A "Star" antiferromagnet: a polymeric iron(III) acetate that exhibits both spin frustration and long-range magnetic ordering.
A "Star" antiferromagnet: a polymeric iron(III) acetate that exhibits both spin frustration and long-range magnetic ordering.
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DOI：
10.1002/anie.200701954
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发表时间：
2007-08
影响因子：
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通讯作者：
Yan‐Zhen Zheng;M. Tong;Wei Xue;Wei-Xiong Zhang;Xiao‐Ming Chen;F. Grandjean;G. Long
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文献类型：
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作者：
Yan‐Zhen Zheng;M. Tong;Wei Xue;Wei-Xiong Zhang;Xiao‐Ming Chen;F. Grandjean;G. Long
制备接近理论模型的新的几何自旋受阻的磁性材料是一个挑战。尽管Mermin-Wagner定理表明，在零开尔文时，长程磁有序可以存在于二维空间，但即使在这种温度下，有序也可能被量子涨落或几何挫败所破坏。理论研究表明，自旋1/2海森堡反铁磁体的基态最有可能是半经典有序的。然而，几何受挫和量子涨落的相互作用已经被发现在两种类型的晶格中产生了没有半经典长程有序的顺磁基态。这些晶格中的第一个是著名的Kagom+晶格(T8)，第二个是所谓的“星形”晶格(T9；方案1)，这可能是量子准磁网的一个新例子。5]这些三角形在Kagom+晶格中是角共享的，而在星形晶格中被桥分开，这意味着它们的次近邻交换作用是不同的。5]Kagom+晶格中的磁性J交换路径都是等价的，而星形晶格中的三角内JT路径弱于三角间JD路径。与Kagom+型反铁磁晶格和相关的几何自旋受挫晶格的快速发展形成对比的是，到目前为止，似乎还没有关于具有真正星形晶格的化合物的报道。具有超交换路径的三角形团簇，例如广泛使用的M3(m~3-O)团簇，其中M可以是Fe、Fe、Co、Ni、Cu、V或Cr，可用于产生受阻晶格，包括所需的磁受阻星晶格。这个星形点阵可以用顶点记法描述为3.12(参见支持信息中的方案S1)，这是一个均匀的、具有大空洞的三维连接的二维网络。必须使用喜欢以平面方式结合的三连接节点亚单位，例如碱性阳离子铁(III)羧酸簇合物[Fe3(m~3-O)(MO_2CR)_6L_3]，其中L可以是水、甲醇或吡啶，以避免三维连接。这些羧酸簇合物是很好的潜在构筑块，因为它们容易制备，更喜欢平面成键，并且R基团和L配体可以很容易地改变。阳离子[Fe3(m-O)(m-O2CR)6L3]+部分以前作为六个或三个连接的节点(见辅助信息中的方案S2)形成三维或零维多孔骨架，这取决于羧酸盐的性质，它可以被二羧酸盐完全或部分取代；L配体通常作为末端配体保留。虽然到目前为止还没有已知的例子，但应该有可能用其他桥联双齿配体取代位于三角形[Fe3(m-O)(m-O2CR)6L3]+阳离子平面上的L配体，这些配体既能更好地调节反铁磁性相互作用，又能产生二维星形晶格。在这里，我们报道了使用双齿醋酸酯桥联配体将[Fe3(M3-O)(MOAc)6]+阳离子连接在一起形成具有所需星形晶格的新化合物[Fe3(m3-O)(m-OAc)6(H2O)3][Fe3(m3-O)(m-OAc)7.5]2·7H2O(1)。1在293和90K下的单晶X射线衍射研究表明，孤立的[Fe3(m3-O)(m-OAc)6(H2O)3]+阳离子(图1)占据了乙酸桥联的[Fe3(m3-O)(m-OAc)7.5]1/2阴离子层堆积形成的十二方通道；[Fe3(m-O)(m-OAc)6(H2O)3]+阳离子与[Fe3(m-O)(m-OAc)6(H2O)3]+离子之间的二面角。比较了Kagom(T8，左)和星形(T9，右)晶格的磁性J交换路径。
The preparation of new geometrically spin-frustrated magnetic materials that approximate theoretical models is a challenge. Although theMermin–Wagner theorem indicates that long-range magnetic order can exist in two dimensions at zero Kelvin, order can be destroyed either by quantum fluctuations or geometric frustration even at this temperature. Theoretical studies indicate that the ground state of a spin-1/2 Heisenberg antiferromagnet is most likely to be semiclassically ordered. However, the interplay of geometric frustration and quantum fluctuations has been found to give rise to a paramagnetic ground state without semi-classical long-range order in two types of lattice. The first of these lattices is the famous Kagom+ lattice (T8) and the second is the so-called “star” lattice (T9; Scheme 1), which may serve as a new example of a quantum paramagnet. 5] The triangles are corner-sharing in the Kagom+ lattice whereas they are separated by a bridge in the star lattice, which means that their next-nearest-neighbor exchange interactions are different. 5] The magnetic J exchange pathways in the Kagom+ lattice are all equivalent, whereas the intra-triangular JT pathway in the star lattice is weaker than the inter-triangular JD pathway. In contrast to the rapid development of Kagom+-type antiferromagetic lattices 7] and related, geometrically spin-frustrated lattices, there appears to date to be no report of a compound with a genuine star lattice. Triangular clusters with superexchange pathways, such as the widely employedM3(m3-O) clusters, whereMmay be Fe , Fe, Co, Ni, Cu, V, or Cr, can be used to generate frustrated lattices, including the desired magnetically frustrated star lattice. This star lattice can be described in vertex notation as 3.12 (see Scheme S1 in the Supporting Information), a lattice that is a uniform, three-connected twodimensional net with large voids. Three-connected node subunits that prefer to bond in a planar fashion, such as the basic cationic iron(III) carboxylate cluster [Fe3(m3-O)(mO2CR)6L3] , where L may be water, methanol, or pyridine, must be used to avoid three-dimensional connections. These carboxylate clusters are good potential building blocks because they are easily prepared, prefer planar bonding, and the R groups and L ligands can easily be varied. The cationic [Fe3(m3-O)(m-O2CR)6L3] + moiety has previously served as a sixor three-connected node (see Scheme S2 in the Supporting Information) to form either threeor zero-dimensional porous frameworks depending upon the nature of the carboxylate, which may be either fully or partially substituted by dicarboxylates; the L ligands are usually retained as terminal ligands. Although no example is known to date, it should be possible to substitute the L ligands located in the triangular [Fe3(m3-O)(m-O2CR)6L3] + cation plane with other bridging bidentate ligands that are better at both mediating antiferromagnetic interactions and producing a two-dimensional star lattice. Herein, we report the use of bidentate acetate bridging ligands to link [Fe3(m3-O)(mOAc)6] + cations together to form [Fe3(m3-O)(m-OAc)6(H2O)3][Fe3(m3-O)(m-OAc)7.5]2·7H2O (1), a new compound with the desired star lattice. Single-crystal X-ray diffraction studies of 1 at 293 and 90 K revealed that isolated [Fe3(m3-O)(m-OAc)6(H2O)3] + cations (Figure 1) occupy the dodecagonal channels formed by the stacking of acetate-bridged [Fe3(m3-O)(m-OAc)7.5] 1/2 anionic layers; the dihedral angle between the triangular [Fe3(m3-O)(m-OAc)6(H2O)3] + cations and the [Fe3(m3-O)(mScheme 1. A comparison of the Kagom (T8, left) and star (T9, right) lattices with indication of the magnetic J exchange pathways.