Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [FeIIMII(CN)4] (MII = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [FeIIPtII(CN)4] to form the material [Fe­(H2O)2Fe­(DPSe)2(Pt­(CN)4)2]·3EtOH (DPSe = 4,4′-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron­(II) environments, [FeIIN6] (Fe1) and [FeIIN4O2] (Fe2), (2) as a consequence of axial water coordination to Fe2, there are “holes” in the [FeIIPtII(CN)4] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group. The magnetic properties are defined by this unusual topology such that only Fe1 is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (T1/2↓ = 120 K and T1/2↑ = 130 K) of this iron­(II) site; Fe2 remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; T(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (T1/2↓ = 60 K and T1/2↑ = 66 K).将近似线性的双吡啶或单吡啶配体与[FeIIMII(CN)4]（其中MII为Pt、Pd、Ni）4,4网格层结合所得到的预期三维（3D）与二维（2D）拓扑结构，已得到充分证实。本文报道了将弯曲型双吡啶连接配体与[FeIIPtII(CN)4]结合，制备得到材料[Fe(H2O)2Fe(DPSe)2(Pt(CN)4)2]·3EtOH（其中DPSe为4,4'-二吡啶硒醚）的磁学与结构效应。结构表征揭示出一种近似三维霍夫曼（Hofmann）拓扑的非常规连接模式，其具有如下特征：(1) 存在两种截然不同的二价铁局部配位环境，分别为[FeIIN6]（Fe1）与[FeIIN4O2]（Fe2）；(2) 由于Fe2轴向结合了水分子，部分氰根配体以端基而非桥联形式存在，导致[FeIIPtII(CN)4]4,4网格层中出现“孔洞”；(3) 相邻网格层仅通过每两个DPSe配体中的一个进行桥联，另一个则以仅结合一个吡啶环的端基配体形式存在。该材料的磁学性质由这一非常规拓扑结构决定，仅Fe1处于适合发生高自旋-低自旋自旋转变的配位环境中。磁化率测试数据表明，该二价铁位点存在完整且具有磁滞效应的突变自旋转变，其升降温半转变温度分别为T1/2↓=120 K与T1/2↑=130 K；而Fe2始终保持高自旋状态。该材料还表现出光磁响应现象（这在霍夫曼类材料中并不常见），呈现光诱导激发自旋态俘获（light-induced excited spin-state trapping, LIESST）效应，其俘获温度T(LIESST)=61 K，并伴随由磁滞回线证实的双稳态行为，对应的半转变温度为T1/2↓=60 K与T1/2↑=66 K。