J. Am. Chem. SOC.1993,115, 11784-11788
11784
Preparation, Characterization, and Reactions of Novel Iron( 111) Porphyrin Dication Complexes Hiroshi Tsurumaki, Yoshihito Watanabe,' and Isao Morishima' Contribution from the Division of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto 606, Japan Received May 26, 1993'
Abstract: A dication complex of (meso-tetramesitylporphynato)iron(III) (FeII'TMP) has been prepared by the reaction of FelIITMP N-oxide with trifluoroacetic acid in toluene at low temperature. The electronic structure of the complex is characterized to be ferric high spin with rhombic symmetry on the basis of iodometric titration and UV-vis, 2HNMR, and EPR spectroscopies. A value of BID, which shows rhombicity of the heme environment, was estimated to be 0.29 by EPR measurement. An unusually high EID value indicates that the symmetry of the ligand field around the iron is rather similar to those of nonheme ferric complexes, and the porphyrin ring could exhibit large nonplanarity. While tetraphenylporphyrin dication complexesof Zn, Mg, and Ni are known to react with methanol to afford the corresponding isoporphyrins, the dication complex of FeIIITMP was found to react with methanol to yield FeIIITMP N-methoxide, possibly due to the sterric barrier of the o-methyl groups of mesitylene. The FeIIITMP dication is not thermally stable and is readily reduced to the FeIIITMP r-cation radical even at -25 "C in toluene. The dication complex of (mesotetrakis(2,6-dichlorophenyl)porphynato)iron(III) (FeII'TDCPP) was also prepared by the same reaction. The preparation of the dication complex of Fe(II1) porphyrin demonstrates possible manipulation of the two-electron-oxidized equivalent in various forms by Fe porphyrin, Le., the O=Fe(IV) porphyrin ?r-cationradical, O=Fe(V) porphyrin, Fe(II1) porphyrin N-oxide, and the Fe( 111) porphyrin dication.
Introduction Recent model studies of cytochrome P-450 by using iron complexes of tetraphenylporphyrin derivatives have served to elucidate the detail of the active species responsible for the oxygenation of foreign compounds, i.e., a two-electron-oxidized oxo-iron complex, equivalent to compound I of peroxidases.' On the other hand, Fe(II1) porphyrin N-oxide had been proposed for a possible description of the active species on the basis of the chemical and spectroscopic behavior of Fe(II1) porphyrin-carbene adducts.2 However, the candidacy of the N-oxide has been ruled out by recent preparationof its modelcomplex, FeIIITMP N-oxide (la).3a*b In our effort to transform l a to the reactive species, we have found the reaction of la and acid gives a new FeTMP complex (3a), which is formally two electron oxidized from FeIIITMP. Preliminary work indicates 3a being the FelI1 TMP d i ~ a t i o n . ~ Interestingly,an Fe(II1) porphyrin dication intermediate has been recently suggested as a candidate of compound 0, a precursor of compound I, in the reaction of H202with horseradish peroxidase;5 however, UV-vis spectra of compound 0 are different from that of 3a. Metalloporphyrin dication complexes of Mg, Zn, and Ni have been prepared by electrochemical oxidation and characterized
* Abstract published in Advance ACS Abstracts, November 15, 1993. (1) (a) Guengerich, F. P.; Macdonald, T. L. Acc. Chem. Res. 1984,17, 9-16. McMurry, T. J.; Groves, J. T. In Cytochrome P-450:Structure, Mechanism and Biochemistry; Ortiz de Montellano, P. R., Ed.; Plenum: New York, 1986;pp 1-28. (b) White, R.E.; Coon, M. J. Annu. Rev. Biochem. 1980,50, 315-356. (c) Groves, J. T.; Haushalter, R. C.; Nakamura, M.; Nemo, T. E.; Evans, B. J. J. Am. Chem. SOC.1981,103, 2884-2886. (d) Groves, J. T.; Watanabe, Y. J . Am. Chem. SOC.1986,108,7834-7836. (e) Yamaguchi, Y.;Watanabe, Y.; Morishima, I. J. Am. Chem. SOC.1993,115, 4058-4065. (f) Higuchi,T.; Shimada, K.; Maruyama, N.; Hirobe, M. J . Am. Chem. SOC.1993, 115,7551-7552. (2)Mansuy, D.;Morgenstern-Badarau, I.; Lange, M.; Gans, P. Inorg. Chem. 1982,21, 1427-1430. (3) (a) Groves, J. T.; Watanabe, Y. J . Am. Chem. Soc. 1986,108,78367837. (b) Groves, J. T.; Watanabe, Y .J. Am. Chem. SOC.1988,110,84438452. (c) Mizutani, Y.; Watanabe, Y.; Kitagawa, T. Submitted for publication. (4)Watanabe, Y.;Takehira, K.; Shimizu, M.; Hayakawa, T.; Orita, H.; Kaise, M.J. Chem. Soc., Chem. Commun. 1990,18, 1262-1264. (5) (a) Baek, H.K.; Van Wart, H. E. Biochemistry 1989,28,5714-5719. (b) Baek, H. K.; Van Wart, H. E. J. Am. Chem. SOC.1992,114,718-725.
