Resonance Raman characterization of iron-chlorin complexes in

J. Phys. Chem. 1986, 90, 6105-61 12

6105

Resonance Raman Characterization of Iron-Chlorin Complexes in Various Spin, Oxidation, and Ligation States. 1. Comparative Study with Corresponding Iron-Porphyrin Complexes Y. Ozaki: K. Iriyama: H. Ogoshi,*T.Ochiai,*and T. Kitagawa*g Division of Biochemistry, The Jikei University School of Medicine, Nishi-Shinbashi, Minato- ku, Tokyo 105, Japan, Department of Chemistry and Chemical Engineering, The Technological University of Nagaoka, Nagaoka, Niigata 949-54, Japan, and Institute for Molecular Science, Okazaki National Research Institutes, Myodaiji, Okazaki 444, Japan (Received: April 7, 1986)

Resonance Raman (RR) spectra of (octaethylch1orinato)iron [Fe(OEC)] complexes in various spin, oxidation, and ligation states have been measured and compared with those of the analogous complexes of (octaethy1porphyrinato)iron [ Fe(OEP)] in an attempt to gain new insight into chlorin chemistry. Regarding the methine-bridge stretching modes, the uI9 frequencies were different between the two kinds of complexes while the ul0 and u3 frequencies were close between them. For the C,C, stretching modes, the frequency separation between the v2 and vI1 modes became markedly larger for the Fe(0EC) complexes. When these frequencies were plotted against the center-to-pyrrole nitrogen distance, both the Fe(0EC) and Fe(0EP) complexes gave straight lines and their inclinations were similar between the OEC and OEP complexes except for the u19 band. Ferrous low-spin complexes deviated from the straight lines, but the deviations were appreciably smaller for the OEC complexes. Frequencies of the u4 mode were lower by 2-6 cm-' for Fe'"(0EC) complexes and higher by 1-5 cm-I for Fe"(0EC) complexes compared with those of the corresponding Fe(0EP) derivatives. As a result, downward shifts of the v4 mode upon reduction of the iron ion, which have been attributed to the increased t-back-donation from the d,(Fe) to the t*(macrocycle) orbitals, are considerably smaller for the OEC complexes. Frequencies of the ul0, ull, and u4 modes of ferrous low-spin complexes decreased linearly with the increase of pK, of axial ligands, but the linear correlations were much less steep with the Fe"(0EC) complexes than with Fe"(0EP) ones. These observations for the ferrous complexes suggested that the t-back-donation is less significant for the Fe"(0EC) complexes than for the Fe"(0EP) ones.

Introduction Metallochlorins, in which the C& bond of one of four pyrrole rings of metalloporphyrins is saturated (Figure l), have recently received wide-spread interest because the chlorin chromophores have been found not only in chlorophylls but also in a variety of heme proteins.'" These findings raised a question as to what kind of differences between iron porphyrins and iron chlorins would render the latter complexes better suited to a prosthetic group of some heme proteins. In order to answer this question it is essentially important to elucidate what features of chlorin chemistry differ from porphyrin chemistry, and in fact various physicochemical studies have been undertaken with model compounds such as octaethylchlorin (OEC) and tetraphenylchlorin (TpC).7-'2 Resonance Raman (RR) spectroscopy has been used as a powerful tool for probing a structure of heme proteins, their model compounds, and chlorophylls.1F18 These findings raised a question as to what kind of differences between iron porphyrins and iron chlorins would render the latter complexes better suited to a prosthetic group of some heme proteins. R R bands of Cu(0EC) in the !300-1700-cm-' region were previously assigned on the basis of frequency shifts upon 7.6-deuteration of meso carbons and I5N substitution of pyrrolic nitrogens, and some characteristic features in R R spectra of metallochlorins were pointed out. Since then R R studies on metallochlorins21*22and chlorin-containing prot e i n ~ " , ~have ~ - been ~ ~ performed by several groups. However, so far no RR data have been observed for ferrous chlorin model compounds, and hence the redox dependence of the R R spectra has never been discussed despite its importance to the catalytic activities a t the iron center. Furthermore, R R data in the lowfrequency region and knowledge about the effects of axial ligands remain to be explored. Accordingly, we have undertaken a more thorough R R study of metallochlorins. As the first of the series, this paper deals with the R R spectra in the high-frequency region (1200-1700 cm-') of Fe(0EC) complexes having various spin, oxidation, and ligation states, characterizing the R R spectra of the Fe(0EC) complexes in comparison with those of the (octaThe Jikei University School of Medicine. $The Technological University of Nagaoka. 8 Institute for Molecular Science.

ethy1porphyrinato)iron [Fe(OEP)] analogues. The following paper24will report the R R spectra in the low-frequency region (1) (a) Lemberg, R.; Barrett, J. Cytochromes; Academic: New York, 1973; p 217 and references therein. (b) Yamanaka, T.; Okunuki, K. In Microbial Iron Metabolism; A Comprehensive Treatise; Neilands, J. B., Ed.; Academic: New York, 1974; p 349 and references therein. (2) (a) Walsh, T. A.; Johnson, M. K.; Barber, D.; Thomson, A. J.; Greenwood, C. J. Inorg. Biochem. 1981, 14, 15 and references therein. (b) Huynh, B. H.; Liu, M. C.; Moura, J. J. G.; Moura, I.; Ljungdahl, P. 0.; Munck, E.; Payne, W. J.; Peck, H. D., Jr.; Der Vartanian, D. V.; LeGall, J. J. Biol. Chem. 1982, 257, 9576 and references therein. (3) (a) Jacob, G.S.;Orme-Johnson, W. H. Biochemistry 1979,18, 2967; (b) 1979, 18, 2975. (4) (a) Morell, D. B.; Chang, Y.; Hendry, I.; Nichol, A. W.; Clezy, P. S . In Structure and Function of Cytochromes;Okunuki, K., Kamen, M., Sekuzu, I., Eds.; University of Tokyo Press: Tokyo, 1968; p 563. (b) Brittain, T.; Greenwood, C.; Barber, D. Biochim. Biophys. Acta 1982, 705, 26 and references therein. (c) Andersson, L. A.; Loehr, T. M.; Lim, A. R.; Mauk, A. G.J. Biol. Chem. 1984, 259, 15340 and references therein. (5) (a) Klebanoff, S. J.; Clark, R. A. In The Neutrophil: Function and Clinical Disorders; North-Holland: Amsterdam, 1978. (b) Sibbett, S . S.; Hurst, J. K. Biochemistry 1984,23, 3007 and references therein. (c) Babcock, G. T.; Ingle, R. T.; Oertling, W. A.; Davis, J. C.; Averill, B. A.; Hulse, C. L.; Stukens, D. J.; Bolscher, B. G. J. M.; Wever, R. Biochim. Biophys. Acta 1985, 828, 58 and references therein. (d) Ikeda-Saito, M.; Argade, P. V.; Rousseau, D. L. FEES Lett. 1985, 184, 52. (6) Davis, J. C.; Averill, B. A. J . Biol. Chem. 1981, 256, 5992. (7) Ogoshi, H.; Watanabe, E.; Yoshida, Z.; Kincaid, J.; Nakamoto, K. Inorg. Chem. 1975, 14, 1344. (8) (a) Spaulding, L. D.; Andrews, L. C.; Williams, G.J. B. J . Am. Chem. SOC.1977, 99, 6918. (b) Ulman, A.; Gallucci, J.; Fisher, D.; Ibers, J. A. J . Am. Chem. Soc. 1980, 102,6852. (c) Gallucci, J. C.; Swepston, P. N.; Ibers, J. A. Acta Crystallogr., Sect. B Struct. Crystallogr. Cryst. Chem. 1982,838, 2134. (d) Strauss, S . H.; Silver, M. E.; Ibers, J. A. J. Am. Chem. Soc. 1983, 105,4108, (e) Strauss, S.H.; Silver, M. E.; Long, K. M.; Thompson, R.G., Hudgens, R. A.; Spartalian, K.;Ibers, J. A. J . Am. Chem. Soc. 1985, 107, 4207.

