9348
J. Phys. Chem. 1992,96,9348-9352
counter distance of two radicals is considerably larger than the corresponding effective reaction distance.
Acknowledgment. We thank Dr. G. V. Buxton for a number of interesting discussions. The work described herein was supported by the Office of Basic Energy Sciences of the US.Department of Energy. This is Document NDRL-3493 from the Notre Dame Radiation Laboratory. NO. H,12385-13-6; OH,3352-57-6; H20,7732-18-5.
Referems aod Notes (1) Pimblott, S.M.; Laverne, J. A.; Mozumder, A.; Green, N. J. B. J . Phys. Chem. 1990,94,488. (2) Monchick, L.; Magee, J. L.; Samuel, A. H. J . Chem. Phys. 1957,26,
935. (3) Mozumder, A,; Magee, J. L. Radiat. Res. 1966, 28, 215. (4) Schwarz, H. A. J . Phys. Chem. 1969, 73, 1928. (5) Kuppermann, A. In Radiation Research; Silini, G.,Ed.; North-Holland: Amsterdam, 1967; p 212. (6) Burns, W. G.; Sims, H. E.; Goodall, J. A. B. Radiat. Phys. Chem. 1984, 23, 143. (7) Laverne, J. A.; Pimblott, S . M. J. Phys. Chem. 1991, 95, 3196. (8) Clifford, P.; Green, N. J. B.; Oldfield, M. J.; Pilling, M. J.; Pimblott, S.M. J. Chem. Soc., Faraday Trans. 1 1986.82, 2673. (9) Clifford, P.; Green, N. J. B.; Pilling, M. J. J . Phys. Chem. 1982, 86, 1318.
(IO) Green, N. J. B.; Pilling, M. J.; Pimblott, S.M. Radiat. Phys. G e m . 1989, 34, 105. (1 1) Clifford, P.; Green, N. J. B.; Pilling, M. J.; Pimblott, S.M. J . Phys. Chem. 1987. 91. 4411. (12) Green, N. J. B.; Pilling, M. J.; Pimblott, S.M.; Clifford, P. J . Phys. Chem. 1989, 93, 8025. (13) Green,N. J. B.; Pimblott, S. M. J. Phys. Chem. 1990, 94, 2922.
Noyes, R. M. Prog. React. Kiner. 1961, 1, 129. Clifford, P.; Green, N. J. B. Mol. Phys. 1986,57, 123. Green,N. J. B. Mol. Phys. 1988, 65, 1399. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Rars, A. B. J. Phys. Chem. ReJ Data 1988.17, 513. (18) Laidler, J. K. Chemical Kinetics; Harper & Row: New York, 1987. (19) von Smoluchowski, M. 2.Phys. Chem. 1917, 92, 129. (20) Rice. S. A. In Comprehensive Chemical Kinetics. mffusron-Limited Reactions; Bamford, C. H., Tipper, C. R. H., Compton, R. G., Eds.; E l d e r : Amsterdam, 1985. (21) Naqvi, K. R.;Mork, K. J.; Waldenstrom, S.J . Phys. Chem. 1980, (14) (15) (16) (17)
84, 1315. (22) Hart, E.J.; Anbar, M. The Hydrated Electron; Wiley-Interscience: New York, 1970. (23) Elliot,A. J.; McCracken, D. R.; Buxton, G. V.; Wood, N. D. J. Chem. Soc., Faraday Trans. 1990,86, 1539. (24) Saltiel, J.; Atwater, B. W. Adu. Photochem. 1988, 14, 1. (25) Verma, N. C.; Fcssenden, R. W. J. Chem. Phys. 1973, 58, 2501. (26) Sehestcd, K.; Christensen, H. Radiat. Phys. Chem. 1990, 36, 499. (27) Green, N. J. B.; Pimblott, S. M.; Brocklehurst, B. J. Chem. SIX., Faraday Trans. 1991,87,2427. Pimblott, S.M.; Green, N. J. B.;Brocklehurst, B. J. Chem. Soc., Faraday Trans. 1991,87, 3601. (28) Collins, F. C.; Kimball, G.E. J. Colloid Interface Sci. 1949,4,425. (29) Berg, 0. G. Chem. Phys. 1978, 31, 47. (30) Feller, W. An Introduction to Probability Theory and Its Applications; Wiley: New York, 1968; Vol. 1. (31) Harrison, J. M. Brownian Motion and Stochastic Flow Systems; Wiley: New York, 1985. (32) Ripley, B. D. Stochastic Simulation; Wiley: New York, 1987. (33) Abramowitz, M.; Stegun, I. A. Handbook of Mathematical Functions; Dover: New York, 1970. (34) Pimblott, S.M.D.Phil. Thesis, Oxford University, 1988. (35) Ahrens, J. H.; Kohrt, K. D.; Dieter, U. ACM Trans. Math. Soft. 1983, 9, 255. (36) Pimblott, S.M.; Pilling, M. J.; Green. N. J. B. Radiat. Phys. Chem. 1991, 37, 317.
