J. Phys. Chem. 1989, 93, 1311-1319 The gas and dilute liquid lack the added contribution of the second carbonyl group and magnetic anisotropies associated with the C-H ~ ~ ~ solely on and C=O bonds may drastically reduce A V . Based the DMF results, it is difficult to explain why the methylene group limiting shift does not increase rather than decrease in liquid vs gas DEF. Apparently, dimerization and solvent interactions affect the molecule's geometry, perhaps twisting the methyl groups back toward and into the plane of the C=O bond. The relative methine and methyl gas- and liquid-phase shift differences for DIPF are consistent with that expected from geometry considerations. Fritz et aI.l4 have investigated a series of N,N-diisopropylamides and thioamides RC(X)N(is~propyl)~ and determined their conformations based on liquid 13Cchemical shifts. Their results indicate that, of the four possible low-energy conformations possible for the isopropyl groups, only two exist in equilibrium, both with the methine protons in or close to the amide plane, but oriented 180' from each other. The conformer in which the methine H is eclipsed by the larger group (R or C=X) is more stable. Because in formamides the C=O group is largest, the cis-methine proton is near and in the plane of the carbonyl bond and would be expected to exhibit a very large Av. Geometry-optimized molecular orbital calculations predict that the conformation with both methine protons oriented in the plane, but away from the carbonyl moiety, is of lowest energy (Figure 6b). In the gas phase because of the absence of intermolecular effects, DIPF would be more likely to assume this low-energy configuration; thus, the gas-phase methine Av should be lower due to the greater distance of this proton from the shielding
1311
carbonyl bond. Also, the methyl protons should have a larger Av than in the liquid because of their closer proximity to the bond, but still less than the methine because they lie outside the plane. Lastly, the AG* values listed in Table I11 show that the barrier to rotation in formamides decreases with increasing size of the alkyl substituents on the amide nitrogen. The differences in the barriers of the first three formamides, however, are quite small. Berg and BlumZ0have determined that N,N-di-tert-butylformamide (DTBF) in [2H8]toluenehas an internal rotation barrier of 13.7 kcal mol-' (285 K, 360 MHz, Av = 0.5 ppm), a dramatic decrease over the barriers in DMF, DEF, and DIPF. They suggest that this is due to the lack of a conformation for DTBF in which no methyl group lies in the formyl plane. One methyl group must always interact sterically eclipsing either the oxygen or formyl hydrogen atom. We have been unable to observe any broadening of the single 18-proton tert-butyl peak in the gas-phase at 273 K (300 MHz). Below this temperature, the peak disappears due to limited vapor pressure. Acknowledgment. We are glad to acknowledge the National Science Foundation (CHE 85-03074 and CHE 83-50 1698(PYI)), the National Institute of Health (GM 29985-04), and the Alfred P. Sloan Foundation for the support of this research. We thank Michael M. Folkendt for assistance in acquiring 500-MHz spectra for DEF and Brian D. Ross for the DIPF spectra. Registry No. DEF, 617-84-5; DIPF, 2700-30-3. (20) Berg, U.; Blum, Z. J . Chem. Res. (S) 1983, 206-207.
Resonance Raman Vlbrational Analysis of Cu", Fe"', and Co"' Porphyrin ?r Cation Radicals and Their Meso-Deuteriated Analogues W. Anthony Oertling, Asaad Salehi, Chi K. Chang,* and Gerald T. Babcock* Department of Chemistry and M S U Shared Laser Laboratory, Michigan State University, East Lansing, Michigan 48824 (Received: June 21, 1988)
Analysis of the near-UVresonance Raman enhanced vibrations in the 1000-1700-cm-' range of Cu", Fer", and Co"' complexes of octaethylporphyrin and etioporphyrin I R cation radicals and of the corresponding complexes with *Hsubstituted at the porphyrin methine carbon positions is presented. Comparison of analogous spectra of octaethylporphyrin vs etioporphyrin I complexes is used to identify vibrations involving the pyrrole b-carbon atoms and thus serves as an alternative to isotopic labeling at this position. The results are consistent with and provide an extension of our earlier vibrational assignments, which were based on structural trends and depolarization ratios of the Raman bands in the 1450-1700-cm-' region (Oertling, W. A.; Salehi, A.; Chung, Y . C.; Leroi, G. E.; Chang, C. K.; Babcock, G. T. J . Phys. Chem. 1987, 91, 5887-5898). Our results reveal that the wavenumbers of the cctaethylporphyrin C,C,, C,N, and C,Cb internal stretching coordinates decrease slightly while those of the c b c b stretching coordinates increase when the macrocycle is oxidized by one electron. The magnitude of the wavenumber shifts caused by methine carbon deuteriation and b-carbon substitution are comparable for the neutral and R cation species. This suggests that although the force constants are changed (- 1-3%) upon oxidation, the overall potential energy distributions of the normal coordinates of the R cation radical remain similar to those of the parent compounds, most likely reflecting similar macrocycle stereochemistry for neutral and oxidized species in CH2CI2solution. Our analysis suggests that the optical and resonance Raman spectral properties of these complexes strongly reflect the conformation of the oxidized porphyrin ring. This implies that the distinct optical spectra of the cobaltic cctaethylporphyrin R cations, which were thought to arise from differences in ground electronic state orbital occupancy, may arise instead from differences in porphyrin stereochemistry,and all of the cation radical complexes studied here may belong to the 2Aluground-state configuration.
Introduction Synthetic metalloporphyrin R cation radicals (MP'+) serve as models for transient species present in several biologically relevant redox processes including heme enzyme catalysis and photosynthesis. In earlier work we established the utility of the resonance Raman (RR) technique to detect porphyrin-centered oxidation in metalloporphyrin (MP) complexes and demonstrated that interpretable structural trends occur in the electronic and vibrational spectra of oxidized, metal-substituted octaethylporphyrin radicals 0022-3654/89/2093-131 l$Ol.SO/O
(MOEP'+).',2 In particular, the vibrational frequencies of the oxidized macrocycle core in the 145C-1700-cm-' range were found to be inverse linear functions of the porphyrin center to pyrrole nitrogen distance, or core size, much the same as in the neutral (1) Salehi, A,; Oertling, W. A,; Babcock, G. T.; Chang, C. K . J . A m . Chem. SOC.1986, 108, 5630-5631. (2) Oertling, W. A,; Salehi, A,; Chung, Y . C.; Leroi, G. E.; Chang, C. K.; Babcock, G. T. J . Phys. Chem. 1987, 91, 5887-5898.
