (4.49), 471 (sh, 4.83), 381 (4.32), 332 (4.16) nm; IH NMR (90 MHz, CDCl3) 6 8.53 (dd, lOH), 7.99 (dd, 10H); 13C NMR (100 MHz, CDC13) 6 187.8, 146.0, 132.3, 127.6, 123.7. Anal. Calcd for C ~ H ~ O S I O C, B ~41.12; F ~ ~H, : 1.73. Found: C, 40.95; H, 1.88. The above procedure was also used for preparation of the other tetracation salts. 1-(4’,5’-Bis(mtbylthi0)-1 ’ , 3 ’ - ~ ~ 1 - 2 ’ - y U ) - ~ ~ 5 ~ ~ ( 1”,3”-benzoaithiol-2”-~lldene)eyclopentp~l Tetrakia(kxrfluorophosphate) (lb4+.4PFs-). 96% yield; a red solid; 1H NMR (90 MHz, CDC13)6 8.71-8.45 (m, 8H), 8.16-7.96 (m, 8H). 2.84 (s, 6H); l3C NMR (100 MHz, CDCl,) 6 187.3, 186.8, 173.4, 158.6, 146.1, 144.8, 132.4, 132.1, 127.6, 127.3, 124.8, 122.3, 120.2,20.3. Anal. Calcd for C ~ ~ H Z Z S I C, ~ P31.62; ~ F ~H,~ 1.54. : Found: C, 31.49; H, 1.65.
Kano et al.
254 The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 1-(4’,5’-DbtbyC 1’,3’-dithiol-2’-ylkk)-2,3,4,5~( l”$”b e n z o d i t b i 0 l - 2 ~ ’ - y ~ d e w ) c y c l oTetrnkis(hexafluor0~~~yl phosphate) (lc4+*4PF6-). 86% yield; a red solid; IH NMR (90 MHz, CDC13)6 8.69-8.43 (m, 8H), 8.20-7.91 (m, 8H), 2.74 (s, 6H); ‘3CNMR(l00MHz,CDC13) 6 187.3,187.1,173.6,158.5, 145.9, 144.8, 132.4, 132.2, 127.6, 127.3, 124.2, 122.2, 121.0, 15.1. Anal. Calcd for C38H22SI~4F24:C, 33.10; H, 1.61. Found: C, 32.67; H, 1.60. Electrochemical Measurements. Cyclic voltammetry and controlled potential electrolysis were performed with a Yanagimoto P-lo00 potentiostat in conjunction with a Graphtec WX2400 X-Y recorder. In potential step chronoamperometry and rotating disk voltammetry, respectively, a Hokuto Denko HB104 function generator and a Yanagimoto SM6S2 synchronous motor were attached to the above system. All electrochemical measurements were carried out at 25.0 f 0.1 OC with a threeelectrodesystem. A positive feedback circuit was used for ohmic drop compensation in cyclic voltammetric measurementsof scan rates faster than 100 mV s-I. The potentials were measured against a double-junction saturated calomel reference electrode (SCE). The working electrode was a planar platinum electrode (1-mm diameter sealed in a soft glass tube with a diameter of 6.5 mm) and the auxiliary electrode consisted of a coiled platinum wire. The platinum working electrode was polished with 0.05I m alumina powder (Union Carbide) before each electrochemical measurement. In controlled-potential electrolysis, two coiled platinum wire electrodeswere used as both the working electrode and the counter electrode and were separated with sintered glass in a two-compartment cell. The solvent for the electrochemical experiments was PhCN containing 0.1 M TEAP as a supporting electrolyte. The other details have been described in a previous paper.7 Digital Simulation.The normal explicit finite difference digital simulation technique1*J9 was employed to calculate the Nernstian voltammetric waves for one-step, two-step, and four-step n-electron transfers. The program was checked first by comparing its output with theory. For the m-step n-electron transfers (m = 2, 4), thecheckwasdonefor twocases. Inonecase, thefirstoxidation potential was set so as to be sufficiently more positive than the other@), where the voltammogram is exactly that of a one-step n-electron