(9) Stolzenberg, A. M.; Strauss, S.H.; Holm, R. H. J. Am. Chem. SOC. 1981, 103, 4763 and references therein.

(10) Chang, C. K.; Hanson, L. K.; Richardson, P. F.; Young, R.; Fajer, J. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 2652 and references therein. (11) Ward, B.; Chang, C. K.; Young, R. J. Am. Chem. SOC.1984, 106, 3943. (12) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 817. (13) Warshel, A. Annu. Rev. Biophys. Bioeng. 1977, 6 , 273. (14) (a) Kitagawa, T.; Ozaki, Y.; Kyogoku, Y. Adu. Biophys. 1978, 11, 153. (b) Kitagawa, T.; Ozaki, Y. Struct. Bonding (Berlin), in press.

0022-3654/86/2090-6105!§01.50/0 0 1986 American Chemical Society

6106 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986

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Ozaki et al.

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Figure 1. Structure and atomic numbering of metallooctaethylporphyrh (a) and metallo tram-octaethylchlorin (b). The x, y, and z axes defined as shown in this figure are used for symmetry representation of M(0EP) and M(0EC).

(100-900 cm-') of Cu(0EC) and the Fe(0EC) complexes. Experimental Procedures

Fe"'(0EP)X (X = F, C1, Br, or I),2Sa Felll(OEP)OH,ZSb [Fem(0EP)],0,2" F$1(OEP)C104:5d Cu(OEP),= Fem(OEC)X,' and C U ( O E C ) ~were synthesized with the methods described elsewhere. Fe"'(0EP) (CH,OH),, Fe"'(0EP) (C2HSOH),, Fe1"(OEP)(Me2S0), (Me2SO: dimethyl sulfoxide), and Fe"'(OEP)(TMSO), (TMSO: tetramethylene sulfoxide) were prepared by dissolving Fe"'(OEP)C104 into a 1:l (v/v) mixture of CH2C12/CH30H, CH2C12/C2H50H, CH2C12/Me2S0, and CH2C12/TMS0,respectively. To obtain Fe111(OEP)(Im)2(Im: imidazole), Fe1"(OEP)(2-MeIm) (2-MeIm: 2-methylimidazole), and Fe"'(OEP)( 1,2-MqIm) (1,2-MqIm: 1,2dimethylimidazole), excess amounts of recrystallized Im, 2-MeIm, and 1,2-Me21mwere mixed with Fe"'(0EP)Br in CH2C12, respectively. Fe"'(OEP)( 1-MeIm), (1-MeIm: 1-methylimidazole) and Fe"'(OEP)(n-C4H9NH2), were produced by adding excess 1-MeIm or n-butylamine to Fe"'(0EP)Br in CH2C12, respectively. Fe1"(OEC)(CH30H),, Fe"'(OEC)(Me,SO),, Fe"'(OEC)(Im),, Fe1"(OEC)(2-MeIm), Fe"'(OEC)(1,2-Me21m), Fe"'(OEC)(lMeIm)2, and Fe"'(OEC)(n-C4H9NH2), were prepared from Fe'"(0EC)X in CHzC12with the same methods as those employed for obtaining the corresponding porphyrin analogues. To prepare Fe"(OEP)(Im),, an aqueous solution of sodium dithionite and imidazole in phosphate buffer was overlaid on the CHzClzsolution of Fe"'(0EP)Br under anaerobic conditions and the cell was vigorously shaken. Similar procedures were adopted to prepare all other ferrous porphyrin and chlorin complexes studied here. All solvents were spectroscopic grade and were used without further purification. Raman spectra were measured with a JEOL-400D Raman spectrometer equipped with a cooled RCA-31034a photomultiplier. The excitation sources used were Kr (Spectra Physics, Model 164), He/Cd (Kinmon Electrics, Model CDRSOMGE), and Ar (NEC GLG3200) lasers. For the measurements a cylindrical cell was (15) Felton, R. H.; Yu, N.-T. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1978; Vol. 3, Part A, p 347. (16) Asher, S. A. Methods Enzymol. 1981, 76, 371. ( 17) Spiro, T. G. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA 1983; Vol. 2, p 91. (18) Lutz, M. Adv. Infrared Raman Spertrosc. 1984, 11, 211. (19) Ozaki, Y.; Kitagawa, T.; Ogoshi, H. Inorg. Chem. 1979, 18, 1772. (20) Ozaki, Y.; Iriyama, K.; Kitagawa, T.; Ogoshi, H.; Ochiai, T. Rev. Port. Quim. 1985, 27, 340. (21) Hanson, L. K.; Chang, C. K.; Ward, B.; Callahan, P. M.; Babcock, G. T.; Head, J. D. J. Am. Chem. SOC.1984,106, 3950. (22) Andersson, L. A.; Loehr, T. M.; Chang, C. K.; Mauk, A. G. J. Am. Chem. Soc. 1985, 107, 182. (23) (a) Cotton, T. M.; Timkovich, R.; Cork, M. S . FEES Lett. 1981,133, 39. (b) Ching, Y.; Ondrias, M. R.; Rousseau, D. L.; Muhoberac, B. B.; Wharton, D. C. FEES Lett. 1982, 138, 239. (24) Ozaki, Y.; Iriyama, K.; Ogoshi, H.; Ochiai, T.; Kitagawa, T., following paper in this issue. (25) (a) Ogoshi, H.; Watanabe, E.; Yoshida, 2.;Kincaid, J.; Nakamoto, K. J. Am. Chem. SOC.1973, 95, 2845. (b) Cohen, I. A. J Am. Chem. Sor. 1969,91, 1980. (c) Buchler, J. W.; Schneehage, H. H. Z . Nuturforsch., E: Anorg. Chem., Org. Chem. 1973, 28, 433. (d) Ogoshi, H.; Sugimoto, H.; Yoshida, Z . Bull. Chem. Soc. Jpn. 1981,54,3414. (e) La Mar, G. N.; Eaton, G. R.; Holm, R. H.; Walker, F. A. J. Am. Chem. SOC.1973, 95, 63.

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Figure 2. 488.0-nm excited polarized R R spectra of Fe"'(0EP)F (A), Fe'"(0EP)Br (B), Fe"'(0EP)I (C), Fe"'(0EC)F (A'), and Fell1(0EC)Br (B') in CHzClz and the 488.0-nm excited RR spectrum of Fe"'(0EC)I (C') in CHzCl2. Through all the R R spectra presented in this paper the Raman bands marked with S are due to solvent. Instrumental conditions are as follows: power, 180 m W slit width, 7 cm-'; scan speed, 25 cm-'/min; time constant, 4.0 s.

placed in a water-jacketed cell holder kept at 5 O C , and laser light was introduced from the bottom under the 90° scattering geometry. The spectra presented here were the same as those obtained with a spinning cell (1800 rpm). Peak frequencies were calibrated with indene and the estimated errors of frequencies were less than 1 cm-' for well-resolved bands. Absorption spectra were recorded with a Hitachi 124s spectrophotometer. Results

