J. Phys. Chem. 1981, 85,2830-2832
2830
Reduction and Demetalation of Silver Porphyrins in Aqueous Solutions' Ani1 Kumar and P. Neta' Radiation Laboratoty and Department of Chemistry, University of Notre Dame, Notre L%m,Indiana 485513 (Received: March IO, 1981; In Final Form: M y 18, 1981)
The reduction of silver porphyrins and the demetalation of the reduced species in aqueous solutions were studied by steady-state and pulse radiolysis. Ag"'TPPS is reduced to Ag"TPPS by ea; with k = 1.0 X 1O'O M-l s-l. Ag"TPPS is reduced by ea; ( k = 1.6 X 10") and by (CH&&OH(k = 6 X lo8M-' s-l) to Ag'TPPS. Ag'TPPS reacts with water (k = 5 X lo49-l) to give HAgITPPS and the latter also reacts with water ( k = 1.3 X lo3 s-l) to yield Ag+ + HzTPPS as final products. H2TPPS is also reduced by ea< rapidly, k = 1.5 X 10'O M-' s-'. The anion radical (H,TPPS)-. transfers an electron to AgnTPPS somewhat slowly, k I 5 X loe M-l s-l, but the process occurs efficiently under y-radiolysis conditions.
Introduction Stable complexes of Ag(I1) and Ag(II1) with porphyrins have been prepared and characterizedS2 On the other hand, silver(1) porphyrins are unstable and demetalate rapidly. Furthermore, Ag+ ions undergo oxidation in the presence of free base porphyrins to yield silver(I1) porp h y r i n ~ . ~ ?Ag"TPPS ~ (TPPS = tetrakisb-sulfonatopheny1)porphyrin) was found to disproportionate in acid solutions to yield Ag"'TPPS, Ag+, and free p ~ r p h y r i n . ~ The mechanism of this reaction involves the intermediate formation of HAg'TPPS and its rapid demetalation. We have utilized the kinetic spectrophotometric pulse radiolysis technique to study the rapid reduction of Ag"TPPS and Ag"'TPPS and the demetalation of AgITPPS. Two distinct consecutive steps of protonation of Ag'TPPS were observed in neutral solution, with k = 5 X lo4 and 1.3 X lo3 s-l, which lead to the formation of free base porphyrin. Experimental Section AgnTPPS, AgmTPPS, and HzTPPSin the form of their sodium salts were purchased from Man-Win Coordination Chemicals (presently Mid-century Chemical Co.). Their absorption spectra indicate that they are of high purity (see below). Water was purified by a Millipore Milli-Q system and all the other chemicals were Baker Analyzed reagents. Solutions were prepared freshly before irradiation and were kept in the dark. They were deoxygenated by bubbling with pure nitrogen. When required, the solutions were bubbled instead with NzO, which reacts with e, - to produce OH (NzO + eaq- N2 + OH-+ OH). kteady-state radiolysis was carried out in a Gammacell 220 6oCosource with a dose rate of 3 X 1017eV g-' min-'. The spectra of reactants and products were measured with a Cary 219 spectrophotometer. Pulse radiolysis was carried out with an ARC0 LP-7 linear accelerator using 7-MeV electron pulses of 5-13s duration. The dose per pulse supplied energy to produce 2-3 M Mtotal radical concentration. Kinetic spectrophotometry with signal averaging was done with a revised version of the computer-controlled apparatus described previ~usly.~All experiments were carried out at room temperature, 21 f 1 "C.
