Synthesis, Structure, and Magnetic Properties of a [Mn22] Wheel-like Single-Molecule Magnet

Abstract

The synthesis and magnetic properties of the compound [Mn₂₂O₆(OMe)₁₄(O₂CMe)₁₆(tmp)₈(HIm)₂] 1 are reported. Complex 1 was prepared by treatment of [Mn₃O(MeCO₂)₆(HIm)₃](MeCO₂) (HIm = imidazole) with 1,1,1-tris(hydroxymethyl)propane (H₃tmp) in MeOH. Complex 1·2MeOH crystallizes in the orthorhombic space group Pbca. The molecule consists of a metallic core of 2 Mn^IV, 18 Mn^III, and 2 Mn^II ions linked by a combination of 6 μ₃-bridging O^2- ions, 14 μ₃- and μ₂-bridging MeO^- ions, 16 μ-MeCO₂^- ligands, and 8 tmp^3- ligands, which use their alkoxide arms to bridge in a variety of ways. The metal−oxygen core is best described as a wheel made from [Mn₃O₄] partial cubes and [Mn₃O] triangles. Variable-temperature direct current (dc) magnetic susceptibility data were collected for complex 1 in the 1.8−300 K temperature range in a 1 T applied field. The χ_MT value steadily decreases from 56 cm³ K mol^-1 at 300 K to 48.3 cm³ K mol^-1 at 30 K and then increases slightly to reach a maximum value of 48.6 cm³ K mol^-1 at 15 K before dropping rapidly to 40.3 cm³ K mol^-1 at 5 K. The ground-state spin of complex 1 was established by magnetization measurements in the 0.1−2.0 T and 1.80−4.00 K ranges. Fitting of the data by a matrix-diagonalization method to a model that assumes only the ground state is populated and incorporating only axial zero-field splitting (DŜ_z²), gave a best fit of S = 10, g = 1.96 and D = −0.10 cm^-1. The ac magnetization measurements performed on complex 1 in the 1.8−8 K range in a 3.5 G ac field oscillating at 50−1000 Hz showed frequency-dependent ac susceptibility signals below 3 K. Single-crystal hysteresis loop and relaxation measurements indicate loops whose coercivities are strongly temperature and time dependent, increasing with decreasing temperature and increasing field sweep rate, as expected for the superparamagnetic-like behavior of a single-molecule magnet, with a blocking temperature (T_B) of approximately 1.3 K.