J. Phys. Chem. 1984, 88, 31 18-3120
3118
Magnotta. An alternate explanation is that NO, has an absorption cross section of the order of magnitude cm2 between 290 and 300 nm. In this case the observed atomic oxygen (ref 4, p 139) is that expected from the photolysis of the NO3in equilibrium with the NO2 and N2Os in the system. All things considered, it seems advisable at this time for atmospheric photochemical modelers to assign a quantum yield of 1 to channel 3, NO2 NO3,even though a small quantum yield
+
from channel 1 cannot be excluded. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U S . Department of Energy under Contract No. DE-AC03-76SFO0098. Registry No. NO3, 12033-49-7; N205,10102-03-1; N,, 7727-37-9; 02, 7782-44-7.
Kinetics of Demetallation of Manganese( I I ) Porphyrins in Aqueous Solutions' K. M. Morehouset and P. Neta*t Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556, and National Bureau of Standards, Washington, D.C. 20234 (Received: November 22, 1983)
Radiolytic reduction of Mn"'TPPS, MnII'TMPyP, and Mn'I'TPyP in acid solutions produces the manganese(I1) porphyrins which then undergo demetallation. The rates of demetallation were monitored by kinetic spectrophotometry at various [H+]. The results show that MnIITPPS hydrolyzes to H4TPPS2+in a process whose rate is dependent on [H'] at 20-60 mM, with k -1 X lo6 M-' S-' , but dependent on ["I2 at [H'] < 10 mM, with k 4 X lo7 M-2 s-'. The two pyridyl porphyrins 5 X lo4 M-2 s-l. demetallate much more slowly, with rate constants at 0.1-1.0 M H' dependent on [H+I2,with k
-
Introduction Manganese porphyrins are of interest since it is believed that some Mn complex is involved in photosynthesis,2 and it has been proposed that these porphyrins would be a good model system for a catalyst to effect photooxidation of water to o ~ y g e n . ~ The -~ characterization and photochemistry of several water soluble manganese porphyrins have been It has been shown that these compounds are good candidates for charge accumulation since they have several oxidation states (Mn", Mn"', Mn", Mn") available. We have recently studiedg the kinetics of the redox reactions of several manganese porphyrins by pulse radiolysis and obseryed the transformations between MnIVP,Mn"'P, Mn"P, and MnIIP-. In the course of this work, as well as in previous studies,331*12it was noticed that Mn" porphyrins undergo demetallation in acid solutions. The kinetics of these processes were followed for several compounds by different techniques. Electrochemical experiments'O with Mn-hematoporphyrin indicated that the reH+ had a first-order dependence on acid conaction Mn"P centration over most of the range studied and a rate constant of k = 2.9 X lo6 M-' s-l was found at pH 3.79. More recently, Hambright12 has found that the acid solvolysis of Mn"TMPyP (2) and Mn"TMPyP (3) (the 2-pyridyl and 3-pyridyl isomers) followed first-order dependence on [H+] at high acidities and second order at lower acidities. It has been recently shown that the pulse radiolysis technique can be effectively used to study kinetics of demetallation of metalloporphyrins both in a q u e o ~ s and ' ~ nonaque~us'~ solvents. In this paper we present a pulse radiolytic investigation of the kinetics of demetallation of three manganese(I1) porphyrins in aqueous solutions.
+
Experimental Section The metalloporphyrins were obtained from Midcentury Chemical Co. in the form of Mnrl'TPPS (sodium tetrakis(4sulfonatopheny1)porphyrin) MnII'TMPyP (tetrakis(N-methyl-4pyridy1)porphyrin chloride), and MnII'TPyP (tetrakis(4pyridy1)porphyrin). They are all monomeric in dilute aqueous solution^>'^-'^ and their concentrations were determined spectrophotometrically on the basis of the reported extinction coefUniversity of Notre Dame. *National Bureau of Standards.
