J . Phys. Chem. 1992,96, 1266-1270
1266
Electron-Transfer Dynamics in Hlghly Reduced States of Simple and Superstructured Metalloporphyrlns Elodie Anxola&h&re, Doris Lexa, and Jean-Michel Sav4ant* Luboratoire d'Electrochimie MolPculaire de I'UniversitC de Paris 7 , Unite Associse au CNRS No. 438, 2 place Jussieu, 75251 Paris Cedex 05, France (Received: July 26, 1991; In Final Form: September 24, 1991) The standard rate constantsof the Fe(I)-/Fe("O")2-couple in a series of four simple and basket-handle superstructured porphyrins have been measured by means of fast cyclic voltammetry at mercury and gold ultramicroelectrodes. Analysis of the experimental data by the Marcus-Hush model revealed that the main rate-controlling factor of these very fast electron-transferreactions is solvent reorganization. The presence of secondary amide groups borne by the basket-handle structure and located in the close vicinity of the metalloporphyrin center largely facilitates the reaction from a thermodynamic viewpoint. This facilitation of the reaction is not counterbalanced by any significant contribution of the fluctuational reorganization of the NHCO dipoles thanks to their attachment to the basket-handle chains. A few complementary experiments were carried out with zinc and copper porphyrins where the same general trends were observed. The role of supermolecular structures on the thermodymamics of electron-transfer and ligand-exchange reactions of iron porphyrins has been investigated in a series of previous studies.' The largest effects of superstructures of the basket-handle or picket-fence type attached to a tetraphenylporphyrin ring at the ortho positions of the phenyl groups were found in cases where the reaction involves the introduction of one negative charge into the complex. The magnitude and even the sign of the effect depend upon the nature of the groups that link the carbon chains to the periphery of the porphyrin ring. When these are ether groups, the reaction becomes more difficult whereas it is facilitated by the presence of secondary amide group. The basic factors revealed by these observationsrelate to the interactions between the charged porphyrins and their environment. In both families the superstructure sterically protects the charged species from solvation. This factor tends to render reactions in which one negative charge is introduced into the porphyrin more difficult than in the absence of superstructure. In the case of NHCO-linked superstructures, however, this negative effect is largely overcompensated by the positive interadions between the NHCO dipoles and the charge borne by the metalloporphyrin. In the electron-transfer reaction that converts the iron(I)2 into the iron('0")2 porphyrin the magnitude of these interactions reaches at least 400 meV in terms of free energy.Id Such a modulation of the porphyrin reactivity by means of through-space interactions with the NHCO groups is reminiscent of the influence that the same groups belonging to the protein chains may exert on the reactivity of prosthetic groups in metalloproteins. (1) (a) Lexa, D.; Momenteau, M.; Rentien, P.; Rytz, G.; SavQnt, J.-M.;
Xu, F. J. Am. Chem. Soc. 1984,106,4755. (b) Lexa, D.; Momenteau, M.; SavQnt, J.-M.; Xu,F. Inorg. Chem. 1985,24, 122. (c) Croisy, A.; Lexa, D.; Momenteau, M.; SavQnt, J.-M. Organomefallics 1985,4,1574. (d) Gueutin, C.; Lexa, D.; Momenteau, M.; SavQnt, J.-M.; Xu, F. Inorg. Chem. 1986,25, 4294. (e) Lexa, D.; Momenteau, M.; SavQnt, J.-M.; Xu,F. J. Am. Chem. Soc. 1986,108,6937. (f) Lexa, D.; Momenteau, M.; SavQnt, J.-M.; Xu,F. Inorg. Chem. 1986,25,4857. (g) Lexa, D.; SavQnt, J.-M. J. Phys. Chem. 1987, 91, 1951. (i) Lexa, D.; Momenteau, M.; SavQnt, J.-M.; Xu, F. J . Electroanal. Chem. 1987,237. 131. 6)Lexa, D.; SavCnt, J.