Models of the Manganese Catalase Enzymes. Dinuclear Manganese(III) Complexes with the [µn₂(µ-O)(µ-O₂CF)₂]²⁺ Core and Terminal Monodentate Ligands: Preparation and Properties of [Mn²O(O₂CR)₂X₂(bpy)₂] (X = Cl⁻, N₃⁻, H₂O)

Procedures are reported that allow access to dinuclear Mn^III complexes possessing the [Mn₂O(µ-O₂CR)2]²⁺ core. The complexes have the general formulation [Mn₂O(O₂CR)₂X₂(bpy)₂] (X = Cl⁻, N₃⁻, H₂O; bpy = 2,2′-bipyridine) and are potential models of the Mn catalase enzymes. Treatment of MnCl₂/bpy/acetic acid reaction mixtures in MeCN with NBuⁿ₄MnO₄ in MeCN leads to subsequent isolation of [Mn₂O(OAc)₂Cl₂(bpy)₂]·AcOH·H₂O (1). Analogous reactions allow the preparation of [Mn₂O(O₂CPh)₂Cl₂(bpy)₂] ·2H₂O (2) and [Mn₂O(O₂CEt)₂Cl₂(bpy)₂]·3EtCO₂H·H₂O (3). In the presence of N₃⁻, the complex [Mn₂O(O₂CPh)₂(N₃)₂(bpy)₂] (5) is obtained; use of AcO⁻ and a greater MnO₄⁻ amount yields [Mn₂O₂(N₃)₄(bpy)₂] (6). Complex 1 can also be prepared from a reaction in which a solution of Cl₂ in MeCN is employed as the oxidizing agent instead of NBuⁿ₄MnO₄. If, however, aqueous HOAc is employed as the reaction medium, oxidation with an excess of Cl₂ leads to [Mn₂O(OAc)₂(H₂O)₂(bpy)₂](ClO₄)₂ (7). The three Mn₂ units are extremely similar and differ only in the identity of the terminal ligands X (Cl⁻, N₃⁻, or H₂O). They each contain a triply-bridged [Mn₂(µ-O)(µ-O₂CR)₂]²⁺ core with chelating bpy and terminal X groups completing near-octahedral geometry at each Mn atom. In each case, the X group and an oxygen atom from a bridging RCO₂⁻ group lie on a Jahn-Teller elongation axis (high-spin d⁴ Mn^III). Complexes 1, 2, 3, and 5 have been studied by cyclic voltammetry in DMF; they each display a quasi-reversible oxidation at ∼0.4 V (1, 2, and 3) and 0.18 V (5) vs ferrocene, assigned to the 2Mn^III/Mn^IIIMn^IV couple. Variable-temperature solid-state magnetic susceptibilities of 1 and 5 were measured in the temperature range 5.0 to ca. 330 K. The effective magnetic moment per Mn₂^III (µ_eff) for 1 decreases gradually from 6.33 µ_B at 327.7 K to 5.85 µ_B at 100 K and then more steeply to 2.09 µ_B at 5.0 K. For 5, µ_eff increases steadily from 6.96 µ_B at 320 K to a maximum of 8.12 µ_B at 30 K and then decreases to 7.45 µ_B at 5.0 K. The data were fit to a model that included an isotropic Heisenberg exchange interaction, an isotropic Zeeman interaction, and axial zero-field splitting terms for both ions. For complex 1, a good fit was found with J = −4.1 cm⁻¹, g = 1.88, D₁ = D₂ = −0.07 cm⁻¹, and 0.8% by weight of a paramagnetic S = 2 impurity. For complex 5, the corresponding values are J = +8.8 cm⁻¹, g = 1.86 and D₁= D₂ = 0.3 cm⁻¹; the quality of the fit is less than that for 1, and this was concluded to be due to the presence of intermolecular exchange interactions propagated by the intermolecular hydrogen-bonding network observed in the crystal structure of 5·MeCN·4H₂O. Thus, 5 is ferromagnetically coupled and has an S = 4 ground state. The J values for all available complexes containing the [Mn₂O(O₂CR)₂]²⁺ core are compared, and a rationalization is suggested for the differences between 1/7 (negative J) and 5 (positive J). The relevance of these results to Mn catalase are discussed as well as to the observed difference in sign of the J values for deoxyhemerythrin (negative J) versus deoxy-N₃⁻-hemerythnn (positive J).