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~
~
Results and Discussion Ag"TPPS, Ag"TPPS, and HzTPPS were studied in dilute aqueous solutions (3 X 104-1 x lo4 M), at low ionic strength ( lo4 M buffer), and usually containing 1-2% alcohol. Under these conditions the silver porphyrins exist predominantly in monomeric forms and the free base porphyrin is mostly monomeric at the lower range of con~entration.~The absorption spectra of these compounds have been r e p ~ r t e d . ~We ~ ~ have J examined 5 X lo4 M solutions of each of the porphyrins and found the = 413,515, 552, following parameters. For H,TPPS, A, 579, 634, nm, e = 4.95 X lo5, 1.68 X lo4, 7.14 X lo3, 6.39 X lo3, 3.52 X lo3 M-' cm-l, respectively. For Ag"TPPS, A, = 421 and 539 nm with t = 3.15 X lo5 and 1.47 X lo4 M-l cm-l. For AgII'TPPS, ,A, = 420 and 535 nm with e = 1.59 X lo5 and 1.22 X lo4 M-I cm-', respectively. Most of these parameters are in very good agreement with the previous o n e ~ . ~ However, i~J the extinction coefficients of the Soret bands are slightly lower than those reported by Kri~hnamurthy.~ Figure 1 shows the spectrum of Ag"TPPS in aqueous t-BuOH solution before and after y irradiation with various doses. The isosbestic points show that a clean reduction process takes place. This process results in the formation of H2TPPS. The final spectrum shown indicates -90% conversion of AgnTPPS to H2TPPSwith an initial G value of -3, i.e., the silver porphyrin is reduce by a;! and H but does not react with the CH2C(CHJ2OHradical produced from t-BuOH + OH. Upon further irradiation H2TPPS begins to protonate (by the H+ ions produced in the radiolysis) as indicated by the growth of the 644-nm absorption due to H4TPPS2+ (not shown in the figure). Experiments with i-PrOH instead of t-BuOH gave identical spectra but with appro4mately twice the yield, G -6, indicating that the (CHJ2COH radical produced from i-PrOH + OH reduces the silver porphyrin efficiently. Irradiation of Ag"ITPPS solutions under the above conditions showed initially some conversion into AgnTF'PS, but, before substantial reduction was achieved, the Ag"TPPS reacted further to produce HzTPPS. The final result is a quantitative two-electron reduction, but the intermediate spectra did not exhibit clear isosbestic points. Apparently, the reaction of Ag"TPPS with ea; and
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(1) The research described herein wm supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2232 from the Notre Dame Radiation Laboratory. (2) Po, H, N. Coord. Chem. Reu. 1976,20, 171. (3) Krishnamurthy, M. Znorg. Chem. 1978, 17, 2242. (4)Patterson, L. K.; Lilie, J. Int. J.Radiat. Phys. Chem. 1974,6, 129. 0022-3654/81/2085-2830$01.25/0
(5) Krishnamurthy, M.; Sutter, J. R.; Hambright, P. J . Chem. SOC., Chem. Commun. 1976,13. (6)Fleischer, E. B.; Palmer, J. M.; Srivastava, T. 5.;Chatterjee, A. J . Am. Chem. Soe. 1971,93, 3162. (7) Reynolds, W. L.; Scbufman, J.; Chan, F.; Brasted, R. C., Jr. Int. J. Chem. Kinet. 1977,9, 771.
0 1981 American Chemical Soclety
The Journal of Physical Chemistty, Vol. 85, No. 19, 1981 2831
Reduction and Demetalation of Silver Porphyrins
1.6 n 51
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1.2 .E
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X- nm Figure 1. y radiolysis of AgI'TPPS. Aqueous solution containing 2.15 X M AgI'TPPS, 0.1 1 M t-BuOH, and 1.2 X lo-' M sodium tetraborate at pH 8.7 was bubbled with N2and irradiated in a source. The lnitlal spectrum exhibits the maxima at 421 and 539 nm. The final spectrum exhibits the maxima at 413,515,552,579,and 634 nm and is assigned to H2TPPS produced by the radiolysis. Intermediate spectra, shown by dotted lines, were recorded after irradiation with 1.5 X lo1' and 3.0 X lo1' eV g-', and the final spectrum after irradiatlon with 4.5 X 10" eV g-'.