Ferric High-Spin Derivatives. Figure 2 compares the 488.0-nm excited RR spectra of Fe"'(0EP)X with those of Fe"'(0EC)X (X = halide). In the spectra presented in this paper, solid and broken lines represent the parallel and perpendicular polarization components, respectively. Vibrational assignments of RR bands are based on the experimental and theoretical analysis for Ni(OEP)26,27and Fe(0EP) derivatives.28 RR bands at 1629(dp), 1568(ap), and 1493(p) cm-' of Fe"'(0EP)Br in CH2C12(p, dp, and ap denote the polarized, depolarized, and anomalously polarized lines, respectively) can be assigned to the modes involving mainly C,C, stretching vibrations (via, vL9,and v3, respectively) while bands at 1581(p) and 1559(dp) cm-' are due to the modes having a substantial C,C, stretching character (v2 and vll, respectively). An intense band at 1374(p) cm-' is assigned to the mode involving largely the C,N stretching coordinate (v4). The RR spectra of Fe"'(0EP)X are in general very similar to that of Fe*1'(OEP)C1.28 However, it is noted that the v I 9 mode of Fe"'(0EP)I is located at a considerably lower frequency compared with those of Fe'"(OEP)F, Fem(OEP)Br,and Fe"'(OEP)Cl(1569 cm-', not shown). Vibrational assignments for RR bands of Fe"'(0EC)X can be made by comparing their spectra with that of Cu(OEC), for which the assignments have been proposed on the basis of isotopic frequency shifts upon meso -y,&deuteration and ISN substit~tion.'~ RR bands at 1630(p), 1570(ap), and 1494(p) cm-I of Fe"'(0EC)Br are assigned to the modes containing mainly the C,C, stretching motions (close to vIo, ~ 1 9 ,and vj, respectively), and those at 1589(p+ap) and 1530(p) cm-' can be attributed t o the modes arising mainly from the C,C, stretching vibrations (close to v2 and ull. respectively). Upon excitation at 406.7 nm, the parallel c6mponent of the 1589-cm-l band was largely intensified. Ac(26) (a) Kitagawa, T.; Abe, M.; Ogoshi, H. J. Chem. Phys. 1978,69,4516. (b) Abe, M.; Kitagawa, T.; Kyogoku, Y. Ibid. 1978, 69, 4526. (27) Kitagawa, T.; Abe, M.; Kyogoku, Y.;Ogoshi, H.; Sugimoto, H.; Yoshida, Z . Chem. Phys. Lert. 1971, 48, 55. (28) (a) Kitagawa, T.; Ogoshi, H.; Watanabe, E.; Yoshida, Z. Chem. Phys. Lett. 1975, 30,451. (b) Kitagawa, T.; Ogoshi, H.; Watanabe, E.; Yoshida, 2. J. Phys. Chem. 1975, 79, 2629.

The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6107

Resonance Raman of Iron-Chlorin Derivatives

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Figure 3. Polarized R R spectra of Fe"'(OEP)(Z-MeIm) (A) and Felt'(OEP)(1,2-MeJm) (B) in CH2CI2and R R spectra of Fe"'(OEC)(2MeIm) (A') and Fe11'(OEC)(1,2-Me21m)(B') in CH2CI2excited by the 488.0-nm line. The Raman band'marked with double asterisks is due to free 1,2-Me21m (B). Instrumental conditions are the same as those in Figure 2.

cordingly, it is categorized as a polarized band. The perpendicular component a t 1589 cm-I is probably associated with a different ap band. The C,N symmetric stretching mode (close to v4) is seen at 1371 cm-I for Fe"'(0EC)Br. RR frequencies of Fe"'(0EC)Br and Fe"'(0EC)I are very close to those of Fe"'(OEC)C1,I9 although those of FelI1(OEC)F are slightly higher. Comparison of the spectra of Fe"'(0EC)X with those of Fe"'(0EP)X reveals that the C& , stretching modes (v2 and v l l ) show a large frequency change upon saturation of one C,C, bond while the C,C, stretching frequencies (vl0, ~ 1 9 ,and v3) change little. In the 1340-1390-cm-' region, new polarized bands are observed near 1350 and 1380 cm-' for Fe"'(0EC)X and the v4 frequency is slightly lower for Fe"'(0EC)X than for FeIU(OEP)X. So far the R R studies on the five-coordinate ferric high-spin complexes have been limited to halide and p-oxo derivatives. To examine the high-spin complexes with one imidazole ligand in the axial position, we treated 2-MeIm and 1,2-MeJm complexes. Absorption spectra of FeI1'(OEP)(2-MeIm) and Fe"'(0EP)(1 ,2-Me21m) in CH2C12confirmed that these complexes adopt the five-coordinate high-spin state. Figure 3 shows the 488.0-nm excited polarized R R spectra of Fe111(OEP)(2-MeIm) (A) and Fetl'(OEP)( 1,f-MeIm) (B) and the 488.0-nm excited R R spectra of Fe111(OEC)(2-MeIm)(A'), and Fe"'(OEC)(1,2-Me21m) (B'). The RR spectra of Fe"'(OEP)(Z-MeIm) and Fe1I1(OEP)(1,2MeJm) are very similar to each other but distinguishable from those of Fe"'(OEP)F, Fe"'(OEP)Cl, and FeII'(0EP)Br in the 1500-1 700-cm-I region; frequencies of RR bands are lower by 1-4 cm-' for the former two than for the latter three and the relative intensities of the 1556-cm-' band of the former group are noticeably stronger than those of the latter group. The effects of ligation of 2-MeIm or l,2-MezIm on the iron ion are different between the OEP and OEC complexes.29 The absorption spectra of FeI1'(OEC)(2-MeIm) and Fe"'(0EC)(1 ,2-Me21m) appeared to be a superposition of spectra of the low-spin and high-spin species when the concentration ratio of imidazole to porphyrin is higher than 10,. However, the RR spectra measured under the conditions that the high-spin species is dominant are very close to those of Fe"'(0EC)X. Only the v l l mode is largely shifted to a higher frequency for FeT1'(OEC)(2-MeIm) and Fe"'(OEC)( 1,2-Me2Im)(1540 cm-') than for Fe"'(0EC)X (1 530 cm-I). Figure 4 shows the 488.0-nm excited polarized RR spectra of Fe1"(0EP)(CH3OH), (A) and Fe111(OEP)(Me2S0)2(B) and the 488.0-nm excited RR spectra of Fe11'(OEC)(CH30H)2(A') and Fe111(OEC)(Me2S0)2(B'). The two OEP complexes are sixcoordinate ferric high-spin derivatives.3w33 Nevertheless, they (29) To be published elsewhere in more detail.

(30) Zobrist, M.; La Mar, G . N. J . Am. Chem. SOC.1978, 100, 1944.

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Figure 4. 488.0-nm excited polarized R R spectra of Fe"'(0EP)(CH,0H)2 in a 1:l mixture (v/v) of CH30H/CH2C12 (A) and Fe"'(OEP)(Me,SO), in that of (Me2SO)/CH2CI2(B) and the 488.0-nm excited R R spectra of Fe11'(OEC)(CH30H)2in a 1:l mixture (v/v) of CH3OH/CH2Cl2 (A') and Fe"'(OEC)(Me2S0)2 in that of Me,SO/ CHlC12 (B'). The Raman band with an asterisk contains contributions from both sample and solvent (B). Instrumental conditions are the same as those in Figure 2.