transfer. In the other case, thejth oxidation potential was set sufficiently more positive than the (j+ 1)th one (j = 1, 2,3), where a voltammogram is exactly that of a one-step (mX n)-electron process. In considering nonlinear diffusion effects (edge effect) in slow-scan voltammetry, mass transfer was assumed to be governed by spherical diffusion. The radius of the postulated spherical electrode (r) was set as a fitting parameter, while the electrode area was fixed to the geometric one (7.85 X cm2). A damping Gauss-Newton method was combined to the above program for a nonlinear least-squaresmethod.20 The final check of the program was performed by applying it to the analysis of a cyclic voltammogram of cytochrome c3 in the 1iterat~re.l~ The refined parameters were practically identical with the reported ones. The other procedures of the digital simulation and the curve-fitting analysis have been described in a previous paper.21
Results and Discussion
Synthesis. The symmetrical (la) and two unsymmetrical ( l b and IC) 1,3-benzodithiol[Slradialenea were obtained according to Scheme 1. Thus, the reduction of 3 with an exof LiAlH4 in THF at 0 O C gave the correspondingalcohol (5) in 93% yield, which was converted to tctrakis( 1.3-benzodithiol-2-ylidene)cyclopentylium tetraflumoborate (6)by reaction with HBF4-OEt2 in 95% yield. When 6 was reacted with either 1,3-benzodithiol-, 4,s-dimethylthio-1,Edithiol-, or 4,5-dimethyl-l,3-dithiol-2-y1triphenylphosphoranes in THF at -78 OC, followed by reaction with an exct88 of triethylamine, the expected la, Ib, and IC were isolated as orange solids in respective yields of 64, 55, and 50%.
SCHEME 1
9
5
The 13C NMR spectra of l b and ICshow nonequivalence in chemical shifts of two different 1,3-benzodithio1-2-ylidene groups, although the difference is very small. Compounds 1a-C were oxidized with NOBF4 or NOPF6 (10 equiv) in CH3CN at 0 O C , and the corresponding tetracations (la4+,lb4+,and IC‘+) were successfully obtained as the BF4- or PF6- salt. Electrochemistry of la. Our previous papers have shown that 4, an ethylene analogue of tetrathiafulvalene, exhibits two successive and reversible one-electron waves,I7 and that 3 (a compound structurally related to la) as well as its octacarbomethoxy derivativeundergo two well-separated reversible twoelectron transfers.15 In contrast, the corresponding [4]radialene (2) and itsoctacarbomethoxyderivativeexhibita reversible, singlewave, two-electron transfer followed by two irreversible oneelectron tran~fers.1~ In marked contrast to 2-4, la exhibited only a single reversible cyclic voltammogram in PhCN at 0.390 V vs SCE in the potential range from -0.4 to 1.7 V. Representative voltammograms of l a and the related compounds are shown in Figure 1. The anodic peak current of la increased practically linearly with the square root of the scan rate ( v ) up to 100mV s-1. Chronoamperometryduringa potential step from 0.2 to 0.5 V revealed a diffusion-controlled oxidation process: the current (i) decreased linearly with a decrease in the reciprocal of the square root of time ( t ) up to at least t = 6 s. Rotating disk voltammetrygave a sigmoidal curve. The limiting current ( i b ) increasedlinearlywith thesquare root of the rotation speed
(@I.
