EPR Spectra from “EPR-Silent” Species: High-Field EPR Spectroscopy of Manganese(III) Porphyrins David P. Goldberg, ² Joshua Telser, ‡ J. Krzystek, § Antonio Garrido Montalban, ⊥ Louis-Claude Brunel, § Anthony G. M. Barrett, ⊥ and Brian M. Hoffman* ,² Department of Chemistry, Northwestern UniVersity EVanston, Illinois 60208 Chemistry Program, RooseVelt UniVersity Chicago, Illinois 60605 Center for Interdisciplinary Magnetic Resonance National High Magnetic Field Laboratory Florida State UniVersity, Tallahassee, Florida 32310 Department of Chemistry Imperial College of Science, Technology and Medicine London, UK SW7 2AY ReceiVed April 14, 1997 EPR spectroscopic methods at conventional microwave frequencies (X-band: ∼9 GHz (0.3 cm -1 ); Q-band: ∼35 GHz (1.2 cm -1 )) have long played a central role in defining the structural and electronic environments of half-integer spin (Kramers) paramagnets. In general, these methods are not applicable to “EPR-silent” systems with integer-spin ground states where the zero-field splitting (zfs) is larger than the microwave quantum, and in particular where the zfs interaction approaches axial symmetry. 1 High-spin manganese(III) (d 4 ,S ) 2) is archetypical of such non-Kramers ions. Mono- and polynuclear Mn(III) is of central importance in biological systems such as superoxide dismutase, 2 catalase, 3 and photo- system II, 4 while Mn(III) porphyrins 5 and phthalocyanines 6 have been used as building blocks in the construction of molecule- based magnets. High-frequency, high-field EPR (HF-EPR; ν > 90 GHz) methods have proved to be effective complements to conven- tional studies of Kramers systems. 7 We illustrate here their value for probing non-Kramers centers, 8-10 with an HF-EPR investigation of the Mn(III) ion incorporated into the three com- plexes depicted in Scheme 1: Mn(TPP)Cl (1), Mn(ODMAPz)- Cl (2), and Mn(ODMAPz)DTC (3). 11 The first of these is a classical metalloporphyrin, and the other two are tetraazapor- phyrin (porphyrazine) complexes newly prepared as part of our broader effort to synthesize novel porphyrazine macrocycles. 12 All three compounds are EPR-silent at conventional microwave frequencies. The HF-EPR spectrometer employed generates numerous microwave frequencies (25-3000 GHz (∼100 cm -1 )) and is capable of continuous field sweeps over a broad range (0-17 T). 13,14 Sets of field-dependent spectra at multiple frequencies were collected at 4 K from samples of 1-3, 15 so as to generate full field-frequency relationships of their EPR transitions. 16 Figure 1 is a selection from the data for 1 at four different frequencies. The spectra show one main feature whose resonant field (H r ) shifts linearly with the microwave quantum (hν), as shown by the field-frequency plot of Figure 2a. 17 The spectra for 2 and 3 are similar, and for all three molecules the frequency dependence of this transition can be well fit to the linear relationship gH r ) hν - ∆ with a value of ∆ between 7 and 8 cm -1 (see Figure 2a). The magnetic properties of an ion with S ) 2 can be described by the standard spin Hamiltonian comprised of Zeeman and zfs terms, H ) H‚g‚S + D(S z 2 - S(S + 1)/3) + E(S x 2 - S y 2 ) (eq 1). 1 The energy levels for H 0 parallel or perpendicular to the principal zfs (z) axis of an S ) 2 system can be calculated through use of analytic solutions 18 to eq 1; their field dependence ² Northwestern University. ‡ Roosevelt University. § Florida State University. ⊥ Imperial College of Science, Technology and Medicine. (1) Abragam, A.; Bleaney, B. Electron Paramagnetic Resonance of Transition Ions; Dover Publications, Inc.: New York, 1986. (2) Fridovich, I. Annu. ReV. Biochem. 1995, 64, 97-112 and references therein. (3) Dismukes, G. C. Chem. ReV. 1996, 96, 2909-2926. (4) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem ReV. 1996, 96, 2927-2950. (5) Miller, J. S.; Calabrese, J. C.; McLean, R. S.; Epstein, A. J. AdV. Mater. 1992, 4, 498-501. (6) Miller, J. S.; Vazquez, C.; Calabrese, J. C.; McLean, R. S.; Epstein, A. J. AdV. Mater. 1994, 6, 217-221. (7) (a) Gerfen, G. J.; Bellew, B. F.; Griffin, R. G.; Singel, D. J.; Ekberg, C. A.; Whittaker, J. W. J. Phys. Chem. 1996, 100, 16739-16748. (b) Mo ¨bius, K. Appl. Magn. Reson. 1995, 9, 389-407. (c) Smirnova, T. I.; Smirnov, A. I.; Clarkson, R. B.; Belford, R. L. J. Phys. Chem. 1995, 99, 9008-9016. (d) Coremans, J. W. A.; Poluektov, O. G.; Groenen, E. J. J.; Canters, G. W.; Nar, H.; Messerschmidt, A. J. Am. Chem. Soc. 1994, 116, 3097-3101. (e) Lebedev, Y. S. Appl. Magn. Reson. 1994, 7, 339-362. (f) Lynch, W. B.; Boorse, R. S.; Freed, J. H. J. Am. Chem. Soc. 1993, 115, 10909-10915. (8) We note an early far-IR study of metalloporphyrins. Brackett, G. C.; Richards, P. L.; Caughey, W. S. J. Chem. Phys. 1971, 54, 4383-4401. (9) Rentschler, E.; Gatteschi, D.; Cornia, A.; Fabretti, A. C.; Barra, A. L.; Shchegolikhina, O. I.; Zhdanov, A. A. Inorg. Chem. 1996, 35, 4427- 4431. (10) Barra, A. L.; Caneschi, A.; Gatteschi, D.; Sessoli, R. J. Am. Chem. Soc. 1995, 117, 8855-8856. (11) Abbreviations used are as follows: TPP, 5,10,15,20-tetraphenylpor- phyrinato; ODMAPz, 2,3,7,8,12,13,17,18-octakis(dimethylamino)por- phyrazinato; DTC, diethyldithiocarbamato. (12) Mani, N. S.; Beall, L. S.; Miller, T.; Anderson, O. P.; Hope, H.; Parkin, S. R.; Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. J. Chem. Soc., Chem. Commun. 1994, 2095-2096. (13) Krzystek, J.; Sienkiewicz, A.; Pardi, L.; Brunel, L. C. J. Magn. Reson. 1997, 125, 207-211. (14) Brunel, L. C. et al. Manuscript in preparation. (15) Compound 1 was purchased from Porphyrin Products, Inc., Logan, UT. Full details of the preparation of 2 and 3, including single-crystal structure determinations, are described elsewhere. Goldberg, D. P.; Montalban, A. G.; White, A. J. P.; Williams, D. J.; Barrett, A. G. M.; Hoffman, B. M. Inorg. Chem. Submitted. Sample size 1 to 3 mg. (16) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Barra, A. L.; Brunel, L. C.; Guillot, M. J. Am. Chem. Soc. 1991, 113, 5873-5874. (17) The field plotted (H r) is that of the peak maximum (derivative zero- crossing point in Figure 1). The experimental line shapes are not well- defined because they are mixtures of absorption and dispersion and may reflect passage effects. Figure 1. HF-EPR spectra obtained at 4 K for 1 at selected frequencies. Scheme 1 8722 J. Am. Chem. Soc. 1997, 119, 8722-8723 S0002-7863(97)01169-4 CCC: $14.00 © 1997 American Chemical Society