Photosynthetic water oxidation center: spin frustration in distorted cubane MnIVMnIII3 model complexes

Abstract

Four MnMn complexes have been prepared as model complexes for the S state of the water oxidation center (WOC) in photosystem II. All of these complexes are prepared by the reaction of a η-oxide Mn complex with MeSiCl which leads to a disproportionation to give the MnMn complex and an Mn product. The reaction of Mn(OCCH)·-2H O with MeSiCl followed by addition of imidazole gives (HIm )[MnOCl(OCCH )(HIm)]-CHCN (1) where HIm+ is the imidazolium cation. Reaction of [MnO(OCCH)6(py)](ClO ) or [Mn(OCCH)(py) ](ClO) with MeSiCl leads, respectively, to [MnOCl(OCCH)(py )]-CHCN (2) and [MnOCl(OCCHCH (py)]-CHCN (4). A similar procedure as for 2 but followed by addition of imidazole yields [MiOCl(OCCH) (HIm)]CHCN(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, and Z = 8. A total of 2513 unique data with F > 2.33σ(f) were refined to values of R and R of 8.10 and 8.70%, respectively. The central [Mn(μ-O)(μ-Cl) core of the anion in complex 1 consists of a Ma, pyramid with the Mn ion at the apex, a μ-Cl ion bridging the basal plane, and a μ-O ion bridging each of the remaining three faces. The Mn ion has six oxygen atom ligands, three from the three μ-O ions and three from the bridging acetates. Two of the Mn ions have Mn(Cl)(μ3-Cl)(M3-O)μ-O CCH) coordination spheres; the third Mnion has one of the terminal Cl ligands replaced by an imidazole ligand. The complex (MnOCl(OCCH) (py)]-CH,CN (2) crystallizes in the hexagonal space group R3̄ with (at-155°C)a = b = c= 13.031 (4) Å, α = β= γ = 74.81 (2)°, K = 2015.93 Å, and Z = 2. A total of 1458 unique data with F > 3.0σ(f) were refined to values of R and R of 3.71 and 4.17%, respectively. The MnMn OCl core in complex 2 b essentially superimposable with that of complex 1. Complex 2 has crystallographkally imposed C3 symmetry. The other two complexes, [MnOCl(OCCHCH )(py)]-CHCN (4) and [MnOCl(O)(HIm) ]-CHCN (5), also crystallize in the R3̄ 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)°, V = 2068.53 Å, and Z = 2. A total of 1425 unique data with F > 3.0σ(F) were refined to values of R and R 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, α = β= 90°, γ = 120.0°, V = 5722.68 Å, and Z = 6. A total of 1156 unique data with F > 3.0σ(F) was refined to values of R and R of 5.75 and 5.90%, respectively. The MnMn OCl core of these complexes is compared with the core of S-State model complexes which have the Mn (μ-O) butterfly structure. It is suggested that increasing the oxidation state from S to S state is coupled to an increase in oxide content. A strong Mn-O stretching IR band at 580-590 cm is identified as characteristic of MnMn OCl cubane complexes. No reversible waves were observed in the electrochemistry of these complexes. However, H NMR and Beers law dependence studies show that complex 1 remains intact in DMF as do complexes 2 and 4 in CHCl and CHCl. 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/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 Meff/molecule drops relatively abruptly. The data were fit to a theoretical model to give exchange parameters J(Mn⋯Mn of -20.8 to -30.3 cm and J of +8.6 to 11.3 cm. The ground state for all complexes is a well-isolated S = 9/2state. 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, 30.0and 48.0 kG for complex 4. These data were fit by a matrix diagonalization approach with Zeeman and axial zero-field (DŜ) interactions to verify a S = 9/2) ground state with D ≃ +0.3 cm. The nature of the spin frustration in these MnMn OCl 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 = 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 MnMn OCl cubane complexes is discussed with the goal of modeling the S state of the WOC.