Supramolecular Entropy Gains. Simple and Superstructured Iron Porphyrins Elodie AnxolrMb&re,laDoris Lexa," Michel Momenteau,lb and Jean-Michel Saviht**la Laboratoire d%lectrochimie MolCculaire de I'UniversitC de Paris 7, UnitP AssociCe au CNRT No. 438, 2 place Jussieu, 75251 Paris Cedex 05, France, and the Institut Curie, Section de Biologie, UnitC Inserm 219, 91405 Orsay. France (Received: May 1 I , 1992; In Final Form: August 3, 1992)
-
The determination of the enthalpy-energy balance of the reaction Fe(1)in simple and superstructured iron porphyrins makes apparent that entropy factors play a crucial role in the modulation of the reactivity by the molecular superstructures. In the case of the ether-linked structures, the decrease of reactivity mainly derives from the rigidification of the molecular superstructure in the reaction. With secondary amide-containiigstructures, the increase in reactivity derives essentially from a decrease of the enthalpy-entropy compensation: solvent-change interactions are substituted by NHCO dipole charge interactions, and the resulting increase in entropy arises from the fact that the NHCO dipoles fluctuate much less than solvent molecules. The incrrase in reactivity brought about by the NHCOcontaining superstructumoffas a remarkable example of a system where entropy expenses allowed during the synthesis of the molecular superstructures result in entropy gains in the reactivity of the active molecular center.
During the past few years we have investigated the electrontransfer and coordination properties of iron porphyrins bearing superstructures of the basket-handle or picket-fence type? It has particularly been noted that the presence of secondary amide group borne by these superstructures induce# remarkable changes in reactivity of the central metalloporphyrin moiety that are r e " t of the modulation of the reactivity of prosthetic groups imbedded in a protein structure such as in enzymes and metallopr~teins.~ The work reported below was an attempt to examine if entropy expenses allowed during the synthesis of the superstructured molecules could be utilized for obtaining entropy gains in the chemical functioning of the reacting center. The most spectacular changes in reactivity brought about by the presence of secondary amide groups held by the basket-handle or picketfence superstructurea in the vicinity of the porphyrin ring col~ms the reactions in which a negative charge is introduced in the metalloporphyrin moiety. As discovered by examining the reactivity of the same porphyrins in which the basket-handle chains 0022-3654/92/2096-9348S03.#/0
are anchored by means of ether rather than secondary amide groups, one function of the superstructure is to sterically hinder solvation. Thus in a reaction represented as
MP"
+ X-
MPX("+')-
(X-being an electron or ligand), the reactivity is decreased by the presence of the superstructure. When secondary amide groups are present in the close vicinity of the reacting center, the reactivity increases. Steric hindrance to solvation still exists, but its effect is annihilated and overcompensated for by the through-space interactions between the negative charges and the NHCO dipoles. The set of secondary amide groups borne by the superstructure may thus be viewed as a "local solvent" stabilizing negatively charged species by virtue of its electrophilicproperties. In usual solvents, the energy of stabilization of ions by interaction with the solvent dipoles has a negative entropy counterpart, due to the decrease of the solvent fluctuations, that tends to diminish the stabilizing effect. The NHCO dipoles attached to the superQ 1992 American Chemical Society
Supramolecular Entropy Gains
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9349
1.320 I
I
I
I
I
I
I
I
253
258
263
288
213
276
283
288
VK 293
Figure 2. Variation of the formal potential of the F~(I)-/Fc("O")~(in couple with temperature for the porphyrin [O-(CH2),2-O]2-CT DMF 0.1 M TBAP).