"+ , ...\ h;-k

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Figure 5. 488.0-nm excited polarized RR spectra of Fe11'(OEP)(Im)2(A) and Fe111(OEP)(n-C4H9NH2)2 (B) in CH2CI2and the 441.6-nm excited R R spectra of Fe"'(OEC)(Im), (A') and Fe"'(OEC)(n-C4H9NH2), (B') in CH2C12. Instrumental conditions are the same as those in Figure 2 except for the excitation source (488.0 nm, 180 mW for (A) and (B) and 441.6 nm, 40 mW for (A') and (B')).

do not exhibit an identical spectrum; the RR spectrum of Fell1(OEP)(TMSO), (not shown) was similar to that of Fe"'(OEP)(Me2S0)2,which is coincident with those reported by Spiro et aL3, and ourselves,33 while the R R spectrum of Fe"'(OEP)(C2H50H), (not shown) was similar to that of FeT1'(OEP)(CH,OH),. The visible absorption spectra of the four complexes are also classified into two groups. In Figure 4, the v 3 band is missing, but it was clearly identified at 1491 cm-I for the alcoholic complexes and at 1483 cm-' for the sulfoxide complexes upon excitation at 441.6 nm. Thus, the alcoholic complexes give Raman lines generally at higher frequencies than the sulfoxide complexes, and the amount of the upshift is appreciably larger for vl0 and v 3 (8-12 cm-') than for vI9, v,, and v l l (2-5 cm-I). By analogy to the OEP complexes, Fe"'(OEC)(CH,OH), and Fe"'(OEC)(Me,SO),

a r e expected to adopt t h e six-coordinate

high-spin state. In fact, their RR spectra are distinct from those of the five-coordinate high-spin complexes shown in Figures 2 and 3. Frequencies of the vl0 and v 3 modes are higher for Fe"'(31) Mashiko, T.;Kastberm, M. E.; Spartalian, K.; Scheidt, R.W.; Reed, C.A. J . Am. Chem. SOC.1978, 100,6354. (32)Spiro, T.G.;Stong, J. D.; Stein, P. J . Am. Chem. SOC.1979, 101,

2648. (33) Teraoka, J.; Kitagawa, T. J . Phys. Chem. 1980.84, 1928.


Ozaki et al.

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Figure 6. Comparison of the 488.0-nm excited polarized RR spectra of Fe"(OEP)(Z-MeIm) (A) and Fe"(OEC)(Z-MeIm) (A') in CH2CI2. Raman bands with double asterisks are due to free 2-MeIm in CH2CI2 (A'). Instrumental conditions are the same as those in Figure 2.

(OEC)(CH30H)2than for Fe111(OEC)(Me2S0)2similar to the OEP complexes. However, their differences are much smaller in the chlorin complexes than in porphyrin complexes. In addition, the v4 modes of Fe111(OEC)(CH30H)2(1372 cm-I) and Fe"'(OEC)(Me2S0)2(1371 cm-') are somewhat lower than those of their porphyrin counterparts. Ferric Low-Spin Derivatives. Figure 5 shows the 488.0-nm excited polarized R R spectra of Fe111(OEP)(Im)2 (A) and Fe111(OEP)(n-C4H9NH2)2 (B) and the 441.6-nm excited R R spectra of Fe"'(OEC)(Im), (A') and Fe"1(OEC)(n-C4H9NH2)2 (B'). The vl0 and v3 frequencies are almost the same between the OEP and OEC complexes. However, the vI9 frequencies of the OEC complexes (1 576-1 579 cm-I) are lower by 10 cm-I than those of the OEP complexes. The v2 and v l l modes of the chlorin complexes are found near 1600 and 1549 cm-I, respectively. Accordingly, as in the case of the high-spin complexes shown in Figures 2-4, the saturation of one COCObond causes a small upward shift of the v2 mode and a large downward shift of the vll mode for the low-spin complexes. The chlorin complexes give two bands in the v4 region in contrast with a single intense band for the porphyrin complexes. Both are strongly enhanced upon excitation at 406.7 nm. The 1372-cm-I line is assigned to the v4 mode. It is noteworthy that the effects of axial ligands upon the R R spectra are very small for the ferric low-spin derivatives of both porphyrin and chlorin; the 1-MeIm complex gave the same RR spectrum as the Im and n-C4H9NH2complexes. Ferrous High-Spin Derivatives. Fe"(OEP)(S-MeIm) is known to adopt the five-coordinate high-spin s t r ~ c t u r e . ~ ~ Figure -~' 6 presents the 488.0-nm excited polarized RR spectra of Fell(OEP)(2-MeIm) (A) and Fe"(OEC)(Z-MeIm) (A'). Upon excitation at 406.7 nm, the parallel components of the latter at 1579 and 1365 cm-I were markedly intensified. Accordingly, they are categorized as polarized bands. The perpendicular component at 1579 cm-' is probably associated with a different ap band. On the basis of the polarization properties, the Raman lines of Fe"(OEC)(Z-MeIm) at 1608(p), 1579(p), 1559(ap), 1535(p) and

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(34) Spiro, T. G.; Burke, J. M. J . Am. Chem. SOC.1976, 98, 5482. (35) Hoard, J. L.; Scheidt, W. R. Proc. Natl. Acad. Sci. U.S.A. 1973, 70, 3919; 1974, 71, 1578. (36) Stein, P.; Mitchell, M.; Spiro, T. G . J . Am. Chem. SOC.1980, 102, 7195. (37) Desbois, A.; Henry, Y . ;Lutz, M. Biochim. Biophys. Acta 1984, 785, 148.

Figure 7. 488.0-nm excited polarized RR spectra of Fe"(OEP)(4CHO(PY))~(A), Fe"(OEP)(Py)2 (B), Fe"(OEP)(Im), (C), Fe"(OEP)(n-C4H9NH2)2(D), Fe"(OEC)(4-CHO(Py))2 (A'), Fe"(0EC)(Py), (B'), Fe11(OEC)(Im)2(C'), and Fe"(OEC)(n-C,H9NH2)2(D') in CH2CI2and the 441.6-nm excited RR spectrum of Fe"(OEC)(Im), (C") in CH2CI2.The Raman band with double asterisks is due to free Im in CH2CI2(C) and those marked with b are assignable to bound-pyridine modes (B and B': see ref 24). Instrumental conditions for the 488.0-nm excited spectra and the 441.6-nm excited spectrum are the same as those in Figure 2 and Figure 5 , respectively.

1477(p) cm-I are assigned to the vIo, v2, ~ 1 9 ,vI1, and v3 modes, respectively. The v4 mode of FeI1(OEC)(2-MeIm) at 1365 cm-I is very weak and observed at a slightly higher frequency than that of FeI1(OEP)(2-MeIm). R R spectra of Fe"(OEP)( 1,2-MeJm) and Fe"(OEC)( 1,2-Me21m)closely resembled those of the 2-MeIm complexes, respectively. In contrast to the case of ferric chlorins, evidence for the formation of six-coordinate complex was not obtained for 2-MeIm and 1,ZMqIm complexes of ferrous chlorins even with highly concentrated 2-MeIm (or 1,2-Me21m) solutions. Ferrous Low-Spin Complexes. The 488.0-nm excited polarized R R spectra of Fe11(OEP)(4-CHO(Py))2(Py: pyridine) (A), Fe"(OEP)(Py), (B), Fe"(OEP)(Im), (C), and Fe"(OEP)(nC4HgNHJ2 (D) are compared with those of Fe"(OEC)(4CHO(Py)), (A'), Fe"(OEC)(Py), (B'), Fe11(OEC)(Im)2(C'), and Fe11(OEC)(n-C4H9NH2)2 (D') in Figure 7, where a part of the spectrum of Fe11(OEC)(Im)2 (C") excited at 441.6 nm is also included. R R spectra have been measured also for Fell( O E P ) ( ~ - C N ( P Y ) ) Fe11(OEP)(y-Pic)2 ~, (y-Pic: y-picoline), Fe11(OEP)(4-NH2(Py))2,38 Fe11(OEP)(4-CH2=CH(Py))2, and Fe"(OEP)(Pip), (Pip: piperidine) and their OEC counterparts. The vl0, vlg, v3, v2 and vl1 modes of Fe11(OEC)(Im)2 can be identified at 1628(p), 1583(ap), 1494(p), 1601(p), and 1545(p) cm-I, respectively. Similar to the high-spin complexes, the parallel component of the 1601- and 1367-cm-l bands of Fe11(OEC)(Im)2 became more intense upon excitation at 441.6 and 406.7 nm, and accordingly the bands are assigned to v2 and v4, respectively. As in the case of ferrous high-spin derivatives the v4 mode of Fe"(OEC)(Im), is very weak in the 488.0-nm excited spectrum and found at somewhat higher frequency (1367 cm-') compared with that of the corresponding porphyrin complex (1364 cm-I). It is (38) Fe"(OEP)(4-NH2(Py))2 and Fe"(OEC)(4-NH2(Py))2 in CH2CI2are considered to consist of high-spin and low-spin species;29RR bands due to the low-spin species are much more dominant in their 488.0-nm excited RR spectra, while their 441.6-nm excited RR spectra contain largely RR bands due to the high-spin species. (39) Coffey, S . , Ed. Chemistry of Carbon Compounds, 2nd ed.; Elsevier Scientific: Amsterdam, 1976.



