High Spin and Spin-Crossover Two-Dimensional Coordination Polymers Containing Fe^II(tetrazol-2-yl)₄(solv)₂ (solv = Ethanol, Acetonitrile) Cores Linked by Flexible/Elastic Spacers

1,6-Di(tetrazol-2-yl)hexane (1, hbtz) in the reaction with Fe(ClO4)2·6H2O performed in acetonitrile or ethanol forms complexes {[Fe(hbtz)2(CH3CN)2](ClO4)2}∞ (2) and {[Fe(hbtz)2(C2H5OH)2](ClO4)2}∞ (3), respectively. Both compounds crystallize in the triclinic system and P1̅ space group. In 2 and 3 the basal plane of the coordination octahedron is formed by four tetrazole rings coordinated through N4 nitrogen atoms. The coordination spheres in 2 and 3 are completed by axially coordinated CH3CN or C2H5OH molecules, respectively. In 2 and 3 ligand molecules bridge neighboring iron(II) ions in two directions resulting in two-dimensional (2D) networks. 3 is paramagnetic in the range 5−300 K whereas 2 undergoes the thermally induced HS ⇆ LS transition (SCO) with T1/2↑ =T1/2↓ = 128 K. Both complexes have very similar architecture with (4,4) network topology; however, the temperature dependence of ligand bridge parameters in 2 differs markedly from the ones observed for 3. In 2 a lowering of temperature from 293 to 100 K causes a reduction of the distances between bridged iron(II) ions at 0.26 and 0.30 Å accompanied by the shortening of the distances between N4,N4′ donor atoms at about 0.04 and 0.09 Å indicating elasticity of the ligand molecules. In 3 at 250 K iron(II) ions are linked by ordered and disordered over two positions hbtz bridges. Cooling to 100 K involves a shortening of Fe···Fe and N4···N4′ distances of the ordered bridge at 0.09 and 0.07 Å, respectively. The previously disordered bridge is ordered at 100 K, and only one conformer is present. In this case conformational changes enable a reduction of Fe···Fe and N4···N4′ distances at about 0.14 and 0.14 Å, respectively.1,6-二(四唑-2-基)己烷（1,6-Di(tetrazol-2-yl)hexane，简称hbtz）与高氯酸亚铁六水合物（Fe(ClO4)2·6H2O）分别在乙腈（acetonitrile）或乙醇（ethanol）中反应，生成配合物{[Fe(hbtz)2(CH3CN)2](ClO4)2}∞（记为2）与{[Fe(hbtz)2(C2H5OH)2](ClO4)2}∞（记为3）。两种化合物均结晶于三斜晶系，空间群为P1̅。在2和3中，配位八面体的基面由四个通过N4氮原子配位的四唑环构成，二者的配位球分别由轴向配位的乙腈或乙醇分子完成。配体分子沿两个方向桥联相邻的铁(II)离子，最终形成二维（2D）网络。配合物3在5~300K范围内呈顺磁性，而配合物2则发生热诱导的高自旋（HS）⇌低自旋（LS）自旋交叉（SCO）转变，其升降温转变温度T1/2↑=T1/2↓=128K。两种配合物的骨架结构高度相似，均具有(4,4)拓扑网络，但配合物2中配体桥联参数的温度依赖性与3显著不同。在配合物2中，温度从293K降至100K时，桥联铁(II)离子间的距离分别缩短0.26Å和0.30Å，同时N4、N4′配位原子间的距离缩短约0.04Å和0.09Å，表明配体分子具有弹性。在配合物3中，250K时铁(II)离子通过两种分别处于有序和无序状态的hbtz配体桥联，降温至100K时，有序桥联的Fe···Fe与N4···N4′距离分别缩短0.09Å和0.07Å；原本无序的桥联在100K时转变为有序，仅存在一种构象，此时构象变化使Fe···Fe与N4···N4′距离分别缩短约0.14Å。