-M. In Redox Chemistry and Interfacial Behavior of Biological Molecules; Dryhurst, G., Niki, K., Eds.; Plenum: New York, 1988; pp 1-25. (2) (a) While there is little doubt that iron(1) is the main resonant form of the com lex resulting from the injection of one electron into an iron(I1) porphyrin: the nature of the main resonant form(s) of the complex resulting from the injection of an additional electron is not a settled question.2ej (b) Cohen, I. A,; Ostfeld, D.; Lichtenstein, B.J . Am. Chem. Soc. 1972, 94,4522. (c) Lexa, D.; Momenteau, M.; Mispclter, J. Biochim. Biophys. Acta 1974, 338, 151. (d) Kadish, K. M.; Larson, G.; Lexa, D.; Momenteau, M. J . Am. Chem. Soc. 1975,97,282. (e) Reed, C. A. Ado. Chem. Ser. 1982, No. 201, 333. (f) Srivatsa, G. S.; Sawyer, D. T.; Boldt, N. J.; Bocian, D. F. Inorg. Chem. 1985,24,2133. (8) Hickman, D. L.; Shirazi, A.; Goff, H. M. Inorg. Chem. 1985,24,563. (h) Donohoe, R. J.; Atamian, M.; Bocian, D. F. J. Am. Chem. Soc. 1987,109, 5593. (i) Teraoka, J.; Hashimoto, S.;Sugimoto, H.; Mori, M.; Kitagawa, T. J . Am. Chem. Soc. 1987,109, 180. (j)Mashiko, T.; Reed, C. A.; Haller, K. J.; Scheidt, W . R. Inorg. Chem. 1984, 23, 3192. See also footnote 20 in ref 1. (I) Hammouche, M.; Lexa, D.; Momenteau, M.; SavQnt, J.-M. J . Am. Chem. SOC.1991, 113, 8433.
L
The presence of more and more NHCO groups in basket-handle c h a i i has been shown to exert a cumulative effect, the magnitude of which depends upon the average distance between each NHCO group and the central porphyrin.Ih The ensemble of secondary amide groups borne by the superstructure may thus be viewed as a "local solvent". This analogy certainly holds as far as the energetics of the dipolecharge interactions are concemed but may be questionable as regards to the fluctuational character of the dipole ensemble. Evidence that fluctuations of the NHCO dipoles attached to the superstructure are smaller than those of the molecules of a true solvent has been provided recently from entropy measurements in the Fe(1)- + e- e Fe("0")" equilibrium in a series of simple and superstructured porphyrin^.^ The aim of the work reported below was to address the same question from the angle of electron-transfer dynamics, taking as the main example the Fe(1)- + e- e Fe("O")2- reactions where the thermodynamic effects of the superstructures are the largest in terms of equilibrium standard free energy. A few additional experiments were carried out with zinc(I1) and copper(I1) porphyrins at their first and second reductions into the corresponding anion radicals and dianions, respectively. Solvent reorganization plays an important role in the dynamics of outer-sphere electron transfer? By analogy, an overpotential resulting from the reorganization of the NHCO dipoles would be created in the case where this 'local solvent" behaves as a true solvent as far as fluctuational reorganization is concemed. This would counteract or even cancel the gain ion thermodynamic potential brought about by the NHCO dipoles. By contrast, the thermodynamic gain will be preserved if the fluctuation of the NHCO dipoles is small. There have been relatively few electrochemical studies of the kinetics of electron transfer to metalloporphyrins. They were all, with one exception (namely, that of zinc tetraphenylporphyrin), dealing with less reduced states than the ones we investigated here.s-6 For example, in the case of iron porphyrins, only F e (3). (a) AnxolaMh&e, E.; Lexa, D.; Momenteau, M.; SavCnt, J.-M. Submitted for publication. (b) A brief account of this work can he found in ref lj. (4) (a) Marcus, R. A. J . Chem. Phys. 1956, 24,966. (b) Hush, N. S . J. Chem. Phys. 1958,28,962. (c) Hush, N. S. Trans. Faraday Soc. 1961.57, 557. (d) Marcus, R. A. J . Chem. Phys. 1965, 43, 679. (e) Marcus, R. A. Special Topics in Electrochemistry; Rock, P . A., Ed.; Elsevier: New York, 1977; pp 161-179. ( 5 ) There is some concern that the values of the standard rate constants obtained with conventional size working electrodes might be ~nderestimated'~ due to Ohmic drop effects. This is particularly true of the data gathered in highly resistive media such as methylene chloride but also for large rate constants in less resistive media such as NJVLdimethylformamide (DMF). Compare for example the value obtained here for the Zn(II)/Zn(II)'- couple in this solvent (0.9 cm s-I, vide infra) to that previously obtained in dimethyl sulfoxide (DMSO) (0.04 cm s - ~ a) solvent ~ ~ of very similar polar characteristics.
