Laboratoire des Champs Magnetique Intenses, CNRS, Grenoble, France.
Chemphyschem. 2001 Sep 17;2(8-9):523-31. doi: 10.1002/1439-7641(20010917)2:8/9<523::AID-CPHC523>3.0.CO;2-D.
The synthesis, crystal structure, and magnetic characterization of a high-spin cluster comprising eight iron ions are presented in this contribution. The cluster has formula [(tacn)6Fe8O2(OH)12Br4.3(ClO4)3.7]·6H2O (Fe8pcl), where tacn is the organic ligand 1,4,7-triazacyclononane. It can be considered a derivative of Fe8 Br8 , a cluster whose low-temperature magnetization dynamics has been extensively investigated, in which four of the bromide ions have been replaced by perchlorate anions. The structure of the central core of the two molecules, Fe8O(OH)12(tacn)6, is essentially the same, but Fe8pcl has a higher symmetry (the bromide derivative crystallizes in the acentric P1 space group while Fe8pcl crystallizes in the monoclinic P2(1)/c space group). The magnetic properties of Fe8pcl suggest it is very similar to Fe8Br8 having a S=10 ground state as well. The zero-field splitting parameters were accurately determined by high field-high frequency EPR (HF-EPR) measurements. The two clusters have similar axial anisotropy D but Fe8pcl has a larger transverse anisotropy E: The value of E/D is 0.21 for the perchlorate derivative but 0.19 for Fe8Br8. AC susceptibility measurements revealed the cluster behaves like a superparamagnetic particle. However, due to the occurrence of large terms in the transverse anisotropy, the temperature dependence of the relaxation time cannot be reproduced by a simple Arrhenius law model. As observed in the bromide derivative, below 350 mK the relaxation time becomes temperature independent and indicating that a pure tunneling regime is attained. The comparison of the tunneling rate in the two clusters shows that in the perchlorate derivative the relaxation process is 35 times faster. The observed ratio of the tunneling rates is in reasonable agreement with that calculated from the tunneling splitting, namely the energy difference between the two almost-degenerate lowest levels Ms =±10, in the two clusters.
本文报道了一个由八个铁离子组成的高自旋簇合物的合成、晶体结构和磁性表征。该簇合物的化学式为[(tacn)6Fe8O2(OH)12Br4.3(ClO4)3.7]·6H2O（Fe8pcl），其中 tacn 是有机配体 1,4,7-三氮杂环壬烷。它可以被认为是 Fe8Br8 的衍生物，后者的低温磁化动力学已被广泛研究，其中四个溴化物离子被高氯酸盐阴离子取代。这两个分子的中心核结构Fe8O(OH)12(tacn)6本质上是相同的，但 Fe8pcl 的对称性更高（溴化物衍生物在非中心 P1 空间群中结晶，而 Fe8pcl 在单斜 P2(1)/c 空间群中结晶）。Fe8pcl 的磁性表明它与 Fe8Br8 非常相似，也具有 S=10 的基态。零场分裂参数通过高频高场电子顺磁共振（HF-EPR）测量准确确定。这两个簇合物具有相似的轴向各向异性 D，但 Fe8pcl 的横向各向异性 E 较大：对于高氯酸盐衍生物，E/D 的值为 0.21，而对于 Fe8Br8，则为 0.19。交流磁化率测量表明该簇合物表现为超顺磁粒子。然而，由于横向各向异性中存在较大项，弛豫时间的温度依赖性不能用简单的 Arrhenius 定律模型来再现。与溴化物衍生物一样，在 350 mK 以下，弛豫时间变得与温度无关，表明达到了纯隧道态。两个簇合物中隧道速率的比较表明，在高氯酸盐衍生物中，弛豫过程快 35 倍。观察到的隧道速率比与从隧道分裂计算得到的隧道速率比在两个簇合物中大致一致，即两个几乎简并的最低能级 Ms=±10 之间的能量差。
