3998
Langmuir 1999, 15, 3998-4004
Reduction of Manganese(III) Protoporphyrin IX Dimethyl Ester Studied by Electrochemistry and Surface-Enhanced Raman Scattering Spectroscopy Shi-Ping Chen, Anthony Williams, David Ejeh, Peter Hambright, and Charles M. Hosten* Department of Chemistry, Howard University, Washington, D.C. 20059 Received September 16, 1998. In Final Form: March 22, 1999 The reductive electrochemistry of manganese(III) protoporphyrin IX dimethyl ester, MnIII(PPDME), in an aprotic solvent was studied using cyclic voltammetry, surface-enhanced Raman scattering spectroscopy (SERS), and thin layer potential dependent UV/vis absorption spectroscopy. Good quality SERS spectra are reported for an electrochemically roughened silver electrode in contact with Mn(PPDME) dissolved in acetonitrile. A reversible one-electron reduction was observed at ca. -0.44 V/SCE in the cyclic voltammetric scans on silver, gold, and platinum electrodes. In thin layer UV/vis spectroelectrochemistry, the Soret band at 473 nm at 0.0 V undergoes a 36 nm blue shift to 437 nm when the electrode potential was stepped to -0.5 V. Resonance Raman and SERS spectral frequencies of Mn(PPDME) are assigned and tabulated. The shift of the Soret band along with the downshift in the ν4 oxidation state marker band from 1373 to 1360 cm-1 in the SERS spectra identify the process occurring at ca. -0.5 V to be the reduction of the porphyrin central metal ion from the MnIII to MnII state. Core sizes for MnIII(PPDME) and MnII(PPDME) adsorbed on an anodized silver electrode surface were calculated to be 1.993 and 2.072 Å, respectively. Both the MnII and MnIII complexes are adsorbed as high spin, five-coordinate species. On the basis of relative intensities of the SERS bands, the orientation of Mn(PPDME) adsorbed onto the silver electrode surface is proposed to be face-on.
1. Introduction Metalloporphyrins are ubiquitous in biological systems resulting in extensive scientific investigation of their biophysical and biochemical properties.1-5 They have also inspired electrochemical interest because of their ability to act as catalysts for organic substrates in oxidation and reduction reactions.6-14 Electrochemical studies of synthetic and naturally occurring porphyrins have resulted in the establishment of half-wave potentials for the oxidation and reduction of the porphyrin ring, central metal and substitutents.15-22 A number of workers18-22 * To whom correspondence should be addressed. Telephone: (202) 806-6829. E-mail:
[email protected]. (1) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier Scientific Publishing Co.: New York, 1976. (2) Boucher, L. J. Coord. Chem. Rev. 1972, 7, 289. (3) Dolphin, D. The Porphyrin, Part B; Academic Press: New York, 1979; Vol. 7. (4) Longo, F. R. Advances in Porphyrin Chemistry; Ann Arbor Science Publishers Inc.: Ann Arbor, MI, 1979. (5) Kitagawa, T.; Ozaki, Y. In Metal Complexes With Tetrapyrrole Ligands I; Buchler, J. W., Ed.; Springer-Verlag: New York, 1987; pp 71-114. (6) Barley, M. H.; Takeuchi, K. J.; Meyer, T. J. J. Am. Chem. Soc. 1986, 108, 5876. (7) McMurry, T. J.; Groves, J. T. In Cytochrome P-450: Structure, Mechanism and Biochemistry; Ortiz de Montellano, P., Ed.; Plenum Press: New York, 1986; Chapter 1. (8) Bruice, T. C. In Mechanistic Principles of Enzyme Activity; Liberman, J. F., Greenberg, A., Eds.; VCH Publishers: New York, 1988; Chapter 8. (9) Tabushi, I. Coord. Chem. Rev. 1988, 86, 1. (10) Suslick, K. S.; Watson, R. A. Inorg. Chem. 1991, 30, 912. (11) Traylor, T. G.; Byun, Y. S.; Traylor, P. S.; Battioni, P.; Mansuy, D. J. Am. Chem. Soc. 1991, 113, 7821. (12) Bruice, T. C. Acc. Chem. Res. 1991, 24, 243. (13) Wang, Z.; Pang, D. J. Electroanal. Chem. 1990, 283, 349. (14) Pang, D.; Wang, Z.; Cha, C. Electrochim. Acta 1992, 37, 2591. (15) Boucher, L. J.; Garber, H. K. Inorg. Chem. 1970, 9, 2644. (16) Kadish, K. M.; Xu, Q. Y.; Barbe, J.-M.; Anderson, J. E.; Wang, E.; Guilard, R. Inorg. Chem. 1988, 27, 691. (17) Kelly, S. L.; Kadish, K. M. Inorg. Chem. 1982, 21, 3631.