To determine the total number (n) of electrons involved in the singlewaveredox process of la, potential stepchronoamperometry and rotating disk voltammetry were performed for 2-4 as reference compounds. All these compounds exhibit a diffusion-controlled process for each wave. Values of nD1I2 were evaluated by potential-step chronoamperometry on the basis of Cottrell equation: nD’l2 = i(?rt)’/2/(FAC) where D, A, and C are the diffusion coefficient, the electrode area, and the bulk concentration,respectively.22 Values of nDZ/3 were determined by rotating disk voltammetry on the basis of Levich equation: n d 1 3 = i,i,u’16/(0.62FAo112~ (3) where u is the kinematic viscosity.23 In this work, the geometric surface area (7.85 X 10-3 cm2) was used for A, while u was calculatedtobe 1.3 X 10-2cm2~-~fromtheviscosity(1.33 X le2 g s-1 cm-1) and density (1.01 g cm-3) of PhCN.2‘ Calculated values of nD112 and n P / 3 are summarized in Table 1. By consideringthe two-step one-electron transfer of 4,” its D value can be estimated to be 2.82 X 10-6 cmz s-1. The nD1l2and n D 1 3 values of 3 for the first and second steps are about 2 times the corresponding ones of 4. This supports the conclusion that 3 follows two separated two-electron transfers as reported previously,ls although its cyclic voltammogram (Figure 1) stcms to
Novel Redox Behavior of [51 Radialenes
The Journal of Physical Chemistry, Vol. 98, No. 1 , 1994 255
71 b
IS"*
transfer, although the upward deviations in the peak separation and the slope can be partly related to the edge effect in the mass transfer26 and electrode kinetics, respectively. Digital Simulationof the Redox Process of la. To extract the redox potential for each electron-transfer step of la, a digital simulation technique combined with a nonlinear least-squares method was applied. Here we reasonably presume that the voltammograms at Y = 5-10 mV s-l are governed by thermodynamics as well as mass transfer. Current thinking suggests that electrons are transferred one at a time microscopically.l-10 Therefore, a four-stepone-electron model was applied first. The observedvoltammogramswere well reproduced by this model: -e
la
-v e
0
V
0.8
1.2
-e
+C
+C
E3'
-e
+ la4+
(4)
+C
E 4 '
An example is given in Figure 2 as the solid line. Refined redox potentials were Elo = 0.43 V, E2' = 0.35 V, E30 = 0.45 V, and Ed0 = 0.32 V, although the standard deviations were as large as 0.2 V. The uncertainty in the estimated potential is attributable to significant inversion of El0 and E 2 O (Elo > E20) and of E30 and E4' (E3O > E4°);27the second and fourth electron transfers proceed much more easily than do the first and third ones, respectively. In a practical sense, such a situation should be regarded as a two-step two-electron transfer:zS
f L 0.4
E:".
-e
la*++ la2+ F= ia*3+
1.6
E I V vs. SCE Figure 1. Cyclic voltammograms of (a) la, (b) la4+,(c) 2, (d) 3, and (e) 4in PhCN containingo. 1 M TEAP at 25 O C . Experimentalconditions arc (a) C = 2.50 X 10-4 M,Y = 50 mV s-l, scns = 0.8 gA;(b) C = 2.50 X 10-5 M,Y = 50 mV s-1, sen8 = 0.08 pA; (c) C =: 3.43 X 10-4 M,Y 100 mV s-1, sen8 = 0.8 pA; (d) C = 2.95 X 10-4 M,Y = 50 mV s-I, Scns = 0.4 MA;(e) C = 6.54 X 10-4 M,Y = 50 mV s-I, sens = 0.8 pA.