0 1989 American Chemical Society
1312
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
parent species (MOEP).3-4 Vibrational normal mode assignments in this wavenumber region for the MOEP'+ complexes were suggested that were based on a comparison of these structural trends and depolarization ratios of the R R bands of the cations with those of the neutral parent compounds. From these vibrational mode assignments we postulated that modes involving primarily C,C, stretching motion decrease in wavenumber frequency upon oxidation while those involving primarily pyrrole CbCb atoms increase in wavenumber. For a given C,C, or cbc, stretch, these wavenumber shifts were found to be relatively constant; thus, the lines that describe the core size dependence of the wavenumber frequency of each vibrational mode of the M%EP'+ are essentially parallel to those of the neutral M"0EP but are displaced along the frequency axis by an amount equal to the wavenumber shift that occurs upon oxidation. Using the correlation parameters obtained from the R R spectra of the MrlOEP'+ complexes, we proposed CH2ClZsolution structures and metal coordination numbers for a variety of Co"' and Fe"' porphyrin a cation radical comple~es.~-~ Based on earlier assignments of radical ground state, our structural correlations suggested that the vibrational frequencies of the MOEP" depended little on which of the two porphyrin orbitals, the a,, or aZu,was the site of the oxidation in forming the radical. These model compounds results were also used to interpret the R R spectrum obtained for horseradish peroxidase compound I, a photolabile heme enzyme transient generally considered to contain a porphyrin a cation r a d i ~ a l . ~ , ~ Comparative RR studies of isotopically substituted complexes are necessary to confirm the vibrational assignments suggested for these M P " compounds. Thus, in the present work we discuss R R spectra of Cu", Fe"', and Co"' complexes of octaethylporphyrin (OEP) and etioporphyrin I (EPI) a cation radicals with hydrogen or deuterium present at the porphyrin methine carbon positions. With the normal-coordinate analysis of NiOEP by Kitagawa and co-w~rkers,*~~ the vibrational assignments through ~ for these compounds are presented. the 1000-1 7 0 0 - ~ m -region Using these assignments and predictions of vibrational frequency changes expected upon electron removal from the two highest occupied molecular orbitals in the neutral metalloporphyrin, we suggest that all of the MOEP" and MEPI" complexes we have examined are 2Aluin character.
Materials and Methods The free-phase porphyrins HzOEP and HzEPI were synthesized according to published Insertion of Cu", Fe"', and Co" metals was achieved by standard methods.l2 Exchange of hydrogen (h4) for deuterium (d4) at the meso positions of the porphyrin was accomplished by using the D2SO4/D20method and confirmed by N M R measurements as described earlier." Oxidation of the neutral CUPto the a cation radicals, CuP'+ClO;, was carried out by using the AgC104 methodss Ferric chloride porphyrins, (Cl-)Fe"'P, were oxidized by using phenoxathiin hexachloroantimonate according to previously described proced u r e ~ Cobaltous .~ octaethylporphyrin, CoOEP, was oxidized to the cobaltic porphyrin a cation radicals C O ~ ~ ' O E P ' + ~ C and IO~(3) Spaulding, L. D.; Chang, C. C.; Yu,N.-T.; Felton, R.H. J. Am. Chem. SOC.1975, 97, 2517-2525. (4) Parthasarathi, C.; Hansen, C.; Yamaguchi, S.; Spiro, T.G. J . Am. Chem. SOC.1987, 109, 3865-3871. (5) Salehi, A.; Oertling, W. A.; Babcook, G. T.;Chang, C. K. Inorg. Chem. 1987, 26,4296-4298. (6) (a) Oertling, W. A.; Babcock, G. T.J . Am. Chem. SOC.1985, 107, 6406-6407. (b) Oertling, W. A.; Babcock, G. T.Biochemistry 1988, 27, 3331-3338. (7) (a) Ogura, T.;Kitagawa, T. J. Am. Chem. Soc. 1987,109,2177-2179, (b) Ogura, T.; Kitagawa, T.Rev. Sci. Instrum., in press. (c) Paeng, K.4.; Kincaid, J. R. J . Am. Chem. Soc., submitted. (8) Kitagawa, T.; Abe, M.; Ogoshi, H. J . Phys. Chem. 1978, 69, 451 6-4525. (9) Abe, M.; Kitagawa, T.;Kyogoku, Y . J . Phys. Chem. 1978, 69, 4526-4534. (IO) Wang, C. B.; Chang, C. K. Synthesis 1979, 548-459. (1 1) Fuhrhop, J.-H.; Smith, K. M. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.;Elsevier: Amsterdam, 1975; pp 765-766, 816-817. (12) Falk, J. E. In Porphyrins and Metalloporphyrins; Elsevier: New York, 1964; p 798.
Oertling et al. *
N m
CuOEP
a ) ha
b) d q
CuEPI
d)
900
1100
1300
1500
dq
1700
RAMAN SHIFT (crn-')
Figure 1. Resonance Raman spectra of CH2C12 solutions of cupric porphyrins and their meso-deuteriated analogues. See Materials and Methods for conditions. Solvent bands are marked with an asterisk.
Co"'OEP''2Br- with Fe(C104)3and Br2 reagents, respectively, as described previously.z Freshly distilled CHzClz was used as the solvent for all preparations. Oxidations were monitored by using UV-vis absorption spectral measurements. Samples for resonance Raman measurements were contained in a cylindrical quartz spinning cell a t room temperature. In room-temperature CHzClzsolutions, at the concentrations used here (0.1-0.5 mM), no aggregation of either the neutral or a cation radical species studied in this work occurs.z~13This was confirmed by the concentration independence of the R R spectra we report. Laser emission of 20-30 mW at 363.8 nm was provided by a Coherent Innova 100 argon ion laser. Raman scattering was collected and analyzed with equipment described elsewhere? Accumulation times for all spectra were typically 10 min. Sample integrity before and after Raman experiments was confirmed with UV-vis spectra. As in our previous work, the samples were stable in air and for this reason samples were not routinely degassed.
Results Comparison of the vibrational spectra of the h4 vs d4 derivatives of each M P and M P " complex reflects the involvement of the C, atoms in the individual normal modes. Vibrations of C,C, stretching character are expected to exhibit a wavenumber decrease upon substitution of d4 for h4. On the other hand, comparison of analogous spectra of OEP vs EPI complexes reveals vibrational contributions from CbCb and c,cbstreching and cbs (S = substituent) stretching and bending motions and serves as an alternative to isotopic labeling at the pyrrole b-carbon positions. Comparison of results from neutral vs cation radical species defines changes in the vibrational spectra that are brought about by (13) (a) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin D.; Felton, R. H. J . Am. Chem. SOC.1970,92, 3451-3459. (b) Mengersen, C.; Subramanian, J.; Fuhrhop, J.-H. Mol. Phys. 1976, 32, 893-897.
