Photosynthetic Water Oxidation Center: Spin Frustration in Distorted Cubane MnIVMnIII3 Model Complexes

David N. Hendrickson, Edward A. Schmitt, Huilien Tsai, Qiaoying Li, John S. Bashkin, John B. Vincent, George Christou, Eduardo Libby, Sheyi Wang, Peter D.W. Boyd, John C. Huffman, Kirsten Folting

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Abstract

“Four MnIVMn3III complexes have been prepared as model complexes for the S2 state of the water oxidation center (WOC) in photosystem II. All of these complexes are prepared by the reaction of a μ3-oxide Mn3III complex with Me3SiCl which leads to a disproportionation to give the MnIVMn3III complex and an MnII product. The reaction of Mn(O2CCH3)3·2H2O with Me3SiCl followed by addition of imidazole gives (H2Im)2[Mn4O3Cl6(O2CCH3)3(HIm)]·3/2CH3CN (1) where H2Im+ is the imidazolium cation. Reaction of [Mn3O(O2CCH3)6(py)3](ClO4) or [Mn3O(O2CCH2CH3)6(py)3](ClO4) with Me3SiCl leads, respectively, to [Mn4O3Cl4(O2CCH3)3(py)3]·3/2CH3CN (2) and [Mn4O3Cl4(O2CCH2CH3)3(py)3]·5/2CH3CN (4). A similar procedure as for 2 but followed by addition of imidazole yields [Mn4O3Cl4(O2CCH3)3(HIm)3]·5/2CH3CN (5). Complex 1 crystallizes in the orthorhombic space group Pbca with (at -158 °C) a = 14.307 (14) Å, b = 14.668 (14) Å, c = 31.319 (36) Å, V = 6572.75 Å3, and Z = 8. A total of 2513 unique data with F > 2.33σ(F) were refined to values of R and Rw of 8.10 and 8.70%, respectively. The central [Mn43-O)33-Cl)]6+ core of the anion in complex 1. consists of a Mn4 pyramid with the MnIV ion at the apex, a μ3-Cl- ion bridging the basal plane, and a μ3-O2 ion bridging each of the remaining three faces. The MnIV ion has six oxygen atom ligands, three from the three μ3-O2- ions and three from the bridging acetates. Two of the MnIII ions have Mn(Cl)23-Cl)(μ3-O)2(μ-O2CCH3) coordination spheres; the third MnIII ion has one of the terminal Cl- ligands replaced by an imidazole ligand. The complex [Mn4O3Cl4(O2CCH3)3(py)33/2CH3CN (2) crystallizes in the hexagonal space group R3 with (at -155 °C) a = b = c = 13.031 (4) Å, α = β = γ = 74.81 (2)°, V = 2015.93 Å3, and Z = 2. A total of 1458 unique data with F > 3.0σ-(F) were refined to values of R and Rw of 3.71 and 4.17%, respectively. The MnIVMn3III3Cl core in complex 2 is essentially superimposable with that of complex 1. Complex 2 has crystallographically imposed C3 symmetry. The other two complexes, [Mn4O3Cl4(O2CCH2CH3)3(py)•3]•5/2CH3CN (4) and [Mn4O3Cl4(O2CCH3)3(HIm)3]•5/2CH3CN (5), also crystallize in the R3 space group. The unit cell of complex 4 has (at -143 °C) a = b = c = 13.156 (6) Å, α=β = γ = 74.56 (3)°, γ = 2068.53 °3, and Z = 2. A total of 1425 unique data with F > 3.0σ(F) were refined to values of R and Rw of 5.265 and 5.44%, respectively. The unit cell of complex 5 has (at -145 °C) a = b = 15.656 (6) Å, c = 26.947 (9) Å, a = ft -90°, 7 = 120.0°, V = 5722.68 Å3, and Z = 6. A total of 1156 unique data with F > 3.0σ(F) was refined to values of R and Rw of 5.75 and 5.90%, respectively. The MnIVMnIIIO3Cl core of these complexes is compared with the core of S1-state model complexes which have the Mn4III3-O)2 butterfly structure. It is suggested that increasing the oxidation state from S1 to S2 state is coupled to an increase in oxide content. A strong Mn-O stretching IR band at 580-590 cm-1 is identified as characteristic of MnIVMn3III3Cl cubane complexes. No reversible waves were observed in the electrochemistry of these complexes. However, 1H NMR and Beers law dependence studies show that complex 1 remains intact in DMF as do complexes 2 and 4 in CH2Cl2 and CHCl3. Magnetic susceptibility data are presented for complexes 1, 2, and 4 at 10.0 kG in the 5-300 K range. The value of μeff/molecule at room temperature increases with decreasing temperature to give a maximum at 60 K for 1 and 2 and 15 K for 4. Below these temperatures μeff/molecule drops relatively abruptly. The data were fit to a theoretical model to give exchange parameters J34(MnIV…MnIII) of -20.8 to -30.3 cm-1 and J33 of +8.6 to 11.3 cm-1. The ground state for all complexes is a well-isolated ST = 9/2 state. This was confirmed by variable field magnetization studies: ~2-40 K at fields of 24.8, 34.5, and 44.0 kG for complex 2; ~2-15 K at fields of 10.0, 30.0v and 48.0 kG for complex 4. These data were fit by a matrix diagonalization approach with Zeeman and axial zero-field (DŜZ2) interactions to verify a ST = 9/2 ground state with D e≃ +0.3 cm-1. The nature of the spin frustration in these MnIVMn3IIIO3Cl cubane complexes is analyzed in detail. It is shown what other ground states may be possible for such a complex. Variable-temperature X-band EPR data are presented for polycrystalline and frozen glass samples of complexes 1, 2, and 4. Q-band spectra are also given for solid samples. A detailed map of expected X-band resonance fields plotted versus the axial zero-field splitting parameter is derived for a complex with ST = 9/2 ground state. The experimental EPR spectra are shown to be qualitatively in agreement with these calculated resonance fields. The electronic structure of the four MnIVMn3IIIO3Cl cubane complexes is discussed with the goal of modeling the S2 state of the WOC.

Original languageEnglish
Pages (from-to)2455-2471
Number of pages17
JournalJournal of the American Chemical Society
Volume114
Issue number7
DOIs
Publication statusPublished - 1992 Mar 1

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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