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 (H3tmp) in MeOH. Complex 1·2MeOH crystallizes in the orthorhombic space group Pbca.
The molecule consists of a metallic core of 2 MnIV, 18 MnIII, and 2 MnII ions linked by a combination of 6 μ3-bridging O2- ions, 14 μ3- and μ2-bridging MeO- ions, 16 μ-MeCO2- ligands, and 8 tmp3- 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 χMT 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 (DŜz2), 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 (TB) of approximately 1.3 K.

本研究报道了配合物[Mn₂₂O₆(OMe)₁₄(O₂CMe)₁₆(tmp)₈(HIm)₂]（记为1）的合成与磁学性质。
配合物1通过将[Mn₃O(MeCO₂)₆(HIm)₃](MeCO₂)（其中HIm为咪唑（imidazole））与1,1,1-三(羟甲基)丙烷（H₃tmp）在甲醇（MeOH）中反应制得。配合物1·2MeOH晶体属于正交晶系空间群Pbca。
该分子的金属核包含2个Mn⁴⁺、18个Mn³⁺与2个Mn²⁺离子，由6个μ₃-桥连O²⁻离子、14个μ₃-与μ₂-桥连甲氧基（MeO⁻）离子、16个μ-乙酸根（MeCO₂⁻）配体以及8个tmp³⁻配体桥连而成；tmp³⁻配体通过其烷氧基臂以多种方式实现桥连作用。该金属-氧核可被最优描述为由[Mn₃O₄]半立方单元与[Mn₃O]三角形单元构成的轮状结构。
在1.8~300 K温度区间、1 T外加磁场下，对配合物1采集了变温直流（direct current, dc）磁化率数据。χ_MT值从300 K时的56 cm³·K·mol⁻¹持续降至30 K时的48.3 cm³·K·mol⁻¹，随后小幅上升，在15 K时达到48.6 cm³·K·mol⁻¹的最大值，之后快速下降至5 K时的40.3 cm³·K·mol⁻¹。
通过在0.1~2.0 T与1.80~4.00 K区间内进行磁化强度测量，确定了配合物1的基态自旋。采用仅考虑基态布居且仅引入轴向零场分裂（axial zero-field splitting, DŜ_z²）的模型，通过矩阵对角化方法对数据进行拟合，得到最优拟合参数：S=10，g=1.96，D=−0.10 cm⁻¹。
在1.8~8 K温度区间、50~1000 Hz振荡的3.5 G交流（alternating current, ac）磁场下，对配合物1进行交流磁化率测量，结果显示3 K以下存在频率依赖的交流磁化率信号。
单晶磁滞回线与弛豫测量结果表明，其磁滞回线的矫顽力具有强烈的温度与时间依赖性，随温度降低与场扫描速率提升而增大，符合单分子磁体（single-molecule magnet）的超顺磁类行为特征，其阻塞温度（blocking temperature, T_B）约为1.3 K。