J . Phys. Chem. 1988, 92, 1464-1468
1464
both the nonconjugated and conjugated diene complexes are much H ~is) ~not . a kinetic effect more stable than C ~ S - C T ( C O ) ~ ( CIf~this but a reflection of relative bond strengths, then it is significant because nonconjugated dienes are often treated as two ethylene units for the purposes of orbital analysis. Experiments on the gas-phase kinetics of Cr(C0)4(.r14-butadiene)are presently under way in our laboratory. Note Added in Proof: Since this work was accepted in final form, we note the publication of a paper reporting synthesis of
tran.~-Cr(C0).,(C~H~)~, which is found to be stable at room temperatureeZ5 This species is observed to be formed following C W at -50 O C . The mechanism of its photolysis of Cr(C0)5(CzH4) production apparently involves photoisomerization of a small photostationary population of the cis bis(ethy1ene) isomer, which can be anticipated to be present under such conditions of temperature and illumination. By contrast, in our system, cis-Cr(C0)4(C2H4)z is formed and decays in the dark between laser pulses, precluding photoisomerization to the trans isomer.
(23) The data of ref 9 can be interpreted by an alternate mechanism (see ref 24) that does not give the unimolecular rate constant for the 9' to q2 conversion of Cr(C0)4(94-butadiene). Thus the solution kinetic data are inconclusive. Nevertheless, Cr(CO),(q'-butadiene) clearly decays faster than Cr(C0)4(94-norbornadiene)or Cr(C0)4(n4-cyclooctadiene) but lives much longer than Cr(COMC,H3,. Future exwriments will attempt to unravel the ,-. gacphase kinetics of Cr(CO),(~'-butadiene). (24) (a) Zingales, F.; Coziani, F.; Basolo, F. J. Organomet. Chem. 1967, 7 , 461. (b) Zingales. F.;Graziani, M.; Belluco, U. J. Am. Chem. SOC.1967, 89, 256.
Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-86 14702.
.
Registry No. C~S-C~(CO)~(C~H,)~, 106682-40-0; Cr(C0)5(C2H4)2, 7 1407-77-7; Cr(C0)6, 13007-92-6; C2H4,74-85- 1. (25) Grevels, F.-W.; Jacke, J.; Ozkar, S.J. Am. Chem. SOC.1987, 109, 7536.
Infrared Spectra of ?r-Cation Radicals of Magneslum, Zlnc, and Cobalt Octrtethytporphyrins Koiehi Itoh,* Keisuke Nakahasi, and Hisao Toeda Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 160, Japan (Received: July 28, 1987)
Infrared spectra in the 1700-250-cm-' region were measured for the bromide salts of the *-cation radicals of MOEP (M = Mg(I1) and Zn(II), OEP = octaethylporphyrin) and for the bromide and perchlorate salts of Co"'OEP+, all of which were prepared by chemical oxidation. By comparing the infrared spectra of the *-cation radicals with those of the parent molecules, assignments were made for the in-plane stretching and bending modes as well as the out-of-plane bending modes of the porphyrin ring of the radicals and the frequency shifts of these modes relative to the unoxidized molecules were determined. Common shift patterns were observed for the Mg(J1) and Zn(I1) radicals, which are known to be in a ZAlustate from the ESR study. These patterns were, however, quite different from the corresponding shift patterns observed for the perchlorate salt of the Co(II1) radical, which is in a zAzustate, suggesting that the measurement of the infrared spectra is one of the straightforward methods for determining the ground electronic structure of the wcation radicals of metalloporphyrins. The infrared spectrum of the bromide salt of the *-cation radical of the Co(II1) complex, whose ground electronic structure is also in a 2Alustate, however, shows features much simpler than those observed for the Mg(I1) and Zn(I1) radicals. This result suggests the existence of a vibronic coupling effect in the Co(II1) radical, which reduces the intensities of some infrared bands due to porphyrin ring modes.
Introduction
A number of experimental evidences have proved the crucial roles played by the *-radicals of metalloporphyrins and chlorophylls in the electron transport and redox processes in living organisms.'*Z The physicochemical properties of the *-radicals have been studied mainly by electrochemical methods with the combined use of ESR and Uv-vis ~pectroscopies.~The resonance Raman scattering spectroscopy has also been applied to the Tcation radicals of bacteriochlorophyll^^*^ and to the *-cation and *-anion radicals of metallop~rphyrins.~-~These studies have (1) Roberts, J. E.; Hoffman, B. M.; Rutter, R.; Hager, L. J. Am. Chem. SOC.1981, 103, 7854-7656. (2) Parson, W. W.; Ke, B. In Photosynthesis: Energy Conversion by Plants and Bacteria; Govindjee, Ed.;Academic: New York, 1983. (3) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. IV, pp 197-256. (4) Lutz, M.; Kleo, J. Biochem. Biophys. Acta 1979, 546, 365-369. (5) Cotten, T. M.; Parks, K. D.; Van Duyne, R. P. J . Am. Chem. SOC. 1980, 102, 6399-6407. ( 6 ) Yamaguchi, H.; Nakano, M.; Itoh, K. Chem. &ti. 1982, 1397-1400. (7) Yamaguchi, H.; Soeta, A,; Toeda, H.; Itoh, K. J. Electroanal. Chem. 1983, 159, 347-359.