+
center and the external solvent by interactions with the secondary amide groups borne by the superstructure. In terms of overall free energy, the largest effects, reaching as much as 7 orders of magnitude in terms of equilibrium constants, have been observed in reactions where the charge passes from -1 to -2, such as the reduction of Fe(1)- porphyrins to Fe("0")2porphyrins. This is the reason we have selected this particular reaction to investigate the possible gains in entropy brought about by the presence of secondary amide groups borne by supramolecular structures. "be study consisted in measuring the standard potential of the reaction Fe(1)- + e- s Fe("O")Zas a function of temperature with simple porphyrins and with basket-handle porphyrins that contained or did not contain secondary amide groups. Besides the comprehension of the effect of these groups, another outcome of the study was a better understanding of the steric hindrance to solvation exerted by the supramolecular structures. The structures shown in Figure 1 together with their designation were investigated.
Results The standard potential of the Fe(I)-/Fe("0")2- couple was obtained as the midpoint between the cathodic and anodic peak potentials in cyclic voltammetry at low scan rate (0.1 V 8-l). In all cases,the starting compound was the iron(II1) porphyrin (under the form of its chloride in most cases and perchlorate in a few cases). The cyclic voltammogram then shows three successive waves corresponding to the couples Fe(III)+/Fe(II), Fe(II)/Fe(I)-, and Fe(I)-/Fe("O")2-. The last of these is chemically and electrochemically reversible at 0.1 V s-', and thus the midpoint between the cathodic and anodic peak tentials can be taken as a measure of the formal potential, ?of the Fe(I)-/Fe("O")" couple. The temperature in the cell containing the glassy carbon working electrode and the DMF solution of the iron porphyrin with a tetraalkylammonium supporting electrolyte was varied in the range -20 to 20 OC, and the formal potential was measured against a reference electrode maintained at 20 OC. This nonINIICO.(CHIIIO.C(mlll2.A~ ~ N I I C ~ ~ ) N H C ~ W C I ! ~ ) ~ L N I I C I 1 ~ . ' h K ' O N I I Iisothermal I-Cl cell arrangement is described in the Experimental Section. The formal potential was found to vary linearly with Figure 1. Simple and superstructured iron porphyrin and their desigtemperature (a typical example is shown in Figure 2). nation. AT,adjacent trans; AC,adjacent cis; CT,cross trans arrange-
ments of the chains.
structure are deemed to be less mobile than solvent molecules. It may therefore be expected that the negative entropy counterpart of the stabilization is decreased or even annihilated by the replacement of the interactions between the negatively charged
Discussion As discussed in the Experimental Section,the portion of a / d T
that might be due to the variation of the liquid junction potential with the temperature of the cell is negligible within experimental error. The value of dEf/dT is thus the reflection of two phe-
AnxolaMhbre et al.
9350 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 TABEL I: Variation of dE'/dT(mV/K) with tbe Size of the Supporting Electrolyte Cation porDhvrin n-BulNt n-Hex4Nt -0.l1 -0.20 TAP -0.47 -0.58 TPP [O(CH2)I 2 0 1 z*T -0.73 0.72 [NHCO(CH2)IoCONH]2 4 T 0.00 o.oo
-
nomena. One is the variation of the standard potential, E O , with temperature. The rate of this variation is the entropy of the Fe(1)Fe('0")" reaction, AS, which is the quantity of interest to our study: E" TAS- AH+ C (1) expressing Eo in VI the enthalpy, AH,in eV, and the entropy, AS, in eV/K. AH is the enthalpy of the Fe(1)- Fe("O")2reaction, and C is a constant representing the absolute potential of the reference electrode. The other source of variation of the formal potential with temperature resides in the fact that the electrostatic interactions between the Fe(1)- porphyrin and the cations of the supporting electrolyte are smaller than those involving the Fe("O")*- porphyrin. The activity coefficient of the former complex, yI,is thus larger than that of the latter, yo. Since y1and yo do not vary considerably with temperature, it is expected from E' = E" + R T In (yl/yo)
0.0 0
+
15
