EPR and magnetic properties of heteronuclear Mn(n)Mg(6-)(n)(O(2)CNEt(2))(12): impact of structural distortions on Mn(II) in weak ligand fields.

The reaction between Mn(6)L(12) and Mg(6)L(12) (L = N,N-diethylcarbamate) results in isolation of heteronuclear complexes Mn(n)Mg(6)(-)(n)L(12). 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 MnMg(5)L(12) is isostructural with Mn(6)L(12) and Mg(6)L(12). Magnetic susceptibility data on the series Mn(n)Mg(6)(-)(n)L(12) (n = 1-6) are consistent with antiferromagnetic Mn.Mn interactions. At low n, the magnetic data demonstrate the formation of magnetically isolated Mn(2+) 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 Mn(2+). The EPR spectrum of Mn(0.06)Mg(5.94)L(12) 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 Mn(2+) 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 Mn(2+) ion with D > 0.3 cm(-)(1) and E/D = 0.33. The wide series is assigned to an axial C(4)(v) pentacoordinate Mn(2+) 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)Mg(6)(-)(n)L(12) to mononuclear Mn(4+) 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.