be quasi-reversible. The D value of 3 was calculated to be 2.62 X 1o-S cm2 s-1. In a similar way, the first wave of 2 was proven to involve a two-electron conversion. Each of the succeeding irreversible waves of 2 at 1.19 and around 1.6 V (at 100 mV s-l) appears to involve a one-electron oxidation. The nD'l2 and n P l 3 values of l a are about 2 times those of the first two-electron wave of 2 or 3, The structural resemblance of l a to 2 and 3 supports the assumption that the D value of l a is close to that of 2 or 3. Therefore, then value of l a is probably 4, and the D value is 2.61 X 1o-S cm2 s-1. To verify the above conclusion, controlled-potentialelectrolysis of 10 mL of 3.0 X 1 W M l a was carried out at 1.0 V with vigorous stirring. The electrolytic charge corresponded to an n value of 3.76, during which the oxidation current decayed down toca. 5% of the initialvalue. Taking the electrolysis factor (95%) into account, n can be corrected to 3.96. Further evidence of the overall four-electron transfer of l a is given by the fact that the (reductive) cyclic voltammogram of la4+ is practically identical with the (oxidative) voltammograms of l a in shape and peak potentials as shown in Figure 1. To our knowledge, this is the first observation of a single-wavefour-electron transfer in organic redox systems. At Y < 20 mV s-l, the peak separation of the cyclic voltammograms of l a was 25 f 1 mV and was practically independent of Y. This value is obviouslylarger than that expected for a direct one-step four-electron process (=59/4 = 14.8 mV29. Furthermore, the value of the slope in a logarithmic plot of the rotating disk voltammogram of l a (E vs log[i/(ilim- i ) ] , where E is the electrode potential) was 35 mV/decade at o = 600 rpm. This is also larger than expected for a one-step four-electron process ( ~ 1 4 . mV/decade23). 8 These results suggest a multistep electron transfer of la rather than a direct one-stepfour-electron
-2e
-2c
+2e
+2c E d
l a + la2+ F= la4+ Em1
(5)
Simulationon the basis of the two-step two-electron model yielded regression curves practically identical with those obtained for the four-step model. The two-electron redox potentials calculated from three independent analyses were 0.394 f 0.003 V for Eml and 0.386 f 0.003 V for E,2. The successful fit with a two-step model supports the assumption that Elo > EzOand E 3 O > Edo. Furthermore, the Emland Em2values fall, respectively, very close tothevaluesof (Elo E2O)/2and(E3' + E40)/2. In thisanalysis, the two-step model is useful for precisely calculating (Elo + Ez0)/2 and (E3' + E4O)/2. In contrast, the calculation based on the direct one-step four-electron model resulted in a poor fit (Figure 2), as seen from the discussion in the previous section. When one assumes that l a has four noninteracting oxidizable centers (case I),) Elo are expected to be 0.3545,0.3796,0.4004, and 0.4255 V for j = 1-4 by taking Eo as 0.390 V on the basis of eq 1 (note here that -(RT/F) in eq 1 should be replaced by +(RT/F) for the oxidation process). Although accurate values of Ejo could not be obtained, the present analysis indicates an inversion in the potentials (El0 > EzO,E 3 O > E4', and (Elo EZ0)/2> (E30 + E4')/2). Therefore, neither case I nor case I1 explains this unusual redox behavior. This conclusion is supported by the fact that the slope of the logarithmic plot (35 mV/decade at w = 600 rpm) is much smaller than that expected for case I (59 mV/decade3). Thus the important conclusion here is that some enhanced interactions (case 111) should participate in the electron transfer of la. Electrochemistry of l b and IC. The novel redox behavior of l a was illustrated by comparing the redox properties of unsymmetrical derivatives of l a (lb and IC). Cyclic voltammograms of l b and IC are given in Figure 3. As in the case of la, l b exhibited a single reversible wave at 0.366 V, while ICexhibited partially overlapped double waves at 0.28 and 0.36 V. By comparing the nD2l3 values of Ib and IC (obtained by rotating disk voltammetry) with that of la, the n value of l b and IC was calculated by assuming their diffusion coefficients are close to that of la. The results are n = 4 for a single wave of l b and n = 2 for each of the double waves of IC. Cyclic voltammograms of the tetracations, lb4+ and IC)+,were practically identical with those of Figure 3 in shape and peak potentials.
+
+
Kano et al.