nificant variations of both quantities with temperature because of mutual compensation of the entropy and enthalpy temperature variations! Thii appears to be particularly true for reactions that entail the creation of an electrical charge as is presently the case. However, the derivation provides a reasonable estimate of AS and AH around the center of the temperature range. Since the variations of entropy and enthalpy we observe when passing from one compound to the other are accompanied by large free energy changes, this estimate is sufficient for the purpose of the semiquantitative discussion of the effects of the superstructures we aim at. The reaction entropies and their variations in the series of compounds are of the order of fractions of meV/K. Although these figures may seem small at first sight, they lead to substantial contributions to the free energy and to its variations in the series as can be seen in the last column of Table 11. As a matter of fact, the changes in reactivity brought about by the superstructures are essentially entropy drive? both with ether-linked and secondary amide containing basket-handle chains even though the effect of these two type of structures are opposite. With the three simple porphyrins, the fact that the entropy of reaction is negative in all cases is a reflection of the ordering of the solvent molecules being more extensive around the Fe("O")zporphyrin than around the Fe(1)- porphyrin. Comparison of the data in this series shows that the variation of the entropic term is substantial and opposed to the variation of the enthalpic term. Comparing, for example, TAP and TPP at 233 K, the free energy of the reaction decreases by 151 meV which decomposes into a 252 meV decrease of the enthalpy and a 101 meV decrease of the entropic contribution. Due to inductive effects, the increase of the interaction energy between the solvent and the porphyrin ions upon passing from Fe(1)- to Fe("0")" is larger in the case of TPP than in that of TAP. This is partly but significantly compensated by a correlated increase of the augmentation of the ordering of the solvent molecules upon passing from Fe(1)- to Fe('O")2-. Since DMF is not an H-bonded solvent, it is tempting
that this effect gives rise to a positive contribution to a / d T . 5 Its existence is apparent in the observation that, with the unprotected po hyrins, the replacement of n-Bu4N+by n-Hex,N+ decreases di? /dT (Table I) as expected from the fact that the interaction between the cation and the porphyrin diminishes as the size of the cation increases. This effect becomes negligible with the more bulky basket-handle porphyrins. With the nHeqN+ cation, the magnitude of this effect can be athated from the extended Debye-Hlickel model:6 2.16 x 10-9 1 Ef-EO
x-'
10
Figure 3. Dependence of the variation with temperature of difference between the formal and standard potential upon the distance of closest approach, a, according to the extended Debye-HUckeI model. a(293 K) = 37.55, a(253 K) = 44.35.8
-
4T)
I
5
(I
(in IS units) x-' being the Debye length, Le., in the present case x-' = 6.29 X 10-12[e(T)T11/2(in IS units) and a the distance of closest a p c h between the cations and the porphyrins in meten. Figure 3 shows the variation of d(E' - Eo)/dT with a. According to the uncertainties in the estimation of u, it can be concluded that the value of d(E' - Eo)/dT falls in the range 0.18-0.22 mV/K.' Correcting the experimental values of dE'/dT by subtraction of 0.2 mV/K to all members of the series provides a satisfactory estimation of the reaction entropy. The resulting values are listed in Table 11. The same table also contains the values of AH - C as defined by eq 1. It has been pointed out that the derivation of the reaction entropy and enthalpy from apparently linear free energy-temperature plots may overlook sig-
-
TABLE Ik Variatioa of the Formal Potential of the Fe(I)-/Fe(yO")2- Couple with Temperatureaand Other T b e r m o d y ~ mCharacteristics i~ of the Fe( 1)- Fe( "Or)*Reaction porphyrin dE'JdP Asb E°C AH-@ Tps' -lo9 -0.20 -0.40 -112, -150 -0.55 -1286 -0.35 -210 -0.77 -972 -0.57 -253 -0.9, -1298 -0.73 -248 -0.71 -0.91 -1199 -243 -0.68 -0.88 -1200 -21s -0.60 -0.80 -1250 -0.20 -888 0.00 -55 -122 -0.45 -845 -0.25 -128 -0.47 -883 -0.27 0.33 0.1, -850 35 -8 -0.0, -800 0.17
+
-
OIn DMF 0.1 M n-Hex4NC104. bEntropy of the reaction Fe(1)Fe("0")2-, in meV/K, derived from dE'/dT after correction from the interactions of Fe(1)- and Fe("O")2- with the tetraalkylammonium cation (see text). 'Standard potential of the Fe(I)-/Fe("O")z- couple at 273 K, in mV. d min meV: enthalpy of the reaction Fe(1)- Fe(YO")z-(see text and eq 1). aEntropycontribution to the reaction Fe(1)- -c Fe(YO")zat 273 K, in meV.
