Supramolecular Iron Metallocubanes Exhibiting Site-Selective Thermal and Light-Induced Spin-Crossover

Treatment of Fe­[BF4]2·​6H2O with 4,6-di­(pyrazol-1-yl)-1H-pyrimid-2-one (HL1) or 4,6-di­(4-methylpyrazol-1-yl)-1H-pyrimid-2-one (HL2) affords solvated crystals of [{FeIII(OH2)6}⊂​FeII8(μ-L)12]­[BF4]7 (1, HL = HL1; 2, HL = HL2). The centrosymmetric complexes contain a cubic arrangement of iron­(II) centers, with bis-bidentate [L]− ligands bridging the edges of the cube. The encapsulated [Fe­(OH2)6]3+ moiety templates the assembly through 12 O–H·​·​·​O hydrogen bonds to the [L]− hydroxylate groups. All four unique iron­(II) ions in the cages are crystallographically high-spin at 250 K, but they undergo a gradual high → low spin-crossover on cooling, which is predominantly centered on one iron­(II) site and its symmetry-related congener. This was confirmed by magnetic susceptibility data, light-induced excited spin state trapping (LIESST) effect measurements, and, for 1, Mössbauer spectroscopy and diffuse reflectance data. The clusters are stable in MeCN solution, and 1 remains high-spin above 240 K in that solvent. The cubane assembly was not obtained from reactions using other iron­(II) salts or 4,6-di­(pyrazol-1-yl)­pyrimidine ligands, highlighting the importance of hydrogen bonding in templating the cubane assembly.将六水合二氟硼酸亚铁(II)（Fe[BF4]2·6H2O）分别与4,6-二(吡唑-1-基)-1H-嘧啶-2-酮(HL1)或4,6-二(4-甲基吡唑-1-基)-1H-嘧啶-2-酮(HL2)反应，可得到溶剂化晶体[{FeIII(OH2)6}⊂FeII8(μ-L)12][BF4]7（其中配合物1对应HL=HL1，配合物2对应HL=HL2）。该中心对称配合物中铁(II)中心呈立方排布，双二齿配体[L]−桥连该立方笼的各条棱边。被包封的[Fe(OH2)6]3+结构单元通过12个O–H···O氢键与配体[L]−的羟基基团相互作用，以此模板化引导整个组装过程。在250 K时，笼内四个不同的铁(II)离子在晶体学上均处于高自旋状态；随着温度降低，这些铁(II)离子会逐步发生高自旋→低自旋交叉转变，该转变主要集中在一个铁(II)位点及其对称等价位点上。这一转变现象通过磁化率数据、光诱导激发自旋态俘获（light-induced excited spin state trapping, LIESST）效应测试得到验证，针对配合物1还补充了穆斯堡尔（Mössbauer）光谱与漫反射光谱数据予以佐证。该金属簇合物在乙腈（MeCN）溶液中具有良好稳定性，且配合物1在该溶剂中于240 K以上仍保持高自旋态。采用其他铁(II)盐或4,6-二(吡唑-1-基)嘧啶类配体进行反应时，均未获得此类立方烷型组装体，这凸显了氢键在模板化构建该立方笼组装体过程中的关键作用。