Cull, Fe"', and Co"' Porphyrin Cation Radicals
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1313 N
:
TABLE I: Normal-Mode Symmetries and PEDS of NiOEP Range' Vibrations in the 1000-1700-~m-~
mode
Y3 Y29 Y4
h 2
+ Yz3
Ug
+ Y23 Uz3 + Yz6 lJ17
Y5 y13
+ Y9
Y14
y30
y5
PED
u(C&) 57, u(C$) 16 v(CaCm) 41, v ( c a c b ) 35 v ( c a c b ) 47, v(cbs)26 u(C,N) 53, 6(C,Cm) 21 U(CaN) 63, Y(CbCb) 13 Azg 6(cbs)57, U(c,c,) 11 d(c,cb)26, U'(cbs)20 6(c$) 84 + d(c,cb)26, v'(Cbs) 20 B2g
B,,
VI I
Yg y22
sym
+ Yg
Alg B2g
t::
+
v'(cacb)26, v'(cbs)20 + 6'(cbs)41, 6'(cac,)31 v(cbs)38, v(c,cb)23 + 6(cbs)23, U(cacm) 16 B I ~ NCrnH) 67, dcacb) 22 Big v(cacb)31, v(cbs)30 Bzg v'(cbs)49, u'(C~N)28 6(CaC,Ca) 36, U(C,N)27 + 6(cbs)57, U(C,C,) 11 Alg Azg V'(C,N) 37, V'(cbs)26 Aig v(cbs)38, v(cacb)23 Alg Alg
t-
z
H
"Taken from Abe et aL9 Only vibrations enhanced with RR excitation at 363.8 nm have been included. oxidation of the porphyrin ring. Thus, the results presented here allow us to test our previous assignments of the cation vibration^.^,^ Figure 1 shows R R spectra obtained with 363.8-nm excitation of h4 and d4 derivatives of both CuOEP and CuEPI. Figure 2 shows analogous spectra of the corresponding porphyrin a cation radical species. Table I describes the potential energy distribution (PED) and D4,+symmetry classification of each of the fundamental and combination modes, which were obtained from the normalcoordinate analysis of Abe et aL9 Table I1 summarizes the vibrational assignments, wavenumbers, and depolarization ratios obtained by using 363.8-nm R R excitation of cupric porphyrin neutral and a cation radical species. As we established earlier,I4 near-UV excitation is essential to avoid spectral artifacts from free-base porphyrin diacid species. From Table I it is apparent that the vibrations in the highfrequency region (1450-1 700 cm-') are predominantly fundamentals of c,c, or CbCb stretching character. Thus, in both CuOEP and CuOEP'+C104- the ulO(CaCm)and v3(C,C,) vibrations are identified by their deuterium shifts of -1 1 and -8 cm-I, respectively. Although the exact frequency of the ul0 vibration in CuOEPd4'+C104- is difficult to determine owing to its coincidence with the feature at 1616 cm-', comparison of Figure 2, parts a and b, clearly indicates a deuterium shift to lower wavenumber in the shoulder located at -1631 cm-I in the spectrum of Cu0EP'+C1O4- upon d4 substitution. Polarized spectra (not shown) allow better resolution of the ul0 vibrations of CuOEP'+C104-. The absence of significant change in the wavenumbers of the features occurring at 1591 and 1568 cm-' in the neutral species (Figure la,b) and 1615 and 1602 cm-' in the cations (Figure 2a,b) is consistent with our earlier assignment of u1,(c,c,) and v2(CbCb), respectively, for these vibrations. The fundamental vibrations of the porphyrin ring in the 1350-1400-~m-~range are predominantly of C,N stretching character. In these Cu complexes, this region is dominated by a polarized band at 1379 cm-' in the neutral spectra and 1362 cm-l in the cation radical spectra which we assign to the AI, fundamental, u4. The depolarization ratio, the absence of a deuterium shift, and the clear spectral analogy to the neutral case form the basis for this assignment for the cation. The above analysis confirms our previous vibrational assignments in the 1350-1700-~m-~region of the metallooctaethylporphyrin a cation radical (MIIOEP") R R spectra.2 To extend our assignments we first consider the neutral CuOEP h4 and d4 spectra appearing in Figure la,b. Vibrational modes involving the porphyrin substituent groups occur in the 1000-1 350-cm-' range. These vibrations may show very large shifts upon deuteriation owing to (1) the involvement of the C,-H bending (14) Oertling, W . A.; Salehi, A.; Chang, C. K.; Babcock, G . T. J . Phys. Chem. 1987, 91, 31 14-31 16.
900
1100
1300
1500
1700
RAMAN SHIFT (cm-'1
Figure 2. Resonance Raman spectra of CH2CI2 solutions of cupric
porphyrin ?r cation radicals and their meso-deuteriated analogues. motions in the normal coordinates and (2) changes in normal-mode composition due to change in vibrational coupling brought about by mass effects incurred by the substitution of d4for h4.9 Both fundamental and combination modes appear in this range; we will discuss the latter vibrations first. With one exception these combination modes occur between 1250 and 1300 cm-'. The weak feature at 1319 cm-' in Figure l a is assigned to the A,, combination u23 + v26. This feature is overlapped by a more intense depolarized vibration at 1318 cm-I in the d4 derivative (Figure lb), which is not visible in the spectra of the h4 complex. Kitagawa et aL8 assign the B2, combination ~ 1 + 7 ~ 2 to 3 a feature that occurs at 1332 cm-I in NiOEP-d4 spectra (1 330 cm-' in CoOEPd4, see below) and is absent in the spectra of the h4 derivative. Consequently, we identify the additional intensity at 1318 cm-I in Figure 1b as coming from this vibration. Although we recognize this feature in all of the d4 neutral and d4cation derivatives, our results do not seem consistent with the PED for this mode presented earlier (see be lo^).^ The polarized feature at 1260 cm-I is assigned to the A,, combination, u5 ug. This band appears to shift to 1264 cm-' in CuOEPd,; however, Abe et aL9 report no wavenumber change in this vibration upon deuteriation. We attribute both the wavenumber increase and apparent change in depolarization ratio observed here to increased spectral contributions from an A,, combination mode, either u9 + u23 or u I 5 + u33, in the d4relative to the h4 complexes. These modes are reported at 1264 and 1280 cm-I, respectively, in NiOEPd4.8 At lower energy the A,, combination Vg + us appears as a shoulder at 1 133 cm-I in CuOEP. This feature also displays an apparent frequency increase in the d4derivative, in contrast to that reported by Kitagawa et al.* However, this discrepancy in our data may also be explained by variable weak contributions from a nearby A,, fundamental, located at 1 120-1 125 cm-I in MOEP-h4 compounds. In this case, decreased spectral con-
-
+
-
1314 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Oertling et al.
TABLE II: RR Frequencies (cm-') and Depolarization Ratios" for h , and d4 Cupric Porphyrin Neutral and .* Cation Radical Complexes CuOEP CuOEP'+CIOdCuEPI CuEPI'+CIOd-
mode
h4
1637 (0.8) 1591 (0.3) 1568 (0.8) 1503 (0.3) 1405 w 1379 (0.3)
VI0 p2
VII
v3 y29
v4 u17
Y23 ~5
+ y23
+ U26 + ~g
1319 W 1260 (0.3) 1214 (0.9)
y13 V14
y30
u6
+
1157= (0.75) 1133 (0.7) 1107 vw 1027 w (0.5)
yg
US
US
d4 1626 (0.8) 1591 (0.3) 1568 (0.8) 1495 (0.3) 1405 w 1379 (0.2) 1308 (0.7) 1318 (0.7) 12646 (0.5) -950 w 1185 (0.7) 1158 (0.8) 1 138b (0.7) 1107 vw 1028 (0.7)
d4
h4
1631 sh (0.6) 1615 (0.3) 1602 (0.5) 1499 (0.4) 1391 w 1362 (0.3) 1317 w 1260 (0.3) 1216 (1.1) 1157 1136 1098 1026
(dp) (0.4) (0.7) (0.4)
d4
h4
?
1614 (0.4) 1601 (0.7) 1491 (0.3) 1391 w 1362 (0.2) 1334 (0.7) ?
1260 (0.4) -955 w 1179 (1.0) 1157 (0.8) 1136 (0.3) 1098 (0.6) 1026 (0.5)
1640 (0.8) 1597 (0.2) 1574 (0.7) 1505 (0.2)
1629 1597 1574 1497
1378 (0.2)
1378 1317 1317 1266
1317 w (0.6) 1260 (0.3) 1223 (0.8) 1162 (0.7) 1136 (0.2) 1100 w 1002 (0.2)
1190 1161 1140b 1100 w 1003
d4
h4
1632 sh (0.7) 1621 (0.4) 1606 (0.8) 1500 (0.3) 1395 sh 1361 (0.3) ? sh 1261 (0.4) 1223
1159 (1.0) 1136 (0.6) 1091 (0.7) 1002
?
