4300
J . Phys. Chem. 1991, 95,4300-4307
field as predicted from the CIDEP theories, the contribution of Psr- in the X-band region is predicted to be a:c = 1:0.03-1:0.05. Consequently, by use of these parameters, the relative contribution of PTMincluding PsT- in the X-band region is found to be a:b:c = 1:0.35:0.03-1:0.38:0.05. This result implies that ST-M in the X-band region at room temperature is negligibly small. In summary, CIDEP spectra have been measured in the L-band microwave frequency range for the first time. CIDEP of acetone in 2-propanol shows E/A* in the X-band region (Eo 330 mT) at room temperature, while CIDEP in the L-band region (Bo 50 mT) shows the E*/A pattern. The opposite net polarization
- -
is explained by the decrease of T M and the increase of ST-M as the magnetic field decreases. From the relative polarization pattern of the second-order splittings, the hyperfine-dependent part of ST-M is found to be important for the mechanism of ST-M. If the frequency of the loop gap resonator could be varied over a wide range and CIDEP could be measured a t various magnetic fields, much more information on the mechanism of CIDEP and the nature of the intermediate radical pair could be extracted from the magnetic field effect on CIDEP. Registry No. (CH3)2C0, 67-64-1; (CH3),CHOH, 67-63-0.
EPR and ENDOR Studies of the Metallooctaethylporphyrln r-Cation Radical P. 0. Sandusky, W. A. OertlingJ C. K. Cbang,* and G. T. Babcock* Department of Chemistry and The LASER Laboratory, Michigan State University, East Lansing, Michigan 48824 (Received: October 4, 1990; In Final Form: December 18, 1990)
Two classes of @-substitutedmetalloporphyrin *-cation radicals are distinguishable by their visible spectra: the green form complexes typified by Mg'UEP', and the gray form complexes represented by CoUUEP+2C1Oi. Previously it was thought that the gray form and the green form complexes represented, respectively, the 2A2uand ZAluground-state configurations. The present consensus in the literature now holds that both classes have predominately ZAluground states. Despite this, however, we find the gray form and green form complexes differ markedly in terms of their magnetic resonance properties. The gray form complexes have isotropic g values atypically high for simple S = metalloporphyrin radical s p i e s . Further, application of high-resolution techniques including Q-band EPR and ENDOR spectroscopy reveals that Co '*OEP+2CIO{ differs from Mg"OEP'+ in having a larger axial g anisotropy, a much larger ISN hyperfine coupling, and a significantly smaller meso-proton coupling. Analysis of the gray form g tensor indicates that the larger nitrogen coupling derives from a small contribution from molecular orbitals of e,, symmetry that are formed from the nitrogen lonepair orbitals. The contribution of this nitrogen lone-pair centered excited state is much smaller in the green form complexes than in the gray form species. We interpret this behavior to indicate a different metal-nitrogen lone-pair interaction and a different ring conformation in the green form and gray form complexes. This conclusion is further supported by evident differences in the meso-proton hyperfine coupling mechanism and by variations in the Raman-active skeletal mode vibrational frequencies. The intrinsic difference between the gray form and green form complexes, therefore, appears to be a matter of ring conformation, rather than a profound difference in the ground-state electronic configuration.
Introduction Metalloporphyrin *-cation radicals are species of considerable biological importance.' Magnesium chlorin or magnesium bacteriochlorin cation radicals occur as the primary products of the photochemical event in all plant, algal, and eubacterial photosynthetic reaction centers? And ferry1 protoporphyrin IX cation radicals are found as the Ycompound I" intermediates in the enzymatic cycles of several peroxidases and catalase^.^ The Gouterman four-orbital model predicts two neardegenerate porphyrin A system HOMOS of alu and a2"~ymmetries.~EPR studies have established that meso-substituted metalloporphyrins form radicals with 2A2, ground states showing large spin densities localized on the nitrogens and meso-carbons (Table I, Figure I).* The literature conceming the ground-state configuration in radicals derived from @-substituted complexes (e.g., metallooctaethylporphyrins) has until recently been far less definitive. Complications in this area arose in part because metallo OEP systems form two different classes of *-cation radicals. Zn"OEP'+ and Mg"OEP'+ together with the chloride and bromide adducts of Co"'OEP'+ and Ru"(CO)OEP+ form a class of complexes, green in color, with visible spectra similar to that of catalase compound I (Figure 2B). The perchlorate and tetrafluoroborate ligated complexes of Col"OEP'+, Cu"OEP+, and Rd'(CO)OEP'+ are gray and have characteristic optical spectra resembling that of horseradish peroxidase compound I (Figure 2A).6 Initially it was postulated that the green and gray forms of the OEP radical Resent address: Inorganic and Structural Chemistry Group, Lca Alamos National Laboratory, Los Alamos, N M 87545.