0022-3654/92/2096- 1266%03.00/0 0 1992 American Chemical Society
Electron Transfer in Metalloporphyrins
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1267
h
TPP
TAP
Figure 1. Porphyrins used in this work and their conventional designations. M: Fe, Zn, Cu.
(III)/Fe(II) couples were investigated. Axial ligands play then an important role in the dynamics of electron transfer. This is not the case here since there is no significantaxial ligation of both members of the Fe(I)-/Fe(UO")Z-redox couple. Since the solvent reorganization is small in view of the large size of the reacting molecules and since cis-ligand reorganization is also expected to be small, the absence of axial ligation should result in large electron-transfer rate constants for all investigated porphyrins, unless the reorganization of the NHCO happens to be an important factor. This is the reason why we employed cyclic voltammetry with small working electrodes (in the 10-pm-diameter range) which allow the determination of large rate constants (up to several cm s-l) thanks to the minimization of the Ohmic drop they permit.7a In spite of the dramatic decrease of the Ohmic drop as compared to the value it has with conventional size electrodes (diameter in the millimeter range) at the same scan rate, the Ohmic drop is not negligible and should be corrected. The correction can be carried out after the recording of the current-potential curve up to scan rate in the megavolt per second range of scan rate.7b It can also be performed automatically by means of a positive feedback d e ~ i c e . ~Scan ~ . ~rates in the range of hundreds of kilovolts per second can be reached in this way.7d We followed this second procedure since it is less tedious than the first and since the maximal scan rates are sufficient for the determination of the aimed rate constants. In most cases we used both a gold and a mercury electrode in order to check the outer-sphere character of the electron-transfer process from the lack of dependence of the observed kinetics from the electrode material. Experimental Section Chemicals. The DMF (from Carlo Erba) was distilled under vacuum and dried over neutral alumina before use. The supporting electrolyte, NEt4CI04(Fluka, purum), was recrystallized thrice in a 1:2 ethyl acetate/ethanol mixture and dried under vacuum (6) (a) Martin, R. F.; Davis, D. G. Biochemistry 1968, 7, 3096. (b) Kadish, K. M.; Davis, D. G. Ann. N.Y.Acad. Sci. 1973, 204, 495. (c) Constant, L. A.; Davis, D. G. J. Electrmnal. Chem. 1976, 74,85. (d) Kadish, K. M.; Larson, G. Eioinorg. Chem. 1977, 7,95. (e) Kadish, K. M.; Sweetland, M.; Cheng, J. S. Inorg. Chem. 1978, 17,2795. (f) Richard, M. J.; Schaffer, C. D.; Evilia, R. G. Electrochim. Acta 1982, 27,979. (8) Kadish, K. M.; Su, C. H. J. Am. Chem. Soc. 1983,105,177. (h) Feng, D.; Schultz, F. A. Inorg. Chem. 1988,27,2144. (i) Mu, X. H.; Schultz, F. A. Inorg. Chem. 1990,29, 2877. (7) (a) Andrieux, C. P.; Hapiot, P.; SavCnt, J.-M. Chem. Rev. 1990,90, 723 and references therein. (b) Andrieux, C. P.; Garreau, D.; Hapiot, P.; SavCnt, J.-M. J. Electroanal. Chem. 1988, 248, 447. (c) Amatore, C.; Lefran, C.; PflOger, F. J . Electroanal. Chem. 1989, 270, 43. (d) Garreau, D.; Hapiot, P.; SavQnt, J.-M. J . Electroanal. Chem. 1990, 289, 73.
