Consistent simulation of X- and Q-band EPR spectra of an unsymmetric dinuclear Mn₂^{II, III} complex

Simulation of X- and Q-band electron paramagnetic resonance (EPR) spectra of an unsymmetric dinuclear [ Mn₂^{II, III} L ( μ - OAc )₂ ] ClO₄ complex (1), (L is the dianion of 2-{[N,N-bis(2-pyridylmethyl)amino]methyl}-6-{[N-(3,5-di-tert-butyl-2-hyd roxybenzyl)-N-(2-pyridylmethyl)amino]methyl}-4-methylphenol) was performed using one consistent set of simulation parameters. Rhombic g-tensors and hyperfine tensors were necessary to obtain satisfactory simulation of the EPR spectra. The anisotropy of the effective hyperfine tensors of each individual ⁵⁵Mn ion was further analyzed in terms of intrinsic hyperfine tensors. Detailed analysis shows that the hyperfine anisotropy of the Mn^III ion is a result of the Jahn-Teller effect and thus an inherent character. In contrast, the anomalous hyperfine anisotropy of the Mn^II ion is attributed as being transferred from the Mn^III ion through the spin exchange interaction. The anisotropy parameter for the Mn^II is deduced as D_II = -1.26 ± 0.2 cm^-1. This is the first reported D_II value for a Mn^II ion in a weakly exchange coupled mixed-valence Mn₂^{II, III} complex with a bis-μ-acetato-bridge. The d_xy¹ d_yz¹ d_zx¹ d x² - y²¹ electronic configuration of the Mn^III ion in 1 is revealed by the negative sign of its intrinsic hyperfine tensor anisotropy, Δa_III = a_z - a_x,y = -46 cm^-1. Lower spectral resolution of the Q-band EPR spectrum as compared to the X-band EPR spectrum is associated to large line width broadening of the x- and y-components in contrast to the z-component. The origins of the unequal distribution of line width between the z- and x-, y-components are discussed.