TABLE k RR Frequencies of the vIo, vI1, and v4 Modes (cm-') of Ferrous OEP and OEC Complexes

OEP complexes Fe"(OEP)(4-CN(Py)), . Fe11(OEPj(4-CHO(Py))2 Fe"(0EP) (Py), Fe1I(OEP)(y-Pic), Fe"(OEP)(4-CH2=CH(Py)), Fe"(OEP)(4-NH2(Py)), Fe"(OEP)(Im), Fe"(OEP)(Pip), Fe"(OEP)(n-C4H9NH2), Fe"(OEP)(Z-MeIm) FeT1(OEP)(1,2-Me21m) - I , -

VI0

v11

v4

1624 1624

1557 1556

1370 1369

1621 1619

1550 1549

1367 1366

1621 1619 1620 1621 1618 1608

1550 1546 1543 1544 1539 1548 1548

1367 1365

1607

1364 1363

1362 1362

1361

OEC complexes Fe"(OEC)(4-CN(Py)), Fe"(0EC j(4-cHo(Pyj), Fe"(0EC) (Py), Fe"(OEC)(y-Pic), Fe"( OEC)(4-CH2=CH(Py)), Fe"(OEC)(4-NH2(Py)), Fe"(OEC)(Im), Fe"(OEC)(Pip), Fe"(OEC)(n-CAH9NH2), Fe"(OEC)(Z-MeIm) Fe"(OEC)(1,2-Me21m)

VI0

v11

v4

1629

1546 1546

1369 1369

1546 1545

1368 1367

1545 1543 1545 1544

1367 1367 1367

1540 1535

1367 1365

1536

1366

1629 1629

1628 a

1627 1628 1626 1626 1608 1608

1367

Hidden by a band due to the vinyl group of bound 4-vinylpyridine. noteworthy that the vl0 mode shows a large upward shift (8 cm-l) upon going from Fe11(OEP)(Im)2to Fe"(OEC)(Im),. A bound-Py mode is seen near 1597 cm-' for both F ~ I I ( O E P ) ( P Y and )~ Fe11(OEC)(Py)2but will be discussed in the following paper.24 Table I summarizes the vl0, u,,, and u4 frequencies of all the ferrous low-spin porphyrin and chlorin complexes examined in this study. The vl0, v I 1 , and u4 frequencies of ferrous low-spin porphyrin complexes are very sensitive to replacement of axial ligands (see Figure 7 and ref 34). In Figure 8 are plotted the ulo, vI1, and v4 frequencies of various ferrous low-spin complexes vs. pKa values of the axial ligands. Since the pKa values are varied by changing only the para substituent of pyridine, the frequency change is considered to reflect purely the effect of acidity of the axial ligands. Although the points are somewhat scattered, it can be concluded that the more basic ligand yields the lower frequency for the ul0, v l l , and v4 modes. On the other hand, the R R frequencies of the same series of complexes of Fe"(0EC) are much less sensitive to pKa. This is consistent with the close similarities in R R frequencies of Fe11(OEC)(4-CHO(Py))2, Fe"(OEC)(Py),, Fe"(OEC) (Im)z, and Fe"(0EC) ( ~ I - C , H ~ N H(Figure ~ ) ~ 7).

Discussion Assignments and General Features of Skeletal Modes. The RR spectral characteristics of metallochlorins in comparison with metalloporphyrins explored so far can be summarized as follows:1"22 (1) an increase in the number of R R bands; (2) altered polarization properties and an increase in the number of polarized bands; (3) pronounced intensity enhancement of R R bands in the 1000-1350-cm-' region upon excitation near the Q band. These features are considered to arise from the lowered symmetry of the chlorin chromophore. In our previous treatment,I9 M(0EC) was regarded to have approximately C, symmetry for explaining the correlation of polarization properties of Raman lines between M(0EP) with D4* symmetry and M ( 0 E C ) with lowered symmetry. Recently Andersson et a1.22 proposed an effective C2 symmetry on the basis of the crystal structure of Fe*1(OEC)8din interpreting R R spectra of metallochlorins. Rigorous symmetry of M(0EC) is in fact lower than C , and probably depends upon the spin state and the coordination number of the iron ion. In the case of porphyrin, the higher frequency vibrations were interpreted with D4,,symmetry satisfactorily despite the fact that rigorous symmetry is lower than it. Since this paper aims at comparing M(0EC) with M(OEP), we keep the terminology of C , symmetry unless serious contradiction appears. Accordingly, the A,,(p) and Bl,(dp) modes of M ( 0 E P ) are correlated with the totally symmetric Al(p) modes in M(0EC) while the A2,(ap) and B,,(dp) modes of M(0EP) are correlated with the B2(ap or dp) modes in M(0EC). Degeneracy of the Raman inactive E, vibrations of M(0EP) is removed for M(0EC) and they split into the Raman-active A, and B2.species of C,. The out-of-plane modes (A2 and B, species) will not be discussed here. Eight C,C, stretching modes (4A1 + 4B2) are expected for M(OEC), although four of them corresponding to the E, mode of D4,,are considered to be weak in Raman intensity. The active four modes correspond to vIo, ~ 1 9 uZ8, , and v3. While the u28 band has never been observed for OEP derivatives, it was previously identified at 1546 cm-' for Cu(OEC).19 The 1546-cm-I band of

16251I-1620

I 1555 -

E" f 1550 -

f

% 1545

-

1365

PKn

Figure 8. RR frequencies of the vIo (A), v I 1 (B), and v4 (C) modes of

Fe"(0EP) (4-CN(Py)), Fe"( OEP)(+CHO( Py)),, Fe"( OEP)(Py),, Fe"(OEP)(y-Pic),, and Fe' ,0EP)(4-NH2(Py)), and their OEC counterparts in CH2C12vs. pK, values3' of 4-CN(Py), CCHO(Py), Py,y-Pic, 4-NH,(Py), respectively: (0) OEP complexes; ( 0 ) OEC complexes. Cu(0EC) was regarded as a superposition of an a p band as~ )a p band due to signable to the C,C, stretching mode ( v ~and the C,C, stretching mode ( v I 1 ) . l 9 The 1530-cm-' band of Fe'"(0EC)Br corresponds to uB. The four C,C, stretching modes are clearly observed for all OEC complexes studied. The polarization property of the ul0 mode can practically be used as diagnosis for distinguishing metallochlorins from metalloporphyrins;",22 p for the former and dp for the latter. The ul0 and vj frequencies are similar between the two complexes but the v 1 9 frequencies exhibit some differences (Figures 4-6). Two Raman-active C,C, stretching modes (v2 and v 1,) belong to an identical symmetry group for M(0EC). Both modes were expected to show a large downward shift upon saturation of one C,C, bond. Actually, however, the v2 and v l l modes exhibited an upward and a downward shift, respectively. The frequency separation between v2 and uII is relatively small and nearly constant (21-33 cm-') for Ni(OEP),26Cu(OEP), and Fe(0EP) derivatives except for ferrous low-spin complexes for which the effect of ?r-back-donation is expected. In contrast, the separation is very large for the OEC complexes, varying from 44 [Fe11(OEC)(2-

Ozaki et al.