before use. Iron, zinc, and copper TPP (see Figure 1 for the conventional designations of the various porphyrins used in this work) were from commerical origin (Aldrich) and were used as received. The synthesis and characterization of the TAP and basket-handle porphyrins have been described previously.* The solutions of the porphyrins in DMF with 0.4 M supporting electrolyte were deoxygenated with argon prior to the recording of the current-potential curves and maintained under an argon atmosphere during the experiment. In the case of TPPFe and TAPFe, the dissolution of the porphyrin was carried out under argon in order to avoid the formation of the p-oxo dimer. The water used in the preparation of the mercury working electrode was distilled in a Millipore apparatus. The sulfuric acid used in the procedure for the fabrication of the mercury microelectrode was from BDH Co. (Aristar grade). Electrodes. As working electrode we used either a 17-pmdiameter gold disk prepared as described in ref 9 or a mercury electrode prepared according to the following procedure inspired from procedures used by Maran and Faulkner.lo Mercury is deposited electrolytically onto a platinum disk (from Tacussel, nominal diameter 10 pm). The problem is to obtain a mercury deposit preventing contact between the background platinum and the bathing solution. The platinum disk is first polished with 0.02-pm alumina and then exposed to 20 successive constantcurrent (1 pA) oxidation-reduction cycles in a 1:l:l HzSO4/ HNO3/HZ0mixture. The platinum disk is then immersed into a deoxygenated 0.5 M solution of sulfuric acid, and the potential of the electrode is scanned between -1.19 and 1.26 V vs SCE first at 1 V s-l and then at 0.1 V s- (20 cycles in total). The electrode is finally immersed in a HgS04 ( 5 X 10" M), H2S04(0.5 M) solution, and the potential is scanned between -0.19 and 0.76 V vs SCE at 0.03 V s-' (five cycles). The concentration of HgS04 is then raised to lr3 M, and the potential is scanned between 4.19 and 0.46 V vs SCE. It is observed that the onset of the current corresponding to proton discharge shifts progressively toward more and more negative values. The process is stopped when this potential reaches 4 . 7 4 V vs SCE. The microelectrode is then immersed into a mercury pool in the same solution and a constant-current (1 pA) electrolysis carried out during 1 h. In all these operations the counter electrode was a platinum wire and the reference electrode a Hg/Hg,S04, 0.5 M H2S04electrode (potential 0.44 V positive to SCE). The mercury microelectrode is finally examined with a microscope to check the continuity of the mercury deposit and its diameter determined (20 pm is a typical value). The gold working electrode was polished (0.02-pm alumina) and rinsed before each run whereas the mercury electrode was simply M s e d and immersed into mercury for a few seconds before each run. In the cyclic voltammetric experiments in DMF, we used a two-electrode c o f i g ~ r a t i o nwith ~ ~ a silver wire as quasi-electrode reference. Its potential was measured before and after each experiment and showed no significant drift. Instrumentation. The potentiostat, current measurer, and positive feedback device were the same as previously described as well as the function generator and the digital ~scilloscope.~~ The current-potential curves were accumulated 10 times in the digital oscilloscope during each experiment. The Ohmic drop compensation was achieved by injection of more and more positive feedback until sustained oscillations appear on the cyclic voltammetric trace. The feedback was then slightly decreased before the recording of the current-potential curve. This procedure poses no difficulty with the gold electrode. This is not the case with (8) (a) Momenteau, M.; Mispelter, J.; Loock, B.; Bisagni, E. J . Chem. SOC.,Perkins Trans I 1983, 189. (b) Momenteau, M.; Mispelter, J.; Loock, B.; Lhoste, J. M. J . Chem. Soc., Perkin Tram I 1985,221. (c) Momenteau, M.; Loock, B. J . Mol. Catal. 1980, 7, 315. (9) Andrieux, C. P.; Garreau, D.; Hapiot, P.; Pinson, J.; SavCnt, J.-M. J. Electroanal. Chem. 1988, 243, 321. (10) (a) Maran, F. Istituto di Chimica Fisica, Padova, Italy, private communication. (b) Faulkner, L. R. University of Illinois at UrbanaChampaign, IL, private communication.
AnxolabWre et al.
