1412
J. Phys. Chem. 1982, 86, 1412-1414
Constralned Complexes of Manganese(I I ) Tetraphenylporphyrln in Rlgld Solution S. Konlshl,' M. Hoshlno, and M. Imamura The Institute of Physlcel and Chemical Research, Wako-shl, Sailem 351, Japan (Receive& September 11. 1981)
Optical absorption and ESR spectra revealed that chloro- and bromo(tetraphenylporphyrin)manganese(III) (C1Mn"'TPP and BrMnIUTPP)in 2-methyltetrahydrofuran (MTHF) solution were reduced to Mn"TPP at 77 K by y irradiation. The MnUTPP species derived from ClMnII'TPP and BrMnII'TPP were found to be different from each other and also from a chemically prepared MnnTPP in their absorption and ESR spectra, although all of the Mn"TPP species were in the high-spin state ( S = 5/2). These spectroscopic data are interpreted in terms of the formation through electron capture during irradiation of constrained complexes of Mn"TPP in which Mn"TPP is forced to retain the halide ions in the axial position because of the rigidity of the low-temperature solvent matrix. On the other hand, the chemically prepared Mnl*TPPis regarded as having a solvent molecule weakly coordinating in the axial position.
Introduction Redox reactions of metalloporphyrins and their related compounds have been increasingly investigated,' partly with regard to their involvement in biological systems and partly in connection with their potential use as photocatalysts for solar energy storage. Since studies of redox reactions are usually made in fluid solutions at ambient temperature, unstable reaction intermediates cannot be detected by conventional techniques. The radiation-induced one-electron redox reactions of solute molecules in rigid solutions have been well established2-' and have an advantage to trap reaction intermediates in low-temperature solvent matrices. We have reported an application of this method to the spectroscopic detection of unstable cobalt(I1) prophyrinsp6and cobaloximes,' some of which retained an anion in the axial position because of the rigidity of solvent matrix and were termed constrained complexes. As an extension of the studies on redox reaction of metalloporphyrins in rigid solutions, the present paper reports the optical and ESR detection of the constrained complexes of manganese(I1) porphyrin. Manganese porphyrins have attracted considerable attention because of the possibility of their acting as catalysts for the photochemical liberation of oxygen from water and also as oxygen carriers.* There have been some reports on their chemicalgJOand photochemicalll redox reactions in fluid solution and also a review work by Boucher.12 Experimental Section CIMnInTPP and BrMnmTPP were prepared according ~
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(1)Felton, R. H.In 'The Porphyrins"; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. V, Chapter 3. (2) Hamill, W. H. In "Radical Ions"; Kaiser, E. T.; Kevan, L., Eds.; Wiley-Interscience: New York, 1968; Chapter 9. (3)Seki, H.; Shida, T.; Imamura, M. Biochim. Biophys. Acta 1974, 372, 100. (4) Hoshino, M.; Ikehara, K.; Imamura, M.; Seki, H.; Hama, Y. Photochem. Photobiol. 1981,34, 75. (5) Konishi, S.; Hoshino, M.; Yamamoto, K.; Imamura, M. Chem. Phys. Lett. 1980, 72, 459. (6)Konishi, S.;Hoshino, M.; Imamura, M. J. Phys. Chem. 1980,84, 3437. (7) Hoshino, M.; Konishi, S.; Terai, Y.; Imamura, M. Inorg. Chem. 1982, 21, 89.
(8)Hoffman, B. M.; Weschler, C. J.; Basolo, F. J . Am. Chem. SOC. 1976,98,5473. (9) Loach, P. A,; Calvin, M. Biochemistry 1963, 2, 361. (10)Calvin, M. Reo. Pure Appl. Chem. 1966,15, 1. (11)Engelsma, G.;Yamamoto,A,; Markham, E.; Calvin, M. J. Phys. Chem. 1962,66, 2517. (12)Boucher, L. J. Coord. Chem. Rev. 1972, 7, 289. 0022-365418212066-1412$01.25/0
to the literature13and p d i e d by column chromatography using Sephadex LH-20. Sample solutions of MnmTPP for y irradiation were prepared on a vacuum line by distilling MTHF, which had been degassed and stored over sodium-potassium alloy, from the storage vessel into a glass apparatus containing the solid sample. Chemical reduction of CIMnInTPP to Mn"TPP was conducted with sodium borohydride in a mixed solvent of chloroform and methanol. After the removal of the solvent, MTHF was introduced to the reaction vessel to prepare the MnUTPP solution in MTHF. Since MnnTPP is sensitive to oxygen and moisture, the whole process was carried out under vacuum. The concentration of the sample solution was of the order of 10-3-10-' M. The cobalt-60 y irradiation of the sample solution was performed at 77 K at dose rate of ca. 4.5 X lo4 rd/min for 10 min. Optical absorption spectra were measured with a Cary Model 14 spectrometer. ESR spectra were obtained with a JEOL JES-FE3AX spectrometer operating in the X band with 100-kHz modulation. Irradiated sample solutions were photobleached before optical measurements in order to eliminate absorption by the trapped electrons in the solvent matrix. Results Optical Absorption Spectra. Figure 1shows visible and near-infrared absorption spectra of an MTHF solution of CIMnmTPP at 77 K. The solid-line spectrum is nearly the same as that taken at room temperature. The room-temperature spectrum of a MTHF solution is very close to the spectrum of a benzene solution but different from that of an ethanol solution, which shows a blue shift by ca. 20 nm compared to the spectrum of a MTHF or benzene solution. The spectrum of BrMnnTPP in MTHF is almost identical with that of C1Mn"'TPP both at room temperature and at 77 K. The spectrum measured after y irradiation is shown by the broken line. Only slightly changes are observed in the visible region after irradiation. The irradiated solution, however, exhibits two new absorption bands in the near-infrared region. The spectrum of an MTHF solution of BrMnmTPP after irradiation was almost the same as that of CIMnmTPP. However, a small blue shift of the two absorption bands in the near-infrared region was observed for the irradiated solution of BrMnmTPP compared to that of CIMnmTPP. When the irradiated solution was once warmed to room temperature and brought back to (13)Adler, A. D.;Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem. 1970,32, 2443.
