10432
J. Am. Chem. SOC.1992, 114, 10432-10440
Ground-State Electronic Structure of the Dimer-of-Dimers Complex [ (Mn202)2(t ~ h p n ) ~ ] ~Potential +: Relevance to the Photosystem I1 Water Oxidation Catalyst Martin L. Kirk,? Michael K. Chan,*William H. Armstrong,***,$ and Edward I. Solomon**+ Contribution from the Department of Chemistry, Stanford University, Stanford, California 94304, and the Department of Chemistry, University of California, Berkeley, California 94720. Received June 3, 1992
Abstract: The tetrameric dimer-of-dimerscomplex [(Mn202)2(tphpn)2]4+ possesses structural features and a parallel polarization EPR spectrum which are similar to those observed for the SIstate of the photosystem I1 water oxidation catalyst. We have
by magnetic susceptibility and isothermal saturation probed the ground-state electronic structure of [ (Mn202)2(tphpn)2]4+ magnetization measurements to understand the origin of the EPR spectrum in terms of the intra- and interdimer magnetic exchange interactions and the ground-state zero-field splitting. Both [Mnll'Mn'V] dimers are treated as having an effective spin S'= which are coupled (J' = +38.8 cm-I) to yield a zero-field split (D= +1.8 cm-I, E = -0.15 cm-l) triplet ground state. The fictitious J'has been related by vector algebra methods to the real J that describes the exchange through the alkoxide This treatment shows that the effective ferromagnetic interaction between oxygen atoms of the tphpn ligands (J' = -4Jalkoxidc). the two S' = cores has its origin in an alkoxide mediated antiferromagnetic exchange interaction. The difference in the magnitude of the singlet-triplet splitting obtained via the effective model and that obtained using a model describing the full exchange symmetry of the tetramer is on the order of 1%. It has been determined that the zero-field splitting is not of single ion or dipolar origin but results from pseudodipolar coupling. We have shown that the electronic structure of [(Mn202)2(tphpn)2]4' parallels the geometric structure of the complex, and these results are presented in light of their relevance to the manganese water oxidation catalyst.
rhombic S = 1 spin-Hamiltonian with g = 2, and zero-field Introduction splitting (ZFS) parameters of D = 0.125 cm-'and E = 0.025 The active site for water oxidation in photosystem I1 (PSII) The approach we and others* are taking to further elucidate consists of a polynuclear manganese aggregate that has been the structure of the PSII polynuclear manganese water oxidation studied extensively with physical techniques such as magnetic catalyst (MnWOC) involves the synthesis and characterization susceptibility, electron paramagnetic resonance (EPR), X-ray of plausible small molecule analogs. In recent years these synthetic absorption (XAS), and electron spin echo envelope modulation efforts have provided a wide variety of polynuclear manganese (ESEEM) spectroscopies. The water oxidation catalyst (WOC) oxo aggregates of potential relevance to the biological system. undergoes four sequential one-electron oxidations before the Compounds containing from two to twelve manganese atoms have liberation of molecular oxygen and regeneration of the catalyst.' These five oxidation levels are known as the Kok S states2 and are labeled S0-S4. Although the actual structure of the water (1) (a) Joliet, P.; Barbieri, G.; Chabaud, R. Photochem. Phorobiol. 1970, oxidation catalyst is not yet defined, spectral data place limitations 10, 309-329. (b) Joliet, P.; Joliet, A,; Bouges, B.; Barbieri, G. Phorochem. on structures that may be proposed for some of the S states. Photobid. 