J. P h p . Chem. 1982, 86, 4537-4539
(P-MV+.) in the case of the metalloporphyrins. We have demonstrated in this and previous studies that the reactivity of the photoexcited triplet state of porphyrins and chlorophylls with radicals is substantially higher than that of the corresponding ground states. This behavior, not limited to the porphyrin moiety only, was found also with pyrenet whose photoexcited triplet exhibits similar chemical reactivity toward transient radicals. The experimental results clearly indicate that PRAP spec-
4537
troscopy is most suitable in studying reactions of photoexcited states with short-lived radicals. With regard to stable radicals such as MV'., the advantage of employing this method, over conventional laser photolysis, is less substantial. Nevertheless, in the PRAP mode the interference due to a possible absorption overlap between the photoexcited triplet and that of the stable radical can be avoided as the triplet state is prepared prior to the radical production.
Electron Spin Resonance and Optical Detection of Manganese(I V ) Tetraphenylporphyrin S. Konlshl,' M. Hoshlno, and M. Imamura The Institute of Physical and Chemical Research, Hirosawa, Wako-shl, Saitama, 351 Japan (Received: May 18, 1982; I n Flnal Form: July 21, 1982)
Radiation-chemicalone-electron oxidation of (tetraphenylporphinato)manganese(III)(CIMnInTPP)was carried out in tetrachloroethane (TCE) at 77 K. The ESR and optical absorption spectra of y-irradiated solutions revealed that oxidation occurs not at the porphyrin ring but at the central metal, giving manganese(IV)porphyrin. The spin state of the manganese(IV)porphyrin was assigned to be 3/2 from the analysis of the highly anisotropic ESR spectra.
Introduction Physical and chemical properties of metalloporphyrins and related compounds have been increasingly investigated with the aim of understanding their functions in biological systems and also of utilizing them as mediators for chemical and photochemical redox reactions. Manganese porphyrins are particularly interesting in this respect because of their possible involvement in the photosynthetic liberation of oxygen from water and also because of the possibility that they act as catalysts for electron transfer reactions. There have been experimental studies on their chemica11v2and photochemical3 redox reactions in fluid solution and a review by B o u ~ h e r .A ~ detailed theoretical study on their electronic structures has also been reported r e ~ e n t l y . ~The possible valence states of manganese in porphyrin complexes are considered to be 2+, 3+, and 4+. Among these valence states, 3+ is the most stable. The reduction of manganese(II1) porphyrins yields manganese(I1) porphyrins, which are stable only in the absence of oxygen and water. No reliable evidence for the existence of manganese(1V) porphyrins, however, has never been reported and a few papers r e p ~ r t evidence ~.~ for the existence of the cation radicals of manganese(II1)porphyrins produced in fluid solutions. Such reports suggest that manganese(1V) porphyrins do not exist or they are unstable in fluid solutions, in which the chemical or electrochemical oxidation of manganese(II1) porphyrins is carried out. If oxidation could be done in low-temperature matrices, manganese(1V) porphyrins might be stabilized. We
have a good method to perform such an experiment, i.e., one-electron oxidation of solute molecules by radiolysis of alkyl halide matrices.h9 The oxidation takes place through electron abstraction from solute molecules by solvent cation radicals produced by ionizing radiation. When radiolysis is carried out at liquid-nitrogen temperature, oxidation proucts are usually stabilized because of the rigidity of the low-temperature matrices. The application of this method has been reported for the formation of cation radicals of chlorophylls1° and copper" and lead12 porphyrins. As another successful application of this technique to trap labile metalloporphyrins, this paper reports the first clear evidence for the existence of the manganese(1V) porphyrin.
(1)Loach, P. A.; Calvin, M. Biochemistry 1963,2,361-71. (2)Calvin, M. Reu. Pure. Appl. Chem. 1965,15,1-10. (3)Engelsma, G.;Yamamoto, A.; Markham, E.; Calvin, M. J . Phys. Chem. 1962,66, 2517-31. (4)Boucher, L. J. J . Coord. Chem. Reo. 1972,7, 289-329. (5)Mishra, S.;Chang, J. C.; Das, T. P. J. Am. Chem. SOC.1980,102, 2674-81. (6)Furhop, J.-H.; Kadish, K. M.; Davis, D. G. J. Am. Chem. SOC.1973, 95,5140-7. (7)Shimomura, E. T.;Phillippi, M. A.; Goff, H. M.; Sholz, W. F.; Reed, C. A. J. Am. Chem. SOC.1981,103,6778-80.
(8)Hamill, W.H."Radical Ions"; Kaiser, E. T., Kevan, L., Eds.; Wiley-Interscience: New York, 1968;Chapter 9. (9)Grimson, A.; Simpson, G. A. J. Phys. Chem. 1968, 72, 1776-9. (10)Seki, H.; Arai, S.; Shida, T.; Imamura, M. J. Am. Chem. SOC.1973, 95,3404-5. (11)Konishi, S.;Hoshino, M.; Imamura, M. J . Am. Chem. SOC.1982, 104,2057-9. (12)Hoshino, M.; Konishi, S.; Ito, K.; Imamura, M. Chem. Phys. Lett. 1982,88,138-41. (13)Adler, A. D.; Long, F. R.; Kampus, F.; Kim. J. J. Inorg. Nucl. Chem. 1970,32,2443-5.