256 The Journal of Physical Chemistry, Vol. 98, No. I, 1994 TABLE 1: Values of nDI2 and nD/3 of la, 2, 3, and 4 wave
2
la
-
first second third
6.52(0.20
first second
7.48
3
-
103n~l/2 5 3.20 (0.20 0.6) 4.91 (0.20- 1.25) 6.82(0.20 1.65)
0.50)
4
--
--
3.17 (0.20 0.7) 6.56(0.20 1.45)
1.60(0.20 0.55) 3.40(0.20 0.80)
3.82 7.64
2.04 4.08
1040/3b
3.67 C
Estimated by potential step chronoamperometry (see eq 1) in cm S-I/~. The values in the parentheses denote the step potentials (in V vs SCE). The second and third waves were most likely complicated by the adsorption of the (irreversible) three-electron oxidized product on the electrode surface. a
* Estimated by rotating disk voltammetry (see eq 2) in cm4/'
TABLE 2 Redox Potentials of la-c Calculated by DTgital Simulation of Two Mechanisms: (1) Two-step Two-Electron and (2) Four-Step One-Electron Transfers'
..,i
compound mechanism la two-step four-step l b two-step four-step l c two-step four-step
-0.21 0.2
0.3
0.5
0.4
0
E I V vs. SCE
Figure 2. Background-corrected cyclic voltammogram of la (2.01 X 10-4 M) at Y = 5 mV s-1 (0). The solid line represents the regression curve calculated on the basis of the reversible four-step one-electrontransfer model using the following data: Elo = 0.434 V, Ezo = 0.353 V, EJO = 0.453 V, E 4 O = 0.317V, C = 1.76 X 10-4 M,r = 3.5 X lt3 cm. The residual sum of squares (RSS)is 3.44 X A2.A practically identicalcurve was obtained based on the reversibletwo-steptwo-electron model with E,I = 0.394V, E,? = 0.386V, C = 1.75 X 10-4 M,and r = 3.5 X cm (RSS= 2.88 X 1 t I 5A2). represents the regression curve calculation based on the reversible one-step four-electron transfer mechanism using the following data: Eo = 0.390V, C = 1.84 X 10-4 M, r = 2.7 X 10-3 cm with an RSS of 1.38 X A2.In all the simulations, the calculation interval was 1 mV and 2.6 X 1V cm2 s-1 was used for the D value.
+
Em1
Ed
El0
EzO
0.394
0.386
0.43
0.35
0.2
0.3
E / V vs.
0.4
0.5
SCE
Figure 3. Cyclic voltammograms of (a) lb and (b) ICat Y = 5 mV s-I. Experimental conditions are (a) C = 1.69 X 10-4M,sens = 0.08 pA; (b) C = 1.13 X 10-4 M,sens = 0.04 pA.
The thermodynamics of l b and ICwere also studied by the digital simulation technique. As in the case of la, the voltammograms of l b and ICcan be reproduced accurately by both the four-step and two-step models, Simulated curves obtained using the two models are not distinguishable from each other. Refined
0.45
0.32 0.358
0.374 0.43
0.32
0.42
0.29 0.360
0.282 0.32
0.24
0.42
0.30
V vs SCE.
redox potentials are summarized in Table 2. The error associated with the calculation of the four one-electron redox potentials is large. This error is attributable to significant inversion in the potentials (E,' > E2' and E3' > E4') as illustrated by accurate simulation based on the two-step model. The values of (E,' + Ez0)/2 and (E3' + E4")/2 can be determined (with standard deviations of 1-3 mV) to be E,1 and Em2,respectively, using the two-step model. The unusual and common feature of 1,3benzodithiol [5]radialenes (1a-c) is the ability to transfer three electrons at virtually no cost in energy after the first electron is transferred (case 111), though E,2 - Emlof ICis somewhat larger compared with those of l a and lb. Energetics of Redox Processes of la-c. In the [5]radialenes 1, the radical intermediates 1*+and l*3+ should be thermodynamically unstable due to the disproportionation reactions: 21'+ 1 + 1 2 + and 2l*3+ * 12+ l 4 + . In contrast, the dication intermediate 12+is stabilized by the comproportionation reacton l a + la4+* 21a2+. The equilibrium constant ( K ) , defined by K = exp[(2F/RT)(Em2 - Eml)],is 0.54 for la. The potential of the first two-electron redox process, Eml,decreases by 0.02 V in lb, and decreases by 0.09 V in IC (Table 11). This sensitive change clearly suggests participation of the replaced 1,3-dithiol group in the redox process. That is, increasing the electrondonating