1971, 14, 287-305. Specifically, the extended fine structure region of the XAS (2)Kok, B.; Forbush, B..; McGloin, M. Phorochem. Phorobiol. 1970,II, 457-475. spectrum clearly indicates that there are at least two -2.7 8, (3)(a) Renger, G. Angew. Chem., Inr. Ed. Engl. 1989,26,643-660. (b) Mn...Mn interactions for the S2state.3-5 A longer M w M n Babcock, G. T. In New Comprehensive Biochemistry: Photosynthesis: Aminteraction at -3.3 8,has also been proposed from the EXAFS esz, J., Ed.; Elsevier: Amsterdam, 1987;pp 125-158. (c) Govindjee; Kamalthough this may result from a Mn-Ca interaction. XAS bara, T.; Coleman, W. Photochem. Phorobiol. 1985,42, 187-210. (d) Dismukes, G. Photochem. Phorobiol. 1986,43,99-115.(e) Brudvig, G. W. In studies in the edge and pre-edge region support an oxidation state Metal Clusters in Proleins; Que, L., Jr., Ed.; ACS Symposium Series 372; assignment of Mn(III,IV,IV,IV) for the manganese aggregate at American Chemical Society: Washington, DC, 1988;pp 221-237. (f) Babthe S2 level.sb The principle EPR signals associated with the cock, G. T.; Barry, B. A,; Debus, R. J.; Hoganson, C. W.; Atamian, M.; manganese site in functional PSII include, for the Kok S2state, McIntosh, L.; Sithole, I.; Yocum, C. F. Biochemistry 1989,28,9557-9565. (4)(a) George, G. N.; Prince, R. C.; Cramer, S.P. Science 1989,243, a 'multiline" signal (at least 19 hyperfine components) centered 789-791. (b) Kirby, J. A.; Robertson, A. S.; Smith, J. P.; Thompson, A. C.; at g = 2 and a second lower field signal at g = 4.lS3Fine structure Cooper, S. R.; Klein, M. P. J. A m . Chem. SOC.1981,103, 5529-5537. (c) on the g = 4.1 signal, recently detected by Kim et aL6 indicates McDermott, A. E.; Yachandra, V. K.; Guiles, R. D.; Cole, J. L.; Dexheimer, that it is also associated with a polynuclear center. The g = 2 S. L.; Britt, R. D.; Sauer, K.; Klein, M. P. Biochemistry 1988,27,4021-4031. ( 5 ) (a) Guiles, R. D.; Zimmermann, J.-L.; McDermott, A. E.; Yachandra, and g = 4.1 EPR signals can be interconverted and have been V. K.; Cole, J. L.; Dexheimer, S. L.; Britt, R. D.; Weighardt, K.; Bossek, U.; interpreted in terms of an odd-electron configuration. When the Sauer, K.; Klein, M. P. Biochemistry 1990,29,471-485. (b) Guiles, R. D.; manganese aggregate is at the SIlevel, an even-electron configYachandra, V. K.; McDermott, A. E.; DeRose, V. J.; Zimmermann, J.-L.; uration should then be present. X-ray edge studies a t SIare Sauer, K.; Klein, M. P. In Current Research in Photosynrhesis; Battscheffsky, M., Ed.; Kluwer Academic Publishers: Netherlands, 1990; Vol. 1, pp consistent with a Mn(III,IV,III,IV) oxidation state although a Mn(III,III,III,III) oxidation state cannot be ruled 0 ~ t . The ~ ~ * ~ 789-792. (c) Sauer, K.; Yachandra, V. K.; Britt, R. D.; Klein, M. P. In Manganese Redox Enzymes; Pecoraro, V. L., Ed.; VCH Publishers: New EXAFS data show no discernible structural change occurs between York, 1992;pp 141-175. SIand S2.5b9CDexheimer and Klein have recently detected a (6)Kim, D. H.; Britt, R. D.; Klein, M. P.; Sauer, K. Biochemistry 1992, 31, 541-547. parallel polarization mode EPR signal associated with the man(7)(a) Dexheimer, S.L.; Klein, M. P.;Sauer, K. In Current Research in ganese active site at the SIoxidation level.' The broad derivative Photosynthesis; Battscheffsky, M., Ed.; Kluwer Academic Publishers: signal, which is centered at g = 4.8, has been simulated using a Netherlands, 1990;Vol. 1, pp 761-764. (b) Dexhcimer, S.L.; Klein, M. P. ~
Stanford University. *Universityof California. 4 Current address: Department of Chemistry, Boston College, Chestnut Hill, MA 02167.