0022-3654/9l/2095-4300$02.50/0
TABLE I: Calculated *-System Spin Densities for Metalloporphyrin Cation Radicalsg
c a
'8 GnCw
nitrogen metal
0.098 0.027 0.000 0.000 0.000
0.102 0.023 0.000 0.000 0.000
0.005 0.019 0.151 0.043 0.009
0.005 0.014 0.135 0.069
"Taken from Edwards and Z e ~ n e r . ' ~
represented the ZAluand ZAzuground states,k,6 respectively, and this idea received considerable support in the literature.' However (1) Dolphin, D.; Felton, R. H. Acc. Chem. Res. 1974, 7 , 26-32. (2) (a) Norris, J. R.; Sheer, H.; Katz, J. J. In The Porphyrins; Dolphin. D.. Ed.;Academic Press: New York, 1979; Vol. IV, pp 159-195. (b) Lubitz, W.; Lendzian, F.; Plato, M.; Mobius, K.; Trankle, E. Springer Ser. Chem. P h p . 1985, 45, 164-173. (3) (a) Frew, J. E.; Jones, P. In Aduantages in Inorganic and Bioinorganic Mechanism; Academic Press: New York, 1984; Vol. 3, pp 175-215. (b) Hewson, W. D.; Hager, L. P. In The Porphyrins; Dolphin, D., Ed.;Academic Press: New York, 1979; Vol. VII, pp 295-332. (4) Gouterman, M. J . Mol. Spectrosc. 1961, 6, 138-163. (5) (a) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.;Academic Press: New York, 1979; Vol. IV, pp 197-256. (b) Fajer, J.; Borg, D. C.; Forman, A.; Felton, R. H.; Vegh, L.; Dolphin, D. Ann. N.Y.Acad. Sci. 1973, 206, 349-364. (c) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J . Am. Chem. SOC.1970, 92, 3431-3439. (6) Dolphin, D.; Forman, A.; Borg, D. C.; Fajer, J.; Felton, R. H. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 614-618.
0 1991 American Chemical Society
Metallooctaethylporphyrin Cation Radical
The Journal of Physical Chemistry, Vol. 95, NO. 11, 1991 4301
b
Ri
R2
OEP: R,=
R2= CH2CH3
Ello: RjmCH3, R2= CH2CH3
Figure 1. Structures of metallooctaethylprphyrin and metalloetioporphyrin. The a-,p-, and meso-carbon positions are indicated.
co(lll)OEP*+ 2~104Room Temperature 100K
A
-
----
\
I
500
600
I
roo
nm Figure 2. Visible spectra of room-temperature and 100 K glassed metallo OEP+ complexes: (A) CO~~~OEP'+ZCIO,in 1:1 CH2C12:toluene;(B) Mg"OEP'+ in 1:l CH30H:toluene. Peaks at 540 and 580 nm in spectrum B are from a small amount of unreacted MgI'OEP. Samples were contained in 4-mm EPR tubes, and the ordinate scale represents absorbance in arbitrary units. The increase in absorbance at 100 K is due to a contraction of the sample volume during glass formation. The initial room-temperature porphyrin concentrations were 100 pM.
