Polarized Resonance Raman Spectroscopy Reveals Two Different

Robert J. Lipski , Esko Unger , Wolfgang Dreybrodt , Valeria Militello , Maurizio Leone , Reinhard Schweitzer-Stenner. Journal of Raman Spectroscopy 2...
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J. Phys. Chem. B 1999, 103, 9777-9781

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Polarized Resonance Raman Spectroscopy Reveals Two Different Conformers of Metallo(II)octamethylchlorins in CS2 Robert J. Lipski,† Esko Unger,† and Reinhard Schweitzer-Stenner*,†,‡ Institute of Experimental Physics, UniVersity of Bremen, PO. Box 330440, 28334 Bremen, Germany, and Department of Chemistry, UniVersity of Puerto Rico, Rio Pedras Campus, PO. Box 23346, San Juan, PR 00931-3346 ReceiVed: June 9, 1999; In Final Form: August 31, 1999

We have for the first time measured and analyzed the Raman spectra of the model hydroporphyrins nickel(II) and copper(II) 2,2,7,8,12,13,17,18-octamethylchlorin in CS2. A detailed spectral analysis of the fingerprint region of nickel(II) chlorin revealed that a band at 1654 cm-1 is composed of two sublines at 1654 and 1662 cm-1. A novel normal coordinate analysis based on a transferrable force field derived from nickel(II) porphin, propane, and 2,2-dimethylpropane revealed that the respective normal mode is comparable with the porphyrin mode ν10 despite significant localization effects due to the reduction of a pyrrole ring. The resonance excitation profile of the low-frequency subline of ν10 is red-shifted with respect to that at higher frequencies. Hence, the two sublines can be interpreted as resulting from the coexistence of a nonplanar (ruffled) and a more planar conformer. The analysis of the ν10 band of the copper(II) octamethylchlorin revealed that it is also composed of two sublines. The frequencies obtained are 1639 and 1645 cm-1. Thus, evidence is provided that copper(II) chlorins can exist in a nonplanar conformation. These results underscore the notion that even in the absence of any steric interactions between substituents and the presence of metals with an optimal ionic radius pyrrole reduction significantly destabilizes the π-electron system of the porphyrin macrocycle.

Introduction Metallodihydroporphyrins (metallochlorins) serve as prosthetic group in a variety of proteins that participate in electron transfer processes and catalysis. Modification of the basic tetrapyrrole structure results in significant alterations of photophysical, redox, and ligand properties of the metallochlorins vs metalloporphyrins.1,2 The reduction of one of the pyrrole rings reduces the symmetry of the macrocycle from D4h to C2V. Moreover, numerous crystallographic studies revealed strong macrocycle ruffling in particular of meso- and β-substituted chlorins and highly reduced hydroporphyrins.3 In this context, investigations by Ulmann et al.,3h Gallucci et al.,3a and Suh et al.3d on the meso-substituted Ni(II) tetramethylchlorin showed that the macrocycle is forced into a ruffled conformation of D2d symmetry, whereas Ni(II) tetramethylporphyrin appears planar. Two different conformations, a planar and a ruffled one, were also reported by Stolzenberg et al. for crystallized Ni(II) β-oxooctaethylporphyrin.2c Moreover, as shown by Strauss et al.,3g β-substituted Fe(II) octaethylchlorin is also ruffled whereas Cu(II) octaethylchlorin is expected to be planar8,10d because the appropriate size of this metal is generally thought to prevent the pyrroles from mutual interaction. This, however, seems not to be a general rule for Cu(II) hydroporphyrins since Senge et al. reported different conformations of the chlorin macrocycle in the same unit cell of (rhodochlorinato-15-acetic trimethyl ester) copper(II).3i Nonplanar distortions are of functional relevance for normal4a,b as well as for hydroporphyrins.2e It is therefore important to check whether they also occur in solution and how they depend * To whom all correspondence should be addressed. Currently at University of Puerto Rico. Phone: 787-764-0000-(2417). Fax: 787-7568242. E-mail: [email protected]. † University of Bremen. ‡ University of Puerto Rico.

