J. Am. Chem. SOC.1988, 110, 742-748
742
Resonance Raman Spectra of Dioxygen Adducts of Cobalt Picket Fence Porphyrins J. Odo,18 H. Imai,lb E. Kyuno,lband K. Nakamoto*la Contribution from the Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233, and Department of Pharmaceutical Science, School of Pharmacy, Hokuriku University, 3 Ho Kanagawa-machi, Kanazawa 920-1 1 , Japan. Received May 4 , 1987
Abstract: The resonance Raman (RR) spectra (406.7-nm excitation, ca. -90 "C) of dioxygen adducts of atropisomers of picket
fence porphyrins, Co(T PP)(B)02 and Co(T,SP)(B)02, were measured in toluene with a variety of base ligands (B). The internal mode of the socent (toluene) was resonance-enhanced when the picket fence has a "semiprotected" cavity (e.g., a'and a2-atropisomers) and when the v ( 0 2 ) is matched closely to the frequency of toluene by choosing a proper base ligand. These observations confirmed our previous notion that vibrational energy transfer can occur when the bound dioxygen is directly associated with the solvent molecule and the frequency matching condition is satisfied. The marked low-frequency shift of to the corresponding Co(a4-T,,PP)(B)02 was attributed to the strengthening the v ( 0 2 ) in going from Co(a4-TpivPP)(B)02 of the N--H.-O2 hydrogen bond between the o-acetamido NH group of the picket fence and the bound dioxygen in the latter complex. When the NH protons of the o-acetamido groups of Co(a4-T,,PP)(4-PhPy)02 were deuteriated, the v(C0-02) at 520 cm-l and b(C000) at 267 cm-' were shifted 2-3 cm-l to lower frequencies. The v ( 0 J of C O ( ~ ~ - T , , ~ P P ) ( ~ - M ~ I ~ ) O ~ at 1137 cm-I was shifted to 1142 cm-'when the temperature was raised from ca.-90 OC to room temperature. These observations provided further support for the N-H(o-acetamido)-O2 bonding proposed by other workers. The v ( 0 J of eleven modified picket fence porphyrins were measured to examine the effect of changing the picket structure on such hydrogen bonding. In all cases,the v ( 0 2 ) of these compounds were observed between those of Co(a4-TpPP)(B)02 (1 148 cm-') and Co(a4-T,J'P)(B)02 (1 137 cm-I) (B = 1-MeIm). It was concluded that the major factor in determing the v ( 0 2 ) of these picket fence porphyrins is the strength of the N--H(o-acetamido)-O2 bond which varies in a subtle manner depending upon the size and shape of the picket. Introduction of protic groups such as OH and NH2 in the picket gave only small low-frequency shifts of v(Oz) relative to those having no protic groups. These results suggest that 'protected porphyrins" other than picket fence type porphyrins must be employed to mimic bound O2 in native heme cavities more closely.