0022-365418812092-1464$01,50/0
proved that the resonance Raman spectra show characteristic features compared to the spectra of the parent molecules. Especially, Kim et aL8 reported the resonance Raman spectra of the *-cation radicals of MOEP [OEP = octaethylporphyrin; M = Zn(II), Cu(II), Ni(II)] and Co"'OEP+X (X = Br- and C104-) and proved that there is a definite correlation between the frequency shifts of the porphyrin ring vibrations caused by the rradical formation and the ground electronic states of the radicals, Le., 2Aluor 2A2u. (The D4* point symmetry is assumed for the (8) Kim,D.; Miller, L. A.; Rakhit, G.; Spiro, T. G. J . Phys. Chem. 1986, 90,33213-3325. Oextling et al.'b suggested that some of the resonance Raman bands from the *-cation radicals reported by Kim et a1.8 are ascribable to the corresponding porphyrin free base diacid salts which are formed through demetalization caused by irradiation with the laser excitation light (406.7 nm). In spite of this fact it is still true that the frequency shifts of some resonance Raman bands from the *-cation radicals compared to those of the resonance Raman bands from the corresponding parent metalloporphyrins give direct information with regard to the symmetry of their ground electronic state (a," or a d . (9) (a) Salehi, A.; Oertling, W. A.; Babcock, G. T.; Chang, C. K. J. Am. Chem. SOC.1986,108,5630-5631. (b) Oertling, W. A,; Salehi, A.; Chang, C. K.; Babcock, G. T. J. Phys. Chem. 1987, 91, 31 14-31 16.
0 1988 American Chemical Society
r-Cation Radicals of Mg, Zn,and Co Octaethylporphyrins
The Journal of Physical Chemistry, Vol. 92, No. 6,1988 1465
metalloporphyrins.) Thus, an identification of the radical type is quite straightforward in the resonance Raman spectra. The infrared spectra of the r-cation radicals of metalloporphyrins have been extensively surveyed by Shimomura et a1.,I0 who indicated that the radicals containing tetraphenylporphine as a ligand show a characteristic feature near 1280 cm-' and those containing OEP show a feature in the 1650-1500-cm-' region. Although this study confined its attention to a limited frequency region, it clearly demonstrated that the features can be used to prove the formation of the *-cation radicals of the metalloporphyrins. The infrared spectra of metalloporphyrins and their r-cation radicals give rise to bands whose vibrational modes are quite different from those of the Raman spectra; e.g., the infrared spectra give strong out-of-plane porphyrin ring modes below 900 cm-' which are usually weak in Raman spectra. Therefore, it is expected that the investigation of the infrared spectra of the 1600 1LbO 1200 I000 800 600 LOO *-cation radicals in a much wider frequency region gives more Wavenumber [ cm -'I information about how the vibrational spectra are correlated to Figure 1. Infrared spectra of Mg"OEP (A) and Mg"OEP'+Br- (B) at the ground electronic structures of the radicals. In this paper we room temperature (A, KBr disk; B, film cast on a KBr plate). measured the infrared spectra in the 1700-25O-cm-' region for the r-cation radicals of Mg"OEP, Zn"OEP, ColIIOEP+Br-, and CO"~OEP+CIO~(Mg"OEP'+Br-, Zn"OEP'+Br-, C O ~ ~ ' O E P ' ~ + 2Br-, and C O " ' O E P ' ~ + ~ C ~ Oand ~ - ) compared the spectra with those of the corresponding parent molecules.
