Flexible and Dynamic Thermal Behavior of Self-Catenated [{Ni3(H2O)3(Bpa)4}(V6O18)]·8H2O Constructed from 10-c Heterometallic Inorganic–Organic Clusters

The hydrothermal treatment of Ni­(NO3)2·6H2O, NaVO3, and Bpa (1,2-Di­(pyridyl)­ethane) (C12H12N2) at 120 °C during 3 days leads to green single crystals of the title compound. The single crystal X-ray diffraction reveals that [{Ni3(H2O)3(Bpa)4}­(V6O18)]·8H2O crystallizes in the monoclinic system, P21/c space group, with a = 13.5536 (2), b = 19.0463 (2), c = 27.7435 (3) Å, β = 112.3880 (10)°, V = 6622(3) Å3, with R1­(obs) = 0.0558, wR2­(obs) = 0.1359, for 10278 observed reflections. The complexity of the crystal structure is based on different points, as the existence of: both “gauche” and “trans” conformations of the organic ligand, the [V12O36]−12 cycles, formed by 12 corner-sharing VO4 tetrahedra, and, finally, the combination of both three-dimensional metal–organic and inorganic substructures, giving rise to a self-catenated highly connected net. The crystallization water molecules are semi-encapsulated in the channels along the [100] direction, and their loss gives rise to a dynamical and reversible structural contraction. Moreover, after the removal of the crystallization water molecules, the compound exhibits a negative thermal behavior in the 85–155 °C temperature range, and irreversible structural transformation due to the loss of coordinated water molecules up to 200 °C. The IR and UV–vis spectra were determined for the as-synthesized sample, after the removal of crystallization water molecules and after the irreversible transformation due to the loss of coordinated water molecules. The thermal evolution of χm was adjusted to a magnetic model considering an isotropic dimer plus two Ni­(II) d8 isolated octahedra.

将硝酸镍六水合物（Ni(NO3)2·6H2O）、偏钒酸钠（NaVO3）与配体Bpa（1,2-二(吡啶基)乙烷，C12H12N2）在120℃下进行水热处理3天，可得到目标化合物的绿色单晶。单晶X射线衍射（single crystal X-ray diffraction）分析显示，[{Ni3(H2O)3(Bpa)4}(V6O18)]·8H2O结晶于单斜晶系（monoclinic system）P21/c空间群（space group），晶胞参数为a=13.5536(2) Å、b=19.0463(2) Å、c=27.7435(3) Å，β=112.3880(10)°，晶胞体积V=6622(3) ų；针对10278个可观测衍射点，其R1(obs)=0.0558，wR2(obs)=0.1359。该晶体结构的复杂性源于多方面特征：有机配体同时存在“旁式（gauche）”与“反式（trans）”两种构象；由12个共顶点VO4四面体构成的[V12O36]^12-环；最终，三维金属有机亚结构与无机亚结构的结合，形成了自穿插的高连通网络。结晶水分子半包裹于沿[100]方向的孔道中，其脱除会引发动态且可逆的结构收缩。此外，脱除结晶水分子后，该化合物在85~155℃温度区间表现出负热膨胀行为；当温度升至200℃时，配位水分子的脱除会引发不可逆的结构转变。分别对原合成样品、脱除结晶水分子后的样品，以及因配位水分子脱除发生不可逆结构转变后的样品，完成了红外（IR）与紫外-可见（UV–vis）光谱的表征。磁化率χm的热演化行为通过各向同性二聚体加上两个孤立八面体配位的Ni(II) d8离子的磁模型进行了拟合。