Photosynthetic Water Oxidation Center: Spin Frustration in Distorted Cubane Mn IV Mn III 3 Model Complexes

David N. Hendrickson, Edward A. Schmitt, Hui-Lien 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 Mn IV Mn3 III complexes have been prepared as model complexes for the S 2 state of the water oxidation center (WOC) in photosystem II. All of these complexes are prepared by the reaction of a μ 3 -oxide Mn 3 III complex with Me 3 SiCl which leads to a disproportionation to give the Mn IV Mn 3 III complex and an Mn II product. The reaction of Mn(O 2 CCH 3 ) 3 · 2 H 2 O with Me 3 SiCl followed by addition of imidazole gives (H 2 Im) 2 [Mn 4 O 3 Cl 6 (O 2 CCH 3) 3 (HIm)]·3/2CH 3 CN (1) where H2Im + is the imidazolium cation. Reaction of [Mn 3 O(O 2 CCH 3 ) 6 (py) 3 ](ClO 4 ) or [Mn 3 O(O 2 CCH 2 CH 3 ) 6 (py) 3 ](ClO 4 ) with Me 3 SiCl leads, respectively, to [Mn 4 O 3 Cl 4 (O 2 CCH 3 ) 3 (py) 3 ]·3/2CH 3 CN (2) and [Mn 4 O 3 Cl 4 (O 2 CCH 2 CH 3 ) 3 (py) 3 ]·5/2CH 3 CN (4). A similar procedure as for 2 but followed by addition of imidazole yields [Mn 4 O 3 Cl 4 (O 2 CCH 3 ) 3 (HIm) 3 ]·5/2CH 3 CN (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 Å 3 , and Z = 8. A total of 2513 unique data with F > 2.33σ(F) were refined to values of R and R w of 8.10 and 8.70%, respectively. The central [Mn 43 -O) 33 -Cl)] 6+ core of the anion in complex 1. consists of a Mn 4 pyramid with the MnIV ion at the apex, a μ 3 -Cl - ion bridging the basal plane, and a μ 3 -O 2 ion bridging each of the remaining three faces. The Mn IV ion has six oxygen atom ligands, three from the three μ 3 -O 2- ions and three from the bridging acetates. Two of the Mn III ions have Mn(Cl) 23 -Cl)(μ 3 -O) 2 (μ-O 2 CCH 3 ) coordination spheres; the third Mn III ion has one of the terminal Cl - ligands replaced by an imidazole ligand. The complex [Mn 4 O 3 Cl 4 (O 2 CCH 3 ) 3 (py) 33 / 2 CH 3 CN (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 Å 3 , and Z = 2. A total of 1458 unique data with F > 3.0σ-(F) were refined to values of R and R w of 3.71 and 4.17%, respectively. The Mn IV Mn 3 III 3 Cl core in complex 2 is essentially superimposable with that of complex 1. Complex 2 has crystallographically imposed C3 symmetry. The other two complexes, [Mn 4 O 3 Cl 4 (O 2 CCH 2 CH 3 ) 3 (py)• 3 ]•5/2CH 3 CN (4) and [Mn 4 O 3 Cl 4 (O 2 CCH 3 ) 3 (HIm) 3 ]•5/2CH 3 CN (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 R w 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 Å 3 , 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 Mn IV Mn III O 3 Cl core of these complexes is compared with the core of S 1 -state model complexes which have the Mn 4 III3 -O) 2 butterfly structure. It is suggested that increasing the oxidation state from S 1 to S 2 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 Mn IV Mn 3 III 3 Cl cubane complexes. No reversible waves were observed in the electrochemistry of these complexes. However, 1 H NMR and Beers law dependence studies show that complex 1 remains intact in DMF as do complexes 2 and 4 in CH 2 Cl 2 and CHCl 3 . Magnetic susceptibility data are presented for complexes 1, 2, and 4 at 10.0 kG in the 5-300 K range. The value of μ eff /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 μ eff /molecule drops relatively abruptly. The data were fit to a theoretical model to give exchange parameters J 34 (Mn IV …Mn III ) of -20.8 to -30.3 cm -1 and J 33 of +8.6 to 11.3 cm -1 . The ground state for all complexes is a well-isolated S T = 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 (DŜ Z 2 ) interactions to verify a S T = 9/2 ground state with D e≃ +0.3 cm -1 . The nature of the spin frustration in these Mn IV Mn 3 III O 3 Cl 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 S T = 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 Mn IV Mn 3 III O 3 Cl cubane complexes is discussed with the goal of modeling the S 2 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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