1268 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 (a)
2
1
2
EvrSCE(V) l
l
t
l
t
l
1
1
1
1
1
IVSSCE(V1 1
I
I
3
4
I
I
3
4
log v (V.1")
6
log v(v.1-1 I
5
Figure 3. Cyclic voltammetry of the Fe(I)-/Fe('O")2- couple of the a-(C12),-CT-TPP Fe(1II)OH porphyrin, neutralized by HClO.. Variation of the anodic to cathodic peak potential distance with the scan rate on a mercury (a) and a gold (b) electrode. The full lines represent the theoretical Up-log u variations for ksap/D'/2(s-'l2) = 603 (a, upper curve), 813 (a, lower curve), 398 (b, upper curve), and 708 (b, lower curve). TABLE I: Apparent Strpdvd Rate Combats of Electron Transfer h Side .IldSuperst"d M e t & " u11-
porphyrin TAP-Fe Fe( I)-/Fe('O")2e- (C 12) &T-TPP-Fe Fe( I)-/Fe('O")2TPP-Fe Fe( I)-/Fe('0")2a-(C 12),-CT-TPP-Fe Fe( I)-/Fe("On)2TPP-Zn Zn(II)/(Zn(II)*Zn(II)'-/Zn(II)2a-(C12),-CT-TPP-Zn Zn(II)/Zn(II)*Zn(II)*-/Zn(II)za-( C 12),-CT-TPP-Cu Cu(II)/Cu(II)*cu(II)~-/cu(II)~-
(11) (a) Nicholson, R. S.Anal. Chem. 1965, 37, 1351. (b) Nadjo, L.; SavCnt, J.-M. J. Electroanal. Chem. 1973, 48, 113.
Hg
Au
s-I
Hg
Au
866
577
3.0
1.50
1.00
433
3.0
849
896
4.5
1.80
1.90
976
577
3.0
1.30
1.10
562 850
600 590
2.0
0.79 1.20
1.04 0.83
704
372 372
1.18 2.8
0.62 0.62
352 414
0.69 2.3
0.53 0.63
458
0.75
The values of ksaP/D1/2 were derived from these AE -log v plots for all four iron porphyrins and kslP derived using t i e values of D obtained from slower scan cyclic voltammetric experiments on conventional microelectrodes (Table I). Repeated determinations showed that the accuracy of ks*P is ca. 0.4 cm s-l. In the case of the e-(ClZ)&T-TPPFe porphyrin the determination on mercury of was made impossible by adsorption of the reactant on the electrode. For all the other iron porphyrins the values obtained on gold and mercury are the same within experimental uncertainty. A few additional experiments were carried out in the same way with zinc and copper porphyrins. We investigated both reduction waves since Zn(I1) and Cu(I1) are usually considered not to be axially ligated. The results are displayed in Table I. We used the Marcus-Hush model of outer-sphere electron transfer for analyzing the above data and, particularly, for un-
Electron Transfer in Metalloporphyrins
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1269
TABLE II: Anolysie of ExperiWaW Data by mea^ of the Marcus-Hush Model of Outer-Spbere Electron Transfer
porphyrin TAP-Fe Fe( I)-/ Fe(Y0")2e-(C12)2-CT-TPP-Fe Fe(I)-/Fe(yO")2TPP-Fe Fe( I)-/Fe("O")2a-(ClZ),-CT-TPP-Fe Fe( I)-/Fe("O")" TPP-Zn Zn(II)/Zn(II)*Zn(II)*-/Zn(II)2a-(C 1 2) 2-CT-TPP-Zn Zn(II)/Zn(II)*Zn(II)*-/Zn(II)2a-(C12)2-CT-TPP-Cu cu(II)/cu(II)*Cu( II)*-/Cu( II)2-
AG * = AG
AG*oY
-429
-Qd,
mV
mV
eV
ZX cms-'
Hg
Au
Hg
meV Au
184
195
43
54
AG*0,tb,
meV
R,A
d, A
5.40
7.25
0.096
-1.77
79
30
0.141
2.2
5.65
8.50
0.098
-1.95
84
22
0.130
1.9
5.15
6.75
0.099
-1.65
76
33
0.149
2.3
180
179
31
30
5.65
8.50
0.098
-1.53
72
19
0.127
1.8
183
189
56
62
5.15 5.15
6.75 6.75
0.099 0.099
-1.31 -1.70
70 79
30 34
0.114 0.150
2.4 2.4
202 192
196 201
88 62
82 51
5.65 5.65
8.50 8.50
0.098 0.098
-1.215 -1.565
66 75
17 19
0.106 0.127
1.85 1.85
186
202 202
72
88 75
5.65 5.65
8.50 8.50
0.098 0.098
-1.12 -1.54
64 75
16 19
0.106 0.126
l.& 1.85
199
206 202
93
100 75
eV
198
68
- AG *O,th.