Recently, a number of model compounds have been synthesized to mimic biological functions of hemoglobin (Hb) and myoglobin (Mb). In 1975 Collman et aL2 first prepared the picket fence porphyrih Fe(T~,,PP),which can reversibly bind O2in the presence of a base ligand (B) at room temperature. Since then, the structural and bonding properties of its dioxygen adducts, Fe(TPiIPP)(B)O2,and its Co(I1) analogues have been studied extensively by using a variety of physicochemical method^.^ As to vibrational spectra, Collman and co-workers4first located the v ( 0 2 ) of these adducts at 1163-1 150 cm-' in IR spectra. Caughey et aLSaobserved two oxygen-isotope-sensitive bands at 1155 and 1107 cm-' in the I R spectrum of H b 0 2 and Alben et al.5battributed its origin to Fermi resonance between the unperturbed v ( 0 2 ) (- 1135 cm-') and the first overtone of the ~ ( F e - 0 ~(2 ) X 567 cm-'). Later, Tsubaki and Yu6 observed three oxygenisotope-sensitive bands at 1153, 1137, and 1103 cm-' in the resonance Raman (RR) spectra of CoHb02 and CoMbO,; they attributed the latter two bands to resonance interaction between a porphyrin mode at 1 123 em-' and the ~(0,) at 1122 cm-I, which originates in the O2adduct hydrogen bonded to the distal histidine, and the first band at 1153 cm-' to the v ( 0 2 ) of the O2adduct which is free from such hydrogen-bonding interaction. The fact that the latter frequency is close to that of the picket fence porphyrins was regarded as evidence to support their assignments. The appearance of two oxygen-isotope-sensitive bands (1158 and 1144 cm-') in the R R spectrum of Co-
Chart I
(1) (a) Marquette University. (b) Hokuriku University. (2) Collman, J. P.; Gagne, R. R.; Reed, C. A.; Halbert, T. R.; Lang, G.; Robinson, W. T. J. Am. Chem. SOC.1975, 97, 1427. (3) Collman, J. P.; Halbert, T. R.; Suslick, K. S. O2 Binding to Heme Proteins and their Synthetic Analogs, In Metal Ions in Biology; Spiro, T. G., Ed.; John Wiley: New York, 1980; Vol. 2, p 1. (4) Collman, J. P.; Brauman, J. I.; Halbert, T. R.; Suslick, K. S. Proc. Natl. Acad. Sei. U.S.A. 1976, 73, 3333. (5) (a) Caughey, W. S.; Choc, M. G.; Houtchens, R. A. Biochemical and Clinical Aspects of Oxygen; Caughey, W. S., Ed.;Academic: New York, 1978; pp 4 and 18. (b) Alben, J. 0.;Bare, G. H.; Moh, P. P.Biochemical and Clinical Aspects of Hemoglobin Abnormalities; Caughey, W. S., Ed.; Academic Press: New York, 1978; p 607. (6) Tsubaki, M.; Yu, N.-T. Proc. Natl. Acad. Sci. U.S.A. 1981,78,3581.
(TPivPP)(1,2-DiMeIm)02was also attributed to the equilibrium mixture of two conformers although their structures were not kn0wn.l The nature of bound O2inside the picket fence is governed by several factors including the size, shape, and polarity of the pocket, and its interaction with the protic group of the picket. The main objectives of this investigation are threefold. First, we will study how the bound 02-solvent (toluene) interaction varies in a series of atropisomers shown in Chart I. We8 observed previously that
- -
9 b a4
0
a3
P
-
-
-
0002-7863/88/1510-0742$01.50/0
6 cis-2
trans-a2
R,
c=o
(7) Mackin, H. C.; Tsukbaki, M.; Yu, N.-T. Eiophys. J. 1983, 41, 349.
0 1988 American Chemical Society
RR Spectra of 0,Adducts of Co Picket Fence Porphyrins
Am. Chem. Soc., Vol. 110, No. 3. 1988 743
in "unprotected porphyrins" such as Co(TPP-d8), marked enhancement of toluene modes occurs by Soret excitation when the v ( 0 J is shifted close to the toluene band by chooosing a proper base ligand. We noted, however, that this vibrational energy transfer does not occur in "completely protected porphyrins" such as Co(a4-Tp,PP) even if the frequency matching condition is met. It is, therefore, of great interest to compare the degree of the bound OZ-toluene interaction in the a4,a3,cis-a2, and trans-d seriesG9 Second, we will examine the effect of changing the picket structure on the hydrogen bonding between bound O2 and the o-acetamido