property of the substituents (R) attached on the 1,3dithiol group (Le., CH3 > SCH3 > benz0,2*-~~) shifts the Em1 value to less positive potential. The first two-electron oxidation occurs preferentially at the ethanediylidene(1,3-benzodithiol)(R,R-1,3-dithiol)(R, R = SCH3, CH3, benzo) moietiestogenerate the corresponding two 1,3-dithiolyliumgroups. If there were no interaction between the 1,3-benzodithiol moieties of the symmetrical compound la, EzOshould be more positive than El' by 25.1 mV (case I),3 The Coulomb repulsion would cause a further increase in Ezo (case I I ) . I s ~ .Therefore, ~ some conformational change must occur to lead to this enhanced interaction (case 111). The molecular structure of 1is still unknown since a single crystal is not available for X-ray structure analysis. However, it is reasonable to assume that the central five-membered ring has a half-chair conformation and the adjacent 1,3-dithio1-2-ylidene groups are twisted from the central ring, judging from the X-ray structure of 315 and pentakis(isopropy1idene)~yclopentane.~After the first two-electron oxidation, the resultant two 1.3-dithiolylium groups would be further twisted relative to the central cyclopentene
+
0.1
E4'
E3O
Novel Redox Behavior of [51Radialenes
The Journal of Physical Chemistry, Vol. 98, No. 1, 1994 257
ring in order to avoid the Coulomb repulsion between the positive charges, as depicted below. This conformational change would also relax the steric hindrance between the two adjacent 1,3dithiol-Zylidenegroups. Furthermore, the resultant cyclopentene structure can be partially stabilized by the *-conjugation. Presumably,all these effects bring about theenhanced interaction in the first two-electron transfer. The fact that E30 > E40 is also explained by such effects. A similar situation seems to apply to 2 and 3, which undergo an essentially reversible two-electron oxidation at effectively the same potential: R
R
M
"UR
TABLE 3: 'Jc Chemical Shifts of la-c, la4+-4BF4-, 1b4+*4PF6-,1C4+*4PF6-,m d Reference 1,3-Mthioli~01101~' C-2(2',2")* C-4,5(4',5',4",5")* C-6(6')* la 127.0 137.4 123.2 la4+.4BF,187.8 146.0 123.7 lb 129.0 137.4 123.4 128.3 137.3 122.0 127.2 137.2 121.8 137.1 121.7 1b4+*4PF6187.3 158.6 124.8 186.8 146.1 122.3 173.4 144.8 120.2 le 128.3 137.5 123.1 127.5
k4+*4PF6-
UF iH In contrast to E,l, the second two-electron redox potential, as sensitive to the variation of the substituent groups. This can be readily understood considering that the oxidizable part in 12+ is the common tris( 1,3-benzodithio1-2-ylidene)cyclopentene moiety, which has almost the same electronic structures in la-c. Thus the double-wave characteristics of IC are attributable to a much lower redox potential of the 4,sdimethyl-1,Edithiolgroupcompared with 1,3-benzodithiolgroup. It is noteworthy that E,z of 1 is much less positive than ( E 3 O + E40)/2 of 2 (-1.4 V as the peak potential) and Em2 of 3 (1.20 V). This can be explained in terms of the unique electronic structure of l 4 + . The cyclopentadienide structure in l 4 + is aromatic in nature (as shown below), which energetically enhances the generation of 14+.In contrast, for the tetracations of 2 and 3, the cyclobutadiene and cyclopentadienone structures appear in the central rings. Both of these electronic structures are antiaromatic and the resultant z4+and 34+are not as stable as 14+. Therefore, the aromatic stabilization of 14+is responsible for the marked shift in Emz to less positive potential. In addition, all five 1,3-dithiolyliumgroups tend to be twisted from the central five-membered ring so as to decrease the intramolecular Coulomb repulsion and steric hindrance, as in the case of the first twoelectron transfer.
-2e
14+
12+
121.7
121.6
158.5
124.2 122.2
187.3 187.1 173.6 184.8
145.9 144.8 145.6
174.5
155.9
173.3
155.2
121.0
EF;
Em2, is not
R R \=(
137.2
137.1
".I;kH
"'
clo;
Measured in CDCl3 for la+, otherwise in CD3CN.