0022-365416212086-4537$01.25/0
Experimental Section C1MnI"TPP was prepared by the method reported in the literature13 and purified by column chromatography using Sephadex LH-20. Commercially available TCE was used without further purification. Sample solutions of M were degassed by repeated freeze-pump-thaw cycles. The cobalt-60 y irradiation was carried out at a dose rate of ca. 4.5 X lo4 rd/min for 70-100 min. ESR spectra were recorded on a JEOL FE-3AX spectrometer operating in the X band with 100-kHz modulation. Optical absorption spectra were taken on a Cary Model 14 spectrophotometer. Both irradiation and spectral measurements were made by placing sample cells in
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 23, 1982
4538
Konishi et al. Mn(lll)TPP
bl,( u,, aIQ( d,;
y)
Mn(lV)TPP _______
~
1
+
____
~
+ -+-*
es(dxr.dvr) * .
b2s( dx,
)
s=2
S=3/2
S=1/2
Figure 3. Ground-state electron configuration for Mn'I'TPP and Mn'"TPP. Two possible configurations are shown for Mn'"TPP. i
t
C'42
H
Flgure 1. Optical absorption spectra of CIMnII'TPP in TCE taken at 77 K before (- - -) and after (-) y irradlation.
C
---
e
1
2
tkG1
3
4
Flgure 2. ESR spectra of CIMn"'TPP in TCE at 77 K taken after y irradiation: (a) before annealing; (b)after annealing; (c) once warmed to room temperature for a short time. The sharp and strong signal at 3.2 kG in spectrum c is due to impurities in the sample tube.
Dewar vessels filled with liquid nitrogen. Immersing TCE solutions in liquid nitrogen gives a glassy state with fine cracks. For the measurement of absorption spectra, cells with ca. 0.1-mm optical path length were used to reduce light scattering by the cracks.
Results Figure 1 shows the optical absorption spectra of a TCE solution of ClMn'I'TPP taken at 77 K before and after y irradiation. Neat TCE shows only a broad absorption band in the near-UV region after y irradiation. Therefore a new very broad absorption band appearing in the near-IR region after y irradiation is attributed to a one-electron oxidized species of C1Mn"'TPP. Figure 2 shows the ESR spectra of a sample solution measured at 77 K after y irradiation. No signal is observed before irradiation. Figure 2a is the spectrum before the irradiation solution is annealed. It shows one broad signal around the g = 4 region and very strong signals due to solvent radicals and trapped hydrogen atoms in the quartz tube in the g = 2 region. When the solution is slightly warmed from 77 K, it changes to a polycrystalline state from a glassy state and gives the spectrum shown in Figure 2b after being recooled to 77 K. This spectrum shows a split of one broad signal observed before annealing of the solution into six lines and a relatively weak signal of the solvent radicals at high field. The solution once warmed to room temperature for a brief period and brought back to 77 K gives the spectrum shown in Figure 2c. A decrease in interval of the six-line splitting at low field accompanies the decrease in intensity of the signal and a new six-line splitting is observed at high field. In case the solution is
kept at room temperature for a longer period, e.g., 0.5 min, it no longer gives signals after being recooled to 77 K. This fact indicates that the oxidized species of C1Mn"'TPP is labile in fluid solution.
Discussion As mentioned in the preceding section, the new absorption band appearing in the near-IR region after y irradiation is attributed to an oxidized species of C1Mn"'TPP. When metalloporphyrins are oxidized, there are two possible cases: one is the oxidation of the porphyrin ring and the other that of the central metal. The former essentially gives the cation radical of a conjugated organic molecule, whereas the latter yields metalloporphyrins with a higher valence state. The absorption spectra of metalloporphyrin cation radicals do not depend greatly on the central metals and are similar to each other. The cation radicals of metallotetraphenylporphyrins show one or two bands with a relatively small width within the wavelength region up to 150 nm longer than the f i t visible band of the parent m ~ l e c u l e s . ' ~ ~ The ' ~ absorption spectrum observed after y irradiation in Figure 1 does not conform to these general results. On the other hand, the absorption spectra of metalloporphyrins with different valence states of the same central metal are strongly dependent on the nature of the metal.15 In view of these facts, the oxidized species of MnII'TPP is better ascribed not to the cation radical of Mn'"TPP but to Mn'"TPP. This assignment is further supported by the ESR spectra of the irradiated solution. The ESR spectrum shown in Figure 2a gives little information about the oxidized speceis in the g = 2 region because of the very strong signals of hydrogen atoms and solvent radicals. It shows, however, a broad signal around the g = 4 region. No such signal is expected for the cation radical of MnI'ITPP, since it is essentially an organic radical. The spectrum of the annealed solution shown in Figure 2b strongly indicates that the oxidized species is Mn'"TPP; the six equally spaced lines are attributed to the hyperfine structure due to 55Mnwith a splitting constant of 83 G. Another six-line structure with a splitting constant of 89 G is observed in the g = 2 region as shown in Figure 2c for the solution warmed to room temperature for a brief period. The entire spectrum shown in Figure 2c is analyzed in terms of an axially symmetric effective g tensor, g,, = 2.00 and g, = 3.93 and the hyperfine splitting constants, A,,= 89 G and A , = 64 G, whereas only the perpendicular components of the g and A tensors are obtained from the spectrum shown in Figure 2b; the values are g, = 4.01 and A , = 83 G,respectively. Such highly anisotropic ESR spectra cannot arise from organic radicals and are attributed to MnrVTPP. We will first discuss the (14)Dolphin, D.;Muljiani, Z.; Rousseau, K.; Borg, D. C.; Fajer, 3.; Felton, R. H. Ann. N . Y.Acad. Sci. 1973,266, 177-200. (15)Gouterman, M. In T h e Porphyrins"; Dolphin, D.; Ed.; Academic Press: New York, 1978;Vol. 111, Chapter 1.
