EPR and Magnetic Properties of Heteronuclear Mn n Mg6- n (O2CNEt2)12:  Impact of Structural Distortions on Mn(II) in Weak Ligand Fields

The reaction between Mn6L12 and Mg6L12 (L = N,N-diethylcarbamate) results in isolation of heteronuclear complexes Mn n Mg6 - n L12. A series was prepared with different doping factors n by varying the Mn/Mg ratio in the crystallization solutions. Single-crystal X-ray diffraction shows that MnMg5L12 is isostructural with Mn6L12 and Mg6L12. Magnetic susceptibility data on the series Mn n Mg6 - n L12 (n = 1−6) are consistent with antiferromagnetic Mn···Mn interactions. At low n, the magnetic data demonstrate the formation of magnetically isolated Mn2+ centers. This was confirmed by measurement of the EPR spectrum at a doping factor n = 0.06 in solution, as a powder, and as single crystals. These show hyperfine interactions consistent with isolated Mn2+. The EPR spectrum of Mn0.06Mg5.94L12 exhibits a dominant signal at g eff = 4, and a wide series of less intense signals spanning 200−6000 G in the X-band regime. This unusual behavior in a weak-field Mn2+ complex is attributed to the substantial distortions from cubic ligand field geometry in this system. The g eff = 4 signals are attributed to a C 2-symmetric hexacoordinate Mn2+ ion with D > 0.3 cm-1 and E/D = 0.33. The wide series is assigned to an axial C 4 v pentacoordinate Mn2+ site with D = 0.05 cm-1. Comparison of the g eff = 4 signals to the g = 4.1 signals exhibited by the tetramanganese complex in photosystem II belies the fact that they almost certainly arise from different spin systems. In addition, the similarity of the spectrum of Mn n Mg6 - n L12 to mononuclear Mn4+ complexes suggests that considerable care must be exercised in the use of EPR as a fingerprint for the manganese oxidation state, particularly in manganese proteins where molecular composition may not be precisely established.