have investigated the effect of electron-donating or -withdrawing groups on the half-wave potentials with the greatest effect being observed when the substituents were attached to the β-pyrrole positions rather than on one of the four meso positions.15,16 The importance of manganese porphyrins lies in their catalytic ability for the transformation of organic compounds8-10,23 and the range of available oxidation states of the central metal.18-19,24,25 Water-soluble and water-insoluble manganese porphyrins in which the central manganese atom possesses a formal charge from +2 to +5 have been reported.25 Attention has focused on mimicking the action of manganese enzymes in the catalytic processes of biological systems26,27 with the intent of unraveling the catalytic mechanisms9-10,13,23 and biological relevance of manganese porphyrins to active sites of various enzymes.8 Recently, the ability of manganese porphyrins to act as redox catalysts in the activation of dioxygen in acetonitrile23 has been investigated. Olefin oxidation by molecular oxygen28 and iodosobenzene29 have (18) Harriman, A. J. Chem. Soc., Dalton Trans. 1984, 141. (19) Bettleheim, A.; Ozer, D.; Weinraub, D. J. Chem. Soc., Dalton Trans. 1986, 2297. (20) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976, 98, 3484. (21) Kadish, K. M.; Lin, X. Q.; Han, B. C. Inorg. Chem. 1987, 26, 4161. (22) D’Souza, F.; Villard, A.; Van Caemelbecke, E.; Franzen, M.; Boschi, T.; Tagliatesta, P.; Kadish, K. M. Inorg. Chem. 1993, 32, 4042. (23) Gutierrez-Granados, S.; Bedioui, F.; Devynck, J. Electrochim. Acta 1993, 38, 1747. (24) Kaaret, T. W.; Zhang, G.-H.; Bruice, T. C. J. Am. Chem. Soc. 1991, 113, 4652. (25) Paula, M. M. S.; Franco, C. V. J. Coord. Chem. 1995, 36, 247. (26) Battioni, P.; Bartoli, J. F.; Leduc, P.; Fontecave, M.; Mansuy, D. J. Chem. Soc., Chem. Commun. 1987, 791. (27) Leduc, P.; Battioni, P.; Bartoli, J. F.; Mansuy, D. Tetrahedron Lett. 1988, 29, 205. (28) Perree-Fauvel, M.; Gaudemer, A. J. Chem. Soc., Chem. Commun. 1981, 874.
