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Alternating current (ac) magnetic susceptibility data are presented for six distorted cubane complexes of the composition [MnIVMnIII 3O3X]. Each of these complexes has a well isolated S = 9/2 ground state. There is zero-field splitting (ZFS) in the ground states where D, the axial ZFS parameter, is found to be in the range of −0.27 to −0.38 cm-1. As a result of the big spin ground state and appreciable magnetic anisotropy, an out-of-phase ac magnetic susceptibility signal is seen for each of the six Mn4 complexes. The out-of-phase ac susceptibility signal reflects slow magnetization relaxation which is taken to indicate that individual molecules are acting as magnets. Alternating current susceptibility data are presented for a frozen glass of one of the Mn4 complexes to confirm that the out-of-phase ac signal is associated with isolated molecules. The factors that influence whether a given complex can function as a single-molecule magnet are described. The above Mn4 complexes represent only the second type of molecules that exhibit enough magnetic anisotropy to function as single-molecule magnets.

We have calculated the electronic structures of five different manganese-oxo dimer complexes using density functional methods combined with the broken symmetry and spin projection concepts. The number of carboxylate, oxo, and peroxo bridging ligands was varied, and the terminal ligands were triazacyclononane (TACN). The formal Mn oxidation states varied from Mn(III)(2) and Mn(III)Mn(IV) to Mn(IV)(2). These complexes have been synthesized and their X-ray structures and magnetic properties measured previously. We have calculated the Heisenberg spin coupling parameters J and resonance delocalization parameters B for all of these systems. Despite the very small energy differences involved, there is a good correspondence between calculated and experimental Heisenberg J parameters. We have analyzed potential changes in the calculated effective Heisenberg coupling J(eff) for the mixed-valence Mn(III)Mn(IV) complexes when partial or complete delocalization due to the B parameter is taken into account. These changes depend also on the energy of the relevant intervalence band. Surprisingly, in the two mixed-valence systems studied, the high spin S = (5)/(2) state lies below S = (7)/(2). This is consistent with spin coupling between Mn with site spins S(1) = 1, S(2) = (3)/(2), corresponding to intermediate spin Mn(I) and Mn(II) respectively, instead of the coupling expected from the formal oxidation states, S(1) = 2, S(2) = (3)/(2) from high spin Mn(III) and Mn(IV). The spin and charge distributions in the broken symmetry ground states are also consistent with intermediate spin S(1) = 1, S(2) = (3)/(2). The calculated charge distributions show strong metal-ligand covalency. In fact, as the formal oxidation states of the Mn sites increase, the net Mn charges generally show a slow decrease, consistent with a very strong ligand --> metal charge transfer, particularly from &mgr;-oxo or &mgr;-peroxo ligands. TACN is a better donor ligand than carboxylate, even when calculated on a per donor atom basis. The ligand atom charge transfer order is peroxo >/= oxo >> TACN > acetate. The TACN > acetate ordering is expected from the spectrochemical series, but the strong charge transfer and strong metal-ligand covalency of peroxo and oxo ligands with the Mn sites cannot be simply related to their positions in the spectrochemical series. In the Mn(IV)(2)(&mgr;-O)(2)(&mgr;-O(2))(TACN)(2), each peroxo oxygen has a small charge (-0.3), much less than found for each &mgr;-O atom (-0.7). The high-spin S = 3 state lies quite low in energy, 8 kcal/mol from our calculations and about 4 kcal/mol based on the experimental Heisenberg spin coupling parameters. Potential molecular oxygen dissociation pathways involving a spin S = 1 state are discussed. Effective ligand field diagrams are constructed from the calculated energy levels which display the competition between spin polarization splitting and the ligand field t(2g)-e(g) splitting and allow comparisons of electronic structure among different complexes. The electronic structure and spin couplin...

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