MeIm)] to 62 cm-I [Felll(OEC)I]. The larger splitting in M(OEC) is probably caused by the larger vibrational coupling between two pyrrole rings, and the size of the coupling would depend on metal species, spin and oxidation states, and coordination number. Interestingly, the u l I frequency is notably sensitive to an axial ligand for five-coordinate FelI1(OEC) derivatives, varying from 1527 for Fe1I1(0EC)I to 1540 cm-l for Fe1"(0EC)(2-MeIm). This sort of large frequency variation has not been recognized for OEP complexes and therefore is a specific feature to the OEC complexes. The frequency change of the uI1mode is not parallel with the changes with the C,C, stretching modes and accordingly is not associated with a structural change such as a core-size expansion or doming. It may be caused by a change of an electronic state that has larger density at C, atoms and at the same time is susceptible to influence by an axial ligand. One of the candidates is the highest filled a,, orbital, which is antibonding about the C,C, bonds but nonbonding about the C,C, and C,N bonds. The u4 mode of Cu(0EC) (1372 cm-I) showed a downward shift by 6 cm-' upon 15Nsubstitution, and from this value the vibrational displacement of nitrogen atoms due to the u4 mode was estimated to be 0.01 1 &I9 which is identical with that for Ni(OEP).27 This suggests that the origin of the vibrational mode of u4 is similar between the OEC and OEP complexes. However, the u4 frequencies of C U ( O E C ) ' ~and Ni(OEC)I9 are lower by 8 and 11 cm-l than those of Cu(0EP) and Ni(OEP),27 respectively. A similar trend is observed for the Fe"' complexes, although the difference is smaller (2-6 cm-I). In contrast, the u4 frequencies of ferrous chlorins are higher than those of ferrous porphyrins. As a result, the downward shifts of the u4 mode upon reduction of the iron atom are considerably smaller for the chlorin complexes than for the porphyrin complexes: -4 and -9 cm-l for Fe(0EC) and Fe(0EP) complexes, respectively. The frequency lowering of the u4 mode for ferrous low-spin porphyrins has been attributed to weakening of the C,N bonds caused by the increased x-back-donation from the d,(Fe) to the x*(porphyrin) orbital^.^^^^^^^ If the same interpretation is adopted, it implies that the increase of x-back-donation upon reduction of the iron ion is not so large in Fe(0EC) complexes as in Fe(0EP) ones or alternatively the antibonding character of the x * orbital regarding the C,N bonds is not so strong in the Fe(0EC) complexes as in the Fe(0EP) ones. The latter depends upon the nodal pattern of the LUMO, which might be significantly different between M(0EP) and M(0EC). The recent M O calculations on free-base porphyrin and chlorin, however, showed that the nodal pattern of the x,* orbital (LUMO orbital) of the chlorin is very similar to those of the x * orbitals (degenerate LUMO orbitals) of the porphyrin regarding the C,N bonds.42 Accordingly, we prefer to the former alternative at the moment. The appearance of a few polarized bands near the u4 region is also a characteristic feature of chlorin R R ~ p e c t r a . ' ~On , ~the ~ basis of the 15N isotope shift,I9 the 1362-cm-, line of Cu(0EC) was assigned to uI2,which has substantially the C,N stretching character.26 This band is hardly observed in the RR spectra of M(0EP). In the RR spectrum of FeI1'(OEC)Br, bands are seen at 1379 and 1347 cm-I (Figure 2). The band at 1347 cm-' is assignable to the u12mode, because the mode appears at somewhat lower frequency than the u4 mode for Cu(OEC).I9 The band at 1379 cm-l is considered to arise from the E, mode ( ~ 4 , ) which became Raman active due to lowering of symmetry. Both ~ 4 and 1 u I 2 modes are clearly recognized in the R R spectra of various Fe(0EC) derivatives. The Raman line of Fe"'(0EC)Br at 1400 cm-l (dp) is assignable to u20 (Azg,C,N stretching + C,Et stretching) or ~ 2 (BZg, 9 mainly C,C, stretching) mode. This band is clearly observed for the 488.0- and 441.6-nm excited R R spectra similar to the uZ9 mode of Ni(OEP).% On the other hand, the uzo mode of Ni(0EP) (40) Spiro, T. G.; Strekas, T.C. J . Am. Chem. SOC.1974, 96, 338. (41) (a) Kitagawa, T.;Iizuka, T.;Saito, M.; Kyogoku, Y . Chem. Lett. 1975,849. (b) Kitagawa, T.; Kyogoku, Y.; Iizuka, T.;Saito, M. J. Am. Chem. Soc. 1976, 98, 5 169. (42) Nagashima, U.; Takada, T.;Ohno, K., to he published.

1500

(b)

1550

1600

1650

RAMAN SHIFT (cm9

I I

I

1

I

was scarcely seen in the 488.0-nm excited RR spectrum.26 Therefore, the 1400-cm-l band of Fe"'(0EC)Br corresponds more likely to the u29 mode. Two ap bands of Fe"'(0EC)Br at 1308 and 1297 cm-' might be due to the u2, and u5 + u26 modes, respectively, as in the case of Ni(0EP). Correlations between Raman Frequencies and Core Sizes. Raman frequencies were correlated with core sizes first by Spaulding et al.43for the u I 9band and was later extended to the uIo, v3, and v2 bands.32,44,45Recently it was found for M(PP) (PP: protoporphyrin IX) that all the skeletal modes above 1450 cm-I show a linear dependence on the Ct-N (center-to-pyrrole nitrogen) distance, with inclinations proportional to the extent of the C,C, stretching character.46 OEP complexes would be more suitable to examine such correlation, because X-ray crystallographic molecular orbital calculations,4s and normal coordinate analysis26have been carried out for M(0EP). In addition, comparison of the correlations between the OEP and OEC complexes (43) Spaulding, L. D.; Chang, C. C.; Yu, N.-T.; Felton, R. H. J . Am. Chem. SOC.1975, 97, 2517. (44) Huong, P. V.; Pommier, J.-C. C. R. Seances Acad. Sci., Ser. C 1977, 285, 5 19. (45) Callahan, P. M.; Babcock, G . T. Biochemistry 1981, 20, 952. (46) Choi, S.; Spiro, T.G.; Langry, K. C.; Smith, K. M.; Budd, D. L.; La Mar, G . N. J . Am. Chem. Soc. 1982, 104, 4345. (47) Scheidt, W. R.; Gouterman, M. In Iron Porphyrins; Lever, A. B . P., Gray, H. B.,Eds.; Addison-Wesley: Reading, MA 1983; Vol. 1, p 89. (48) Loew, G. H. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; Vol. 1, p 1. (49) (a) Cullen, D. L.; Meyer, E. F., Jr. J . Am. Chem. Soc. 1974,96,2095. (b) Collins, D. M.; Countryman, R.; Hoard, J. L. J . Am. Chem. SOC.1972, 94, 2066. (c) Radonovich, L.J.; Bloom, A,; Hoard, J. L. J. Am. Chem. SOC. 1972, 94, 2073. (d) Hoard, J. L. Science (Washington, D.C.) 1971, 174, 295. (e) Hoard, L. J.; Scheidt, W. R. Proc. Natl. Acad. Sci. U.S.A.1973, 70, 3919.