N H group of the picket which has been proposed by several workers.lWl3 It is of our particular interest to obtain spectroscopic evidence for such interaction and to study the effect of changing the picket structure on it. A series of a 4 - t yCo picket fence porphyrins containing six different R groups have been prepared for this purpose. Finally, we will explore the possibility of hydrogen bonding between bound O2 and the protic R group of the T,,PP picket shown in Chart I. A series of seven a3d-type complexes in which a denotes the T,,PP picket and a' represents a T,,,PP picket containing the OH or N H 2 group have been prepared for this p ~ r p o s e . ' ~ Model J~ building studies suggested that such interactions would mimic the N-H(dista1 His).-02 hydrogen bonding found in MbOZI6and HbO2I7better than that of a4-type picket fence porphyrins. In the present work, we focused our attention on the v(Oz)of these Co picket fence porphyrins since the v ( 0 2 )is well-recognized as a sensitive probe in detecting subtle changes in the O2 environment.' Resonance enhancement of the v ( 0 2 )of analogous Fe complexes has not been succasful except for those containing axial thiolate ligands.''J9 This has been attributed to the small oscillator strength of the F d ZCT transition or to the unavailability of laser lines in the 780-1300-nm region where such transition is predicted.z0
3-AcPy
3,s-CiPy
2-BrPy
Experimental Section Compounds. (meso-Tetrakis[a4-o-(neopentylcarboxamido)phenyl]porphyrinato)cobalt(II), Co(a4-Tn$P), and (meso-tetrakis[a4-o-(pivalamido)phenyl]porphyrinato)cobalt(II), Co(a4-TPi,PP), and their atropisomers were prepared by the method reported previously? Deuteriation of the o-acetamido protons of Co(a4-T,,PP) was made by dissolving it in CH,OD and evaporating the solvent in vacuo. This process was repeated several times to ensure complete deuteriation. The four other a4-type complexes containing aprotic R groups and seven cu3cu'-type complexes in which a represents the T n 2 Ppicket and d denotes a picket containing a protic R group (Table I) were also prepared as described p r e v i o u ~ l y . ~The ~ , ' ~bases, 2-bromopyridine (2-BrPy), 3-acetylpyridine (3-AcPy), 3,5-dichloropyridine (3,5-C1Py), 4-phenylpyridine (4-PhPy), 1-methylimidazole (1-MeIm), 4-cyanopyridine (4-CNPy), and pyridine (Py), were purcharsed from Aldrich Chemical Co., Milwaukee, WI. These compounds were purified by distillation or recrystallization. The solvent, toluene, was distilled from metallic sodium. The gases, I6O2and '"02 (85%), were purchased from Airco and Monsanto Research Corp., respectively. Spectral Measurements. The resonance Raman spectra were recorded on a Spex Model 1403 double monochromator equipped with a Spex (8) Kincaid, J. R.; Proniewicz, L. M.; Bajdor, K.; Bruha, A,; Nakamoto, K. J. Am. Chem. SOC.1985, 107, 6715. (9) Imai, H.; Nakata, K.; Nakatsubo, A.; Nakagawa, S.; Uemori, Y.; Kyuno, E. Synth. React. Inorg. Met.-Org. Chem. 1983, 13, 161. (10) Mispelter, J.; Momenteau, M.; Lavalette, D.; Lhoste, J.-M. J. Am. Chem. SOC.1983, 105, 5165. (1 1) Lavalette, D.; Tttreau, C.; Mispelter, J.; Momenteau, M.; Lhoste, J.-M. Eur. J. Biochem. 1984, 142, 555. (12) Walker, F. A.; Bowen, J. J. Am. Chem. SOC.1985, 107, 7632. (13) Jameson, G. B.; Drago, R. S . J. Am. Chem. Soc. 1985, 107, 3017. (14) Imai, H.; Nakatsubo, A.; Nakagawa, S.; Uemori, Y.; Kyuno, E. Synth. React. Inorg. Met.-Org. Chem. 1985, IS, 265. (15) Imai, H.; Sekizawa, S.; Kyuno, E. Inorg. Chim. Acta 1986, 125, 151. (16) Phillips, S . E. V.;Schoenborn, E. P. Nature (London) 1981,292, 81. (17) Shaanan, B. Nature (London) 1982, 296, 683. (18) Chottard, G.; Schappacher, M.; Ricard, L.; Weiss, R. Inorg. Chem. 1984, 23, 4557. (19) Bangcharoenpaurpong, 0.;Rizos, A. K.; Champion, P. M.; Jollie, D.; Sligar, S. G. J. Biol. Chem. 1986, 261, 8089. (20) Case, D. A.; Huynh, B. H.;Karplus, M. J. Am. Chem. SOC.1979, 101,4433.