'"04 R
symmetry is slightly lower than Cs, since there is a small difference in the chemical shifts of C-2, C-4, C-5, and C-6 carbons in the two 1,3-benzodithiol-2-ylidene groups adjacent to and apart from the 4,5-dimethylthio-l,3-dithiol-2-ylidene group in l b or the 4,sdimethyl-1,3-dithiol-2-ylidenegroupin IC. Obviously, the carbons of the 1,3-dithiol groups in 14+ have chemical shifts quite close to those of the corresponding 1,3-dithiolylium ions, Le., la4+ (1 87.8,146.0), lb4+(1 87.3,186.8,146.1,144.8) and 1c4+(187.3, 187.1, 145.9, 144.8)/1,3-benzodithiolylium ion (184.8, 145.6); lb4+(1 73.4,158.6)/4,5-dimethylthio-l,3-dithiolyliumion (174.5, 155.9); lc4+ (173.6, 158.5)/4,5-dimethyl-1,3-dithiolyliumion (173.3 , 155.2). This implies the presence of one positive charge on each of the five 1,3-dithiol groups in 14+.Furthermore, no lower field shift is observed for the C-6(6') carbons in a comparison of14+and1: la4+(123.7)/1a(123.2);lb4+(124.8, 122.3,120.2)/ l b (123.4, 122.0, 121.8, 121.7); lc4+ (124.2, 122.2, 121.0)/lc (123.1,121.7,121.6). This might contradict the expectation that, in general, the carbons attached at the 2-position of 1,3dithiolyliumion are shifted to lower field. However, if one negative charge resides in the central five-membered ring, then this abnormal chemical shift change is understandable. From the above results, 14+indeed has a unique structure of cyclopentadienide substituted with five 1,3-dithiolylium groups. Concluding Remarks
P
34+
1 C NMR Spectra of la'+, lb'+, and IC'+. Details of the electronic structures of 14+'swere obtained from the I3C NMR spectral data. The observed chemical shifts of these compounds, as well as those of 1and the 1,3-dithiolyliumreference salts, are summarized in Table 3. The 13CNMR spectrum of la4+*4BF4in CDBCNshowed five signals at 6 123.7, 127.6, 132.3, 146.0, and 187.8, indicating that la4+is of Cs symmetry on the NMR time scale. For lb4+.4PF6- and lc4+.4PF6-, the molecular
The single-wave, four-electron transfer of la, a first case in organic redox systems, is driven both by the aromatic stabilization of cyclopentadienide in the tetracation state and by a large twist groups from each other. This of the 1,3-benzodithiol-2-ylidene twist lowers the Coulomb repulsion between the positive charges generated in the course of the oxidation and relaxes the steric hindrance. In particular, the aromatic stabilization facilitates the second two-electron transfer process and shifts it markedly to less positive potential, resulting in the overlap with the first two-electron-transfer process. Conversely, the appearance of the
2SS The Journal of Physical Chemistry, Vol. 98, No. 1, 1994
antiaromatic cyclobutadiene and cyclopentadienone structures in the tetracations of 2 and 3, respectively, makes the processes of 2*3+- e z4+and 32+ - 2e 34+ much harder. The above two effects are particularly remarkable in the redox processes of 1,3-dithiol-substituted[nlradialenes. The present findingsuggests the importance of taking both conformational and electronic structure changes into account in order to understand the detailed redox behavior of nonplanar *-electron conjugated systems. Detailed kineticstudy of theconformationalchange is in progress. Acknowledgment. We gratefully acknowledge the support of this research by Grant-in-Aid for Scientific Research on Priority Area "Molecular Magnetism" (No. 04242104) from the Ministry of Education, Science and Culture of Japan. References and Notes M.J. Electroanal. Chem. 1973, 47, 115 . (2) Bard, A. J. Pure Appl. Chem. 1971,25, 379. (3) Flanagan, J. B.; Margel, S.;Bard, A. J.; Anson, F. C. J. Am. Chem. Soc. 1978, 100, 4248. (4) Richardson, D. E.; Taube, H. Coord. Chem. Rev. 1984, 60, 107. ( 5 ) Eberson. L. Ado. Phys. Org. Chem. 1982, 18, 79. (6) Pross, A. Ace. Chem. Res. 1985, 18, 212. (7) Kubota, T.; Kano, K.; Uno,B.; Konse, T. Bull. Chem. Soc.Jpn. 1987, 60, 3865. (8) Pierce, D.T.; Geiger, W. E. J. Am. Chem. Soc. 1989, 111, 7636. (9) Wipf, D. 0.;Wehmeyer, K. R.; Wightman, R. M.J. Org. Chem. 1986, 51, 4760. (IO) Kano, K.; Uno,B. Anal. Chem. 1993, 65, 1088. (1 I ) Yoshida, Z.; Sugimoto, T. Angew. Chem. 1988, 100, 1633; Angew. Chem., Int. Ed. Engl. 1988, 27, 1573. (1) Ammar, F.; Savhnt, J.