-
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9351
Supramolecular Entropy Gains to resort to simple models of solvation such as Born model? This would predict that (eZT3K = 41, de/dT -0.17')
k x)
AS (meV/K) = -0.73 AH (meV) = -7000(
-- -
k)
where ro and rI (in A) are the radii of the hard spheres equivalent to the Fe("0")' and Fe(1)- complexes respectively. The observed variations of the entropic contribution to the difference in free energy is obviously much larger than predicted by the Born's model pointing to a large effect of the short distance ion-solvent interactions in the present case. As noted before?- the ether-linked basket-handleporphyrins are, in terms of free energy, significantly more difficult to reduce than TAP for which the inductive effect of the ether groups is of comparable magnitude. This effect was deemed to result from the steric protection against solvation offered by the basket-handle Thus if the protection volumes were approximately the same at the Fe(1)- and Fe(yO")z- level, the free energy of solvation should be diminished as compared to TAP more at the Fe("0")" level than at the Fe(1)- level. That this interpretation provides an oversimpWied picture of the phenomenon was already suspected upon comparison of the results obtained in DMF with those obtained in a solvent that solvates anions more aggressively, namely, N-methylacetamide:k the effect was observed to decrease opposite to the predictions of the constant protection volume model. It was then concluded that N-methylacetamidesolvates the anions so strongly that the protection offered by the chains becomes less at the Fe(Y0")2- level than at the Fe(1)- level. The question further arises whether these factors could not be already acting in the case of less anion solvating solvents such as DMF. If the constant protection volume model was correct for DMF, the enthalpy of the reaction should increase as well as, correlatively, the entropic contribution, upon passing from TAP to the ether-linked basket-handle porphyrins. As seen in Table 11, this is clearly not the case. With all four ether-linked basket-handle porphyrins the reaction entropy is distinctly smaller than with TAP. The values found for the reaction enthalpy show that, in terms of energy, the reactivity is decreased for the two CT porphyrins but only to a modest extent, 30 and 18 meV, respectively. For the less protecting AC and AT structures, the reactivity is slightly increased, by 63 and 57 meV, respectively. In the whole series, the effect of the protection offered by the chains at the Fe(1)- level is thus wiped out by a decrease of the protection offered by the chains at the Fe("O")z- level. In terms of simple solvation enthalpy-entropy compensation as observed with the simple porphyrins, one would then expect that the entropic contribution to the reaction free energy should be about the same for the ether-linked basket-handle porphyrins and for TAP. This is obviously not the case: what makes the ether-linked basket-handle porphyrins more difficult to reduce than TAP in terms of free energy is overwhelmingly the variation of the entropic contribution. This variation is therefore not mainly related to the ordering of the solvent molecules themselves. Among the various elements of the system that may contribute to variations of its partition function without changing to a large extent its potential energy, the best candidates are the chains themselves since they may occupy many spatial configurations of similar energy (the straps are long enough for not creating steric constraints in the porphyrin ring). In other words, despite the protection offered by the chains and because the protection decreases from Fe(1)- to Fe(yO")z-, the increasing ordering of the solvent molecules upon passing from Fe(1)- to Fe("O")z- is not very different with the ether-linked basket-handle porphyrins and with TAP. The difference between the two cases is the ordering of the chains upon passing from Fe(1)- to Fe('O")2- in the etherlinked basket-handle porphyrins as opposed to the case of TAP. This falls exactly in line with the above conclusion, based on