ravelling the question of the possible participation of the interactions between the NHCO dipoles and the negative charge borne by the iron porphyrin to the reorganization energy of the Fe(I)-/Fe('O")" electron transfer. We first assume that the dynamics of the electron transfer is solely under the dependence of solvent reorganization. The Hush-Marcus model relates the standard free energy of activation for an electrochemical reaction to the reorganization of the solvent according to the following equation4.'
where NAis the Avogadro number, eo is the electron charge, Dpp and Ds are the optical (square of the refractive index) and static dielectric constants of the solvent, respectively, R is the radius of a hard sphere equivalent to the reactant in terms of Born solvation energy, and d is the distance between the center of the reactant and the electrode surface. When d = R the above relationship tends toward the Marcus formulation with full development of the image force effects whereas when d >> R we find the Hush formulation where the image force effects are neglected. We estimated the values of R and d for the four porphyrins (Table 11) as follows. For the TPP and TAP porphyrins, R was derived from an estimate of the volume of the molecule on molecular models. This procedure is more uncertain for the two basket-handle porphyrins. When passing from TAP to e-(C12)2-CT-TPP, where the inductive effects of the ether substituents are very close, a -180-mV shift of the standard potential of the Fe(I)-/Fe('O")" couple has been observed and assigned to the increase of the radius of the equivalent hard sphere. Thus, in the framework of the Born model of solvation PTAP
AG :*
AG*O,OXP
EO, V VS SCE
- Pa(C12)rCT-TPP ._
= 1
2
Re-(CI2)2-CT-TPP
RTAP
Rc.(cl =m.Tpp was found equal to 5.65 A starting from R T A p = 5.40 (Ds = 36.714). The same value was taken for a(C12)2-CT-TPP since the basket-handle chains have the same length in both cases. The values of d were obtained, for all four iron porphyrins, from molecular models as an average of the various orientations the molecules can take to get into contact with a plane. We thus find that the d values are larger than the R ~~~~
(12) Peover, M. E.; Powell, J. S.J . Electroanal. Chem. 1969, 20, 427. (13) Kojima, H.; Bard, A. J. Am. Chem. Soc. 1975.97, 6317. (14) Delahay, P. Double layer and Electrode Kinetics; Wiley: New York, 1965.
values. The values of AGlo were then derived from eq 1 (Table 11). They are almost the same for all four compounds as results from a compensation between the variation of the solvation radius and that of the closest approach distance in the series. The next step was to estimate the work term due to the effect of the double layer so as to obtain an estimate of the standard activation free energy AG*o,* that could be compared with the experimental value AG*O,exp.Since the reaction involves a singly and a doubly negatively charged oxidized and reduced form of the reactant, respectively, still assuming that the transfer coefficient is 0.514 AG*O,th = AG*o - (CY- Z)& where the free energies of activation are expressed in electronvolts, f$d is the difference between the electrical potential at the center of the molecule and the electrical potential of the solution, and z is the charge of the oxidized from the redox couple. f#Jd was determined from the potential difference at the outer Helmholtz plane, classically denoted 42 according to 4 d 42 exp[-K(d - RNEt4+)1 where K-I is the Debye length (3.3 A) and RNE,,+ the radius of the hard sphere equivalent to a tetraethylammonium cation (from molecular models RNEk+ = 4 A). 42was obtained from previous determinati~ns.'~J~ The ensuing values of f$d and then of AG*O,th are listed in Table 11. AG*o,ap is obtained from the values of k$P according to
Z being given by the Schmoluchovski equation Z = (kT/2rm)Il2 (m is the mass of the reactant). As seen in Table 11, the values of AG*o,ththat take only into account the solvent reorganization energy are systematically smaller than the experimental values but only by a small amount (ranging from 30 to 70 meV). This might be due to some internal reorganization concerning the cis ligation of the iron atom by the porphyrin ligand.16 (15) (a) The values of q5z in ref 13 concern a 0.5 M solution of n-NBu,CIO, in DMF whereas in our case the supporting electrolyte was NEt,CIO, at a concentration of 0.4 M. It has been shown that the Gouy-Chapman theory of the diffuse double layer is closely followed by solution of both supporting electrolytes in DMF.Isb Thus, the values of q52 for both of them should be very close at the same concentration. We corrected for the variation in concen= RT/Fln (CJCz). (b) Fawcett, W. R.; Ikeda, tration using" (&, B. M.; Sellan, J. B. Can. J . Chem. 1979, 57, 2268. (16) Upon electron transfer the iron atom may get a little more out of the porphyrin plane.
