Feng Patrick L, Beedle Christopher C, Koo Changhyun, Wernsdorfer Wolfgang, Nakano Motohiro, Hill Stephen, Hendrickson David N
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093-0358, USA.
Inorg Chem. 2008 Apr 21;47(8):3188-204. doi: 10.1021/ic702241m. Epub 2008 Mar 7.
A family of distorted heterometallic cubanes, [Mn (III) 3Ni (II)(hmp) 3O(N 3) 3(O 2CR) 3], where O 2CR (-) is benzoate ( 1), 3-phenylpropionate ( 2), 1-adamantanecarboxylate ( 3), or acetate ( 4) and hmp (-) is the anion of 2-pyridinemethanol, was synthesized and structurally as well as magnetically characterized. These complexes have a distorted-cubane core structure similar to that found in the S = 9/2 Mn 4 cubane family of complexes. Complexes 1, 3, and 4 crystallize in rhombohedral, hexagonal, and cubic space groups, respectively, and have C 3 molecular symmetry, while complex 2 crystallizes in the monoclinic space group Cc with local C 1 symmetry. Magnetic susceptibility and magnetization hysteresis measurements and high-frequency electron paramagnetic resonance (HFEPR) spectroscopy established that complexes 1-4 have S = 5 spin ground states with axial zero-field splitting (ZFS) parameters ( D) ranging from -0.20 to -0.33 cm (-1). Magnetization versus direct-current field sweeps below 1.1 K revealed hysteresis loops with magnetization relaxation, definitely indicating that complexes 1-4 are single-molecule magnets that exhibit quantum tunneling of magnetization (QTM) through an anisotropy barrier. Complex 2 exhibits the smallest coercive field and fastest magnetization tunneling rate, suggesting a significant rhombic ZFS parameter ( E), as expected from the low C 1 symmetry. This was confirmed by HFEPR spectroscopy studies on single crystals that gave the following parameter values for complex 2: gz = 1.98, gx = gy = 1.95, D = -0.17 cm (-1), B 4 (0) = -6.68 x 10 (-5) cm (-1), E = 6.68 x 10 (-3) cm (-1), and B 4 (2) = -1.00 x 10 (-4) cm (-1). Single-crystal HFEPR data for complex 1 gave g z = 2.02, gx = gy = 1.95, D = -0.23 cm (-1), and B 4 (0) = -5.68 x 10 (-5) cm (-1), in keeping with the C 3 site symmetry of this Mn 3Ni complex. The combined results highlight the importance of spin-parity effects and molecular symmetry, which determine the QTM rates.
合成了一系列畸变的异金属立方烷[Mn(III)₃Ni(II)(hmp)₃O(N₃)₃(O₂CR)₃]，其中O₂CR⁻为苯甲酸根(1)、3 - 苯基丙酸根(2)、1 - 金刚烷羧酸根(3)或乙酸根(4)，hmp⁻为2 - 吡啶甲醇的阴离子，并对其进行了结构和磁性表征。这些配合物具有与S = 9/2的Mn₄立方烷配合物家族中发现的结构相似的畸变立方烷核心结构。配合物1、3和4分别结晶于菱面体、六方和立方空间群，具有C₃分子对称性，而配合物2结晶于单斜空间群Cc，具有局部C₁对称性。磁化率和磁化滞后测量以及高频电子顺磁共振(HFEPR)光谱表明，配合物1 - 4具有S = 5的自旋基态，轴向零场分裂(ZFS)参数(D)范围为 - 0.20至 - 0.33 cm⁻¹。低于1.1 K时，磁化强度与直流磁场扫描曲线显示出具有磁化弛豫的磁滞回线，明确表明配合物1 - 4是单分子磁体，通过各向异性势垒表现出磁化量子隧穿(QTM)。配合物2表现出最小的矫顽场和最快的磁化隧穿速率，表明存在显著的菱形ZFS参数(E)，这与低C₁对称性预期一致。对单晶进行的HFEPR光谱研究证实了这一点，该研究给出了配合物2的以下参数值：gz = 1.98，gx = gy = 1.95，D = - 0.17 cm⁻¹，B₄(0) = - 6.68×10⁻⁵ cm⁻¹，E = 6.68×10⁻³ cm⁻¹，以及B₄(2) = - 1.00×10⁻⁴ cm⁻¹。配合物1的单晶HFEPR数据为gz = 2.02，gx = gy = 1.95，D = - 0.23 cm⁻¹，以及B₄(0) = - 5.68×10⁻⁵ cm⁻¹，与该Mn₃Ni配合物的C₃位点对称性一致。综合结果突出了自旋 - 宇称效应和分子对称性的重要性，它们决定了QTM速率。
