Modeling the photosynthetic water oxidation center. Preparation and properties of tetranuclear manganese complexes containing [Mn4O2]6+,7+,8+ cores, and the crystal structures of Mn4O2(O2CMe)6(bipy)2 and [Mn4O2(O2CMe)7(bipy)2](ClO4)

Abstract

An inorganic model approach to the photosynthetic water oxidation enzyme has been initiated, and synthetic entry into tetranuclear Mn complexes containing [Mn4O2]6+,7+,8+ cores has been achieved. They have been obtained by bipyridine (bipy)-mediated conversion of trinuclear [Mn3O]-containing species, with the product oxidation level governed by the exact identity of the [Mn3O] reagent employed. Treatment of Mn3O(O2CMe)6(py)3 with ∼3 equiv of bipy in MeCN yields Mn4O2(O2CMe)6(bipy)2 (1) in 91% yield. Complex 1·2CHCl3 crystallizes in triclinic space group P1 with (at -160°C) a = 13.883 (3) Å, b = 10.592 (2) Å, c = 8.848 (1) Å, α = 91.18 (1)°, β = 72.14 (1)°, γ = 71.44 (1)°, V = 1163.84 Å3, and Z = 1. A total of 3064 unique data with F > 3σ(F) were refined to values of R and Rw of 3.23 and 3.75%, respectively. The molecule lies on an inversion center and contains a planar Mn4 rhombus with two μ3-O atoms, one above and one below the Mn4 plane. The resulting [Mn4O2]6+ core is mixed valence (2MnII, 2MnIII) and can be considered as fusion of two Mn3O units by edge-sharing. Peripheral ligation is by six μ2-O2CMe and two terminal bipy groups to yield a complex with imposed Ci symmetry. Treatment of Mn3O(O2CR)6(py)2(H2O) (R = Ph, 3-Me-Ph) with ∼3 equiv of bipy in MeCN yields Mn4O2(O2CR)7(bipy)2 (R = Ph (2) or 3-Me-Ph (3)) containing MnII, 3MnIII. Similarly, treatment of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et, Ph) with ∼3 equiv of bipy in MeCN yields [Mn4O2(O2CR)7(bipy) 2](ClO4) (R = Me (4), Et (8), Ph (9)) containing 4MnIII. Use of 4,4′-Me2-bipy instead of bipy results in the corresponding complex [Mn4O2(O2CMe)7(4,4′-Me 2-bipy)2] (ClO4) (5). Complex 4 has been found to undergo facile carboxylate substitution when more acidic carboxylic acids are added; addition of PhCOOH or HCOOH to CH2Cl2 solutions of 4 yields 9 and [Mn4O2(O2CH)7(bipy)2] (ClO4) (6), respectively. Complex 9 can also be synthesized directly by reaction of NBun4MnO4 with Mn(O2CMe)2·4H2O in pyridine solution in the presence of PhCOOH, bipy, and NBun4ClO4. Complex 4 crystallizes in triclinic space group P1 with (at -155°C) a = 21.133 (11) Å, b = 11.428 (5) Å, c = 11.839 (6) Å, α = 102.12 (2)°, β = 119.72 (2)°, γ = 78.20 (2)°, V = 2410.61 Å3, and Z = 2. A total of 6294 unique data were refined to values of R and Rw of 9.05 and 8.94%, respectively. Complex 4 contains an [Mn4O2]8+ core which is not planar as found in 1 but exhibits an Mn4 butterfly arrangement with both μ3-O atoms on the same side of the molecule. Peripheral ligation is again by μ2-O2CMe and terminal bipy groups, but now there is an additional MeCO2- ligand bridging the two "hinge" Mn atoms of the butterfly to yield C2 symmetry. Complexes 1 and 4 display both "short" and "long" Mn⋯Mn separations, 2.779 (1), 3.288-3.481 (1) Å and 2.848 (5), 3.312-3.385 (5) Å, respectively. Variable-temperature, solid-state, magnetic-susceptibility studies have been performed on representative complexes 1 and 4 in the temperature range 5-300 K. The observed susceptibility data have been fitted to models involving isotropic exchange interactions between the high-spin manganese ions in the clusters. In the case of MnII2MnIII2 complex 1, fitting the data gave J13 = -3.12 cm-1 for the interaction between the two di-μ-oxo-bridged MnIII ions and J(MnII-MnIII) = -1.97 cm-1. With these parameters, complex 1 has an S = 2 ground state with six other spin states within 15 cm-1. Fitting the data for MnIII4 complex 4 gave for the di-μ-oxo-bridged MnIII pair J13 = -23.5 cm-1, and for the other MnIII-MnIII interaction J = -7.8 cm-1. The ground state for 4 has S = 3 with two lowest lying excited states being two energetically degenerate S = 2 states at ∼15 cm-1 above the S = 3 ground state. The nature of the ground and low-lying spin states for 1 and 4 were confirmed by using magnetization measurements at fields up to 48 kG and temperatures down to 1.8 K. The change in the magnitude of spin-spin interaction between the two central di-μ-oxo-bridged MnIII ions in 1 and 4 can be related to the differing single-ion coordination in these two complexes. The electronic difference spectrum of the [Mn4O2(O2CPh)7(bipy) 2]0,+ pair has been found to be extremely similar to the S0 → S1 difference spectrum of the water oxidation site, suggesting the latter involves a MnII → MnIII transition also. Complex 4 in CDCl3 has been found to display a well-resolved 1H NMR spectrum in which all expected proton resonances have been located and assigned. Both σand π spin-delocalization mechanisms appear to be operative. The combined results of this work are discussed with respect to their biological relevance, and it is proposed that complexes containing the [Mn4O2]6+,7+,8+ cores represent potential synthetic analogues of the S-1, S0, and S1 states, respectively, of the water oxidation enzyme.