(CH3)$OH is nearly as rapid as that of AgmTPPS, as will be confirmed below by the pulse radiolysis experiments. Oxidation of Ag+ by OH radicals in NzO-saturated aqueous solutions containing HzTPPS leads to the formation of Ag'ITPPS and Ag"ITPPS successively. However, because of thermal reactions which also produce AgIITPPS, it was not possible to examine the radiolytic effect quantitatively. The kinetics and intermediate spectra in the reduction and demetalation processes were examined by pulse radiolysis. Ag"TPPS was studied in the presence of either t-BuOH in Nz-bubbled solutions or i-PrOH in NzO-saturated solutions. Representative kinetic traces are shown in Figure 2 and transient spectra in Figure 3. Using Nz-bubbled t-BuOH solutions, the broad spectrum of eaqis observed immediately after the pulse and is found to decay over -2 ps. The spectrum is not shown in the figure but its formation and decay are shown in traces 1and 2 in Figure 2. The rate constant for the reaction of ea; with AgnTPPS, calculated from these and several other traces at various concentrations of the porphyrin, is (1.6 f 0.3) X 1O'O M-l s-l. Hydrogen atoms, produced in a low yield (G = 0.55) during the radiolysis, are expected to react with AgnTPPS nearly as rapidly, judging from their diffusioncontrolled reaction with Ag+.8 Therefore, the reaction of H with Ag"TPPS is expected to occur at the same time as the reaction of eap and to yield most probably the same product. Following these reactions, two kinetic steps are observed over 100 ps and 2 ms (traces 3-6, Figure 2). The first step represents a spectral change as shown in Figure 2, and the second step leads to the final product shown in Figure 1, Le., HzTPPS. Both reactions follow first-order rate law and are independent of solute concentration (10-5-104 M). The rate constants are (5 f 1) X lo4and (1.3 f 0.4) X lo3 s-l, respectively, as determined from traces 3-6 in Figure 2 and many other traces at various wavelengths and ~~
~~
(8) Anbar, M.; Farhataziz; Ross, A. B. Natl. Stand. Ref. Data Ser., Natl. Bur. Stand. 1975 No. 51.
51 t,
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Figure 2. Kinetic traces observed upon pulse radlolysis of AgI'TPPS solutions: traces 1-6, 4.3 X lod M Ag'ITPPS, 0.1 Mf-BuOH, pH 8.7 borate buffer, N2bubbled; traces 1, 3,5, 540 nm; 2,630 nm; 4,620 nm; 6,610 nm. Traces 7-10, 1.5 X lo4 M Ag'lTPPS, 0.2 M I-PrOH, pH 8.9 borate buffer, N,O bubbled. Traces 7, 9,540 nm; 8,10,610 nm.
timescales. These rates probably represent pseudo-firstorder reactions of the intermediates with water. In the absence of buffers, where the H+ produced by the radiolysis is not rapidly neutralized, the rates become faster upon increasing the dose per pulse. The increase indicatas more rapid reaction with H+, but this could not be examined quantitatively because of the instability of the metalloporphyrin at lower pH. The results are confirmed by similar experiments in the presence of i-PrOH and NzO. Under these conditions the eaq-reacts with NzO to yield OH. The OH radicals and H atoms react with i-PrOH to yield (CH3)&OH. This radical reduces Ag"TPPS with K = (6 f 1) X lo8 M-l s-l as derived from the rate of bleaching at 540 nm shown by trace 7 in Figure 2 and several other traces at various wavelengths and various solute concentrations. This reduction step yields the same intermediate as that produced by the ea( reaction (Figure 3). However, trace 8 in Figure 2, taken at 610 nm, shows that before the reduction by
2832
The Journal of Physical Chemistry, Vol. 85, No. 19, 1981 18,000[
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9'7
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700
A- nm Figure 3. Transient absorption spectra observed in the pulse radiolysis IO-' M AgI'TPPS, 0.1 M I-PrOH, pH 8.0 borate buffer, N2 bubbled: (A)12 ps and (0)70 ps after the pulse. The extinction coefficients were calculated by using thiocyanate dosimetry and assuming G = 6.
of AgI'TPPS solutions. 3.7 X
(CH&OH is complete, a change in spectrum begins. This change occurs over -100 ps and is again followed by further change over -2 ms, with rate constants of 5 X lo4 and 1.3 X lo3 s-l as found in the above experiment with t-BuOH. It may be, therefore, concluded that the one-electron reduction of Ag'TPPS yields a transient which undergoes two successivereactions to produce fmally HzTPPS. These reactions probably involve protonation by water and may be formulated as follows:
AgI'TPPS
+ e,;
+ HzO + H20
Ag'TPPS HAg'TPPS
(or (CH3)&OH)
-
-
-
Ag'TPPS (1)
+ OHHzTPPS + Ag+ + OHHAg'TPPS
(2)
(3)
Reaction 1 produces Ag'TPPS in which the metal is probably bound to two nitrogens. In this process the 540-nm absorption of AgI'TPPS is bleached and a broad absorption at 550-700 nm is formed. The intermediate produced by reaction 1 does not appear to be the anion radical of AgnTPPS since that species would be expected to exhibit sharper and more intense absorption in the 650-700-nm region as was observed with other metalloporphyrin anion radicals?JO It is reasonable to expect that if the anion radical is, indeed, produced in the initial stage, it would undergo a rapid intramolecular electron transfer to yield the Ag(1) species. Reaction 2 (k = 5 X lo4 s-l) introduces a proton on one of the nitrogens and further removes the Ag(1) from the plane of the porphyrin ring. As a result, absorption maxima around 590 and 630 nm begin to take shape. Reaction 3 (k = 1.3 X lo3 s-l) introduces an additional proton and thus releases the Ag+ completely to produce HzTPPS as shown in Figure 1. This stepwise mechanism resembles that suggested for the acid-catalyzed demetalation of Zn and Mg porphy(9) Neta, P.; Scherz, A.; Levanon, H. J. Am. Chem. SOC.1979, 101, 3624. (10)See also experiments with H,TPPS described below.