on the ionic radii of the metals, the bulkiness of peripheral substituents, and the degree of pyrrole reduction. Results of Andersson et al. on cis- and trans-Ni(II) and Cu(II) octaethylchlorin dissolved in CH2Cl2 show that the Qy absorption bands of cis-Cu(II) OEC and Ni(II) OEC are redshifted with respect to those of the corresponding trans isomers.10d In view of our present knowledge on metalloporphyrins, this could be interpreted as arising from different degrees of the macrocycle’s nonplanarity.4a,e,f While coexisting planar and nonplanar conformers in solution are well established for metalloporphyrins,4 the evidence for respective nonplanar conformers of metallochlorins is rather limited.5,10d Normally, resonance Raman spectroscopy is an ideal tool to address this issue, but at present, it is a matter of a controversial debate, whether some of the high-frequency skeleton modes of chlorins correspond to classical structuresensitive marker modes of porphyrins. This issue was addressed by different research groups who performed normal mode calculations for planar metallochlorins. Prendergast and Spiro reported a normal coordinate analysis of nickel(II) octaethylchlorin (NiOEC) on the basis of the force constants of Ni(II) porphyrin (NiP).6 They predicted that the normal vibration’s eigenvectors of NiOEC remain delocalized so that they are comparable to those of metalloporphyrins.6b In contrast, Boldt et al. reported normal mode calculations on NiOEC carried out by using the semiempirical QCFF/PI method for the determination of force constants.7 Later, Procyk et al. refined these calculations and compared them to results from a purely empirically analysis based on the Raman spectra of several site-specific deuterated CuOEC derivatives.8 These calculations used the structure of Ni(II) octaethylporphyrin (NiOEP) with the Cβ-ethyl substituents represented as 15 amu point masses as starting point. In a second step, they considered the experimentally observed bond properties of chlorins. Both

10.1021/jp9918858 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999

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Lipski et al. indicators of the iron spin and coordination states even for metallochlorins.9 To clarify this conflicting and important issue, we have, for the first time, measured resonance Raman spectra of Ni(II) and Cu(II) 2,2,7,8,12,13,17,18-octamethylchlorin (NiOMC, CuOMC) in CS2 solution. These are even more appropriate model systems than the frequently used metallooctaethylchlorins in that steric interaction between Cβ-substituents, which are known to cause nonplanarity in the case of NiOEP, are mostly avoided. Material and Methods

Figure 1. Structure of Me(II) 2,2,7,8,12,13,17,18-octamethylchlorin (Me ) Ni (NiOMC), Cu (CuOMC)).

Figure 2. Decomposition of the Raman spectra of NiOMC in the fingerprint region, measured parallel (x) and perpendicular (y) polarized with respect to the laser polarization. (A) Decomposition of the fingerprint region. (B) Cutout of the band ν10. The absorption band region of the given excitation wavelength is indicated at the right-hand side of each spectrum. LF and HF label the sublines of ν10 as obtained from the band shape analysis.

studies revealed that a significant number of normal modes with frequencies between 1000 and 1760 cm-1 are rather localized so that they can hardly be compared with porphyrin vibrations. By comparing the normal modes of Fe(II)OEC and Fe(II)OEP, Sun et al. came to the conclusion that pyrrole reduction has only a limited influence on the characteristic properties of the classical marker modes ν2, ν3, ν4, ν11, and ν19.9 They found, for instance, that the frequency shifts of Fe(II)OEC isotopomers are in the range of those found for corresponding iron porphyrins. This led them to conclude that these modes serve as reliable