1621 (0.4) 1605 (0.8) 1492 (0.3) 1395 sh (0.5) 1359 (0.3) 1332 (0.7) ? sh 1261 (0.6) 1183 w (1.0) 1158 (1.0) 1137 (0.8) 1090 (1.0) 1003
Depolarization ratios p , given in parentheses in all tables, were measured with 363.8-nm excitation. bThe apparent wavenumber increase upon deuteriation in these features are due to variable spectral contributions in h4 vs d4 derivatives of nearby Azr vibrations as discussed in text. These are thought to obscure unchanged vibrational frequencies in h4 vs d4 samples for these normal modes. CTheY~~ vibration is overlapped by a solvent band at 1156 em-'; thus its wavenumber value in these tables is an estimate. Abbreviations used: w = weak, vs = very weak, sh = shoulder, p = polarized ( p < 0.75), dp = depolarized ( p = 0.75), ap = anomalously polarized ( p > 0.75).
-
tributions at 1125 cm-I in the d4 relative to the h, complex result in a higher wavenumber estimate for v6 vs. The large wavenumber shifts for v22 upon deuteriation reported by both Abe et al.9 and Gladkov and Solovyov15are consistent with this interpretation. The fundamental vibrations enhanced with 363.8-nm Raman excitation of CuOEP in the 1000-1350-cm-' range include vi3 at 1214 cm-I, ~ 3 at 0 1157 cm-l, and v5 at 1027 cm-l. The ~ 1 vibration 3 undergoes a substantial deuterium shift owing to its 6(C,H) character and is reported at 940 cm-' in NiOEPd4.9 We observe 0 v 5 vibrational freno deuterium-induced changes in the ~ 3 and quencies, in accord with other s t ~ d i e s . The ~ ~ 3 0vibration is coincident with a solvent (CH2C12) vibration in these samples. , these spectra is not Assignment of the B,, fundamental, ~ 1 4 in straightforward. This mode is not typically observed in R R spectra; however, Abe et al.9 calculate its frequency at 1095 cm-' for NiOEP. These authors assign a feature at 1187 cm-' in spectra of NiOEPd, to this mode and predict its frequency in the d4 derivative at 1145 cm-I. The basis for the large d4 shift was attributed to a change in mode composition caused by deuteriation. Similar to the above-mentioned work, we observe a feature at 1184 cm-' in the spectrum of CuOEPd, that is not present in that of CuOEPh4. No band is observable at 1095 cm-' in either case. On the basis of the analysis of Abe et aL9 we assign the feature at 1184 cm-l in Figure l b to ~ 1 4 . The analysis of the cation CuOEP"C10,- spectra in the 1000-1 350-cm-l region (Figure 2a,b) follows from the normalmode assignments for neutral CuOEP (Figure la,b). In the cation, 3 occurs at 1334 cm-' and is visible only the v17 to ~ 2 combination in the d4 spectra, similar to the neutral spectra. However, this mode no longer overlaps the weaker ~ 2 + 3 v26 vibration at 1317 cm-I. Also the combination modes at 1260 and 1136 cm-' display no apparent d4 shifts as was suggested for the neutral species; this simplifies their assignments as indicated in Table 11. The apparent d4 frequency shifts observed for these two combination modes in the RR spectra of the neutral CuOEP were attributed above to spectral contributions from A2, vibrations in the d4 samples that were not present in the spectra of the h4 derivatives. These nontotally symmetric modes are not apparent in the spectra of either the d4 or h, cation radicals owing to the closer proximity of the laser line (363.8 nm) to the Soret absorption maximum of the cation (383 nm) relative to the neutral species (397 nm). That is, for metalloporphyrin RR, A2, modes are enhanced via a Herzberg-Teller mechanism (Albrecht B-term scattering16) and
+
-
(15) Gladkov, L. L.; Solovyov, K. N. Spectrochim. Acta 1986,42A, 1-10. (16) Tang, J.; Albrecht, A. C. In Raman Spectroscopy Theory and Practice; Szymanski, H. A,, Ed.: Plenum Press: New York, 1970; Chapter 2, p 33.
are most easily observed by using Qo,l excitation." Although these B-term contributions are also present with Soret excitation, they are normally obscured by the stronger A-term scattering, which enhances AI, and, by a Jahn-Teller prOcess,I* BI, modes. However, when the Soret region excitation is off-resonance, as in the case of 363.8-nm excitation of neutral CuOEP, the AI, modes (A-term scattering) are less prominent, and the A2, contributions (B-term scattering) may become apparent. The ~ 3 0and ~ 1 4assignments of the cation vibrations are analogous to those of the parent species. The features at 1098 cm-' in the CuOEP'+ClO; spectra (Figure 2a,b) requires some discussion. We correlate this to a weak feature at 1107 cm-I in Soret R R spectra of CuOEP (visible in spectra obtained with higher sensitivity than those in Figure la,b). Although this feature is not predicted by the analysis of Abe et al.,9 Gladkov and Sol o ~ y o vpredict '~ a B,, mode at 1072 cm-l and an A,, mode at 1092 cm-' for both CuOEPh, and CuOEPd,. The analyses of Abe et al? treated the terminal methyl groups of Cb substituents as point masses, whereas that of Gladkov and S o l o ~ y o vconsidered ~~ each substituent atom; thus, the Occurrence of additional cb-s related motions is not surprising. We will call this additional vibration VS.
The wavenumber frequencies, depolarization ratios, and vibrational assignments for CuEPI and CuEPI*+C1O4-also appear in Table 11. The altered peripheral substitution of CuEPI relative to CuOEP changes the strict symmetry classification from D4h to c4h. Both the Al, and the A2,. vibrations in D4,,symmetry correlate to A, vibrations in C,. Likewise BI, and BZgvibrations correlate to B,. Thus, on this basis, we would expect vibrations analogous to the Alg modes of CuOEP to become allowed with Soret R R excitation of CuEPI. However, the absence of the ~ 1 9 vibrations at 1585 cm-l in both Figure l a and Figure I C establishes that spectral effects of symmetry lowering are not apparent under these conditions; thus, D4hsymmetry labels are used in discussing the etioporphyrin species. Comparison of the frequencies for analogous OEP and EPI species in Table I1 allows us to identify vibrations involving the Cb atoms. The vibrations located at 1591 and 1568 cm-I in CuOEP and 1615 and 1602 cm-' in CUOEP'+C~O-~ appear 4-6 cm-l higher in the analogous CuEPI species. Together with the absence of d4 shifts, this clearly establishes the similar v(CbCb) character of these modes in both neutral and cation radical species. The features assigned to ~ 2 9 , v13, ~ 1 4 us, , and v 5 also show significant wavenumber differences in spectra of EPI vs OEP complexes (see Table VI). This is true of both neutral and cation radical species and is in accord with
