Synthesis, structure, and magnetic properties of a [Mn22] wheel-like single-molecule magnet

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 (H(3)tmp) 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 mu(3)-bridging O-2(-) ions, 14 mu(3)- and mu(2)-bridging MeO- ions, 16 mu-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 chi(M)T value steadily decreases from 56 cm(3) K mol(-1) at 300 K to 48.3 cm(3) K mol(-1) at 30 K and then increases slightly to reach a maximum value of 48.6 cm(3) K mol(-1) at 15 K before dropping rapidly to 40.3 cm(3) 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 (D (S) over cap (2)(z)), 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.