Thermal Response, Catalytic Activity, and Color Change of the First Hybrid Vanadate Containing Bpe Guest Molecules

Four isomorphic compounds with formula [{Co2(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, CoBpe 1; [{CoNi­(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, CoNiBpe 2; [{Co0.6Ni1.4(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, NiCoBpe 3; and [{Ni2(H2O)2(Bpe)2}­(V4O12)]­·4H2O·Bpe, NiBpe 4, have
been obtained by hydrothermal synthesis. The crystal structures of
CoBpe 1 and NiBpe 4 were determined by single-crystal
X-ray diffraction (XRD). The Rietveld refinement of CoNiBpe 2 and NiCoBpe 3 XRD patterns confirms that those
are isomorphic. The compounds crystallize in the P1̅ space group, exhibiting a crystal structure constructed
from inorganic layers pillared by Bpe ligands. The crystal structure
contains intralayer and interlayer channels, in which the crystallization
water molecules and Bpe guest molecules, respectively, are located.
The solvent molecules establish a hydrogen bonding network with the
coordinated water molecules. Thermodiffractometric and thermogravimetric
studies showed that the loss of crystallization and coordinated water
molecules takes place at different temperatures, giving rise to crystal
structure transformations that involve important reduction of the
interlayer distance, and strong reduction of crystallinity. The IR,
Raman, and UV–vis spectra of the as-synthesized and heated
compounds confirm that the structural building blocks and octahedral
coordination environment of the metal centers are maintained after
the structural transformations. The color change and reversibility
of the water molecules uptake/removal were tested showing that the
initial color is not completely recovered when the compounds are heated
at temperatures higher than 200 °C. The thermal evolution of
the magnetic susceptibility indicates one-dimensional antiferromagnetic
coupling of the metal centers at high temperatures. For NiCoBpe 3 and NiBpe 4 compounds magnetic ordering is
established at low temperatures, as can be judged by the maxima observed
in the magnetic susceptibilities. CoNiBpe 2 was proved
as catalyst being active for cyanosilylation reactions of aldehydes.

本研究通过水热合成法制备了四种同构化合物，其化学式依次为：1. CoBpe 1：[{Co₂(H₂O)₂(Bpe)₂}(V₄O₁₂)]·4H₂O·Bpe；2. CoNiBpe 2：[{CoNi(H₂O)₂(Bpe)₂}(V₄O₁₂)]·4H₂O·Bpe；3. NiCoBpe 3：[{Co₀.₆Ni₁.₄(H₂O)₂(Bpe)₂}(V₄O₁₂)]·4H₂O·Bpe；4. NiBpe 4：[{Ni₂(H₂O)₂(Bpe)₂}(V₄O₁₂)]·4H₂O·Bpe。通过单晶X射线衍射（XRD）解析了CoBpe 1与NiBpe 4的晶体结构；对CoNiBpe 2和NiCoBpe 3的XRD图谱进行里特维尔德精修后证实，该二者与前述两种化合物同构。该系列化合物均以P1̅空间群结晶，其晶体结构由Bpe配体柱撑的无机层构筑而成，内部包含层内与层间通道，分别容纳结晶水分子与Bpe客体分子；溶剂分子可与配位水分子形成氢键网络。热衍射与热重分析研究表明，结晶水与配位水的脱除发生于不同温度区间，由此引发的晶体结构转变会伴随层间距的显著缩减以及结晶度的大幅降低。对原合成态与热处理后样品的红外（IR）、拉曼（Raman）及紫外-可见（UV–vis）光谱表征结果证实，结构基元与金属中心的八面体配位环境在结构转变后得以保持。对水分子脱除/吸附的可逆性与颜色变化进行测试后发现，当化合物在高于200℃的温度下热处理后，无法完全恢复至初始颜色。磁化率的热演化行为表明，高温下金属中心存在一维反铁磁耦合；对于NiCoBpe 3与NiBpe 4而言，低温下会出现磁有序，这可通过磁化率曲线出现的峰值得以证实。CoNiBpe 2被证实可作为催化剂，对醛的氰硅化反应具有催化活性。