-
(17) Shelnutt, J. A. J . Chem. Phys. 1981, 74, 6644-6657. (18) Cheung, L. D.; Yu, N.-T.; Felton, R. H. Chem. Phys. Lett. 1978.55, 527-530
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1315
Cull, Fell', and Co"' Porphyrin Cation Radicals
TABLE III: RR Frequencies (cm-') and Depolarization Ratios" for h , and d 4 Ferric Chloride Porphyrin Neutral and r Cation Radical Complexes ClFeOEP CIFeOEP'+SbCl[ ClFeEPI ClFeEPI'+SbCl[
mode VI0 y2 VI I y3 y29
y4
y12
VI7 y23 y5
or + y23 + y23 +h6
+ y9
y1 3
h4
d4
h4
d4
h4
d4
h4
d4
1629 1581 1559 sh 1494 1406 1378
1617 (0.9) 1582 (0.4) 1560 sh (0.7) 1487 (0.5) 1404 (0.9) 1375 (0.5)
1624 sh 1602 1573 sh 1487 1391 w
? 1604 (0.4) -1575 sh (0.6) 1481 (0.5) 1391 (dp)
1631 1587 -1560 sh 1495 1410 1375
1618 1586 -1560 sh 1488 1411 1374
1626 (0.6) 1608 (0.3) -1578 sh (0.6) 1490 (0.3) 1396 (0.8)
1613 sh? 1608 (0.4) -1575 sh (0.6) 1480 (0.3) 1396 (0.8)
1364 w
1364 (ap) 1313 (0.9) 1310 sh 1264 (0.9)
1304 (0.9) 1317sh 1261 -955 w 1189 w 11576 (0.6) 1135 sh 1107 vw 1028 (0.6)
1317 w 1262 1210 w
y14
y30 y6
+ US
US y5
1157 1133 sh 1107 vw 1026
1310 1264 1212 1156 1128c 1093 1028 w
? 1158 (0.9) 11326 (1.0) 1093 (0.9) 1031 (0.6)
1358 1302 1317 1267
1319 w 1265 1218 w
1309 (1.0) 1267 w (0.7) 1222 (0.8)
1195 1159 11406 1100 w 1006
1159 1135 1100 w 1005
1358 (0.8) 13 13 (0.9) 1309 sh 1268 w
1156 (1.0) 1125e (0.9) 1083 (1.0) 1003 (0.5)
1156 (1.0) 1128 (0.7) 1085 (1.0) 1002 (0.3)
'Measured with 363.8-nm excitation. bSee note b to Table 11. CTheapparent wavenumber decrease in v6 + us upon oxidation may be due to increasing intensity contributions from a feature at -1125 cm-' ( Y ~ ~in) the cation relative to neutral samples. The y6 + us frequency may be unchanged throughout. x
A,, =363.8nm
*
h
i
900 I
900
1100
I
I
1300
I
1500
I
,
1700
RAMAN SHIFT (crn")
Figure 3. RR spectra of ferric chloride porphyrins in CH2C12. large contributions from C,Cb and CbS stretching character in the latter vibrations as predicted by Abe et aL9 The cupric porphyrin systems discussed above most likely display planar macrocyclic s t r ~ c t u r e s . ~In~ 'CHzClz ~ solutions, the Clod- counterion of the cation species presumably does not ligate to the Cu", which then remains four-coordinate. In contrast, (19) (a) Scholz, W. F.; Reed, C. A.; Lee, Y.J.; Scheidt, W. R.;Lang, G. (b) Gans, P.; Buisson, G.; Duee, E.; Marchon, J.-C.; Erler, B. S.; Scholz, W. F.; Reed, C. A. J . Am. Chem. Soc. 1986,108, 1223-1234. (c) Erler, B. S.; Scholz, W. F.; k, Y.J.; Scheidt, W. R.; Reed, C. A. J . Am. Chem. SOC.1987, 109, 2644-2652. J . Am. Chem. SOC.1982,104,6791-6793.
l
I
1100
~
I300
RAMAN SHIFT
I
I
1500
I
~
1
1700
(cm-')
Figure 4. RR spectra of ferric chloride porphyrin ?r cation radicals in CH2C12. ferric chloride porphyrin systems display five-coordinate metal solution structure^,^*^^ and some buckling or doming of the porphyrin core may be manifest.21 Figures 3 and 4 present R R spectra of the ferric chloride complexes of OEP and EPI and the corresponding K cation radical species. The vibrational assignments of these complexes can now be based on the observed deuterium shifts and are in agreement with those we suggested previ~usly.~ They appear in Table 111 and are largely analogous to those of the cupric porphyrin systems; however, a few spectral (20) Boersma, A. D.; Goff, H. M. Inorg. Chem. 1984, 23, 1671-1676. (21) Spiro, T. G.; Strong, J. D.; Stein, P. J . Am. Chem. SOC.1979, 101, 2648-2655.
1316 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Oertling et al.
TABLE IV: &ret Absorption Maxima for MOEP and MOEP" ComplexesQ abs, nm abs, nm CuOEP 397 CuOEP"C10c 383 ClFeOEP 378 CIFeOEP'+SbClC 356 CoOEP 391 COOEP'~C~O; 376 (MeOH)2CoOEPC104409 CoOEP'+2CIO, 393
A,
=3638nm
CoOEP
~
I-
aThe absorption maxima of the corresponding EPI complexes are within f l nm of the OEP complexes.
differences between the Fe"' and Cu" complexes are worthy of comment. We can use the intensity of the u3 and u5 vibrations as standards to compare the spectra in Figure 1 to those of the analogous complexes in Figure 3. This reveals that while the relative intensities of the u29 and u2 vibrations increase, the relative intensities , and u I l and vlo decrease in the spectra of the ferric of ~ 1 3 u4, relative to the cupric porphyrin complexes. In general, the R R vibrational intensities of the cation radical species closely parallel those of the cupric or ferric neutral porphyrin parent compound. The few apparent contradictions to this can be rationalized by the proximity of the 363.8-nm laser line to the respective Soret absorption maximum. For example, the relative intensify of the u2 band in CuOEP spectra (Figure la) is less than that of CuOEP'+C104- (Figure 2a). However, the Soret band maximum of the former species is much farther from the laser line (2300 cm-') than that of the latter species (1400 cm-I). That is, the uz intensity decreases more sharply than that of the other vibrations, particularly vll, as A,, is moved to the blue side of the Soret maximum. This has been found to be the case for these and other MOEP and MOEP" compoundsa2Table IV lists Soret maxima for the CH2CI2solutions. The relative R R intensity of the u2 vibrations obtained with 363.8-nm excitation is least for ( MeOH)2C01110EPC104-and increases for CuOEP and again for CUOEP'+CIO~-,~ The most drastic difference in relative intensity between spectra of Cu" and Fe"' complexes occurs for the u4 mode, which dominates the spectra of the cupric derivatives (Figures 1 and 2). The relative intensity of this ring-breathing vibration is reduced significantly in the spectra of the ferric complexes, particularly so in the r cation radicals (Figure 4), where it cannot be identified by using 363.8-nm excitation. Analogy to the spectra of the parent ferric porphyrin (Figure 3) suggests assignments of the features at 1360 cm-I in the spectra of the ferric porphyrin K cation radicals (Figure 4) to the u4 vibrations. However, the depolarization ratios measured for the feature at 1360 cm-' in the cation radical spectra are clearly too high for u4, and alternative assignments are proposed in Table 111. We recognize a similar feature in polarized spectra of cupric compounds (not shown);
-
900
I100
1300
1500
1700
RAMAN SHIFT (ern-')
Figure 5. RR spectra of cobaltous octaethylporphyrin in CH2CI2.