Experimental Section Materials. O E P was purchased from Strem Chemicals Co., Inc. Mg"OEP, Zn"OEP, and Co"0EP were synthesized following usual procedures.ll Dichloromethane was refluxed with calcium hydride and distilled just prior to use. Silver perchlorate and bromine were obtained from a commercial source and used without further purification. Argon, which was of extrahigh purity, was obtained from Tomoe Chemicals Co., Inc., and used as received. Chemical Oxidation of Metalloporphyrins. MglaEP'+Br- and ZnI1OEP'+Br- were prepared by following the procedure reported by Fajer et al.12 To an about 0.1 wt %solution of a metal complex in CH2CI2,which was deareated by bubbling argon gas, were mol/L), added aliquots of a Br2 solution in CH2C12(ca. 1.5 X and the oxidation reaction was monitored by absorption spectroscopy. After the absorption spectrum was confirmed to be identical with that reported for either Mgl*OEP'+Br- or Zn"OEP'+Br-, the solution was concentrated by bubbling argon gas. Co"0EP undergoes stepwise one-electron-oxidation reactions on treatment with Br2.I3 On addition of '1, equiv of Br, to a Co"0EP solution in CH2C12the complex is converted to its trivalent complex, Co"'OEP+Br-. Further oxidation of the trivalent complex with another I / , equiv of Br2 yields a a-cation radical, CO"'OEP'~+~B~-. The solutions of the trivalent complex and its r-cation radical were concentrated by bubbling argon gas. A CH2CH2 solution of C O ~ ~ ' O E P ' ~ + ~was C ~ prepared O~by treatment with an excess of AgC1O4.I3 After filtration of AgBr and AgC104 the absorption spectrum of the sample solution was observed, confirming that the spectrum was almost identical with that reported by Dolphin et and that by Hanson et al.I4 The solution was concentrated by bubbling argon gas. All the preparation procedures of the *-cation radicals were performed under an argon gas atmosphere. Measurement of Infrared Spectra. The measurement of infrared spectra was performed on the films of the r-cation radicals which were prepared by casting their concentrated solutions on KBr plates. After the measurement of the spectra the films were dissolved into CH2C12. The absorption spectra of these solutions (10) Shimomura, E. T.;Phillippi, M. A.; Goff,H. M. J . Am. Chem. SOC. 1981, 103,6778-6780. (11) Fuhrhop, J. H.; Mauzerall, D. J . Am. Chem. SOC. 1969, 91, 4174-41 8 1. (12) Fajer, J.; Borg, D. C.; Forman, A,; Dolphin, D.; Felton, R. H. J . Am. Chem. SOC.1970, 92, 3451-3459. (13) Dolphin, D.; Forman, A.; Borg, D. C.; Fajer, J.; Felton, R. H. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 614-618. (14) Hanson, L. K.; Chang, C. K.; Davis, M. S.;Fajer, J. J . Am. Chem. SOC.1981, 103,663-670.
1600
lLb0
1200 1000 800 Wavenumber cm-' I
600
LOO
Figure 2. Infrared spectra of Zn"0EP (A) and Zn"OEP'+Br- (B) a t room temperature (A, KBr disk; B, film cast on a KBr plate).
were identical with those of the corresponding r-cation radicals, which indicated that there occurred neither degradation nor reduction reactions on the film samples during the infrared measurements. Infrared spectra were also measured for the KBr pellets of Mg"OEP, Zn'IOEP, and Co"'OEP+Br-. A Hitachi Co., Ltd., Model 260-50 infrared spectrophotometer was used for all the measurements.
Results and Discussion Figures lB, 2B, and 3, C and D, are the infrared spectra of MgIIOEP'+Br-, Zn"OEP*+Br-, C O I I ' O E P ' ~ + ~ B ~ -and , C O ~ ~ ' O E P ' ~ + ~ respectively. C ~ O ~ - , For comparison purposes the spectra of Mg"OEP, Zn"OEP, Co"OEP, and Co"'OEP+Br- are also presented in Figures l A , 2A, and 3, A and B, respectively. The frequencies of the infrared bands given in Figure lA, 2A, and 3A are virtually identical with those already r e p ~ r t e d . ' ~ Assignments of the Infrared Bands of Mg"OEP, Zn"OEP, and C O ~ ~ O E PComparison . of the spectra in Figures 1B and 2B with those in Figures 1A and 2A indicates that, on conversion to the r-cation radicals, a set of infrared bands of Mg"0EP and Zn"0EP show only a little shift. Most of these bands can be assigned to the vibrations from the peripheral ethyl group, because the oneelectron oxidation is considered to give only a little effect on the electronic structure and the force constants of the ethyl group. Therefore, we can assign the bands near 1455, 1370, 1318, 1055, 1015, and 957 cm-I in Figures 1A and 2A to the ethyl group modes, as summarized in Table I. The infrared spectrum of CoIIOEP in Figure 3A gives features similar to those of Mg"0EP ~~
~
(15) (a) Ogoshi, H.; Masai, N.; Yoshida, 2.; Takemoto, J.; Nakamoto, K. Bull. Chem. SOC.Jpn. 1971, 44,49-51. (b) Urban, M. W.; Nakamoto, K.; Kincaid, J. Inorg. Chim. Acta 1982, 61, 77-81. (c) Kincaid, J. R.; Urban, M. W.; Watanabe, T.; Nakamoto, K. J . Phys. Chem. 1983,87, 3096-3101.
1466 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
Itoh et al. TABLE I: Observed Freaueacies (cin-') and Tentative Assignments of Infrared B I I ~ of ~ SM ~ E P ZAEP, , and CO%EP
Md'OEP ZnI'OEP Co1'0EP 1666 1674 1630 1428 1580 1585 1565 1528 1461 1465 1449 1450 1390 1391 1384 1377 1371 1374 1368 1318 1317 1318 1308 1268 1272 1269 1220 1231 1217 1146 1148 1150 1133 1116 1111 1111 1066 1059 1064 1058 1056 1017 1020 1014 993 919 978 958 956 957 923 91 1 911 846 843 846 839 837 838 834 832 829 747 753 750 728 734 } 735 0
1600
lL00
I
1000 800 Wavenumber [ cm")
1200
"R
727