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86,No. 8, 1982 1413
Constrained Complexes of MnI'TPP in Rigid Solution
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F@m2. ESR spectra of the yirrediated MTHF solutions of cIMn"'TPP (A) and BrMn'*ITPP(B).
77 K, it gave the dotted-line spectrum, which was identical with that of a solution of Mn"TPP prepared chemically. Ita two main bands in the visible region show a blue shift by ca. 10 nm and increase in their peak height compared to the other two spectra, and it exhibits no absorption in the near-infrared region. ESR Spectra. Although manganese(II1) porphyrins are paramagnetic with a high spin d4 c~nfiguration,'~J~ they show no ESR signal. This is due most likely to short spin-lattice relaxation times and large zero-field splittings. The irradiated MTHF solutions of CIMnmTPP and BrMnmTPP, however, exhibit ESR signals in a wide range of magnetic field as depicted in Figure 2. The signals between 3.1 and 3.6 kG are due to trapped hydrogen atoms in a quartz tube and solvent radicals. Both spectra, A and B, show six equally spaced lines between 1.0 and 1.3 kG. The resolution of six lines is much better in spectrum B than in spectrum A. The difference between spectra A and B is much more distinct in the higher magnetic field. Spectrum A shows three signals around 5.5,8.7, and 12.2 kG, whereas spectrum B shows two signals around 7.0 and 12.2 kG. When the irradiated solution was once warmed to room temperature and brought back to 77 K, it gave the spectrum shown in Figure 3. This spectrum is identical with that obtained for the chemically prepared MnnTPP in MTHF solution. It shows seven poorly resolved lines between 0.9 and 1.3 kG and six fairly well-resolved lines between 3.1 and 3.5 kG but shows no signals above 5 kG up to 13.5 kG. (14) Behere, D. V.; Marathe, V. R.; Mitra, S. Chem. Phys. Lett. 1981, 81, 57.
1
1
Discussion Fornation of Constrained Complexes of MnnTPP. The ESR spectra observed at 77 K for the MTHF solutions of manganese(II1) porphyrins after y irradiation are ascribed to manganese(I1) porphyrins. The g = 6.0 signals with a hyperfine structure due to 6SMnnucleus (I = 5/2) are characteristic of a high-spin d5 configuration of manganese(I1) porphyrins. The g = 2.0 signals are thought to be masked by the strong signal of the solvent radical. The radiation-chemical, one-electron reduction of solute molecules in MTHF glass at 77 K has been well established for organic and this technique has also been successfully applied to the reduction of cobalt(II1) complexes."' The manganese(I1) porphyrins produced at 77 K from CIMnmTPP and BrMnII'TPP, however, are different from each other and also from chemically prepared ones in the optical absorption and ESR spectra. These spectral differences among manganese(I1) porphyrins can be best explained in terms of the difference of axial ligands. It has been reported12Js that, when manganese(II1) porphyrins are dissolved in a strongly coordinating solvent like pyridine or methanol, axial ligands are displaced by the solvent molecules and this displacement of axial ligands accompanies changes in the optical absorption spectra. The absorption spectra of C1Mn"'TPP and BrMnnlTPP in MTHF show little temperature dependence, and they are almost the same as those in benzene, but different from those in ethanol. These results indicate that halide anions are not displaced by the solvent molecules and remain located in the axial position in the MTHF rigid solutions of CIMnmTPP and BrMnmTPP. Therefore, the MnnTPP produced by y irradiation from CIMnmTPP or BrMn'"TPP in MTHF rigid solution is considered to hold C1- or Br- in the axial position. The anions, however, are thought to be confiied forcibly in the axial position because of the rigidity of the low-temperature matrix. This means that constrained complexes, Cl-.MnnTPP and Br--MnnTPP are formed in the irradiated rigid solutions of CIMnmTPP and BrMnmTPP. When the rigid solutions of the constrained complexes of Mn"TPP were warmed, the constrained complexes were no longer formed upon recooling the solutions to 77 K. This is considered to be due to displacement of the axial anions by the solvent molecules in the fluid MTHF solutions. Therefore, no difference was observed in the optical absorption and ESR spectra upon recooling the once-warmed solutions of the constrained complexes, and both spectra became identical with those of the MTHF solution of the chemically prepared Mn"TPP. Thus, the observed spectral changes of Mn"TPP are attributed to the differences of the axial ligands. The near-infrared absorption bands observed for the con(15) Boucher, L.J. J. Am. Chem. SOC.1968, 90,6640.