TABLE II: Iron-Porphyrin (Chlorin) Core-Size Correlation Parameters for High-Frequency Skeletal Modes above 1450 em-'

modeb

PEDc

v I ~ ( A ~ ~C,Cm(67) ) vlo(B1 ) C,Cm(49)

Kd 576.3

Ad

4.74 5.30 5.63 7.44 6.93

OEP A(Im); +8

A(2-Me1m)e -3

Kd 331.8

Ad

6.76

OEC A(Im)ze A(2-MeIm)e K" +5 -6 494.3

Ad

5.20

PPQ A(Im)2e A(2-MeIm)e +3 -10

494.9 -12 -5 471.9 5.46 -4 -5 517.2 5.16 -17 -6 346.5 6.33 -5 -8 448.3 5.35 -5 -9 -8 -6 C,Cm(41) 413.8 344.8 6.53 -21 +1 367.1 6.19 +9 +14 vll(B1 ) C,C,9(57) 287.5 -20 -3 +8 +2 390.8 6.03 +9 +2 404.3 5.94 +9 +6 C,C,(60) 322.3 v2(AIJ "From ref 46. bMode numbers and symmetries for Ni(0EP) (ref 26). cPercentage contribution of the major contributor (C,C, or C,C, stretching) to the Ni(0EP) potential energy distribution (PED), according to ref 26. dSlope (cm-'/A) and intercept (A) for the relation D = K ( A - 4, where i~is mode frequency and d is center-to-pyrrole distance (Ct-N). CDeviations(cm-') for imidazole and 2-methylimidazolecomplexes of iron(I1) OEP, OEC, and PP from the frequencies expected on the basis of their core sizes.

vs(AIJ

seems useful for elucidating vibrational properties of OEC complexes. The observed frequencies of M(0EP) and M(0EC) complexes are plotted vs. the Ct-N distances in Figure 9. The Ct-N distances used for the OEP complexes are the same as those employed by Choi et a1.,46 who examined similar correlations for the PP derivatives. The average Ct-N distance of each OEC derivative is assumed to be the same as that of the corresponding OEP complex due to lack of X-ray data.50 This assumption might be justified by the fact that the average Ct-N distances of Fe"(0EP) (1.996 A) and Fe"(0EC) (1.986 A) obtained from the X-ray analysisse are alike. As expected, the frequencies of R R bands above 1450 cm-' of the OEP complexes are linearly correlated with the Ct-N distances and the modes with larger contribution from the C,C, stretching coordinate (vlo, ~ 1 9 ,and v3) are more sensitive to a change in the core size. Table I1 summarizes correlation parameters of OEP, OEC, and P p 6 complexes and the potential energy distribution of each mode. It is evident that the size of the gradients is parallel with the percentage contribution of the C,C, stretching coordinate, and this tendency is more rigorous for the OEP complexes than for the PP complexes. The correlations in Figure 9a are anchored by Ni(0EP) and Fe111(OEP)(Me2S0)2as in the case of the correlations for M(PP) derivatives. Fe"'(OEP)(Im), and Fe"'(0EP)Br fall very near the lines for all modes, but Fe"(OEP)(Z-MeIm) and Fe11(OEP)(Im)2 are significantly deviated from the lines. Such deviations were previously noticed by Choi et al.46 for Fe"(PP) complexes and ascribed to doming of porphyrin for Fe"(PP)(t-MeIm) and to the increased n-back-donation for Fe11(PP)(Im)2. Similar correlations hold also for the OEC complexes. Particularly notable in Figure 9b is that the lines for the vIo and v3 modes of OEC complexes coincide fairly well with those of OEP derivatives, respectively, but for ~ 1 an 9 appreciable discrepancy is seen, indicating that the contribution of the C,C, stretching coordinate to the v19 mode is not so high for the OEC complexes as for the OEP complexes. The deviations from the straight lines are again observed for the two ferrous complexes. The three C,C, stretching modes of FeI1(OEC)(2-MeIm) are deviated downward by 5-8 cm-' from the lines as in the case of FeI1(OEP)(2-MeIm) and FeI1(PP)(2-MeIm) (Figure 9 and Table 11). This type of deviation is considered to arise from a domed structure of por~ h y r i ncharacteristic ,~~ of a five-coordinate high-spin complex. The magnitudes of the deviations are much smaller for Fe"(OEC)(Im), than for Fe11(OEP)(Im)2. This may also suggest that the increased n-back-donation is considerably less in the OEC complexes than in the OEP ones. Additional support comes from the results that the vlo, v l l , and v4 frequencies are less sensitive to the alteration of the pK, value of the axial ligand (Figure 8). Diagnostic Marker Bands for Iron Chlorins. Recently R R spectra have been reported for several proteins containing an iron-chlorin prosthetic g r ~ ~ p . ~ To , ~ interpret ~ , ~ - those ~ , ~R~R spectra it would be helpful to point out some marker bands about the spin, oxidation, and ligation states of the iron ion. It became clear from the extensive comparison of the R R spectra of Fe(0EC) (50) X-ray crystallographic data are available only for Zn(TPC)(Py),8a Ni(TMC),8b*c and Fe"(OEC).Sd.C

and Fe(0EP) complexes that the diagnostic marker bands for iron porphyrins can generally be applied to iron chlorins, but regarding their frequencies slight modification of an empirical rule might be necessary. The v4 band appears at 1371-1374 and 1365-1369 cm-' for the Fe"'(0EC) and Fe"(0EC) complexes, respectively. Accordingly, the v4 band is expected to serve as an oxidation-state marker for the chlorin-containing proteins as for porphyrin-containing proteins. However, it must be kept in mind that the v4 frequencies for the proteins are slightly lower than those for the model compounds; the oxidized and reduced forms of myeloperoxidase and the green heme protein gave the v4 band at 1365-1366 and 1351-1352 cm-', r e s p e ~ t i v e l y .This ~ ~ difference is not peculiar to the iron-chlorin complexes. Similar differences were also noticed for the porphyrin chromophore^;'^ Fe"'(0EP) and Fe"(OEP) complexes give the v4 mode at 1373-1378 and 1361-1370 cm-', respectively, while met-Mb (Mb: myoglobin) derivatives and reduced heme proteins including deoxy-Mb and various cytochromes give the v4 band at 1371-1375 and 1355-1361 cm-I, respectively. Figure 9b demonstrates that all the R R skeletal modes above 1450 cm-' can serve as marker bands for the spin, oxidation, and ligation states of iron chlorins as for iron porphyrins. Among the five bands the vlo band seems to be most useful because the band is relatively isolated. It is noteworthy that met-SMb (SMb: sulfmyoglobin that contains a sulfur-modified iron chlorin) and deoxy-SMb, having the six-coordinate ferric high-spin and the five-coordinate ferrous high-spin hemes, respectively, give the vlo modes at almost the same frequencies as those of Fe"'(0EC)(Me2S0)2and Fe"(OEC)(Z-MeIm), re~pectively.~" Categorization of RR Spectra by Axial Ligands. The R R spectra of iron porphyrins and heme proteins have been categorized in terms of spin, oxidation, and ligation states of the iron ion, but frequencies of R R bands are appreciably distributed within a categorized group. This would reflect the iron-ligand interaction specific to the particular ligand. Such a feature was previously pointed out for only the ferrous low-spin c ~ m p l e x e sbut , ~ ~here we stress that such a ligand-specific effect is widely seen for various spin and ligation states. Figure 2 indicates that the R R frequencies of Fe"'(0EP)I above 1450 cm-' are lower by 2-7 cm-l than those of other Fe"'(0EP)X. A comparison of R R spectra in Figure 2 with those in Figure 3 shows that the R R frequencies above 1500 cm-l are generally lower for Fe"'(OEP)(Z-MeIm) and Fe"'(OEP)( 1,2-MeJm) than for FeIrl(OEP)F and Fe"'(OEP)Br, while v4 frequencies remain unaltered. These frequency lowerings might be caused by repulsion between ligand and pyrrolic nitrogens, which induces core expansion. However, since the frequency change is markedly larger for v2 and v l l bands in the case of imidazole complexes, there would be an additional change in an orbital having higher density at C, atoms. Axial ligand dependence of the R R spectra of Fe"'(0EC)X is not always the same as that of Fe"'(0EP)X. Frequencies of core-size markers are higher for Fe"'(0EC)F than for other Fe"'(OEC)X, but FelI1(OEC)I exhibits no specific feature in contrast with the case of Fe"'(0EP)I. The v l l modes of Fe"'(OEC)(2-MeIm) and Fe"'(OEC)( 1,2-Me21m) are shifted to higher frequency compared with those of Fe"'(0EC)X. Although X-ray crystallographic a n a l y s i ~ pointed * ~ ~ ~ out that S4 ruffling is


