Poly[(2,2‘-bipyridine)tetrakis(imidazolato)diiron(II)]: Structural and Spin-State Phase Transitions and Low-Temperature Magnetic Ordering in a Unique 2-Dimensional Material

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 °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 α → β and β → γ at 151 and 133 K, respectively. On warming, the γ → β and β → α transitions
are observed at 137 and 151 K, respectively; a 4 K thermal hysteresis clearly detectable in the lower temperature
β ↔ γ 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 α-, β-, and γ-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
γ-phase one in three six-coordinate sites corresponds to spin singlet ground-state iron(II); i.e., the overall α → γ
process involves a spin-crossover transition such that at least 1/6 of the iron sites in the γ-phase correspond to
S = 0. This is supported by the low-temperature Mössbauer spectra of the γ-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 Tc ∼ 6.5 K.

聚[(2,2'-联吡啶)四(咪唑根)二铁(II)]通过二茂铁与咪唑在过量2,2'-联吡啶存在下，于130℃的卡里乌斯管（Carius tube）中反应合成。在1000 G的外加磁场下开展的直流磁化率研究显示，该材料从室温降温过程中先后发生两次结构相变，分别在151 K和133 K时标记为α→β与β→γ相变。升温过程中则观测到γ→β和β→α相变，对应的转变温度分别为137 K和151 K；低温β↔γ相变存在清晰可辨的4 K热滞效应。上述结构相变还通过精细的变温交流磁化率与穆斯堡尔谱学（Mössbauer spectroscopy）技术进行了深入研究。分别在294 K、143 K和113 K下完成的单晶X射线衍射（single-crystal X-ray diffraction）研究表明，α、β、γ三种晶型分别含有2个、12个和6个独特的铁原子中心。三种晶型均具有相同的基本结构：由交替的四面体和八面体铁原子通过咪唑根配体单桥连形成二维扩展双层片层（双分子层），八面体配位中心同时由位于片层间隙的2,2'-联吡啶配体完成配位。磁化率与键长数据显示，在γ晶相中，每三个六配位位点中即有一个对应自旋单重态基态的铁(II)；即整体α→γ过程涉及自旋交叉（spin-crossover）转变，γ晶相中至少1/6的铁位点对应自旋量子数S=0。这一结论得到了γ晶相低温穆斯堡尔谱的佐证，谱图显示同时存在S=2与S=0的铁(II)中心。该化合物在低温下发生磁有序转变，伴随产生微弱的剩余磁化强度。结合直流与交流磁化率、穆斯堡尔谱技术的联合表征，该磁相变温度Tc约为6.5 K。