however, it is typically obscured by the intense u4 vibration. It is the apparent absence of the u4 vibration that makes this weak feature visible in the spectra of the (CI-)Fell'P'+SbCI,- complexes. Isotope labeling at the nitrogen should distinguish between the possible assignments proposed. Figures 5 and 6 display R R spectra of CoIIOEP and Co"'OEP'+2X- (X- = C10, or Br-) and vibrational assignments for these species appear in Table V. In contrast to the cupric and ferric porphyrin systems discussed above, one-electron oxidation of CoOEP may occur at either the metal or porphyrin,I and it is the two-electron oxidation products whose spectra appear in Figure 6. The RR spectrum of CO~~'OEP'+~C~O.,(Figure 6a) is similar to that of Cu"OEP*+CIO,- (Figure la); however, the relative intensity of u2 obtained with A, = 363.8 nm is diminished owing to the red-shifted Soret maximum of the cobaltic species (see above). The u4 mode, located at 1360 cm-* in the oxidized CoOEPhl derivative, appears to shift to 1358 cm-I and gain intensity in the d4 spectrum (Figure 6b); however, this is due to 3 the d4derivatives. This feature the feature assigned to uI7 + ~ 2 in occurs at 1346 cm-' (from polarized spectra, not shown) and overlaps the u4 band in Figure 6b. Thus, in Table VI1 we report the u4 frequency unchanged in the d4 derivative. Two-electron oxidation of CoOEPh4 by Brz results in a product distinctly different from that obtained by oxidation with Fe(C104),. The RR spectrum of the latter species appears in Figure 6c, and that of its d4 derivative in Figure 6d. The deuterium shifts obtained support our previous assignments2 for the three highest frequency vibrations in the R R spectrum obtained with A,, = 363.8 nm (see
-
TABLE V: Frequencies (cm-') and Depolarization Ratios" for h 4 and d 4 Cobaltous and Cobaltic Porphyrin and Cobaltic Porphyrin ?r Cation Radical Complexes CoI'OEP CO"'OEP'~~CIO~Co"'OEP''2 Brmode
h4
d4
h4
d4
1647 (0.7) 1599 (0.3) 1575 (0.6) 1512 (0.2) 1409 sh (dp) 1379 (0.2)
1636 (1.0) 1598 (0.4) 1572 (0.9) 1504 (0.2) 1409 sh (dp) 1379 (0.3)
1642 sh 1617 w 1605 (0.8) 1505 w
? 1618 sh 1603 (0.8) 1496 (0.6) 1394 w 1360 (0.4) 1366 vw (ap) 1346 sh (dp)
1320 w (p) 1260 w (0.3) 1219 (0.9)
1330 (0.75) 1318 (0.7) 1263 (0.4)
1360 (0.3)
1261 w 1220 (0.8)
1263 (0.9)
1157 (dp) 1134 (PI
1188 (1.0) 1157 (0.9) 1 1 39b (0.6)
1157 (0.8) 1139 (0.7)
1181 (1.5) 1157 (0.9) 1140 (0.6)
1027 w
1028 w (0.5)
1095 (1.0) 1026 (0.6)
1102 w 1026 (0.6)
"Measured with 363.8-nm excitation. *See note 6 to Table 11.
h4
d4
1649 1611 1603
1639 161 1 1603
1393
1393
1369
1370
1315 w 1259 (0.4) 1217 (0.8)
1309
1309 1265
1156 (0.8) 1137 (0.5) 1127 (0.8) 1108 w 1025 (0.5)
1156 1136
1157 1138
1107 1020
1105 1020
BrCo"'OEPh4 1657 (0.8) 1599 (0.2) 1575 (0.8) 1513 (0.2) 1408 (0.6) 1377 (0.2)
1220
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1317
Cu", FelI1, and Co"' Porphyrin Cation Radicals
TABLE VI: Vibrational Frequency Shifts (cm-') Caused by Methine Deuteriation (Aud), Peripheral Substitution ( A v ~ ) and , Macrocycle Oxidation (Av,) of Cupric Porphyrins
CuOEP
CuEPI
-1 -1 -8 0 0
0 0 -8 0 0
+24 +34 -4 -14
0 0 -8
-11
0
0 -1 -8 0 -2
+16
-2 0
? 0 -26 1
-1 +4" -264
+2 -6 0 +3 -9 -1
0 0 0 0
+1 5"
+
+1
0
+5" ? -1 +4" 0 +1
? 0 ? -1 +1 -1 +1
+24 +32 -5
+6
-17 +15 ? -4 0 -1 -3 0 -9 0
-1
+6 +2
+6 +4 +1
+4 -1 -2
-1 -2
+1 +9
+1
+5 +4 +3 -7 -25'
+4 +2 0 -7 -24
+7
OThese apparent wavenumber shifts may be artifacts as discussed in text. bThe large value of AuSubfor vs appears inconsistent with that found for the combination mode us + u9. Possible explanations are that the feature at 1002-1005 cm-l in the metalloetioporphyrin spectra is not us or that the feature at 1260-1265 cm-I is a distinct fundamental rather than a combination mode. ,X,
= 363 8 nrn t F
ComOEP' 2 C I O i
n I
a ) hq
b) d r
t
LL
, d , 900
I
,
I100
I
1300
,
I
,
1
I500
1700
R A W A N SHIFT (ern-')
Figure 6. RR spectra of cobaltic octaethylporphyrin 7 cation radicals in CH2CI2.
Table V). The elevated uIo (1649 cm-') and lowered u2 (1611 cm-I) wavenumber values relative to the diperchlorate adduct allows resolution of the ul0 mode in both h4 and d4 spectra. Both u3 and u4 are absent in these R R spectra (Aex = 363.8 nm) of the dibromide species (Figure 6c,d). The absence of u4 reveals weak features 1369 and 1394 cm-', similar to the spectra of the (C1-)Fe111P'+SbC14-species discussed above. The PED presented in Table I describes the depolarized vibration at 1300-1 340 cm-' (assigned to uI7 u23 by analogy to the work of Kitagawa et a1.' on NiOEP) as a sum of 6(cbs), Y'(CaCb), and u'(cbs) motions. However, we detect no wavenumber difference between OEP and EPI species for this feature in any of the d4 derivatives. Furthermore, on the basis of these results and those of Kitagawa et al.? the frequency of this vibration
-
+
appears to exhibit a strong dependence on core size, d(C,N), yielding K and A parameters for the neutral porphyrin species of 501 cm-'/A in the relation u = K ( A - d).22 This is surprising because such high K values are seen only for vibrations with high percentages of u(C,C,) character and normally occur at higher wave number^.^^^^ We also note a substantial increase in wavenumber upon macrocycle oxidation for this feature, which in all other cases was attributed to contributions from Y(CbCb) coordinates. Thus, the behavior of this vibration suggests that it involves a mixture of C,C, and CbCb stretching character.
Discussion The above R R results from C,-deuteriated Cb-substituted metalloporphyrin ?r cation radicals are consistent with our earlier structural correlations2 and firmly establish the vibrational normal-mode assignments of the high-frequency Soret excitation R R bands of the MOEP" complexes. This represents a basis fundamental to the interpretation of not only the vibrational but also the electronic and structural aspects of these compounds. Tables VI and VI1 present the wavenumber shifts in the normal modes incurred by the substitution of d4 for h4 in the porphyrin-neutral (Aud) and porphyrin-cation radical (Avd'+) species. The average wavenumber change in these vibrations for both h4 and d4 species upon oxidation is tabulated as Avo,, and the frequency shifts brought about by the substitution of EPI for OEP for the neutral and cation are represented by AuSuband AuSub*+ radical cases, respectively. These tables show that for the metalloporphyrin complexes studied here, Aud = Aud'+ and Avsub = Ausub*+;that is, there is good quantitative agreement in the wavenumber shifts caused by methine deuteriation and changes in peripheral substituents between the neutral parent and cation radical species. As these shifts in vibrational frequency are a sensitive indicator of the relative contributions of several internal coordinates, these similarities strengthen our earlier suggestion that the PEDS of the normal modes of the neutral and cation radical porphyrin species are similarS2 On the basis of these normal-mode assignments we now can generalize about the wavenumber shifts caused by porphyrin oxidation. From Tables I, VI, and VI1 it is apparent that stretching modes that predominantly involve cat,, Cacb, or C,N atom pairs decrease in wavenumber while the frequency of modes whose primary composition involves CbCb stretching increase upon oxidation to the 7 cation radical. Thus, Auox is a function of the PED of a particular normal vibration. If we assume that the geometry and vibrational mode composition of the macrocycle are similar for neutral and cation radical porphyrins, as appears ( 2 2 ) Huong, P. V.; Pommier, J. C. C. R. Acad. Sci. Ser. C 1977, 285, 5 19-522. ( 2 3 ) Warshel, A. Rev. Biophys. Bioeng. 1977, 6, 213.
