Models for the photosynthetic water oxidizing enzyme. 2. Electronic, magnetic, and EPR characterization of a binuclear manganese(II) semiquinone complex

The preparation and characterization of a tetranuclear manganese complex, Mn₄(ASQ)₄(CO)₈, and a binuclear complex into which it dissociates in solution, Mn₂(ASQ)₄L₂ (ASQ = 2-acetyl-1,4-benzosemiquinone), is described. EPR spectroscopy and variable temperature magnetic susceptibility of the solution species indicate that there are two equivalent Mn(II) ions coupled by a ferromagnetic exchange interaction, having J ≥ 23 cm^-1, fostered by two bridging semiquinones. This is evidenced by a paramagnetic ground state with a magnetic moment μ_eff = 6.78, electron spin resonance fine structure consistent with an apparent spin S = 3 and having zero-field splitting parameters of \D\ = 0.103 cm^-1 and |E| = 0.011 cm^-1, a nearly isotropic 11-line hyperfine structure for ⁵⁵Mn with coupling constant \A\ = 45 ± 3 G, and a nearly isotropci g = 2. The quenching of the magnetic moment of the dimer with increasing temperature is interpreted to arise from the ferromagnetic coupling and, near room temperature, from an internal redox equilibrium in which Mn₂(AQL)₄L₂ and possibly Mn₂-(ASQ)₂(AQL)₂L₂ (AQL = 2-acetyl-1,4-benzoquinol dianion) form. This complex is discussed in relation to its suitability as a model for the magnetic properties of the photosynthetic water-oxidizing enzyme.