0022-3654/84/2088-3118$01.50/0
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f i c i e n t ~ . ~ Water , ~ , ' ~ was purified by a Millipore Milli-Q system and all the other chemicals were Baker Analyzed reagents. Solutions were prepared freshly before each experiment and were kept in the dark to minimize photolysis. They were deoxygenated by bubbling with pure N 2 0which serves as an electron scavenger (N20 + eaq- N2 + OH- OH). Spectra were monitored in a Cary 219 spectrophotometer interfaced through an LSI-11 to a PDP-11/55 computer for storage and manipulation of spectra. Steady-state radiolysis was carried out in a 'Wo source (Gammacell 220) with a dose rate of 2 X lOI7 eV g-' m i d . Pulse radiolysis experiments were performed with the computer-controlled apparatus described previously18 with some modifications. Pulses of 8-MeV electrons from an A R C 0 LP-7 linear accelerator were usually of 10-ns duration with doses (300-600 rd) sufficient to produce 2-4 pM of radicals from water. An electronic shutter and the proper interference filters were used to minimize photolytic effects by the analyzing light. Dosimetry was performed by N 2 0 saturated KSCN solutions assuming +
+
(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2525 from the Notre Dame Radiation Laboratory. (2) Sauer, K . Acc. Chem. Res. 1980, 13, 249. (3) Loach, P. A,; Calvin, M. Biochemistry 1963, 2, 361. (4) Porter, G. Proc. R . SOC.London Ser. A 1978, 362, 281. (5) Harriman, A. Coord. Chem. Rev. 1979, 28, 147. (6) Harriman, A.; Porter, G. J. Chern. SOC.,Faraday Trans. 2 1979, 75, 1532, 1543. (7) Carnieri, N.; Harriman, A,; Porter, G. J . Chem. SOC.,Dalton Trans. 1982, 931. (8) Wohlgemunth, R.; Otvos, J. W.; Calvin, M. Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 5111. (9) Morehouse, K. M.; Neta, P. J . Phys. Chem. 1984, 88, 1575. (10) Davis, D. G.; Montalvo, J. G., Jr. Anal. Chem. 1969, 41, 1195. (11) Duncan, LA.; Harriman, A.; Porter, G. J . Chem. SOC.,Faraday Trans. 2 1980, 76, 1415. (12) Hambright, P. J . Inorg. Nucl. Chem. 1977, 39, 1102. Inorg. Nucl. Chem. Lett. 1977, 13, 403. (13) Kumar, A,; Neta, P. J . Phys. Chem. 1981, 85, 2830. (14) Levanon, H.; Neta, P. Chem. Phys. Lett. 1980, 70, 100. (15) Krishnamurthy, M.; Sutter, J. R.; Hambright, P. J. Chem. Soc., Chem. Commun. 1975, 13. (16) Pasternack, R. F.; Huber, P. R.; Boyd, P.; Engasser, G.; Francesconi, L.; Gibbs, E.; Fasella, P.; Venturo, G. C.; Hinds, L. de C. J . Am. Chem. SOC. 1972, 94, 4511. (17) Kalyanasundaram, K.; Neumann-Spallart, M. J . Phys. Chem. 1982, 86, 5163. (18) Patterson, L. K.; Lilie, J. In!. J . Radiat. Phys. Chem. 1974, 6 , 129.
0 1984 American Chemical Society
Kinetics of Demetallation of Manganese(I1) Porphyrins
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3119
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A-nm Figure 1. (A) Absorption spectra of MnII'TPPS (-) and the product of its one electron reduction and demetallation, H4TPPSZ+(---). A M MnII'TPPS, 0.1 M i-PrOH, and 0.01 solution containing 3.3 X M HC104 was saturated with N 2 0 and irradiated in the Gammacell. The optical path length was 2 mm. The dotted line was recorded after 30-s irradiation, and dashed line after 30 s more. (B) Differential absorption spectra observed upon pulse radiolysis of a solution containing 5.8 X M MnII'TPPS, 0.1 M i-PrOH, and 0.06 M HClO,, saturated with NzO. The solid line was obtained from the difference between the spectra of the product and the starting material is A. One unit on the relative absorbance scale corresponds to e = 1000 M-' for species produced with a yield of G = 6.
G[(SCN),-] = 6.0 and e = 7600 M-' cm-' at 480 nm."
Results and Discussion Demetallation of Mn"P was studied by pulse radiolysis by producing this species in situ from Mn"'P and then following the displacement of MnI1 by reaction with acid. Aqueous solutions of Mn"'P with 10% i-PrOH at various acid concentrations were saturated with NzO and then irradiated. In this system all the primary radicals of water radiolysis result in the reduction of Mn"'P to MnIIP, as shown previo~sly.~ HzO eaq-, H, O H (1) eaq-
--
+N20
eaqOH
+ H+
+ (CH3)&HOH
+ (CH,),CHOH (CH,),COH + MnlIIP H
+ OH- + O H
-- ++ - + Nz
(2)
H
HzO
H2
(3)
(CH,)#OH
(4)
(CH3)2COH
(5)
(CH3)2C0
H+
+ Mn"P
(6)
The competition between reactions 2 and 3 depends on the acid concentration. However, either route leads to production of the hydroxyisopropyl radical and eventual reduction of the porphyrin. Direct reaction of H and O H with the porphyrin, which would lead to different products, is unimportant in these solutions because of the low concentration of the porphyrin relative to that of iPrOH. Steady-state radiolysis experiments have shown that Mn"P produced by reaction 6 is stable in neutral and alkaline solution^.^ (19) Schuler, R. H.; Patterson, L. K.; Janata, E. J. Phys. Chem. 1980,84, 2088.
(20) Nwaeme, J.; Hambright, P. Inorg. Chem., submitted for publication.
Morehouse and Neta
3120 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
on [H+] observed here with Mn"TPPS at the higher H+ concentration indicates that the first protonation (reaction 8) is the rate-determining step under these conditions. The square dependence at [H+] < 10 mM suggests that in this range reaction 9 is sufficiently slow to contribute to the kinetic order of the overall reaction. For a more rigorous treatment of the results we may assume that reaction 8 is a preequilibrium and that reaction 9 is immediately followed by reactions 10-12. This leads to the expressionI2 kobsd
=
ks[~+iZ (k-8/k9)
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30.
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40.
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50.
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60.
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A plot Of [H+]'/kObsdVS. [H'] (Figure 3B) yields k8 = 1.2 x lo6 M-' s-' from the slope and k-8/k9 = 2.8 X from the intercept. The value of ks is in good agreement with that derived from Figure 3A and represents the second-order rate constant when [H+] >> 2.8 X M. On the other hand, when [H+]
