Characterization of high spin molecular magnets

This dissertation describes the characterization of magnetic sandwich-type transition metal substituted polyoxotungstates and large Fe 3+ containing clusters with large total spins. A series of isostructural copper trimer polyanions, [Cu3(H2O)3(alpha-XW 9O33)2]n- (n = 12, X = As III, SeIII; n = 10, X = SeIV, Te IV), have been investigated to specifically understand the role of the diamagnetic hetero atom, which has a lone pair of electrons projecting into the magnetic plane, on the overall magnetic properties. We have discovered that the magnetic parameters can be tuned by controlling the hetero atom which is incorporated into the polyanion, though the magnitude of change is small. Further, we have sought to understand the spin-frustration of (Cu2+ ) in general and we have made comparison to the copper tetramer, [Cu 4K2(H2O)8(alpha-AsW9O 33)2]8-. The (Cu2+) 4 polyanion is again spin-frustrated, however, addition of the fourth unpaired electron into the magnetic system changes the spin arrangements. Both antiferromagnetic and ferromagnetic exchange couplings are observed and the ground state spin is determined to be S = 1. We also explored replacement of the transition metal substituted into the core of the polyanion "sandwich". Six Fe3+ ions are incorporated into [Fe6(OH) 3(A-alpha-GeW9O34(OH) 3)2]11- forming a Keggin dimer each containing three Fe3+ centers. High frequency EPR experiments indicate a diamagnetic ground state spin though a broad paramagnetic transition is observed at higher temperature originating from an excited state. The second foci of this dissertation concerns the nature of anisotropy within large Fe 3+-containing clusters with high spin ground states. An assumption has been made that the electron Zeeman parameter, the g tensor, is isotropic and equal to 2.00 for such species. We have investigated a series of similar complexes with very large ground state spins and nominal D,E, and higher order anisotropy. Employment of a variable frequency EPR approach has revealed considerable Zeeman anisotropy in these complexes such that the assumption that g = 2.00 must be reexamined for other such Fe3+-containing species.