Patrick Brian O, Reiff William M, Sánchez Victor, Storr Alan, Thompson Robert C
Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1.
Inorg Chem. 2004 Apr 5;43(7):2330-9. doi: 10.1021/ic030170n.
Poly[(2,2'-bipyridine)tetrakis(imidazolato)diiron(II)] was synthesized by the reaction of ferrocene with imidazole in an excess of 2,2'- bipyridine in a Carius tube at 130 degrees C. Dc magnetic susceptibility studies at an applied field of 1000 G reveal that on cooling from room temperature, the material undergoes two structural phase transitions designated alpha --> beta and beta --> gamma at 151 and 133 K, respectively. On warming, the gamma --> beta and beta --> alpha transitions are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature beta <--> gamma transition. These structural phase transitions have also been studied by detailed, variable-temperature, ac susceptibility and Mössbauer spectroscopy techniques. Single-crystal X-ray diffraction studies done at 294, 143, and 113 K reveal 2, 12, and 6 unique iron centers in the alpha-, beta-, and gamma-forms, respectively. All three forms have the same basic structure involving 2D extended double layer sheets (bilayers) of alternating tetrahedral and octahedral irons singly bridged by imidazolate ligands, with the octahedral centers additionally coordinated by 2,2'-bipyridine ligands that occupy positions between the sheets. Magnetic susceptibility and bond length data reveal that in the gamma-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall alpha --> gamma process involves a spin-crossover transition such that at least 1/6 of the iron sites in the gamma-phase correspond to S = 0. This is supported by the low-temperature Mössbauer spectra of the gamma-phase, which reveal the simultaneous presence of both S = 2 and S = 0 iron(II) centers. The compound magnetically orders, with a resultant small remnant magnetization, at low temperatures. The magnetic phase transition, studied by a combination of dc and ac susceptibility and Mössbauer techniques, occurs at T(c) approximately 6.5 K.
聚[（2,2'-联吡啶）四（咪唑基）二铁（II）]是通过在130℃下于卡里乌斯管中，使二茂铁与咪唑在过量的2,2'-联吡啶中反应合成的。在1000 G的外加磁场下进行的直流磁化率研究表明，从室温冷却时，该材料分别在151 K和133 K经历两个结构相变，分别指定为α→β和β→γ。升温时，分别在137 K和151 K观察到γ→β和β→α转变；在较低温度的β⇄γ转变中可清楚检测到4 K的热滞现象。这些结构相变也已通过详细的变温交流磁化率和穆斯堡尔光谱技术进行了研究。在294 K、143 K和113 K进行的单晶X射线衍射研究分别揭示了α-、β-和γ-形式中2个、12个和6个独特的铁中心。所有三种形式都具有相同的基本结构，涉及由咪唑酸盐配体单桥连的交替四面体和八面体铁的二维扩展双层片（双层），八面体中心还由占据片层之间位置的2,2'-联吡啶配体配位。磁化率和键长数据表明，在γ相中，三个六配位位点中的一个对应于自旋单重基态铁（II）；即，总的α→γ过程涉及自旋交叉转变，使得γ相中至少1/6的铁位点对应于S = 0。γ相的低温穆斯堡尔光谱支持了这一点，该光谱揭示了S = 2和S = 0的铁（II）中心同时存在。该化合物在低温下发生磁性有序排列，产生小的剩余磁化强度。通过直流和交流磁化率以及穆斯堡尔技术相结合研究的磁相变发生在T(c)约6.5 K处。