1318 The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Oertling et al.
TABLE V U Vibrational Frequency Shifts (crn-l) Caused by Methine Deuteriation (Av,), Peripheral Substitution (Av,), and Macrocycle Oxidation (Avo,) of Ferric Chloride Porphyrins' ClFeOEP ClFeEPI mode
A'd -1 2 +1 +1 -7 -2 -3
VI0 y2
YII y3
u29 v4
y12
or
Y17
+ "23
y8
+ y23
AY~" ? +2
0 -6 0
0
+ Y26 4 + u9
0 0 ?
-255
u13 y14
0 +2
u30 u6
+
US u5
+2
-5 +21 +14 -7 -14
A ud -13 -1 0 -7 -1 -1
Ah, -5 +21 +17 -6 -14
AUd"
-13 0 -3 -10 0
A'sub
+2 +5 +5 +1 +5 -2 -6 -2 0 +4
0
0 -1
y23
& I *
+2 -4" 0 +3
+9 -7 +2 +2 ? 0 +4' -14 +2
+11 -9 +2
0
-2 +2 ?
+1 ?
0 +5' 0 -1
0 +3 +2 -1
+8
+4 ?
+6 +2 +2
-3 -10' -16 -3
-2 1
A'sub't
+1 +4 0 +1 +5
0 -1 +4 +10 ? 0 -3 -10 -27
'These apparent wavenumber shifts may be artifacts as discussed in text and note c to Table 111.
to be the case,2 these relatively small changes in vibrational frequency imply that the force constants of the c,-c,, Ca-Cb, and C,-N bonds decrease by 1%, while those of the c b - c b bonds increase by -3% upon oxidation of the porphyrin. These changes in force constant, however, are not large enough to change appreciably the overall character of the normal modes, as reflected by the similar values of the deuterium and substituent related frequency shifts in neutral and cationic species. Comparison of Table VI to Table VI1 reveals that although AVd = AVd" and AV,,b = AV,,b'+ for both the cupric and ferric complexes, there are subtle differences in the magnitudes of these values between the different metal systems. That is, the absolute value of Avd for vl0 is -2 cm-I larger while that of vi3 is -7 cm-I smaller in the ferric compared to the cupric system. Similarly, the values of AvSub for v2, vI1, ~ 1 3 ,and vs possibly show small variations as well. More definite differences are observed for A V ~ : 3 smaller, while the magnitudes of this shift for v l l and vi7 ~ 2 are those of ~ 2 3 v26 and us are larger in the ferric then in the cupric complexes. This implies that while parent and cation species are structurally and vibrationally similar, the vibrations of the fivecoordinate Fe"' complexes have slightly different PEDS than those of the four-coordinate CUI' complexes. These vibrational differences most likely reflect macrocycle structural differences; thus, asymmetric ligation of the metal presumably induces similar macrocycle distortion in the ferric complexes of both parent and porphyrin cation radical species. These should be especially manifest in the vibrations in the lOO-lOOO-cm-' region,24 and studies of the low-frequency R R spectra of these porphyrin cation radical species are in progress. Further evidence that the porphyrin core distortion present in (CI-)Fe"'OEP are also present in the (C1-)Fe1t'OEP'+SbC16- structure is obtained from our earlier correlations of the Raman frequencies in the 1450-1700-cm-' range to the porphyrin core size of MOEP and MOEP" compounds.2 If we assume a center-to-nitrogen distance of 2.01 A for both (C1-)Fe1110EP25and (C1-)Fe1110EP'+SbCls-,26 we find that the R R wavenumbers obtained from these complexes show distinct negative deviations from the lines described by the MOEP and MOEP'+ samples2 These negative deviations may by attributed to macrocycle buckling.21 Other investigators have pointed out that the value of Avo, obtained from vibrational spectra should reflect the electronic ground state of the porphyrin r cation radicaL2' A variety of
-
+
+
(24) Choi, S.;Spiro, T. G . J. Am. Chem. SOC.1983, 105, 3683-3692. (25) Hoard, J. L.; Cohen, G. H.; Glick, M. D. J. Am. Chem. Soc. 1967, 89, 1992-1996. (26) Buisson, G.; Deronzier, A,; Duee, E.; Gans, P.; Marchon, J.-C.; Regnard, J.-R.J. Am. Chem. SOC.1982, 104,6193-6796. (27) (a) Yamaguchi, H.; Nakano, M.;Itoh, K. Chem. Lett. 1982, 1397-1400. (b) Kim, D.; Miller, L. A,; Rakhit, G.; Spiro, T. G . J. Phys. Chem. 1986, 90, 3320-3325.
TABLE VIII: Qualitative Values of Avox, the Wavenumber Shift Due to Porphyrin Oxidation, for Internal Stretching Coordinates of the Metalloporphyrin coordinate
ZAl,,
predicted ZA,,,
obsd for MOEP'+
u(C,C,) u(cacb) 4C,N) Y(CbCb)
negative' negative negative" positive
negative positive positive negative
negative negative negative positive
"Strictly speaking both the Ca-C, and C,-N interactions are nonbonding in the A,, orbital; however, the large coefficient for the acarbon atom suggests that removal of electron density from this orbital will weaken slightly any bonds to this atom.
spectroscopies have been used to assign 2Aluor 2A2ustates to these MOEP" radicals; however, these assignments have generated C~O~considerable controversy. In particular, C O ~ ~ * O E P ' + ~and Co1"OEP'+2Br- were designated as 2A2uand 2Aluradicals, respectively, and their UV-vis absorption spectra were proposed as typical of these electronic states.28 However, when this analysis was extended to compound I enzyme transients, seeming inconsistencies with EPR and Mossbauer spectra were recognized and the validity of classification based on optical spectra was quest i ~ n e d . Furthermore, ~~ the analysis of N M R measurement^^^ has challenged the earlier electronic assignments of these model compounds; in particular a 2Aluassignment has been suggested for CuOEP'+C104- based on the N M R spectrum,31while MCD measurements were interpreted to indicate a 2A2ustate.32 To our knowledge, the species symmetry of NiOEP'+ClO; has not been discussed, and both ZnOEP'+C104- and MgOEP'+C104- are unambiguously assigned to the 2Alustate.33 Thus, if we consider the more recent assignment of a 2Alustate to C u O E P W 1 0 4 to be correct, then only C O ~ ~ ' O E P ' + ~ C remains I O ~ - identified as a 2A2uradical among the MOEP'+ complexes considered here. Table VI11 depicts the sign of Avo, predicted" for several internal coordinates of the metalloporphyrin based on the r orbital diagrams presented by M a g g i ~ r a .Also ~ ~ included are signs of Avox (28) Dolphin, D.; Forman, D. C.; Borg, J.; Felton, R. H. Proc. Nufl.Acud. Sci. U.S.A. 1971, 68, 614-618. (29) Rutter, R.; Valentine, M.; Hendrick, M. P.; Hager, L. P.; Debrunner, P. G. Biochemistry 1983, 22,4169-4114. (30) Morishima, I.; Takanumi, Y.; Shiro, Y. J. Am. Chem. Soc. 1984,106. 7666-7612. (31) Godziela, G. M., Goff, H. M. J . Am. Chem. SOC.1986, 108, 2231-2243. (32) (a) Browett, W. R.; Stillman, M. J. Inorg. Chim. Acfu 1981, 49, 69-77. (b) Browett, W. R.; Stillman, M. J. Biochim. Biophys. Acfa 1981, 660, 1-7. (33).(a) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1979; Vol. 4, pp 197-256. (b) Fujita, E.; Chang, C. K.; Fajer, J. J . Am. Chem. SOC.1985, 107, 1656-7669.