by UV-vis spectroscopy.6 Though the second oxidation potential obtained by cyclic voltammetry of some Fe(II1) porphyrin complexes wasattributed to Fe(II1) porphyrindicationformation,7 their physical properties such as UV-vis, EPR, and NMR spectra have never been observed, possibly due to their instability under the conditions. Thus, preparation, characterization, and elucidation of the structure and reactivities of Fe(II1) porphyrin dication complexes are quite important. Especially, it allows us to fulfill a series of two-electron-oxidized forms of Fe(II1) porphyrin complexes. In this paper, we report the preparation, characterization, and reactionsof the novel FeII'TMP dication and FeIIITDCPPdication complexes.
Results and Discussion Preparation and Characterization of FPTMP N-Oxide (la). FeIlITMP N-oxide (la) was prepared in the reaction of Fell1TMP(0H) and mCPBA in toluene at 0 OC according to the procedure reported b e f ~ r e .While ~ la has been characterized to be ferric high spin bearing the bridged F A N structure based on the magnetic susceptibility (5.4 j ~ ) and ~ resonance ~ . ~ Raman spectro~copy,~~ its EPR signal was not well-defined. In addition, no @-pyrroleproton resonances of la have been identified by lH NMR, because of broadening of signals. In this study, we have successfully prepared an EPR sample of la to observe well-defied signals at g = 9.0, 5.0, 3.8, and 3.5 (Figure la). Though the UV-vis spectrum of the EPR sample solution shows almost complete formation of l a (Figure la, inset), the EPR spectrum gives some signals due to impurities. The signals observed at 7.1, 6.1, and 1.98 are readily attributed to unreacted FelIITMP(OH) and FeIIITMP(m-chlorobenzoate) in comparison with the authentic samples, respectively. These assignments are further (6)(a) Fajar, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J . Am. Chem. Soc. 1970,92,3451-3459. (b) Dolphin, D.; Niem, T.; Felton, R. H.; Fujita, I. J . Am. Chem. SOC.1975,97, 5288-5290. (c) Chang, D.; Malinsky, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 817-824.
(7)Phillippi, M. A.;Shimomura, E. T.; Goff, H. M. Inorg. Chem. 1981, 20, 1322-1325. Lee,W. A.;Calderwood, T. S.: Bruice, T. C. Proc. Narl. Acad. Sci. U.S.A. 1985,82,4301-4305.
0002-7863/93/1515-11784$04.00/00 1993 American Chemical Society
Novel Iron(III) Porphyrin Dication Complexes
5.a
X 4a
WAVELENGTH (nm)
A
II
3.4
Figure 1. (a) EPR spectrum of FelIITMP N-oxide (la) in toluene at 77 K; inset, UV-vis spectrum of the EPR sample solution; (b) an excess amount of pyridine added toa; (c) EPR spectrum of FeII'EDTAp-peroxide in HzO at 77 K.
confirmed by adding an excess amount of pyridine to the EPR solution, causing the replacement of the absorption at 7.1 and 6.1 by those a t 2.43, 2.15, and 1.92, according to the formation of the ferric low-spin complex, Fe111TMP(Py)2(Figure 1b), while the signals observed at 9.0,. 5.0, 3.8, and 3.5 remain unchanged. In addition, a sharp signal at 4.3 comes from a nonheme ferric complex due to the decomposition of la. That the E I D value for l a is estimated to be 0.20 by the g-value-EID diagrams indicates unusually large rhombicity of the ligand field. Very similar rhombicity in the EPR spectrum has been also observed for an Fe1I1EDTAp-peroxide complexg (Figure IC). The amounts of FeIIITMP and nonheme iron(II1) species remaining as impurities in the EPR sample solution were estimated in comparison of the UV-vis and EPRIO spectra shown in Figure 1 with authentic samples to be