0002-7863/92/1514-10432$03.00/0
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J. Am. Chem. SOC.1992, 114, 2821-2826. (8) (a) Pecoraro, V. L. Phorochem. Phorobiol. 1988,48, 249-246. (b) Christou, G. Acc. Chem. Res. 1989,22,328-335. (c) Weighardt, K.Angew. Chem., Int. Ed. Engl. 1989,28, 1153-1 172. (d) Brudvig, G. W.; Crabtree, R. H. Prog. Inorg. Chem. 1989,37, 99-142.
0 1992 American Chemical Society
J. Am. Chem. SOC.,Vol. 114, No. 26, 1992 10433
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wFigure 1. Structure of the dimer-of-dimers complex [ (Mn202)2( t ~ h p n ) ~ ]with ~ ' thermal ellipsoids at 50% probability. Hydrogen atoms have been omitted for clarity. Primed and unprimed atoms are related by inversion symmetry.
been isolated.8b Complexes containing the {Mn2(p-O),)"+core, many of which have been reported: have Mn...Mn distances of approximately 2.7 A and as such this core can be thought of as a substructure of the MnWOC. In the n = 3 core oxidation state (i.e., Mnl[*Mnl") these binuclear cores are strongly antiferromagnetically exchange coupled (J = -1 18 to -159 cm-I; 7f = -2Jsl's2for SI(Mn(1Il)) = and S2(Mn(IV)) = 3/2) producing a s,t = ground state which typically displays g = 2 X-band EPR signals with 16 principal hyperfine component^.^ Qualitatively, the multiline EPR signature for (Mn202)3+species is similar to that of the MnWOC at the S2oxidation level; however, the latter usually has several more lines and is markedly broader. Thus, {Mn202)binuclear species can be thought of as first generation models for the MnWOC. However, based on the EPR spectral data it is unlikely that the biological aggregate is composed of isolated (Mn202Jsubunits. One configuration of manganese ions that is consistent with the EXAFS results at the SIand S2oxidation levels is a dimerof-dimers1° structure consisting of two tethered {Mn202)cores.5 It was conjectured that the heptadentate ligand tphpn (N,N,(9) (a) Hagen, K. S.;Armstrong, W. H.; Hope, H. Inorg. Chem. 1988,27, 967-969. (b) Plaskin, P. M.; Stoufer, R. C.; Mathew, M.; Palenik, G. J. J . Am. Chem. Soc. 1972, 94, 2121-2122. (c) Stebler, M.; Ludi, A.; Burgi, H.-B. Inorg. Chem. 1986, 25, 4743-4750. (d) Collins, M. A,; Hodgson, D. J.; Michelsen, K.; Towle, D. K. J . Chem. SOC.,Chem. Commun. 1987, 1659-1660. (e) Towle, D. K.; Botsford, C. A.; Hodgson, D. J. Inorg. Chim. Acto 1988,141, 167-168. (f) Suzuki, M.; Tokura, S.;Suhara, M.; Uehara, A. Chem. Lett. 1988,477-480. (8) Hoof, D. L.; Tisley, K. G.; Walton, R. A. Inorg. Nucl. Chem. Lett. 1973, 9, 571-576. (h) Goodson, P. A,; Hodgson, D. J . Inorg. Chem. 1989, 28, 3606-3608. (i) Libby, E.; Webb, R. J.; Streib, W.E.; Folting, K.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 1989, 28, 4037-4040. (j)Brewer, K. J.; Liegeois, A.; Otvos, J. W.; Calvin, M.; Spreer, L. 0. J . Chem. Soc., Chem. Commun. 1988, 1219-1220. (k) Suzuki, M.; Senda, H.; Kobayashi, Y.; Oshio, H.; Uehara, A. Chem. Lett. 1988, 1763-1766. (I) Goodson, P. A.; Glerup, J.; Hodgson, D. J.; Michelsen, K.; Pedersen, E. Inorg. Chem. 1990, 29, 503-508. (m) Cooper, S.;Calvin, M. J . Am. Chem. SOC.1977, 99,6623-6630. (n) Brewer, K. J.; Calvin, M.; Lumakin. R. S.: Otvos. J. W.: Sareer. L. 0. Inorp. Chem. 1989. 28. 444614451, Goodson, P. A.; Hodgso;, D. J.;Michelsen, k.Inorg. Chim. Acto 1990. 172. 49-52. (10) The term dimer-of-dimers is used throughout the manuscript to refer to two dimers, each strongly exchange coupled, with a weak exchange coupling between them. ~