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989
Cu", Fe"', and ColI1 Porphyrin Cation Radicals determined from our experimental work. The table illustrates that the qualitative values of Avox observed for all MOEP" compounds we have studied (M = Zn", Cu", Co", Co"', Ni", Fen') are consistent with those expected for ZAlurather than 'AZu radicals. Inspection of the wavenumbers listed in Table V for C O " O E P ~ ~ for this and Co1110EP'+2C10; reveals that Avd, Avd'+ and box system are essentially the same as those listed in Table VI for the CuOEP/CuOEP'+ClO; system. For these examples, a one-to-one correlation of the Soret RR bands (1OOO-1700crn-') of porphyrin neutral and ?r cation radical complexes can be made, and the p values and relative intensities of the corresponding R R bands are similar. The agreement in the vibrational frequency shifts upon deuteriation and oxidation suggest similar flat macrocycle geometries for these two cation species (similar to the parent compounds), in accord with other studies.I9 The similar relative R R intensities and UV-vis absorption ~ p e c t r a ~indicate ~ " ~ a common character of the Soret transition (presumably aZu(?r) e,(?r*)) for all complexes. We postulated earlier2 that these ?r cation radical species formed four-coordinate cupric and six-coordinate cobaltic complexes. Thus, the effect of weak diperchlorate ligation and metal oxidation in the latter compound results in a net stabilization of the e,(?r*) excited state with no effect on the relative az,(a) orbital energy or porphyrin core size.2 Thus, in view of these similar spectral properties these two ?r cation radials most likely belong to the same ground-state symmetry, despite the contradictory reports in the literature. In contrast to the cupric and cobaltic complexes, the usually intense v4 vibration is not identified in the R R spectra of the ferric chloride porphyrin cations, breaking down the one-to-one analogy of the R R active vibrations of the neutral parent and oxidized species. Because we similarly do not observe a v4 (or v3) vibration in the Soret R R spectrum of the Co11'OEP+'2Br- complex (Figure 6), we are currently exploring the possibility that this spectral analogy to the five-coordinate (C1-)Fe"'OEP'+SbC16- implies a similar nonplanar geometry of the porphyrin in the cobaltic dibromide complex.37 RR studies suggest that the parent compound for this ?r cation radical, (Br-)Co"'OEP, displays a distorted porphyrin core g e ~ m e t r y . ~ ,Thus, ~ * it is possible that this mac-
-
~~
~~~~
(34) Maggiora, G . M. J . Am. Chem. SOC.1973, 95, 6555-6559. (35) In dry CH2C13one-electron oxidation by AgC104 of Co"0EP produces Co"'OEP'+2C10~.' Several six-coordinate cobaltic porphyrins were shown to display RR frequencies practically indistinguishablefrom COIUEP.~ Thus, in this case, it matters not that we compare a cobaltous porphyrin to a cobaltic porphyrin cation radical complex to determine Av,. (36) Fuhrhop, J. H.; Mauzerall, D. J . Am. Chem. SOC. 1969, 91, 4174-41 81. (37) In agreement with our earlier speculation,2a recent report of the IR spectra of MOEP" complexes also suggests a ruffled structure for the Co"'OEP''2Br- macrocycle (Itoh, K.; Nakahasi, K.; Toeda, H. J . Phys. Chem. 1988, 92, 1464-1468). However, the vibrational assignments for the IR-active E, modes of MOEP complexes are somewhat controversial (Spiro, T. G. In Iron Porphyrins; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1983; Part 2, 89-159. Kincaid, J. R.; Urban, M. W.; Watanabe, T.; Nakamoto, K. J . Phys. Chem. 1986,90,5646-5650) and less well defined than the Raman-active vibration^.^,^ Thus, until the IR band assignments of these MOEP'+ species are confirmed by isotopic substitution, conclusionsbased on analysis of the IR spectra must be considered tentative.
1319
rocycle conformation is maintained in the oxidized complex, Co1110EP'+2Bi. Just as recent work by Reed and co-w~rkers'~ has stressed the importance of the evaluation of porphyrin stereochemistry as well as ?r orbital occupancy to the analysis of the magnetic properties of metalloporphyrin ?r cation radicals, we emphasize that these structural factors must also be considered in the analysis of the electronic and vibrational spectra of these complexes. It is likely that the absorption spectra of C O ~ ~ ' O E P ' + ~ C and ~O~CoI1'OEP'+2Br-, which were originally thought to be characteristic of the ZA2uand 2Alustates, actually reflect some other aspect of these complexes. We suggest that the stereochemistry of the porphyrin core is reflected by the absorption spectrum and that a planar conformation exists in the solution for the diperchlorate complex, and presumably a nonplanar one is present for the dibromide adduct. We postulate that the resonance Raman spectral properties are also determined by these structural properties and either that they are to some extent independent of 2Alu vs 2A2,,designation or that all of the MOEP" compounds we have examined belong to the same radical ground state, namely, 2Alu. To resolve questions concerning structure, we are studying the low-frequency R R active vibrations of the metal-axial ligand and porphyrin skeleton of these complexes. Because R R spectral differences in the 1350-1400-~m-~region are apparent between those complexes that we consider to possess planar porphyrin skeletons (D4,,)and those thought to display lower symmetry, R R measurements of MOEP" complexes substituted with 15N are in process. These will monitor the vibrational contributions from the v(C,N) internal coordinate, which is sensitive to the electronic interactions of metal and porphyrin. Finally, as a means by which to test the tentative electronic ground-state assignment we have made by Raman analysis, ENDOR measurements are in progress.
Acknowledgment. This work was supported by N I H Grants GM36520 (C.K.C.) and GM25480 (G.T.B.). G.T.B. acknowledges helpful discussion with Professor D. Bocian and Professors T. G. Spiro and J. R. Kincaid for communicating results in advance of publication. Note Added in Proof. Czernuszewicz et al. (Czernuszewicz, R. S.;Macor, K. A.; Li, X.-Y.; Kincaid, J. R.; Spiro, T. G. J. Am. Chem. SOC.,submitted) have also studied the vibrational characteristics of MOEP" systems. Their data and ours are in good agreement, and they conclude, as we do in the present paper, that the ground state for this class of compounds is ZAlu. Registry No. D2,7782-39-0; CuOEP, 14409-63-3; CuOEP+'CIOC, 78520-99-7; CuEPI, 13698-60-7; CuEPI+'CIO4-, 1 1 7652-29-6; CIFeOEP, 28755-93-3; ClFeOEPSbCb-, 100333-81 1; ClFeEPI, 1941 3-49- 1; ClFeEPI+'SbCl,-, 117627-31-3; CoIrOEP, 17632-19-8; Co"'OEP+*2C1O4-, 33058-44-5; BrCo"'OEP, 55845-53-9; Co"'OEP+'2Br-, 32880-79-8; CoOEP+'C104-, 55845-55-1; (MeOH),CoOEPClO;, 103904-50-3.
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(38) Felton, R. H.;Yu, N.-T.; OShea, D. C.; Shelnutt,J. A. J. Am. Chem. SOC. 1974, 96, 3675-3676.
