RESUMO
A new high symmetry Mn(12) oxocarboxylate cluster [Mn(12)O(12)(CN-o-C(6)H(4)CO(2))(12)(CH(3)CO(2))(4)(H(2)O)(4)]·8CH(2)Cl(2) (1) with mixed carboxylate ligands is reported. It was synthesized by the standard carboxylate substitution method. 1 crystallizes in the tetragonal space group I4(1)/a. Complex 1 contains a [Mn(12)O(12)] core with eight CN-o-C(6)H(4)CO(2) ligands in the axial positions, four CH(3)CO(2) and four CN-o-C(6)H(4)CO(2) in equatorial positions. Four H(2)O molecules are bonded to four Mn atoms in an alternating up, down, up, down arrangement indicating a 1 : 1 : 1 : 1 isomer. The Mn(12) molecules in 1 are self-assembled by complementary hydrogen C-H···N bonds formed with participation of the axial o-cyanobenzoate ligands of the adjacent Mn(12) clusters. The lattice solvent molecules (CH(2)Cl(2)) are weakly interacted with Mn(12) units that results in solvent loss immediately after removal of the crystals from the mother liquor. The electronic structure and the intramolecular exchange parameters have been calculated. Mn 3d bands of 1 are rather broad, and the center of gravity of the bands shifts down from the Fermi level. The overlap between Mn 3d bands and 2p ones of the oxygen atoms from the carboxylate bridges is higher than in the parent Mn(12)-acetate cluster. These changes in the electronic structure provide a significant difference in the exchange interactions in comparison to Mn(12)-acetate. The magnetic properties have been studied on a dried (solvent-free) polycrystalline sample of 1. The dc magnetic susceptibility measurements in the 2-300 K temperature range support a high-spin ground state (S = 10). A bifurcation of temperature dependencies of magnetization taken under zero field cooled and field cooled conditions observed below 4.5 K is due to slow magnetization relaxation. Magnetization versus applied dc field exhibited a stepwise hysteresis loop at 2 K. The ac magnetic susceptibility data revealed the frequency dependent out-of-phase (χ(M)'') signals characteristic of single-molecule magnets.