enthalpy data, that the solvent molecules are able to penetrate the protective chain system to a larger extent at the Fe("O")2level than at the Fe(1)- level, thus reducing the number of its accessible spatial configurations. A side remark is that the above interpretation does not imply that the kinetic energy of the chains does not contribute to the reaction enthalpy. However, as discussed before, the detection of the possible resulting variation of the reaction enthalpy with temperature may well be obscured by the compensating term in the entropy contribution. In summary, the ether-linked chains do offer a protection against solvation in the Fe(1)- porphyrin but this is less at the level of the Fe("Or)2- porphyrin. The decrease of entropy from TAP to the ether-linked basket-handle porphyrins is essentially a reflection of the ordering of the basket-handle structure triggered by the increased interaction of solvent molecules with the negative charges upon passing from Fe(1)- to Fe("O")z-. This is the essential cause of the shifting of the standard potential toward negative values observed upon passing from TAP to the etherlinked superstructured porphyrins. The results pertaining to the superstructures containing secondury amide groups in comparison with TPP in which the inductive effects are of the same order of magnitude show that their reduction is substantially easier in terms of free energy. As in the preceding case, the enthalpy of reaction does not vary much from the simple porphyrin (TPPin this case) to the superstructured porphyrins, and the difference in free energy comes essentially from the entropic contribution. The latter however now varies in the opposite direction: it is much larger with the superstructured porphyrins than with the simple porphyrin. This behavior is caused by the replacement of the interactions with the solvent molscules in the close vicinity of the negative charges by interactions with the set of electronically polarizable NHCO dipoles. The increase of the reaction entropy as well as the rough constancy of the reaction enthalpy when passing from TPP to the NHCO-containing superstructures are consequences of the fact that NHCO dipoles fluctuate much less than do the solvent molecules. Comparison between the various NHCO-containing superstructures shows that the enthalpy contribution is more favorable and the entropy contribution l w favorable to the Fe(1)- F a reaction with the two most flexible structures, [NHCO(CH2)loCONH]2-AC and -AT than with the more rigid structures, [NHCO(CH2)loCONH]2-CT and, especially, [NHCO(CH2)3Ph(CH2)3CONH]2.This observation points to the mcept that the flexibility of the chains leads to a more facile orientation of the NHCO dipole allowing the optimization of the dipole charge interactions. The resulting decrease of the enthalpic contribution is however partially counteracted by a negentropy (opposite of the entropy) compensation arising from the increased ordering of the flexible structure from Fe(1)- to Fe("O")2-. Some of this negentropy most probably also arises from the lesser protection against solvation offered by the AC and AT structures. We thus see that the orientation of the dipoles toward the regions of the porphyrin that bear the negative charges plays an important role. The flexibility of the chains allows the optimization of the orientation but the ensuing creation of negentropy diminishes the overall free energy gain. With the more rigid structures, there is lets negentropy compensationlobut the more static orientation of the NHCO dipoles is less favorable to the minimization of the energy upon passing from Fe(1)- to Fe("0")". The ideal situation would be a rigid orientation of the NHCO dipoles that would be precisely suited to the location of the negative charges. Among all the NHCOoontaining structur*i investigated in this study, the [NHCOCH(Ph)NHCO(CH2),CONHCH(Ph)CONHI2€T structure appears as the most favorable in the sense that the entropy compensation is very small and, at the same time, the minimization of the reaction enthalpy is large owing to the presence of four NHCO groups in each of the two straps.
-
Conclusions The comparison between the enthalpy-entropy balance of the Fe(I)-/Fe('0")2- reaction in the ether-linked basket-handle
AnxolabWre et al.
9352 The Journal of Physical Chemistry, Vol. 96, No. 23, 1992
Iastreaoat.tiaL The nonisothermal cell arrangement is depicted in Figure 4. As discussad in detail in ref 12, such an arrangement leads to a negligible contribution (less than 0.03 mV/K) of the thermal junction potential to dEf/dT. This contribution should be even less in a nonaqueous solvent13such as NJV'dimeth ylformamide. A home-built potentiostat equipped with a positive feedback was used together with a function generator from commercial origin (Taccugsel GSTP2). The cathodic and anodic peak potentials were measured on a Tektronix 3A75 digital oscilloscope. The overall accuracy of the peak potential determinations during a series of experiments were the temperature was varied was f l mV.
Acknowledgment. We are indebted to P. Maillard (Institut Curie, Orsay) for the gift of a sample of the NHCOCH(Ph)NHCO(CH2)4CONHCH(Ph)CONH]2-CT Fe"'C1 porphyrin.