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N',N'-tetra(2-methylpyridyl)-2-hydroxypropanediamine) is well set up to promote the formation of such a target structure. After characterizing two structurally distinct tetranuclear species," we isolated a complex with the {Mn2O2):+ core, as reported recently.I2 The structure of [(Mn202)2(tphpn)2]4+ is shown in Figure 1 and consists of two (M~I,O,)~+ core units bridged by the alkoxide oxygens of the tphpn ligand such that a Mn(II1) is linked to a Mn(1V) of an opposite dimer.I3 This complex displays a parallel polarization EPR signal that closely resembles that associated with the SIstate of the MnWOC in PSII.7912In this study, we evaluate this model in the context of what is known about the MnWOC, describe the origin of the parallel polarization EPR resonance, and, in general, develop an overall electronic structure description of [(Mn2o2)2(tPhPn)2J4+. (11) (a) Chan, M. K.; Armstrong, W. H. J . Am. Chem. SOC.1989, 111, 9121-9122. (b) Chan, M. K.; Armstrong, W. H. J . Am. Chem. SOC.1990, 112, 4985-4986. (12) (a) Chan, M. K.; Armstrong, W. H. J . Am. Chem. SOC.1991, 113, 5055-5057. (b) Chan, M. K. Ph.D. Dissertation, University of California, Berkeley, CA, 1991. (13) The [(Mn202)2(tphpn)2]4'cation resides on a crystallographic inversion center (monoclinic space group P2,lc). The two (Mn2(p-O),}core units possess bond distances and angles which are nearly identical to those found in the structurally characterized di-p-oxo dimer complexes (see ref 9). The crystal structure shows an elongation along the Mn"'(l)-N(l) and Mn11'(l)-0(3),lt,,id, bonds with respect to the in-plane bond distances. As a result of this structural distortion and the dominant ligand field created by the in-plane oxo ligands, the ground state of the Mn(1II) ion in this complex should be dX2-,,2(hole formalism) with the d,2 orbital singly occupied. This situation is the same as that found for Mn(II1) in the isolated di-p-oxo dimer compounds.
Kirk et al.
10434 J. Am. Chem. Soc., Vol. 114, No. 26, 1992 Experimental Section
s,=1/2
Preparation of the heptadentate ligand tphpn and [ ( M n 2 0 & (tphpn)2(H20)(CF3S03)2]3+ have been described elsewhere.Il The title was prepared complex [(Mn202)2(tphpn)2](C104),~3H20~2CH,COCH, as previously describedi2 by vapor diffusion of acetone into an acetonitrile solution of [Mn402(tphpn)2(H20)2(CF3S03)2]3+ followed by recrystallization from MeCN/acetone. Magnetic susceptibility data were collected in the temperature range 2.0-300 K and in applied magnetic fields up to 54 kGauss with the use of a Quantum Design Model M P M S SQUID magnetometer. Mercury tetrathiocyanatocobaItate(I1) and a palladium cylinder were employed as duel magnetometer calibrants. Pascal's constants were used to determine the constituent atom d i a m a g n e t i ~ m . ' ~A 17.01-mg sample of powdered [(Mn202)2(tphpn)2] was contained in the small half of a gelatin capsule and capped with a cotton plug. A phenolic guide (clear soda straw) was used to house the sample holder and was fixed to the end of the magnetometer drive rod. The measured diamagnetism of the entire sample holder assembly was approximately 25% of the measured magnetization at 300 K and 3 kOe. Standard library routines for matrix diagonalization and function minimization were used in the analysis of the data.
