Usefulness of in Situ Single Crystal to Single Crystal Transformation (SCSC) Studies in Understanding the Temperature-Dependent Dimensionality Cross-over and Structural Reorganization in Copper-Containing Metal–Organic Frameworks (MOFs)

Two copper-containing compounds [Cu3(μ3-OH)2(H2O)2{(SO3)-C6H3-(COO)2}(CH3COO)], I, and [Cu5(μ3-OH)2(H2O)6{(NO2)-C6H3-(COO)2}4]·5H2O, II, were prepared using sulphoisophthalic and nitroisophthalic acids. The removal of the coordinated water molecules in the compounds was investigated using in situ single crystal to single crystal (SCSC) transformation studies, temperature-dependent powder X-ray diffraction (PXRD), and thermogravimetric analysis (TGA). The efficacy of SCSC transformation studies were established by the observation of dimensionality cross-over from a two-dimensional (I) to a three-dimensional structure, Cu6(μ3-OH)4{(SO3)-C6H3-(COO)2}2(CH3COO)2, Ia, during the removal of the coordinated water molecules. Compound II exhibited a structural reorganization forming Cu5(μ2-OH)2{(NO2)-C6H3-(COO)2}4], IIa, possessing trimeric (Cu3O12) and dimeric (Cu2O8) copper clusters. The PXRD studies indicate that the three-dimensional structure (Ia) is transient and unstable, reverting back to the more stable two-dimensional structure (I) on cooling to room temperature. Compound IIa appears to be more stable at room temperature. The rehydration/dehydration studies using a modified TGA setup suggest complete rehydration of the water molecules, indicating that the water molecules in both compounds are labile. A possible model for the observed changes in the structures has been proposed. Magnetic studies indicate changes in the exchanges between the copper centers in IIa, whereas no such behavior was observed in Ia.本研究以磺基间苯二甲酸与硝基间苯二甲酸为反应物，合成了两种含铜配位化合物：[Cu₃(μ₃-OH)₂(H₂O)₂{(SO₃)-C₆H₃-(COO)₂}(CH₃COO)]（记为I）与[Cu₅(μ₃-OH)₂(H₂O)₆{(NO₂)-C₆H₃-(COO)₂}₄]·5H₂O（记为II）。通过原位单晶-单晶（single crystal to single crystal，SCSC）转变测试、变温粉末X射线衍射（powder X-ray diffraction，PXRD）与热重分析（thermogravimetric analysis，TGA），系统探究了这两种化合物中配位水分子的脱除过程。研究中观测到化合物I在配位水分子脱除过程中发生了维度交叉转变，从二维（2D）结构转化为三维（3D）结构Cu₆(μ₃-OH)₄{(SO₃)-C₆H₃-(COO)₂}₂(CH₃COO)₂（记为Ia），这一结果证实了SCSC转变测试的有效性。化合物II则发生结构重排，生成了含有三聚铜簇(Cu₃O₁₂)与二聚铜簇(Cu₂O₈)的Cu₅(μ₂-OH)₂{(NO₂)-C₆H₃-(COO)₂}₄（记为IIa）。PXRD测试结果显示，三维结构Ia为亚稳态且不稳定，冷却至室温后会恢复为更稳定的二维结构I。化合物IIa在室温下具备更高的结构稳定性。采用改良热重分析装置开展的脱水/复水实验表明，脱除的水分子可完全实现复水，证明两种化合物中的配位水分子均具有解离活性。针对实验中观测到的结构演化现象，本研究提出了一种可行的作用机制模型。磁学表征结果显示，IIa中铜中心之间的磁交换作用发生了显著变化，而Ia中未观测到此类磁行为变化。