References md Notes
F'igure 4. Nonisothermal cell used in this work (1) working electrode (glassy carbon disk of 3-mm diameter from Tokai); (2) thermostated reference electrode (20 "C), Cd-Hg amalgam bathed with a saturated CdC12solution in DMF, separated from another saturated CdC12 solution
in DMF by a glass frit, itaclf separated by another glass frit from the bridge 2' which contains a 0.1 M solution of the supporting electrolyte in DMF; (3) platinum wire counter electrode; (4) thermometer. The cell is thermostated by a methanol circulation through a double-wall jacket and the reference electrode by a water circulation.
porphyrins and that of TAP allows one to uncover the reason that the reaction is more difficult in the fmt case than in the second. Protection against solvation is important in the Fe(1)- complex but much less in the Fe(yO")2- complex owing to the intrusion of solvent molecules attracted in the vicinity of the porphyrin by the interaction with the double negative charge. The enthalpy contribution is thus only slightly increased with the most protactins structures and actually slightly decreased with the legs protecting ones. Under these conditions, what makes the reaction more difficult with the superstructured porphyrins is essentially the decrease in the entropy brought about by the increased ordering of the chains in the FC('O")~- complex arising from the presence of strongly oriented solvent molecules in the vicinity of the porphyrin (this squeezing of the chains by the oriented solvent molecules diminishes the number of possible spatial configurations). In the case of NHCO-containing structures, entropy factors have an opposite effect. A large portion of the interactions with the solvent are replaced by interactions with the surrounding NHCO dipoles. These are fluctuating much less than solvent molecules and, therefore, the entropy compensation of the interaction energy is much less. The reason that the reaction is easier in the presence than in the absence of these NHCO-containing superstructures thus essentially derives from this entropy effect. These findings offer a remarkable example of a system where entropy expenses allowed during the synthesis of the molecular superstructures result in entropy gains in the chemical reactivity of the active center. Experimntal Section Chemicals. TAPFe"'C1 and the ether-linked and secondaryamide Containing baslet-handle F R l Porphyrins (Figure 1) were s y n t h i z d accofding to previously published proced~res.~~9" OEP and TpPFdwl were from commercial origin (Ventron) and used as received. The NJV'dimethyIformamide (from Carlo Erba) was distilled under reduced pressure and dried over neutral alumina (Woelm Super I) before use. The supporting electrolytes, n-Bu4NPF6and n-HeqNC104 (from Fluka) were recrystallized and vacuum dried at 50 OC before use.
(1) (a) Universitt de Paris 7. (b) Institut Curie. (2) (a) Cotlman, J. P.Acc. Chem. Res. 1977,10,265. (b) Smith, P.D.; James, B. R.; Dolphin, D.H. Coord. Chem. Rw. 1981,39,31. (c) Traylor, J. G. Acc. Chem. Res. 1981,14,102. (d) Morgan, B.; Dolphin, D.Srrucrure and Bondfng;Buchler, J. W., Ed.; Springer-Verlag: New York, 1987; Vol. 64,p 115. (3) (a) Lexa,D.;Momenteau,M.;Rentien, P.;Rytz, G.; Savtant, J.-M.; Xu, F.J. Am. Chem. Soc. 1984,106,4755. (b) Lexa, D.;Momenteau, M.; Savtent, J.-M.; Xu,F. Inorg. Chem. 1985, 21, 122. (c) Gueutin, C.; Lexa, D.;Momentau, M.; Savtant, J.-M.; Xu, F.Imrg. Chem. 1986,25,4294. (d) Lexa,D.;Momenteau, M.;Savtant, J.-M.; Xu, F.J. Am. Chem. Soc. 1986, 108, 6937. (e) Lexa, D.;Momenteau, M.;Savhnt, J.