Kumar and Neta rins.11J2 Unlike those cases, however, Ag'TPPS is extremely unstable and can accept protons directly from water molecules. This process is too rapid to be followed by the conventional stopped-flow technique. Pulse radiolysis allows determination of the rates of these rapid demetalation reactions as was also shown previously in other systems.13 Attempts to observe a reverse prccess, i.e., to oxidize Ag' to Ag(I1) in the pulse radioly~is'~ and to observe the kinetics of binding Ag(I1) to the porphyrin, were hampered by the thermal reaction between the solutes. Even rapid mixing of the two components at the entrance of the irradiation cell did not prevent the thermal reaction sufficiently to yield satisfactory pulse radiolysis results. Pulse radiolysis studies with solutions of Ag"'TPPS containing t-BuOH were carried out. The rate of reaction with e,; was determined to be k = (1.0 f 0.2) X 1O'O M-l s-l. The transient absorption recorded after completion of this reaction indicated that Ag'I'TPPS is initially reduced to Ag'ITPPS. However, because the latter reacts with e,; more rapidly than the original compound, the difference spectrum observed in the pulse experiment is slightly distorted as compared with that calculated from the spectra of the two compounds. This problem was also encountered earlier in the steady-state radiolysis experiments. HzTPPS was also found to react with e, - with a diffusion-controlled rate, k = (1.5 f 0.3) X lola M-l s-l. The product is the anion radical (H,TPPS)-. observed at pH ~ i700 : nm, c ~ i8000 : M-l cm-l), or its protonated 11.8 (A, form (H,TPPS). in neutral solution, as discussed previously for a similar ~ y s t e m .The ~ fact that in the y-radiolysis experiments Ag'ITPPS was cleanly and quantitatively converted into HzTPPS (Figure 1)suggests that the anion radicals of the free porphyrin, produced in solution by the partial reaction of H,TPPS with ea; or (CH3),COH, must transfer an electron efficiently to AgI'TPPS. An attempt to determine the rate constant of this electron transfer was made by a pulse radiolysis experiment in which the effect of addition of Ag'ITPPS on the rate of decay of (H,TPPS)-. was examined. From the lack of effect at Ag"TPPS concentrations up to 2 X lo4 M an upper limit of k I5 X lo6 M-' s-l is estimated for this electron transfer. The fact that the reaction occurs under y-radiolysis conditions suggest that k is probably >lo4 M-ls-l. The low rate constant must be partially the result of electrostatic repulsion between the reactants, bearing overall charges of -5 and -4. Conclusions Both the free base porphyrin and the silver porphyrins are reduced by ea; with diffusion-controlled rates. The anion radical (H2TPPS)-. can also reduce Ag'ITPPS. Ag'TPPS is unstable and demetalates by reaction with water into Ag+ and H,TPPS. Two successive steps of protonation with k = 5 X lo4 and 1.3 X lo3 s-l were observed, involving HAg'TPPS as an intermediate. (11) Snellgrove, R.; Plane, R. A. J. Am. Chem. SOC.1968, 90,3186. (12) Shears, B.; Shah,B.; Hambright, P. J. Am. Chem. SOC.1971,93, 776. (13) Levanon, H.; Neta, P. Chem. Phys. Lett. 1980, 70, LOO. (14) Kumar, A.; Neta, P. J. Phys. Chem. 1979,83,3091. J. Am. Chem. SOC.1980,102,7284.