recent NMR? ENDOR? and Ramanlo studies have established that both classes of metallo OEP radicals have predominately 2Alu ground states. Yet, the differences between the gray and green forms are significant. In addition to the different characteristic (7) (a) Edwards, W. D.; Zerner, M. C. Can. J. Chem. 1986, 63, 1763-1772. (b) Dinello, R. K.; Dolphin, D. H. J. B i d . Chem. 1981, 256, 69034912. (c) Hanson, L. K.; Chang, C. K.: Davis, M.S.; Fajer, J. J. Am. Chem. Soc. 1981,103,663-670. (d) Bmett, W.R.;Stillman, M.J. Biochim. Blophys. Acfu 1981,660, 1-7. (e) Gasyna, Z.: Browett, W.R.;Stillman, M. J. Imrg. Chem. 1988.27,4619-4622. (f) Kim, D.; Millar, L. A.; Rakhit, G.; Spiro. T. 0. J. Phys. Chem. 1986, 90,3320-3325. (8) (a) Morishima. I.; Takamuki, Y.;Shiro, Y. J. Am. Chem. Soc. 1984, 106.7666-7672. (b) Godziela, G. M.; Goff,H. M. J. Am. Chem. Soc. 1986, 108,2237-2243. (9) Sandurky, P. 0.;Salehi, A.; Chang, C. K.; Babcock, G. T. J. Am. Chem. Soc. 1989,111,6437-6439. (10) (a) Oertling, W.A.; Salehi, A.; Chang, C. K.; Babcock, G. T. J. Phys. Chem. 1989,93,1311-1319. (b) Oertling, W. A.; Salehi, A.; Chung, Y. C.; Leroi, 0. E.; Chang, C. K.: Babcock. G. T. J. Phys. Chem. 1987, 91, 5887-5898. (c) Czemurzewicz, R.S.;Macor, K. A,; Li. X.Y.; Kincaid, J. R.;Spiro, T. G. J. Am. Chem. Soc. 1989, 1 1 1 , 3860-3869.
optical spectra, the gray and green forms systematically differ both in the energies of their Raman active core vibrational modes'* and in the magnitude and sign of their meso-proton EPR hyperfine Thus the consensus that the ground-state configurations of all metallo OEP'+ species are 2Alu leaves unaddressed the nature of the difference between the gray form and the green form complexes. To understand better the differences between the gray and the green complexes of the metallo OEP radical, we have undertaken a systematic spectroscopic study to compare the two forms. In this paper we present data characterizing the differences in r e p resentative gray and green form complexes in terms of EPR, g tensor, nitrogen hyperfine coupling, a meso-proton hyperfine coupling. In most cases we take C O ~ ~ ~ O E P ' + ~ Cas I Orepre,sentative of the gray form species and compare its properties to those of the prototypical green form Mg"OEP'+. We find that the spectroscopy of the gray and green forms differs significantly with respect to all of the above parameters. There is, for instance, a more pronounced g-tensor anisotropy and a larger nitrogen hyperfine coupling in the gray forms. This suggests contributions from u-system nitrogen lone-pair orbitals to the gray form ground state. There are also some indications of 2A2uexcited-state mixing into the ground state, a possibility recently discussed by Czernuszewicz et al.Ioc Taken as a whole, the data we present here constitute fairly compelling evidence for different ring conformations in the gray and green forms, and we discuss possible origins for these conformational differences. Materials and Methods OEPHz was synthesized by the method of Wang and Chang." OEPHz 99% enriched in ISN was also synthesized by this method, by using IsN-enriched diethyl (hydroxyimin0)malonate as the starting material. Metal insertion and meso-position deuteration were achieved by standard methods.lz The C1-, Br-, and ClO,ligated forms of Co"'OEP+ were prepared from CdloEP by using dilute CH2Cl2solutions of Clz, dilute CH2C12solutions of Br2, and solid Fe111(C104-)3 as oxidants.lob Alternatively, Co1I1OEP'+2C1O4-could be prepared fram C O ~ ~ ~ O E P ' by +~B~the addition of AgCIO,, followed by removal of the resulting AgBr precipitate by centrifugation. Likewise Co1I1OEP'+2BF