ESR and Optical Detection of MnI'TPP
spin state of MnwTPP a d then the cause of the spectral change observed in Figure 2. The ground-state d-electron configurations for manganese porphyrins are given in Figure 3. The four lowest d orbitals of manganese(II1) porphyrins me singly occupied giving a high-spin state. Oxidation of manganese(II1) to manganese(1V) corresponds to the removal of an electron from alF orbital. The resultant d3 ion can have either a high-spin (S = 3/2) or a low-spin (S = 1/2) state. In case the eg orbital becomes far higher in energy than the b2g orbital, the low-spin state is favored. The low-spin state, however, is not expected to give highly anisotropic ESR spectra. In general, the shift from the free-electron g value (2.0023) of a perturbed metal ion takes the form Agaa = aaaX/AE,where X is the spin-orbit coupling constant of the metal, AE an excitation energy, and aaa a numerical factor. There is a useful principle16 for making a qualitative estimate of the g factor of a complex: admixture of excited states in which an electron is promoted from a filled orbital to the half-filled orbital leads to a positive sign for Ag,,, while promotion of an unpaired electron to a vacant orbital gives a negative sign for baa. If MnIvTPP is in a low-spin state, promotion of the unpaired electron from the eg orbital to the alg or blg orbital through the spin-orbit interaction causes a decrease in the g factor from the free-electron value. On the other hand, the largest contribution to the increase in the g factor is expected by promotion of an electron from the b2gto the eBorbital. This contribution, however, is unlikely to be so large that it gives g, = 4,since AE for this excitation is large enough to cause spin pairing. Thus, MnIVTPPis assigned not to the low-spin state but to the high-spin state. Similarity of the ESR spectra of MnwTPP to those of MnnTPP(02)17 and FemTPPI8with a quartet or a quasi-quartet stab gives further support to this assignment, although electron configuration of the metals is different; the former has the (16) Atherton, N. M. "Electron Spin Resonance";Wiley: New York, 1973; Chapter 6 and 7. (17) Hoffman,B. M.; Weschler, C. J.; Basolo, F. J. Am. Chem. SOC. 1976,98, 5473-82. (18)Reed, C. A.; Mashiko, T.;Bentley, S. P.; Kastner, M. E.; Scheidt, W. R.; Spartalian, K.; Lang. G. J. Am. Chem. SOC.1979, 101,2948-58.
The Journal of Physical Chemistry, Vol. 86, No. 23, 1982 4539
Figure 4. Schematic representationfor the formation of deficient and normal Md'TPP.
The circled letter S denotes a solvent molecule.
d3 and the latter two the d5 configuration. We now proceed to discuss the cause of the spectral change in Figure 2. When C1Mn"'TPP is dissolved in a noncoordinating solvent like TCE, C1- occupies one of the axial positions of the complex4 as illustrated in the lefthand formula in Figure 4. The oxidation process of Mn"'TPP to MnIVTPPin TCE solution is though to be as follows: RCl-RC1+ eRCP- + Mn'I'TPP
+ eR- + C1-
--
RCP.
RC1
+ MnIVTPP
where RC1 denotes a TCE molecule. Since Mn'"TPP is produced through electron transfer from Mn'I'TPP to the cation radical of TCE, it lacks one counteranion to fill one of the axial positions, which is occupied by a solvent molecule as shown in the central formula in Figure 4. Moreover, Mn"TPP is likely to be strained because it cannot relax to the stable form fit for the valence state of 4+ due to the rigidity of the solvent matrix. Therefore, MnwTPP produced at 77 K is a strained, deficient complex, which shows the ESR spectrum given in Figure 2a. When the irradiated solution is warmed, the strain will be released and two counteranions will occupy the axial positions as shown in the right-hand formula in Figure 4. The ESR spectra shown in Figure 2, b and c, are ascribed to such Mn"TPP. The difference between the two spectra can be attributed to the difference in the degree of release of the strain and in the extent of coordination by the anions. MnNTPP in the slightly warmed solution is likely to be still strained to some extent and imperfectly coordinated by the anions.