-M.; Xu, F. Inorg. Chem. 1986,25,4857. (f) Lexa, D.;Savtant, J.-M.; Wang, D.L. Orgum metallics 1986, 5, 15428. (g) Lexa, D.;Maillard, P.;Momenteau, M.; Sav&mt, J.-M. J. P h y . Chem. 1987,91,1951. (i) Lexa,D.;Mmenkau, M.; Savtant, J.-M.; Xu, F. J. Elecrroanal. Chem. 1987, 237, 131. (j)Lexa,D.; Savtant, J.-M. Supramolecular Effects in the Redox and Coordination C h e d t r y of Superstructured Iron Porphyrins. In Redox Chemisrry and Interfacial Behwior of Biologic01 Molemles; Dryhurst, G., Niki, K.,Ede.; Plenum: New York, 1988; pp 1-25. (k) Anxolab€h€rc, E.;Lexa, D.;Momenteau, M.;Savtant, J.-M. J. Phys. Chem. 1992, 96, 1266. (4) (a) While therc is little doubt that iron(1) is the main reanant form of the com lex resulting from the injection of one electron into an iroo(I1) porphyrin,4& the nature of the main reanant f m ( s ) of the cclmplex multing from the injection of an additional electron is not a settled question.* (b) cohm,I. A.; 08tfekL D.;Lichtcnstein, B. J. Am. Chem. Soc. 1972,944522. (c) Lexa, D.;Momenteau, M.; Mispelter, J. Biochim. Biophys. Aero 1974, 338,151. (d) Kaduh, K.M.;L a m , G.; Lexa, D.;Momenteau, M.J. Am. Chem. Soc. 1975,97,282. (e) Red, C. A. A h . Chem. Ser. 1!382,H)l, 333. (f) Srivatsa, G. S.; Sawyer, D.T.; Boldt. N. J.; Bocian, D.F.Inorg. Chem. 1985,21,2133. (g) Hickman, D.L.; Shirazi, A.; Goff,H. M. Inorg. Chem. 19M, 2 4 563. (h) Donohot,R. J.; Atamian, M.;Bocian, D.F.J. Am. Chem. Soc. 1987,109, 5593. (i) Teraoka, J.; Hashimoto, S.; Sughoto, H.; Mori, M.;Kitagawa, T. J. Am. Chem. Soc. 1987,109,180. *) Mashiko, T.; Reed, C. A.; Haller, K. J.; Scheidt, W.R. Inorg. Chem. l&, 23, 3192. (k) See a h footnote 20 in ref 41. (1) Hammouchc, M.;Lexa, D.;Momenteau. M.; Savtant, J.-M. J. Am. Chem. Soc. 1991, 113,8455. (5) Aodrieux, C. P.;Savtant, J.-M. J. Elecrroonal. Chem. 1974,57,27. (6) Bockris, J. OM.; Reddy, A. K.N. Modern Elecrrochemisrry; Plenum Ress: New York, 1970; Vol. 1. (7) Note, however, that the values obtained in the case of n-Bu,N+ and simple porphyrins point to a more specific interaction than those dmcribed by the Debye-Hllckel model. (8) M e r , J. E.; Grunwald, E. Rates and Equilibria of Organic Recrctlons; Wiley: New York, 1963; pp 40-56. (9) Jaworaki, J. S . J. Elecrroonal. Chem. 1987,219,209. (10) Although ClW to the range of experimental uncertainty, the slightly positive entropy contribution found for the [NHCQ(CH&Ph(CH&CO~]2 structure may look surprising. It may be rationalized as follows. Fmt, protection against solvation is maximal as suggested by the results found in the aeries of ether-linked superstructure. Second, as revealed by molecular models, the most stable orientation of the NHCO dipoles is more suited to a large interaction with the negative charges borne by the porphyrin in the Fe("O")* complex where a substantial part of the electron density is likely to be delocalized over the periphery of the porphyrin ring. Maximization of thedipok-chargL? htsnotionenergy in tbc latter species m y thus wcll iaMfve the existence of several configurations with different NHCO orientations. (11) (a) Momenteau, M.;Mispclter, J.; Loock, B.; Bisagni, E. J. Chcm. Soc., Perkins Trans. 1 1983,189. (b) Momenteau, M.;Mupeltcr,J.; Loock, B.; L h t e , J. M.J, Chem. Soc., Perkins Trans. 1 1985,221. (e) Momentau, M.; Imck, B. J. Mol. Carol. 1980, 7, 315. (12) de Bethum, A. J.; Licht, T. S.; Swendeman, N. J. Electrochem. Soc. 1959.106,616. (13) Gaboriaud, R.; Letelier, P. J. Chim. Phys. 1975, 72, 357. (14) Garreau, D.;Savtant, J.-M. J. E/ecrroona/.Chem. 1977,99,2786.