Results The temperature dependence of the magnetic susceptibility and the effective magnetic moment are shown in Figure 2. The magnetic moment differs considerably from that observed in Mn(III)/Mn(IV) di-p-oxo complexes, and its temperature dependence can be considered in three regions. At 15 K,there is a maximum in peffwhich rapidly drops off with decreasing temperature, being characteristic of a zero-field split spin system. Between 15 and 150 K the behavior is quite complex. The moment decreases with increasing temperature from 15 K,goes through a broad minimum at -75 K, and then increases with temperature. The initial decrease is suggestive of populating a state or states with lower spin multiplicity than that of the ground state. Above 150 K the moment increases in a manner reminiscent of Mn(II1)-Mn(IV) di-p-oxo dimer complexes. Isothermal magnetization data are plotted as a function of H I T in Figure 3. Complete saturation is not achieved, even at the highest applied field and the lowest temperature. This could be due to the presence of more than one Zeeman component contributing to the Boltzmann population or to the effects of temperature independent paramagnetism. The latter phenomenon results from magnetic field induced mixing of excited-state wave functions into the ground state. The data acquired a t different temperatures are superimpsable, showing no evidence for nesting. This behavior is characteristic of the presence of small substate splittings and negligible field induced mixing between thermally populated spin states.
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v sb=1/2 Figure 4. Schematic representation of the dimer-of-dimers effective exchange formalism. If the interdimer interaction (J2)is assumed to be a t least an order of magnitude smaller than the Mn(II1,IV) di-p-oxo intradimer interaction ( J , ) , the system may effectively be treated as a weakly interacting Sa = Sb = spin system with a singlet-triplet splitting of 2J'.
(tphpn),](ClO,), may effectively be treated as a weakly interacting Sa = Sb = l/2 spin system with a singlet-triplet splitting of 25'. This situation is depicted schematically in Figure 4. We note that in the following section this model is evaluated by solving the complete exchange problem involving both the intradimer (JJ and interdimer (J2)exchange interactions. We use primes to denote spin-Hamiltonian parameters within the effective scheme. Two equivalent approaches may be taken at this point. One may perform operations on the coupled (S,ml basis or the uncoupled ( S amsa,SbmSb(basis. Although the two are related by Clebsch-Gordon coefficients such that (1111 = (1901 = m
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it is convenient to work within the uncoupled basis when orientational averaging is considered and the strong exchange limit (J Analysis >> D g@H)cannot be assumed a priori (Le., the total spin is Effective Exchange Formalism. The dimer cores of not a good quantum number). The spin-Hamiltonian (1) [(Mn202)2(tphpn)2] (C104)4are essentially structurally identical 7f = 2J'x(Sax*Sbx) - 2J;(Say6b,) - 2J'z(S,z.Sbz) + to those found in isolated Mn(III)/Mn(IV) di-p-oxo dimer complexes.9J3 All known Mn(III/IV) di-p-oxo dimer complexes are g:bH'(Sax + S b x ) + g',@H'(Say+ Shy) + g'#H'(Saz + Sbz) strongly antiferromagnetically exchange coupled with doublet(1) quartet splittings (3JJ of approximately 360-480 ~ m - ' .Reduced ~ was used to operate: on the uncoupled basis functions. An ancoupling has been observed in the Mn(II1)-Mn(IV) alkoxideisotropic exchange HamiltonianI6 was used in order to account bridged dimer complex [Mn"'Mn*V[2-OH-3,5-C12-(SALPN)]2for any zero-field splitting (ZFS) of the triplet state. In order (THF)]C1O,,l5 which possesses a 3.65 A Mn-Mn distance and to avoid problems associated with overparameterization during a Jvalue of -10 cm-l. Therefore, the interdimer interaction ( J 2 ) function minimization, the g tensor has been assumed to be isoin [(Mn202)2(tphpn)2](C104)4 should be at least an order of tropic (g: = g; = g:). Diagonalization of the resultant matrix magnitude smaller than that within the di-p-oxo cores (JI)since yielded four energy eigenvalues which were substituted into the the former derives from a long (3.96 A) MnalkoxideMn bridge. thermodynamic expression for the magnetic susceptibility ( 2 ) Due to the large exchange coupling observed in di-p-oxo bridged state is populated at low temperatures. dimers, only the S = In this limit, the dimer-of-dimers complex [ N xcoso = -H (14) (a) Figgis, B. N.; Lewis, J. In Modern Coordination Chemistry; Lewis, J.; Wilkins, R. G. Eds.; Interscience: New York, 1960; Chapter 6, p 403. (b) Konig, E. Magnetic Properties of Transition Metal Compounds; Springer-Verlag: West Berlin, 1966. (c) Weller, R. R.; Hatfield, W. E. J. Chem. Ed. 1979, 56, 652. (15) Larson, E.; Haddy, A.; Kirk, M. L.; Sands, R. H.; Hatfield, W. E.; Pecoraro, V. L. J. Am. Chem. SOC.1992, 114, 6263-6265.
