New single-molecule magnet based on Mn12 oxocarboxylate clusters with mixed carboxylate ligands, [Mn12O12(CN-o-C6H4CO2)12(CH3CO2)4(H2O)4]·8CH2Cl2: Synthesis, crystal and electronic structure, magnetic properties

Abstract

A new high symmetry Mn₁₂ oxocarboxylate cluster [Mn₁₂O₁₂(CN-o-C₆H₄CO₂)₁₂(CH₃CO₂)₄(H₂O)₄]·8CH₂Cl₂ (1) with mixed carboxylate ligands is reported. It was synthesized by the standard carboxylate substitution method. 1 crystallizes in the tetragonal space group I4₁/a. Complex 1 contains a [Mn₁₂O₁₂] core with eight CN-o-C₆H₄CO₂ ligands in the axial positions, four CH₃CO₂ and four CN-o-C₆H₄CO₂ in equatorial positions. Four H₂O molecules are bonded to four Mn atoms in an alternating up, down, up, down arrangement indicating a 1 : 1 : 1 : 1 isomer. The Mn₁₂ 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 Mn₁₂ clusters. The lattice solvent molecules (CH₂Cl₂) are weakly interacted with Mn₁₂ 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 Mn₁₂-acetate cluster. These changes in the electronic structure provide a significant difference in the exchange interactions in comparison to Mn₁₂-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.