4-CNPy
)
I too WAVENUMBER (CM-'1
1
li 50
Figure 1. R R spectra of Co(a4-T,,PP)(B)02 in toluene at ca. -90 OC (406.7 nm excitation). The axial base (B) is 3-acetylpyridine (trace A), 3,5-dichloropyridine (trace B), 2-bromopyridine (trace C), and 4cyanopyridine (trace D). D M l B controller. All the spectra were obtained by using 406.7-nm excitation from a Spectra-Physics Model 164-01 Kr-ion laser, except for those shown in Figure 6 which were obtained with the 441.6-nm line of a Liconix Model 4240 N B H e C d laser. The minibulb techniquesz1were employed to measure the spectra of dioxygen adducts at ca. -90 "C. Proton N M R spectra were measured on a JEOL JMN-MH-100 spectrometer.
Results and Discussion Interaction of Bound Dioxygen with Solvent. Figure 1 shows the R R spectra of Co(a4-T,,,PP)(B)02 in toluene a t ca. -90 O C containing a series of axial base ligands (B). In each case, a single sharp peak was observed at 1144-1 148 cm-', and this band was assigned to the u(Oz)since it is shifted to 1070 cm-' by 160z/1802 substitution. In contrast, the C o ( a 3 - T , ~ P ) ( B ) O series z obtained under similar conditions (Figure 2) exhibits a shoulder band at 1 154 cm-' and its intensity increases as the base ligand is changed in the order 3-AcPy, 3,5-C1Py, 2-BrPy, and 4-CNPy. Toluene exhibits three Raman bandsz2at 1210 (Tl), 1178 (Tz), and 1155
-
(21) Bajdor, K.; Kincaid, J. R.; Nakamoto, K. J . Am. Chem. SOC.1984, 106, 7141.
744 J . Am. Chem. SOC.,Vol. 110, No. 3, 1988
Odo et al.
Q
3
IIO0 I250 WAVENUMBER (CM-') Figure 3. RR spectra of Co(a4-TpivPP)(4-CNPy)Ozand C 0 ( a 3 TpiJW(4,-CNPy)O, in toluene at ca. -90 O C (406.7-nm excitation). Chart I1
Q
-
2-BrPy
4-CNPy I
I
1
I
0
1100 I IO WAVENUMBER (CM-'1 Figure 2. RR spectra of Co(a3-T,J'P)(B)Oz in toluene at ca. -90 OC (406.7 nm excitation). The axial base (B) is 3-acetylpyridine (trace A), 3,5-dichloropyridine (trace B), 2-bromopyridine (trace C ) , and 4cyanopyridine (trace D).
cm-I (T,) whose relative intensity ratio is ca. 6:l:l under our experimental conditions? Thus, the anomalous enhancement of the T3 band observed for the Co(ar3-T,2P)(B)Oz series indicates direct association of the toluene molecule with the bound dioxygen through a partial opening in the cavity. As stated previously,s the second condition for the bound 02-toluene vibrational energy transfer is the frequency matching. This is accomplished by changing the basicity of the axial ligand since the u(Oz)becomes higher as the ligand becomes more acidic.* The RR spectrum of Co(a'-T,,PP)( l-MeIm)02 (pK, 7.20) in toluene consists of and 1154 cm-I (T3) since two separated bands at 1141 (~(0~)) these two bands are far apart (not shown in Figure 2). As the base ligand becomes more acidic, the u(Oz)is shifted closer to the T3band and the latter band becomes stronger. This is clearly seen in Figure 2 where the base ligand is changed in the order 3-AcPy (3.18), 3,5-C1Py (3.4), 2-BrPy (0.90), and CCNPy (1.90). Although the numbers in the brackets indicate the pKa values in aqueous solution,23 their order does not change appreciably in nonaqueous solvents.24 A small base ligand effect on the u ( 0 2 ) is seen in the a4series (Figure 1). However, the enhancement of the T3 band does not occur in this case. The two structures (22) La Lau, C.; Synder, R. G. Spectrochim. Acta 1971, 27A, 2073. (23) Perrin, D. D. Dissociarion Constants of Organic Bases in Aqueous Solution; Butterworths: London, 1965. (24) Bordwell, F. G.; Hughes, D. L. J . Org.Chem. 1982, 47, 3224.
