An electron paramagnetic resonance study of Mn2(H2O)(OAc)4(tmeda)2 (tmeda = N,N,N',N'-tetramethylethylenediamine): a model for dinuclear manganese enzyme active sites

The complex Mn2(H2O)(OAc)4(tmeda)2 (tmeda = N,N,N',N'-tetramethylethylenediamine) is a model for the active site of hydrolase enzymes containing acetate-bridged dimanganese cores. The two high-spin Mn(II) ions are antiferromagnetically coupled, as determined by previous magnetic susceptibility studies (Yu, S.-B; Lippard, S. J.; Shweky, I; Bino, A. Inorg. Chem. 1992, 31, 3502-3504) to yield a spin "ladder" with total spin S = 0, 1, 2, ..., 5 in increasing energy. In this study, the complex was characterized by Q-band and X-band EPR spectroscopy in frozen solution. Analysis of the temperature dependence of these EPR spectra indicates that the primary spectral contribution is from the S = 2 manifold. The EPR spectra were simulated using a full spin Hamiltonian for this manifold of a coupled spin system, which provided the fit parameters J = -2.9 cm-1, g = 2.00, and D2 = -0.060 +/- 0.003 cm-1. An additional multiline EPR signal is observed which is proposed to arise from the total spin S = 5/2 ground state of a Mn(II) trimer of the type Mn3(OAc)6(tmeda)2.