Zhao Hanhua, Berlinguette Curtis P, Bacsa John, Prosvirin Andrey V, Bera Jitendra K, Tichy Shane E, Schelter Eric J, Dunbar Kim R
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842-3012, USA.
Inorg Chem. 2004 Feb 23;43(4):1359-69. doi: 10.1021/ic030144p.
The syntheses and characterization of two new, highly soluble, single-molecule magnets [Mn12O12(CF3COO16(H2O)4].2CF3COOH.4H2O (1) and Mn12O12(CF3COO16(H2O)4].CF3COOH.7H2O (2) are reported. Compound 1 was isolated from the reaction of Mn12O12(CF3COO16(H2O)4] with trifluoroacetic acid in CH2Cl2. Compound 1 crystallizes in the tetragonal space group Ifourmacr; (No. 82) with unit cell parameters a = b = 18.128(3) A, c = 13.048(3) A, V = 4287.9(19) A3, Z = 2 and is isostructural to [Mn12O12(CH3COO)16(H2O)4]. Compound 2 was prepared from the reaction of Mn12O12(CF3COO16(H2O)4] with neat trifluoroacetic acid, and crystallizes in the monoclinic space group P2(1)/n (No. 14) with unit cell parameters a = 15.221(8) A, b = 21.870(12) A, c = 27.217(15) A, beta = 90.53(1) degrees, V = 9060(9) A3, and Z = 4. The dc magnetic susceptibility measurements in the 2-300 K temperature range support a high-spin ground state. The magnetization data collected in the 1-7 T field range from 1.8 to 4.0 K were best fit to the parameters S = 10, g = 2.15, D = -0.65 cm(-1), and E = 0 cm(-1) for 1 and S = 10, g = 1.87, D = -0.34 cm(-1), and E = -0.10 cm(-1) for 2. The ac susceptibility data for compound 1 reveal out-of-phase (chi(m)") signals in the 4-7 K temperature range, whereas the chi(m)" signals for compound 2 appear below temperatures of 4 K. This variation in blocking temperatures is a consequence of the two different crystallographic forms of compounds 1 and 2. Compound 1 exhibits the same structural geometry and distortions found in [Mn12O12(CH3COO)16(H2O)4], while compound 2 is of lower molecular symmetry with two Jahn-Teller axes of distortion being oriented along oxide ligands. This different structural arrangement facilitates a different tunneling pathway that leads to a lower effective barrier for magnetization reorientation for compound 2. The substitution of the acetate ligands by trifluoroacetic acid was monitored by mass spectrometry, which is a convenient tool for judging completion of the substitution process.
报道了两种新型、高溶解性的单分子磁体[Mn12O12(CF3COO)16(H2O)4].2CF3COOH.4H2O (1)和[Mn12O12(CF3COO)16(H2O)4].CF3COOH.7H2O (2)的合成与表征。化合物1是通过Mn12O12(CF3COO)16(H2O)4与三氟乙酸在二氯甲烷中的反应分离得到的。化合物1结晶于四方晶系空间群I4/mcm(编号82)，晶胞参数a = b = 18.128(3) Å，c = 13.048(3) Å，V = 4287.9(19) Å3，Z = 2，且与[Mn12O12(CH3COO)16(H2O)4]同构。化合物2由Mn12O12(CF3COO)16(H2O)4与纯三氟乙酸反应制备，结晶于单斜晶系空间群P2(1)/n(编号14)，晶胞参数a = 15.221(8) Å，b = 21.870(12) Å，c = 27.217(15) Å，β = 90.53(1)°，V = 9060(9) Å3，Z = 4。在2 - 300 K温度范围内的直流磁化率测量结果支持高自旋基态。在1.8至4.0 K、1 - 7 T磁场范围内收集的磁化数据对1而言最佳拟合参数为S = 10，g = 2.15，D = -0.65 cm-1，E = 0 cm-1；对2而言最佳拟合参数为S = 10，g = 1.87，D = -0.34 cm-1，E = -0.10 cm-1。化合物1的交流磁化率数据在4 - 7 K温度范围内显示出异相(χm")信号，而化合物2的χm"信号出现在4 K以下温度。这种阻塞温度的变化是化合物1和2两种不同晶体形式的结果。化合物1呈现出与[Mn12O12(CH3COO)16(H2O)4]相同的结构几何形状和畸变，而化合物2具有较低的分子对称性，两个 Jahn - Teller 畸变轴沿氧化物配体方向排列。这种不同的结构排列促成了不同的隧穿途径，导致化合物2的磁化重取向有效势垒更低。通过质谱监测乙酸根配体被三氟乙酸取代的情况，质谱是判断取代过程完成情况的便捷工具。