Preparation of Samples. Starting with octamethylporphyrin (1), over 2,3,7,8,12,13,18,18-octamethyl-17-oxochlorin (2), and Zn-2,3,7,8,12,13,18,18-octamethyl-17-oxochlorin (3), the free base 2,2,7,8,12,13,17,18-octamethylchlorin (4) was synthesized according to a method developed and refined by Montforts and associates.13a,b (1) 2,3,7,8,12,13,18,18-octamethyl-17-oxochlorin (2) was synthesized from octamethylporphyrin (1) according to the method of Bonett et al.13c The generated raw product was chromatographed on aluminum oxide (activity II-III, ICN Biomedicals) with CH2Cl2 as the eluent. Further purification of 2 failed, and the chromatographed raw product was used for the first metalation step. (2) To obtain Zn 2,3,7,8,12,13,18,18-octamethyl-17-oxochlorin, 2 was disolved in CH2Cl2 under an argon atmosphere and stirred at room temperature overnight with a 4-fold excess of zinc acetate dihydrate and a small amount of triethylamine. The reaction mixture was then mixed with the same amount of water, and substance 3 was extracted with CH2Cl2. The organic phase was filtered from the formated Zn(OH)2, washed with water, and dried over Na2SO4, before the solvent was removed. 3 was chromatographed on silicia (32-63 µm 60A, ICN Biomedicals) with a 1% methanol in CH2Cl2 solution as the eluent. The deep blue-violet product 3 was thereby eluted as the fifth fraction (Rf ) 0.25) and, at room temperature, isothermically recrystallized from CH2Cl2/n-pentane. (3) 2,2,7,8,12,13,17,18-octamethylchlorin (4) was obtained as follows. A solution of 900 mg ZnCl2 and LiAlH4 in tetrahydrofuran (THF) was stirred under an argon atmosphere, and 60 mg of the chlorin 3 in 100 mL THF was added dropwise over 15 min. The mixture was then refluxed for 30 min. A time period above 45 min was avoided to prevent the metalation of the chlorin with aluminia. The solution was cooled to 0° and a NaHCO3 solution was added, filtered from Al(OH)3, washed with H2O, and dried over Na2SO4 The solvent was then removed under vacuum. For demetalation, the residue was dissolved in 200 mL methanol and 400 mL of HCl was added. 4 was extracted with CH2Cl2, neutralized with a saturated NaHCO3 solution, and purified over silicia with 10% MeOAc in CH2Cl2 solution as the eluent. (4) Ni(II) and Cu(II) 2,2,7,8,12,13,17,18-octamethylchlorin were obtained by dissolving 4 in CH2Cl2/MeOH with a 4-fold excess of Ni(II) or Cu(II) acetate and refluxing the solutions overnight under an argon atmosphere. The metalated complexes of 4, NiOMC and CuOMC, were extracted into CH2Cl2, washed with water, and dried over Na2SO4, and the solvent was removed. Both complexes were isothermically recrystallized from CH2Cl2/n-hexane. The dried chlorins were solved in highly purified CS2 (Aldrich, HPLC-grade) and chromatographed on silicia. Resonance Raman Spectroscopy. The resonance Raman spectra were recorded by using an excimer pumped dye laser

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Figure 3. Eigenvectors of the normal modes ν10 and ν2 of Ni(II) octaethylporphyrin and Ni(II) octamethylchlorin as obtained from a novel normal coordinate analysis.

system (Lambda EMG53MSC processing at 308 nm pumps a dye laser Lambda FL2001) and CW laser systems (argon ion laser, Spectra-Physics, model 2020-05) or a krypton ion laser, (Coherent, Innova 90 K). Measurements were carried out in backscattering geometry. Details of the experimental setups used in the present study and the strategy adopted to callibrate the spectra and to correct them for absorption are reported by Unger et al.4e,12 Curve Fitting. To identify the frequency positions, the line profiles, and the line widths (full widths at half-maximum, fwhm) of all Raman lines, small spectral regions of the absorption-corrected and callibrated spectra were decomposed using a line shape analysis program called MULTIFIT. The overlapping Raman lines of each spectral region were fitted simultaneously with a convolution of a line profile (Lorentzian or Voigtian) and the transfer functions of the respective spectrometer. The transfer functions, which depend on the slit widths, have been measured previously.4f,12 The functions are well approximated by Gaussians or a convolution of Gaussians and a boxcar function. We eliminated ambiguities in the line shape analysis to fit each Raman line for all excitation wavelength covering the B, Qx00,01, Qy00,01,02 absorption band