1414
The Journal of Physical Chemistry, Vol. 86, No. 8, 1982
Konishi et al.
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broaden as X increases. (2) For near-axial symmetry (A close to zero), observation of the signals at 1.1and 3.3 kG means that D I 0.2 cm-'. (3) The observation of several A / F / signals at fields between 5 and 15 kG means that D lies between 0.1 and 0.6 cm-'. Furthermore, simulated spectra Figure 4. Schematic representation of the formation and decay profor an S = 5/2 system in the polycrystalline state revealedI8 cesses of the constrained complexes of Mn'ITPP. The circled letter that a certain combination of the D and X values produced X denotes CI- or Br-, and the circled letter S a solvent molecule. a seven-line hyperfine structure instead of a six-line structure for the g = 6.0 signal. The seven-line hyperfine strained complexes may be of charge-transfer character structure was actually observed for the manganese(I1) which gained intensity because of the perturbation by the protoporphyrin complexed with a protein.Ig Thus, the axial anions. The formation and decay processes of the approximate values of D and X can be estimated not only constrained complexes of MnnTPP can be summarized in by fitting the observed spectra to the calculated diagrams the scheme illustrated in Figure 4. but also by taking into accountg the above-mentioned Analysis of ESR Spectra. The ESR spectra of high-spin facts. The estimated values are D = 0.22 cm-I and X N d5 systems can usually be interpreted in terms of the 0 for Cl-.Mn"TPP, and D = 0.46 cm-l and X N 0 for following spin Hamiltonian which neglects the hyperfine Br-.Mn"TPP. For the MTHF coordinated Mn"TPP, D interaction for ease of manipulation: I 0.20 cm-l and X > 0 are evaluated, although the values H ' = PHgS + D[S: - 1/3S(S+ l)]+ E [ S Z 2- Syz] appear less accurate compared to those of the other two Mn"TPP species owing to the absence of the high-field The zero-field splitting parameter D is a measure of axial signals. distortion from cubic symmetry, and E is that of rhombic The D value of the constrained complexes increases in distortion. The parameter, A, is defined as E / D to express the order Cl- < Br-, but X remains close to zero. This result the degree of rhombic distortion. Dowsing and Gibson16 means that the axial distortion increases in the same order made numerical computation based on the above Hamas above. A similar tendency was also found for other iltonian to predict ESR transitions for various values of Mn(I1) complexes." However, the constrained complexes D and X and published a series of diagrams which could still have an essentially axial symmetry regardless the axial be used to obtain D and X values from observed spectra. anions. This indicates that the symmetry of the conThey used the diagrams to interprete the ESR spectra of strained complexes of MnnTPP is the same as that of their some high-spin Fe(III)16 and Mn(II)" complexes. Since precursors, CIMnmTPP and BrMn*TPP. In other words, the ESR spectra were measured for rigid-solution samples the anions remain undislocated from the symmetry axis in the present case, all of the predicted transitions were by the change of the valence state of manganese from +3 not expected to be observed because of the resonance field to +2. dependence on orientation. It is still, however, possible In conclusion, the radiation-chemical formation of the to estimate approximate values for D and X by comparing constrained complexes of Mn"TPP in MTHF rigid glass the observed spectra with the D vs. H graphs. Dowsing has been confirmed by the optical absorption and ESR et made some remarks which emerged from their spectra. The analysis of the ESR spectra of the concalculation. A few of them, which are concerned with strained complexes assures that the anions are confined X-band spectra and relevant to the present spectra, are on the axis of symmetry despite the change of the valence the following. (1)For axial symmetry a strong signal at state of manganese. 1.1kG indicates that X = 0. This band will split or will X
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(16) Dowsing, R. D.; Gibson, J. F. J. Chem. Phys. 1969,50, 294. (17) Dowsing, R. D.; Gibson, J. F.; Goodgame, M.; Hayward, P. J. J. Chem. SOC.A 1969, 187.
(18)Dowsing, R. D.; Ingram, D. J. E. J. Magn. Reson. 1969, 1, 517. (19) Yonetani, T.; Drott, H. R.; Leigh, J. S.;Reed, G . H.; Waterman, M. R.; Asakura, T. J. Biol. Chem. 1970,245, 2998.
