J. Phys. Chem. 1984, 88, 5332-5333
5332
Kinetics of Electron Transfer from Cobalt( I I ) Porphyrins to Various Metalloporphyrin 7r-Radical Cations in Irradiated Carbon Tetrachloride Solutions' J. Grodkowski? J. H. Chambers, Jr.,3 and P. Neta*4 Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, and National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: March 27, 1984)
One-electron oxidation of a series of metallotetraphenylporphyrins (M-TPP) was studied by pulse radiolysis in CCll solutions. The resultant r-radical cations were found to oxidize Co"TPP to Co"'TPP with rate constants of the order of lo5 M-' d, much slower than the diffusion-controlledrates despite the high free energies of these reactions. In the presence of pyridine these electron-transfer reactions were much faster ( 107-10s M-I sd), suggesting that pyridine, as an axial ligand, facilitates the electron transfer between metalloporphyrins.
this medium, radiolytic formation of acid is not expected, as was indeed confirmed by the absence of any demetallation. Furthermore, one-electron oxidation of solutes takes place with higher yields than in most other solvents since M P is oxidized by the primary solvent radical cations and also by the CC1302radicals produced in aerated solutions. Rates and yields of oxidation in this medium have been reported recently,I2 and we have utilized this system to study reaction 1. We find that in the absence of pyridine as axial ligand the reaction is slower by about 2 orders of magnitude.
Introduction Electron transfer reactions between metalloporphyrins (MP) and their radical cations (MP+.) are of interest as models for similar reactions occurring during photosynthesis. The kinetics of these reactions can be conveniently studied by pulse radiolysis, as has been dem~nstrated.~.~ The metalloporphyrin radical cations are produced either in aqueous solution^,^^^ with several oxidizing radicals, or in organic solvents:*8 mainly halogenated hydrocarbons as oxidizing media under ionizing r a d i a t i ~ n . ~Recent , ~ ~ studies using 1,2-dichloroethane (DCE) as solvent have shown that many metalloporphyrins undergo one-electron oxidation under radiol~ s i s . ~However, ,~ the study of these processes was complicated by the radiolytic formation of HCl as a by-product, which in certain cases reacts with M P to cause demetallation. To prevent these side reactions, pyridine was used as a base. Under these conditions, electron transfer reactions between various MP+. and either Co"TPP or chlorophyll a were found to take place with rate constants usually of the order of lo8 M-' s-'.~ These values, however, are not for reaction of the simple metalloporphyrins, i.e. MP+.
+ CO"P
-
MP
+ C0"'P
(1)
but rather for the reaction of their pyridine complexes (monoor dipyridisate)" as discussed beforee5 py-MP+.
+ PY-CO"P
+
py-MP
+ PY-CO"'P
(2)
In order to prevent demetallation and to study the reactions of Ybarenmetalloporphyrins, we have used C C , as solvent. In (1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2592. (2) On leave of absence from the Institute of Nuclear Chemistry and Engineering, Warsaw, Poland. (3) Parts of this work were presented as a prerequisite for the B.S. degree of Kalamazoo College. (4) Address correspondence to this author at the National Bureau of Standards (C216 Bldg 245). ( 5 ) Neta, P.; Grebel, V.;Levanaon, H. J . Phys. Chem. 1981, 85, 2117. (6) Neta, P. J. Phys. Chem. 1981, 85, 3678. (7) Harriman, A.; Richoux, M. C.; Neta, P. J. Phys. Chem. 1983, 87, 4957. (8) Levanon, H.; Neta, P. Chem. Phys. Le??.1980, 70, 100. (9) Arai, S.; Ueda, H.; Firestone, R. F.; Dorfman, L. M. J . Chem. Phys. 1969, 50, 1072. (10) Wang, Y.;Tria, J. J.; Dorfman, L. M. J. Phys. Chern. 1979,83,1946. (1 1) Reaction 2 is formulated with all reactants and products as the monopyridinate, although it is possible that the number of pyridines bound to the metal may change with the change in oxidation state, e.g. py-Co"P gives py2-Coi1'P (Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 13, 2260. Kadish, K. M.; Bottomley, L. A.; Beroiz, D. Inorg. Chem. 1978, 17, 1124). This change was not observed spectrophotometrically as a distinct step on the time scale of the oxidation reaction and is neglected in the formulation of reaction 2.