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(1 6) Abragam, A.; Bleaney, B. In Electron Paramagnetic Resonance of Transition Ions; Dover Publications, Inc.: New York, 1986; pp 521-535.
J . Am. Chem. SOC.,Vol. 114, No. 26, 1992 10435 h
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where xaaeis the molar magnetic susceptibility with the magnetic field direction a t an angle 6 with respect to the principle axis of the molecule. The partial derivatives with respect to the applied magnetic field, d E / d H , were calculated using the HellmanFeynman theorem.17 This method utilizes the eigenvectors obtained directly from the matrix diagonalization to determine d E / d H exactly. Numerical methods for determining this partial derivative are approximate and require function evaluation at two additional magnetic fields about the actual applied field. A numerical integration technique utilizing the Labatto quadratureI8 was employed in the calculation of the powder average molar magnetic su~ceptibility~~ (3) The advantage of this method over the more familiar Gaussian quadrature is the ability to evaluate the function a t the limits of the integral, allowing for the calculation of the parallel and perpendicular components of the magnetic susceptibility. The above formalism assumes only S = states on each dimer interact and thus best describes the low-temperature data since the S 1 3 / 2 states on each dimer are not populated. Therefore, the data in the temperature range 2-40 K (Figure 2 ) were fit with these formulae, and the resulting best-fit curve is shown in Figure 5 . The theoretical curve through the experimental data points was obtained using the parameters J: = +39.6, J $ = +39.3, and J : = +37.6 cm-I, while the g value was not varied and held constant a t 2.0. The anisotropic exchange parameters may be recast into the more familiar isotropic exchange parameter, J', and zerefield splitting parameters, D'and E', using the following relations J' = X ( J : J',, + J r Z ) (4)
+
The effective exchange coupling between the two Sa = Sb= centers is found to be ferromagnetic with a singlet-triplet splitting (25') of +77.7 cm-I. The ground-state triplet is zero-field split with D ' = +1.8 and E ' = -0.15 cm-I. In order to confirm the sign and magnitude of the ground-state zero-field splitting, the parameters obtained from the temperature dependent susceptibility fit were used to model the isothermal (17) Vermaas, A.; Groeneveld, W. L. Chem. Phys. Lerr. 1974, 27, 583-585. ( 1 8) Scarborough, J. P. Numerical Mafhemaricol Analysis; Oxford University Press: New York, 1971; p 159. (19) Marathe, V. R.; Mitra, S. Chem. Phys. Lerr. 1974, 27, 103-106.
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Figure 7. A complete representation of the interactions within the tetramer. The individual Mn centers are labeled by their spins, Si, and the exchange constants, J, and J2,which describe exchange interactions mediated through the oxo ligands and the alkoxide oxygens, respectively.
magnetization data. Since these data were acquired a t temperatures between 2 and 10 K, they are particularly sensitive to the energy level splitting of the triplet ground state. The agreement between theory and experiment is excellent, as evident in Figure 6, and provides solid evidence for the < 1 ,Ol component of the triplet lying lowest in energy in zero magnetic field (Le., D' > 0). There exists an alternative description of the low-temperature (
