Single molecule magnets with m-fluorobenzoate and difluoromethylacetate as polar ligands

Abstract

Two new single molecule magnets (SMMs), [Mn₁₂O₁₂(CHF₂COO)₁₆(H₂O)₄]·2CH₂Cl₂·4H₂O (1) and [Mn₁₂O₁₂(m-FPhCOO)₁₆(H₂O)₄]·2(m-FPhCOOH)·7CH₂Cl₂ (2), based on a disc-shaped {Mn₁₂O₁₂}-framework with difluoromethylacetate (CHF₂COO⁻) and m-fluorobenzoate (m-FPhCOO⁻) as polar ligands were prepared in CH₂Cl₂. Structural analyses by X-ray diffraction at 100 K showed orientationally disordered fluorine atoms in the CHF₂COO⁻ and m-FPhCOO⁻ ligands of crystals 1 and 2, respectively. In crystal 1, the fluorine atoms at the axial coordination sites of the disc-like {Mn₁₂O₁₂}-framework exhibited large thermal parameters even at 100 K, suggesting that thermally activated rotation of the –CHF₂ groups along the C–COO axis occurred in the CHF₂COO⁻ ligands. The SMMs behaviors of crystals 1 and 2 showed magnetization hysteresis at 1.8 K and frequency-dependent in-phase and out-of-phase ac magnetic susceptibilities, characteristic of SMMs. Frequency-dependent dielectric responses as well as two Debye-type relaxation processes at ∼100 and ∼220 K were observed in crystal 1, while such responses were not observed in crystal 2. The activation energy and blocking temperature of the relaxation process for 1 were 12 kJ mol⁻¹ and 48 K, respectively, at ∼100 K, whereas those at ∼220 K were 66 kJ mol⁻¹ and 153 K, respectively. In 1, the two polarization types at ∼100 and ∼220 K were associated with thermally activated fluctuations of the hydrogen-bonded crystalline H₂O molecules and rotation of the –CHF₂ group in the CHF₂COO⁻ ligand.