1270 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992
It is clear from the last row of Table I1 that the presence of the NHCO groups in the close vicinity of the porphyrin ring does not bring about any significant increase of the intrinsic barrier: AG*O,,P- AG*O,ch for the a-(C12)2CT-TPPporphyrin is about the same as for the e-(C12)2-CT-TPP porphyrin. If the NHCO dipoles had behaved like a true solvent with a Dsclose to that of DMF COOPdoes not vary very much from one solvent to the other, being close to 2 in most cases), application of eq 1, with a value of R of the order of 2 A (the average distance between the positive end of the NHCO dipoles and the porphyring ring), would have led to AG*o = 0.270 eV The predicted standard rate constant then would have been about 5 orders of magnitude lower than observed experimentally. The data pertaining to the zinc and copper porphyrins were analyzed in the same manner (Table 11). Although the study could not be as extensive as with the iron porphyrins because of the nonavailability of certain porphyrins or insolubility of others, the conclusions are essentially the same for the second reduction wave. For the first reduction wave we note that the electron transfer is slightly slower than for the second reduction wave. Although the Zn and Cu atoms may not be formally ligated by DMF molecules, interactions of the (Lewis) acid-base type may well exist between them that would lead to a stronger solvation of the Zn(I1) and Cu(I1) complexes than otherwise predicted by the Born-type treatment included in the Hush-Marcus model.
Conclusion In summary,the standard electrochemicalrate constant of the Fe(I)-/Fe("O")2- in simple and superstructured porphyrins of the basket-handle type are large, falling in the 0.5-2 cm s-l range. This measurement could be achieved by use of fast-scan cyclic voltammetry at ultramicroelectrodes with positive feedback compensation of the Ohmic drop. Analysis of the experimental data by means of Marcus-Hush theory of outer-sphere electron
AnxolabEh6re et al. transfer revealed that solvent reorganization is the main controlling factor of the electron-transfer dynamics. In basket-handle superstructures containing NHCO groups in the close vicinity of the porphyrin ring, the NHCO groups behave as a "local solvent" from the thermodynamic viewpoint in the sense that their interactions with the negatively charged porphyrin center shift the standard potential of the Fe(I)-/Fe("0")2- couple to a significant extent. The ensemble of NHCO groups, however, does not behave like a true solvent in the control of the electron-transferdynamics. They indeed do not participate in the nuclear reorganization accompanying electron transfer to any significant extent. The thermodynamic facilitation of the reaction they bring about is thus not counterbalanced by any significant decrease of the electrontransfer kinetics. The effect of the NHCO groups is thus essentially that of fmed dipoles interacting with the negative charge borne by the metalloporphyrin. In this respect, their effect is more similar to that of an electron-withdrawing substituent although through-space rather than through-bond interaction is dealt with. In addition to their intrinsic dipole moment, the polarizability of the NHCO groups appears to come into play although to a modest extent since their effect on the Fe(1)- + e- + Fe("0")2- reaction is a little stronger than their effect on the Fe(I1) + e- Fe(1)reaction.ld Acknowledgment. We thank Dr. Michel Momenteau Institut Curie, Orsay, France) for the generous gift of samples of the superstructured metalloporphyrins used in this work. We are indebted to Dr. Philippe Hapiot (Laboratoire d'Electrochimie Molhlaire, Universit6 de Pans 7,France) for his help and advice in the use of fast-scan voltammetric techniques employing ultramicroelectrodes. We are grateful to Dr. Flavio Maran (Istituto di Chimical Fisica, Padova, Italy) and to Prof. Larry R. Faulkner (University of Illinois at Urbana-Champaign) for disclosing to us their precious recipes for the preparation of mercury microelectrodes as well as to Dr.Michel Froment (CNRS, UPR15, Paris, France) for the permission to use his metallographic microscope for examining the mercury-plated electrodes.