0022-3654/84/2088-5332$01.50/0
Experimental Section Metal complexes of tetraphenylporphyrin (TPP) were obtained . from Midcentury Chemical Co. The other materials and the experimental procedures are identical with those used in the previous s t ~ d i e s . ~ , ' ~ ~
Results and Discussion Steady-state radiolysis experiments with CCl, solutions of ZnTPP did not indicate any demetallation of the porphyrin. Several types of experiments- were carried out, parallel to those reported previously with DCE,8 and unlike the results of those earlier studies, we found no apparent formation of H2TPP or H4TPP2+but only oxidation of the intact ZnTPP. This finding is expected since the radiolysis of CC14 cannot produce acid as does the radiolysis of DCE. Therefore, we can proceed to use CCl, as solvent for radiolytic oxidation of metalloporphyrins. Pulse radiolysis experiments were carried out with a series of M-TPP (M = Zn", Mg", Cd", Pb", Hg", Pt", Cu", Co", Mn"', and VIVO),and in each case the transient differential spectrum of the one-electron-oxidized species was recorded. The spectra were generally similar to those observed previously in DCE sol u t i o n ~and , ~ ~in~ most cases (except Co") they indicate the formation of the ?r-radical cations MP+. characterized by their broad absorption peaks in the 600-700-nm range. The yields of these radicals were found to be relatively low in deaerated solutions but much higher in the presence of oxygen; in the case of ZnTPP reaching a value of G 8 (radicals/100 eV). For the dependence of G on [ZnTPP] in the presence and absence of 02.see Figure 3 in ref 12. As discussed before, the oxidation occurs by two distinct processes: (a) reaction of the solvent radical cations and other primary oxidizing species and (b) reaction of CC1302radicals produced from CCI3 O2in aerated solutions. The former process involves short-lived species and takes place with very high rate constants, while the latter reaction involves the longer lived radical CC1302.
-
+
+ MP
CCI3O2
+
CC1302-
+ MP+*
(3)
Reaction 3 was found to take place with moderate rate constants: ~
(12) Grodkowski, J.; Neta, P. J . Phys. Chem. 1984, 88, 1205.
0 1984 American Chemical Society
One-Electron Oxidation of Metallotetraphenylporphyrins TABLE I: Rate Constants for One-Electron Oxidation of Co"TPP by Various MetalloDorohvrin Radical Cations MP E(MP/MP+.)" k , (in CCl,)b k , (in CCI, P Y ) ~ ~~~
+
~
0.54 0.63 0.71 0.90 1 .oo
MgTPP CiTPP ZnTPP CuTPP VOTPP
1 x 105 -6 X lo5 2 x 105 6 X lo5
3.6 X 10' - 5 x 107 3.7 x 107 1.2 x 108 1.3 X 10'
These oxidation potentials19 were measured in solvents other than CCI4 and in the absence of pyridine. In relation to E(Co"/Co"'TPP) = 0.32 V they can serve to obtain a rough estimate of AE for reaction 1. M-' SKI. (I
VOTPP, 0.5; CuTPP, 0.3; PtTPP, 0.7; MnTPP, 2; ZnTPP, 7; PbTPP, 6; CdTPP, 10; MgTPP, 4; and CoTPP, 10 (in units of lo8 M-' s-l ). The k3 values generally increase with decrease in oxidation potential of the porphyrin as expected. At [MP] = 10-4-10-3 M the formation of MP+. is practically complete within 10-100 ps, after which time one can follow the subsequent reactions of MP+., such as reaction 1. Before proceeding to study the kinetics of reaction 1, some comments should be made on the behavior of Co"TPP in irradiated CC14solutions. The differential spectrum recorded with freshly prepared solutions indicated bleaching of the Co"TPP absorption and formation of CoII'TPP as reported previously in DCE (ref 5 , Figure 1). However, after the solution stands under air for 30 min, some Co"TPP appears to be oxidized to the Co"' form and Co"'P the pulse radiolysis then results in both oxidation of Co"P and of Co"'P Co"'P+.. The latter process was deduced from the appearance of very broad absorption at 600-700 nm. Therefore, in order to avoid this complication, reaction 1 was studied only in deoxygenated solutions. Reaction 2 can be studied only in deoxygenated solutions since the pyridinate complex of CoIITPP is very readily oxidized by 02. The kinetics of reaction 1 were determined by following the decay of the MPf. absorption a t 600-700 nm at various concentrations of Co"P. At low [Co"P] the formation of the Co"'P absorption at 5 5 5 nm also was monitored; however, at higher [Co"P] the absorption by this compound prevented experimental observations at 555 nm. M P was usually present at 10-4-10-3 M, and the Co"P concentration was 3-10 times lower. The rate constants were found to be in the range of (1-6) X lo5 M-' s-l, i.e. much lower than those measured in DCE solutions in the presence of pyridine (-lo8 M-' s - ~ ) . ~In order to determine whether this large difference was owing to the effect of pyridine, we have carried out similar measurements in CCl, solutions containing 1% pyridine. The rate constants k2 obtained in this case are at least 2 orders of magnitude higher than those found in the absence of pyridine (Table I).I3
-
-+
(13) It should be mentioned that a similar experiment with chlorophyll a as an electron donor to ZnTPP*. showed a rate constant of 9 X lo6 MM1s-l in CCl, without pyridine, while the value reportedS for DCE + pyridine solution was 1.5 X 10' M-l s-l. In this case, where oxidation occurs on the ligand, pyridine increases the rate constant by a factor of 17, while in the oxidation of Co"TPP, where oxidation occurs on the metal, pyridine exhibits a stronger effect, an average factor of 200 for k z / k l .