B (A)
Q
A
o2
A
(B)
shown in Chart I1 are probable for the dioxygen adduct of a cr3-porphyrin. However, structure B is less likely since a bulkier would occupy the axial position having base ligand (relative to 0,) band shown less steric hindrance. The doublet feature of the 40,) in Figure 2 cannot be attributed to an equilibrium mixture of these two isomers, since the RR spectrum of the corresponding '*02 analogue exhibits a single peak near 1070 cm-I. Figure 3 compares the RR spectra of Co(a4-TpivPP)(B)02and Co(ar3-TpivPP)(B)Ozin the toluene at ca. -90 "C where B is 4-CNPy. It is noted that the 402)of the former (1 156 cm-I) is 8 cm-I higher than that of Co(a4-T,,PP)(4-CNPy)02 (1 148 cm-I, Figure 1D). Similarly, the u ( 0 2 ) of Co(a3-TpivPP)(4CNPy)02 (1160 cm-l) is 11 cm-I higher than that of C0(a3T2P)(4-CNPy)Oz (1 149 cm-l, Figure 2D). Thus, the frequency order, u ( 0 J < u(T3), observed for the T,,PP is reversed in the TpivPPalthough the T3 band is enhanced in both a3-porphyrins. The origin of the observed shift of u ( 0 2 ) in going from the Tpi,PP to T,,PP complex will be discussed in the subsequent section. Traces A and B of Figure 4 show the RR spectra of Co(cisa2-T,ivPP)(Py)02 and its 4-CNPy analogue in toluene at ca. -90 OC, respectively. In the cis structure, direct interaction between the bound O2and toluene is possible since there is no steric barrier is shifted to prevent such association. As stated earlier, the ~(0,) to higher frequency as the axial base ligand becomes more acidic. In the case of Py (pKa 5.25), the ~(0,) is at 1150 cm-I, and only the T3 band a t 1155 cm-l is enhanced (Figure 4A). However, a more acidic base such as 4-CNPy (pK, 1.90) shifts the u ( 0 2 ) to 1164 cm-I which is between v(TZ) and u(T3). Thus, the T, as well as T3 band is enhanced as seen in Figure 4B. Figure 4C shows the RR spectrum of Co(trans-a2-TpivPP)(4CNPy)OZin toluene at ca. -90 OC. The v ( 0 2 )appears as a weak shoulder near the T3 band although its exact frequency cannot
RR Spectra of O2 Adducts of Co Picket Fence Porphyrins
T
1
J . Am. Chem. SOC.,Vol. 110, No. 3, 1988 745
n
n
n
n
Figure 5. Schematic representations of relative positions of picket fence (cross-dotted and open circles, above and below the porphyrin core plane, respectively), base ligand plane (dotted rectangular boxes, below the porphyrin core plane), and Co-0-0 plane (solid lines, above the porphyrin core plane).
4-CNPy
9
respect to the N,-Co-N, direction prevents electron flow from the base ligand to dioxygen. It is interesting to note that as the oxygen affinity decreases, the ~ ( 0is shifted ~ ) to higher frequency in the series of atropisomers of Co(TpiVPP)(B)02: 80