larger for metallochlorins than for metalloporphyrins, it seems rather unlikely to ascribe those frequency shifts to changes in the core size or in the extent of doming, because the C,C, stretching frequencies of Fe"'(OEC)(Z-MeIm) and Fe'"(OEC)( 1,2-MeJm) are very close to those of Fe"'(0EC)X. The ligation of 2-MeIm or 1,2-MezImto the Fe"' ion brought about the spin equilibrium for Fe(0EC) but only high-spin species for Fe(OEP).29 This implies that the affinity for the second 2-MeIm (or 1,2-MezIm) is much higher for Fe(OEC)(2-MeIm) than for Fe(OEP)(2-MeIm). Probably, the structure of the macrocycle is more flexible for Fe(0EC) than for Fe(0EP). It was pointed out previously33that the vl0 frequency of six-cmrdinate ferric high-spin OEP complexes is very sensitive to the molecular species of axial ligands. The observations in Figure 4 have manifested that not only the vl0 frequency but also the frequencies of all modes above 1450 cm-' are sensitive to the nature of axial ligands. They are generally higher for alcoholic complexes than for sulfoxide complexes. However, the vI9 mode, which should be most sensitive to an alteration of the core size, shifts only by 2 cm-I. Therefore, the frequency changes cannot be attributed simply to a change of the core size. In contrast to the OEP analogues, the frequency differences between the RR bands of Fe11'(OEC)(CH30H)2 and Fe"'(OEC)(Me$O), are smaller; frequency separation between the alcoholic and sulfoxide complexes are 5 and 3 cm-' for vl0 and v3 modes of the OEC complexes, respectively, and 12 and 8 cm-l for vl0 and v3 modes of the OEP complexes, respectively. This is understandable if vl0 and vj frequencies are sensitive to ruffling, because the extent of ruffling is already larger and therefore less altered upon a change of axial ligands in the OEC complexes than in the OEP complexes. For ferric low-spin complexes three ligands were examined: n-C4H9NH2,Im, and 1-MeIm. Choi et a1.52previously found RR (51) A notable example is Ni(TMC), in which the dihedral angles between the planes of opposite pyrrole rings range from 35O to 38O while Ni(TMP) is essentially planar. The core size of the former is slightly smaller than that of the latter (ref 8b,c). (52) Choi, S.; Spiro, T. G. J . Am. Chem. SOC.1983, I O S , 3683.

Ozaki et al. spectral differences between Fe"'(OEP)(Im), and Fe"'(0EP)(CN), in the low-frequency region and ascribed them to the asymmetric structure of Fe"'(OEP)(Im),, which may be induced by the hydrogen bonding of the ligand. Im and 1-MeIm are different regarding the ability to form the hydrogen bond. However, the RR spectra of the three OEP derivatives were much alike and this was also true for OEC derivatives. Probably the effects of hydrogen bonding are too small to be detected in the RR spectrum in the high-frequency region. The v4 frequency of ferric porphyrin has been believed to be insensitive to the spin state." However, the present examinations of the v4 frequencies for a wide variety of ligands demonstrated that the v4 frequencies of low-spin complexes are slightly higher than those of the high-spin complexes (Figures 2-5). This trend seems to be consistent with that observed for heme proteins. Such Raman features should be explained, in future, on the basis of MO calculations.

Acknowledgment. This work was supported by the Joint Studies Program (1984-1985) of the Institute for Molecular Science. Registry No. Fe11(OEP)(4-CN(Py))2,104439-45-4; Fe"(OEP)(4CHO(PY))~,104439-40-9; Fe"(OEP)(Py),, 19496-63-0; Fe"(OEP)(yPic)2, 61 112-14-9; Fe"(OEP)(4-CH2=CH(Py)),, 104463-52-7; Fe"(OEP)(4-NH2(Py)),, 104439-47-6; Fe"(OEP)(Im),, 53401-75-5; Fe"(OEP)(Pip),, 57335-3 3-8; Fe11(OEP)(n-C4HgNH,)2,104439-41-0; Fe1'(OEP)(2-MeIm), 64685-92-3; Fe"(OEP)( 1,2-Me2Im), 7581 1-16-4; Fd1(0EC)(4-CN(Py)),, 104439-46-5; Fe"(OEC)(4-CHO(Py)),, 104439-42-1; Fe"(OEC)(Py),, 104439-35-2; F e " ( o E c ) ( y - P i ~ ) ~ , 104439-36-3; Fd1(OEC)(4-CH2=CH(Py)),,104463-53-8; Fe"(OEC)(4-NH2(Py)),, 104439-48-7; Fe"(OEC)(Im),, 104439-43-2; Fe"(OEC)(Pip),, 104439-37-4; Fe11(OEC)(n-C4H9NH2)2, 104439-44-3; Fe"(OEC)(Z-Melm), 104439-34- 1 ; Fe"(OEC)( 1 ,2-Me21m), 10443927-2; Fe"'(OEP)F, 41697-91-0; Fe"'(OEP)Br, 41697-92-1; Fe"'(OEP)I, 41697-73-8; Fe"'(OEC)F, 78319-96-7; Fe"'(OEC)Br, 54643-23-1; Fe"'(OEC)I, 54643-24-2; Fe1''(OEP)(2-MeIm), 73078-24-7; Fel"(OEP)(1,2-Me21m), 104439-28-3; Fe"'(OEC)(Z-MeIm), 104439-29-4; Fe"'(OEC)( 1,2-Me21m), 104439-30-7; Fet1'(0EP)(CH30H),, 7370401-5; Fe"'(OEP)(Me2S0),, 70991-95-6; Fe"'(OEC)(CH,OH),, 104439-3 1-8; Fe"'( OEC) (Me2SO)2 , 104439- 39-6; Fe"'(0EP) (Im)2, 104439-32-9; Fe111(OEC)(Im)2, 48242-03- 1; Fe11'(OEP)(n-C4H9NH2)2, 104439-33-0; Fe111(OEC)(n-C4HgNH2),, 104463-54-9.

Resonance Raman characterization of iron-chlorin complexes in

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