regions. Thus, we carried out a self-consistent global fit protocol for each spectral region of about 160 spectra, for which only the line intensities and the background intensity were used as fitting parameters.4c-f,12 Finally, the resonance excitation profiles were determined by using the intense 656 cm-1 line of the CS2 solvent as an internal standard. Molecular Mechanics and Force Field Calculations. The normal coordinate analysis of NiOMC was achieved on the basis of a transferable force field. The underlying concept assumes that each atom forms local symmetry units with its nearest neighbors. The number of interaction constants for these units was reduced by symmetry restrictions.11a The force constants for the local unit CCH3 were determined from ethane, while those for the pyrroles and methin bridges were taken from NiP. The force constants for the reduced pyrrole ring were taken from CCH2C and C(CH3)4 of propane and 2,2-dimethylpropane. The ring geometry was taken from NiOEP and was corrected for the bond lengths and angles of the reduced element.11b Results and Discussion Figure 1 shows the octamethylchlorin which exhibits Cs symmetry for a planar macrocycle conformation. Figure 2 shows

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Figure 4. The upper panel shows the resonance Raman excitation profile of the low- (LF) and high-frequency subline (HF) of ν10 for NiOMC in CS2, based on the self-consistent decomposition of the band shape. The lower panel displays the Qy and Qx bands of NiOMC absorption in CS2.

the Raman spectra taken for NiOMC at the indicated wavelengths. We found that the general spectral features resemble those obtained for various metalloporphyrins.10 The assignment of the identified lines was achieved by a normal coordinate analysis of NiOMC on the basis of a transferable force field11b which is described under Materials and Methods. Hence, we found that the normal mode patterns calculated with this set of force constants are indeed significantly changed by the pyrrole reduction in accordance with results reported by Boldt et al.7 and Procyk et al.8b To give an example, Figure 3 compares the respective eigenvectors of ν10 and ν2 for NiOEP and NiOMC. Apparently, the predominantly contributing internal coordinates, i.e., CRCm for ν10 and CβCβ for ν2 are more localized in NiOMC, thus reflecting the distortion of the reduced pyrrole ring. In contrast, however, to what has been suggested by Procyk et al.,8b the general patterns of both modes remain comparable in that nearly the same internal coordinates determine the eigenvectors. Thus, one expects that corresponding modes of NiOMC and porphyrins show a similar dependence on core size and out of plane distortions. The spectral analysis revealed that the fingerprint region of the metallochlorins investigated is composed of strongly overlapping bands as shown in Figure 2. The ν10 band of NiOMC at 1654 cm-1, the frequency of which is greatly sensible for out of plane distortions in metalloporphyrins,4c-e,12 was found to be composed of two sublines as illustrated in Figure 2b. Both sublines have Voigtian line shapes with Lorentzian half-widths of 10 and 7 cm-1 and Gaussian half-widths of 12 cm-1 and 13 cm-1 for the low-frequency line at 1654 cm-1 and the high-frequency line at 1662 cm-1, respectively. Figure 4 shows the resonance excitation profiles (REPs) for both ν10 sublines of NiOMC. The absorption spectrum in the lower panel facilitates the identification of resonance positions. Apparently, the REP of ν10LF is red-shifted with respect to that of ν10HF. In view of our knowledge on metalloporphyrins4 and in accordance with results by Stolzenberg et al., who reported coexisting ruffled and planar conformers in the same unit cell of nickel(II) octaethyloxoporphyrin crystals,2d this indicates that the ν10LF is assignable to a nonplanar, probably ruffled, conformer, while ν10HF arises from a nearly planar conformer. Hence, our results show that NiOMC can adopt nonplanar conformations also in solution. Our molecular mechanics calculations suggest that in contrast to NiOEC steric interactions

Figure 5. Decomposition of the Raman band ν10 of CuOMC in the fingerprint region, measured parallel (x) and perpendicular (y) polarized with respect to the laser polarization. The absorption band region of the excitation wavelength is indicated at the right-hand side of each spectrum. LF and HF label the sublines of ν10 as obtained from the line shape analysis.