The Journal of Physical Chemistry, Vol. 88, No. 22, 1984 5333
The great enhancement of the rates of electron transfer in the presence of pyridine must be owing to the axial ligation of pyridine to the metalloporphyrins. Its ligation with the cobalt porphyrin affects the AG of the reaction since pyridine binds to the Co"' state more strongly than to the Co" state.14 The redox potential of the C O ~ ~ P / C Osystem ~ ~ ' P was found to shift from -0.3-0.4 to -0.2 V (vs. SCE) upon changing from a noncomplexing solvent to pyridine.I5 The effect of pyridine on the redox potential of the MP/MP+. couples may be expected to be somewhat smaller since this redox reaction does not involve the metal itself (where the pyridine binds). The magnitude of this effect was not measured; however, an effect of C1- on the redox potential of ZnTPP of AE 0.2 V has been reported.16 Another effect which may contribute to the rate enhancement is that the pyridines on either or both porphyrins serve as channels for the electron transfer, as suggested for other ligands in the reaction of Co"'P with various reducing agents. l 7 , I 8 The rate constants summarized in Table I are much lower than the diffusion-controlled limit despite the fact that many of the reactions are driven by a considerable difference in redox potential ( A E > 0.4 V).I9 Furthermore, the rate constants are fairly insensitive to variations in redox potentials within each set of data. Therefore, the strong effect of pyridine on the rate of electron transfer is more likely to result from the efficiency of pyridine as an electron channel rather than from its effect on the redox potential. The very low rate constants observed for reaction 1 indicate that if the reaction takes place by an outer-sphere electron transfer mechanism,20it requires a considerable extent of geometric orientation of the reactants and/or inner-sphere reorganization. Pyridine ligation may lower the geometric orientation requirements. However, the apparently low sensitivity of kl and k2 to changes in redox potential may suggest that these reactions are not true outer-sphere electron transfer in the sense that the rate-determining steps may involve the formation of an intermediate complex between Co"P and MP'..
-
Acknowledgment. We thank Dr. D. Meisel for helpful discussions. Registry No. CoTPP, 14172-90-8; MgTPP, 14640-21-2; CdTPP, 14977-07-2; ZnTPP, 14074-80-7; CuTPP, 14172-91-9; VOTPP, 1470563-6. (14) Pasternack, R. F.; Cobb, M. A.; Sutin, N. Inorg. Chem. 1975, 24, 866. (15) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. SOC.1976, 98, 3484. (16) Fajer, J.; Borg, D. C.; Forman, A.; Felton, R. H.; Vegh, L.; Dolphin, D.Ann. N.Y. Acad. Sci. 1973, 206, 349. (17) Pasternack. R. F.; Sutin. N. Inorp. Chem. 1974. 23. 1956. (18) From the lack of clear dependence zf k2,measured in DCE pyridine solutions, on the redox potential, it was previously suggestedS that the ratedetermining step in those electron transfer reactions involves removal or reorientation of pyridine. That suggestion is not supported by the present results and should be reinterpreted as discussed below. (19) Felton, R. H. In "The Porphyrins"; Dolphin, D., Ed., Academic Press: New York, 1978; Vol. 5, Chapter 3 and references therein (see also ref 15). (20) Marcus, R. A. J . Chem. Phys. 1956, 24, 966; 1957, 26, 867, 872.
+