10.1021/la981270m CCC: $18.00 © 1999 American Chemical Society Published on Web 04/29/1999
Study of the Reduction of MnIII(PPDME)
been extensively studied in the presence of manganese porphyrin catalysts. Efficient biomimetic electrochemical systems26,27 have been proposed for the oxidation of hydrocarbons by dioxygen with manganese porphyrin catalysts. Electrochemically generated porphyrin intermediates have been investigated by a number of spectroscopic techniques in an attempt to obtain specific molecular information.5,30 Because SERS can be employed as an in situ probe providing structural and orientative information on molecules adsorbed at roughened metal surfaces31,32 and sols,33 the technique has found application to the study of electrochemical intermediates generated at the surface of a working electrode. SERS spectroscopy has been coupled with potential step voltammetry to characterizeporphyrinsadsorbedatelectrodesurfaces.31,32-35 Purely theoretical predictions of metalloporphyrin Raman band frequencies using normal coordinate analysis have been performed by Abe36 and Li and Zgierski,37 and these results have been used to correlate Raman band frequencies with the oxidation, spin, and ligation states of the porphyrin central metal ion.38-41 As a result, the application of SERS spectroscopy to the study of electrochemically generated porphyrin species allows for the characterization of the intermediates and the effect of adsorption on the molecular species. The reduction of MnII(PPDME) on a mercury electrode has been investigated using linear sweep polarography.15 The MnIII to MnII reduction was observed at -0.35 V versus SCE in acetonitrile solutions. Kelly and Kadish17 presented a systematic study of the effect of the interaction of the solvent and axially coordinated monovalent anions on the electrode reactions of manganese porphyrins. Limited data exist on the physical properties of electrogenerated porphyrin intermediates, adsorbed onto silver electrode surfaces. When the high specificity of Raman spectroscopy is combined with the large enhancement and reduction of fluorescence offered by SERS spectroscopy, a highly detailed profile of electrogenerated porphyrin intermediates can be obtained. In the present paper, we report the spectroelectrochemistry of manganese protoporphyrin IX dimethyl ester, Mn(PPDME), in aprotic media and cyclic voltammetry of Mn(PPDME) on Ag, Au, and Pt surfaces. Potentialdependent ultraviolet/visible absorption spectroscopy and SERS spectroscopy were used to characterize electro(29) Mansuy, D.; Leclaire, J.; Fontecave, M.; Dansette, P. Tetrahedron 1984, 40, 2847. (30) Dolphin, D. The Porphyrin, Part A; Academic Press: New York, 1978; Vol. 3. (31) Hosten, C. M.; Birke, R. L.; Lombarde, J. R. J. Phys. Chem. 1992, 96, 6585. (32) Nabiev, I. R.; Chumanova, G. D.; Manykin, E. A. Sov. Phys. J. 1985, 28, 204. Sanchez, L. A.; Spiro, T. G. J. Phys. Chem. 1985, 89, 763. (33) Feofanov, A.; Janoul, A.; Oleinika, V.; Gromov, S.; Fedorova, O.; Alfimov, M.; Nabiev, R. J. Phys. Chem. 1996, 100, 2154. Cotton, T. M.; Schultz, S. G.; Van Duyne, R. P. J. Am. Chem. Soc. 1982, 104, 6528. (34) Okumura, T.; Endo, S.; Ui, A.; Itoh, K. Inorg. Chem. 1992, 31, 1580. (35) Orita, H.; Shimizu, M.; Nishihara, C.; Hayakawa, T.; Takehira, K. Can. J. Chem. 1990, 68, 787. (36) Abe, M. In Spectroscopy of Biological Systems; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons: New York, 1986; Vol. 13, Chapter 7. (37) Li, X.-Y.; Zgierski, M. Z. J. Phys. Chem. 1991, 95, 4268. (38) Spiro, T. G.; Czernuszewicz, R. S.; Li, X.-Y. Coord. Chem. Rev. 1990, 100, 541. (39) Alden, R. G.; Ondrias, M. R. Adv. Multi-Photon Processes Spectrosc. 1990, 6, 1. (40) Tu, A. T. Raman Spectroscopy in Biology: Principles and Applications; John Wiley & Sons: New York, 1982; Chapter 12. (41) Kitagawa, T.; Mizutani, Y. Coord. Chem. Rev. 1994, 135/136, 685.
Langmuir, Vol. 15, No. 11, 1999 3999
Figure 1. Chemical structure of manganese protoporphyrin IX dimethyl ester.
chemically generated intermediates. Assignment of the band frequencies of the resonance Raman (RR) and SERS spectra of Mn(PPDME) are presented. From these band frequencies, the coordination, oxidation, and spin states of Mn(PPDME) adsorbed on a silver electrode are deduced. Finally, core sizes of MnIII(PPDME) and MnII(PPDME) are calculated using the Raman frequencies of core size sensitive bands. The orientation of Mn(PPDME) at the electrode surface is determined. 2. Experimental Section 2.1. Chemicals. All chemicals were ACS reagent grade and used as received. Manganese(III) protoporphyrin IX dimethyl ester, MnIII(PPDME), was synthesized using standard literature procedures.42,43 The structure of MnIII(PPDME) is shown in Figure 1. Acetonitrile (99.8% anhydrous,