between substituents are unlikely to contribute to nonplanarity. This leads us to conclude that the nonplanarity is mostly assignable to a combined effect of pyrrole reduction and the small ionic radius of the nickel atom. We like to emphasize that the latter alone is not capable of causing any nonplanar distortions.14 Thus, it is the great flexibility of the hydroporphyrin macrocycle which stabilizes nonplanar conformations. Moreover, the Gaussian contributions to the half-width of both spectral components indicate further conformational heterogeneity. One generally expects that Cu(II) hydroporphyrins adopt a planar configuration,8,10d because the optimal ionic radius of the metal prevents the pyrroles from tilting and twisting. Surprisingly, however, we found that also the ν10 band of CuOMC is composed of two sublines at 1639 and 1646 cm-1 with Voigtian line shapes (Lorentzian fwhm both 5 cm-1; Gaussian fwhm 13 and 6 cm-1) as shown in Figure 5. The respective REPs displayed in Figure 6 reveal that, as obtained for NiOMC, the REP of ν10LF is red-shifted with respect to that of ν10HF though to a minor extent. In contrast to NiOMC, the HF subline dominates the band shape of ν10 at all excitation wavelengths. These findings clearly indicate that, while the planar conformer of CuOMC is the preferred conformation in solution, the pyrrole reduction stabilizes the nonplanar (ruffled) conformer in a way that its free energy difference with respect to the planar conformation allows significant population at room temperature. The Gaussian part of the half-widths reflects substates of both conformations. In an earlier study, Anderson et al.10a identified clearly resolvable doublets at 1627/1637 cm-1 and 1645/1660 cm-1 in

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Figure 6. The upper panel shows the resonance Raman excitation profile of the low- (LF) and high-frequency subline (HF) of ν10 for CuOMC in CS2, based on the self-consistent decomposition of the band shape. The lower panel displays the absorption bands of CuOMC in CS2.

the Raman spectra of trans-NiOEC and trans-CuOEC, respectively. This was interpreted as resulting from extensive mixing between the former IR-active Eu mode (ν37) and the Raman active modes ν10 and ν19 as caused by the symmetry lowering of the macrocycle. Such a mechanism can be ruled out as an explanation for the above ν10 doublets for the following reasons. First, such a mixing is not supported by our normal coordinate analysis. Second, one expects qualitatively different depolarization ratio dispersions for the sublines if they really arise from vibrational mixing of modes having different D4h-symmetries. This was not observed (data not shown). Third, a mixing of former B1g, A2g, and Eu modes would produce depolarization ratio dispersions between 0.125 and ∞, while in reality we observed only variations between 0.125 and 0.75, consistent with the notion that C2 symmetry is at least approximately maintained. A full account of the normal coordinates, the excitation profiles, and the depolarization ratios of NiOMC and CuOMC will be given in a forthcoming publication. Taken together, our studies reveal that already the reduction of a pyrrole ring is capable of destabilizing the porphyrin macrocycle so that nonplanar distortions (ruffling) are induced even when Cu(II) is used as the central metal. Ample evidence has already been provided that normal porphyrins are easier to oxidize and harder to reduce when nonplanar distortions are present.15 Holten and associates have shown that ruffling significantly affects the photophysical properties of metalloporphyrins.16 All these findings underscore the notion that the important role which hydroporphyrins play in electron transfer processes predominantly stems from their capability to adopt a nonplanar conformation. A direct and quantitative correlation between nonplanarity and electron transfer capabilities has to await further experiments. Acknowledgment. We thank Dr. Christina Lemke for critically reading the manuscript, Prof. Dreybrodt for helpful discussions and his support of the project, Dr. Anne Windheim for helpful discussions about the synthesis, and Prof. F. P. Montforts, who allowed us to synthesize the samples in his laboratory. R.J.L. is a recipient of a University of Bremen doctoral fellowship in the framework of the program “Modeling of thermal and light induced electron transfer reactions in biological systems”.

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