Single Crystal X- and Q-Band EPR Spectroscopy of a Binuclear Mn₂(III,IV) Complex Relevant to the Oxygen-Evolving Complex of Photosystem II

The anisotropic g and hyperfine tensors of the Mn di-μ-oxo complex, [Mn₂(III,IV)O₂(phen)₄](PF₆)₃·CH₃CN, were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn₂(III,IV)O₂(phen)₄](PF₆)₃·CH₃CN EPR spectra showed distinct resolved ⁵⁵Mn hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn₂(III,IV) di-μ-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystal EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn₂(III,IV)O₂(phen)₄ as shown in the crystallographic data by Stebler et al. (Inorg. Chem. 1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: g_x = 1.9887, g_y = 1.9957, g_z = 1.9775, and hyperfine coupling constants are A^III_x = |171| G, A^III_y = |176| G, A^III_z = |129| G, A^IV_x = |77| G, A^IV_y = |74| G, A^IV_z = |80| G.