GROUND-STATE VARIABILITY IN MU-3-OXIDE TRINUCLEAR MIXED-VALENCE MANGANESE COMPLEXES - SPIN FRUSTRATION

A reexamination of bulk magnetic susceptibility data supplemented by variable-field magnetization and EPR studies definitively established the ground state of the mixed-valence complex [Mn(II)Mn(III)2O(O2CCH3)6(py)3](py) (1) as having S = 3/2, in contrast to the S = 1/2 ground state reported previously. This represents a rare example of a triangular mu-3-oxide metal complex characterized as having an intermediate spin ground state (i.e., ground-state spin value other than the minimum available to the system). Variable-temperature susceptibility data measured at 10.00 kG were fit to a Heisenberg-Dirac-Van Vleck expression with g = 2.05, J = -5.2 cm-1, and J* = -2.7 cm-1, where J characterizes the Mn(II)...Mn(III) interaction and J* the Mn(III)...Mn(III) interaction. Variable-field magnetization data measured at 10.00, 30.00, and 48.00 kG in the range from 1.83 to 20.00 K were fit by full-matrix diagonalization to a spin Hamiltonian appropriate for a S = 3/2 state under the influence of axial zero-field splitting of the form H = DS(z)2. A fit was found for g = 1.98 and D = 3.1 cm-1. X-band EPR spectra measured from 2 to 30 K on a polycrystalline sample of 1 show several transitions in the g almost-equal-to 2 and g almost-equal-to 4 regions; the g almost-equal-to 4 signal decreases in intensity with increasing temperature, consistent with the assignment of a S = 3/2 ground state. Susceptibility data were also reanalyzed for the related mixed-valence complex [Mn(II)Mn(III)2O(O2CPh)6(py)2(H2O)].1/2(CH3CN) (2). On the basis of susceptibility data determined at 10.00 kG and X-band EPR spectra from 4 to 31 K, the ground state for complex 2 is assigned as S = 1/2. Fitting parameters for complex 2 were found to be g = 1.99, J = -6.5 cm-1, and J* = -4.5 cm-1. The results for both complexes are discussed in terms of spin frustration within the Mn3O core. In addition, the observation of multiple EPR-active states is discussed relative to the origin of EPR transitions from the oxygen-evolving complex of photosystem II.