Kano et al. (12) Sugimoto, T.; Misaki, Y.; Yoshida, Z.; Yamauchi, J. Mol. Cryst. Liq. Cryst. 1989, 176, 259. (13) Sugimoto,T.;Mi~,Y.;Kajita,T.;Nagatomi,T.;Ymhida,Z.Angnu. Chem. 1988.100. 1777: Annew. Chem.. Int. Ed. E n d . 1988.27, 1711. (14) Sugimoto,T.;Awaj~H.;Misaki,Y.;Yoshida,-Z.;Kai,Y.;Nakagawa, H.;Kasai, N. J. Am. Chem. Soc. 1985,107,5792. Sugimoto, T.; Awaji, H.; Sugimoto, I.; Misaki, Y.; Yoshida, Z. Synth. Met. 1987, 19, 569. (15)
Sugimoto,T.;Misaki,Y.;Arai,Y.;Yamamoto,Y.;Yoshida,Z.;Kai,
Y.; Kasai, N. J . Am. Chem. Soc. 1988, 110,628. (16) Sugimoto, T.; Misaki, Y.; Kajita, T.; Yoshida, 2.;Kai, Y.; Kasai, N. J. Am. Chem. Soc. 1987,109,4106. (17) Sueimoto,T.;Awaji,H.;Sugimot4, I.; Misaki,Y.;Kawase,T.;Yoneda, S.;Yoshida, Z. Chem. Mater. 1989, I , 535. (18) Feldberg, S.In Electroonalyrical Chemistry; Bard, A. J., Ed.;Marcel Dekker,New York, 1969; Vol. 3, p 200. (19) Sokol, W. F.; Evans, D. H.; Niki, K.; Yagi, T. J. Electroanal. Chem. 1980, 108, 107. (20) Kowalik,J.; Osborne, M. R. Methods in Unconstrained Optimization Problems; Elsevier: Amsterdam, 1968. (21) Kano, K.; Konse, T.; Uno,B.; Kubota, T. In Redox Chemistry and Interfacial Behavior of Biological Molecules; Dryhurst, G., Niki, K., Eds.; Prenum Press; New York, 1987; p 267. (22) Bard, A. J.; Faulkner, L. R. Electrochemical Methods-Fundamental and Applications; John Wiley & Sons: New York, 1980; Chapter 5 . (23) Bard, A. J.; Faulkner, L. R. In ref 22, Chapter 8. (24) Weast, R. C.; Astle, M.J. Eds. CRC Handbook of Chemistry and Physics; 60th cd.; CRC Press: Bocn Raton, FL, 1979; pp F-56, C-182. (25) Bard, A. J.; Faulkner, L. R. In ref 22, Chapter 6. (26) Kano, K.;Mori, K.; Uno, B.; Kubota, T.; Ikcda, T.; Senda, M. Bioelectrochem. Bioenerg. 1990, 23,227. (27) Richardson, D.E.; Taube, H.Inorg. Chem. 1981,20, 1278. (28) Moses, P. R.; Chambers, J. Q.J . Am. Chem. Soc. 1974,96,945. (29) Mizuno, M.;Garito, A. F.; Cava, M. P. J. Chem. Soc., Chem. Commun. 1978, 18. (30) Iycda, M.;Otani, H.;Oda, M.;Kai, Y.; Baba, Y.; Kasai, N. J. Chem. Soc., Chem. Commun. 1986, 1794.
