| The construction of novel and specific functional metal-organic supramolecular compounds through metal-ligand coordination and intermolecular interactions (hydrogen bonds, ?···? stacking, etc.) has become an important research subject in coordination chemistry, supramolecular chemistry and crystal engineering. In this work, we mainly focus on the research of crystal-to-crystal transformation based on our previously investigated crystal transformation system of CuII-3-pyrazinyl-5-(4-pyridyl)-1,2,4-triazole. Three comparable triazole/pyrazole ligands were prepared and characterized, along with two ranges of coordination compounds based on ZnII and CuII, both showing intriguing crystal-to-crystal transformation phenomena. The whole thesis consists of four chapters.Chapter 1 briefly reviews literatures on crystal transformation, and introduces the purpose and significance of this work. In this chapter, the research background is concisely introduced, focusing on i) the relationship between supramoleculor chemistry and crystal engineering, ii) current researches about triazolate complexes, iii) researches about Jahn-Teller effect in transition complexes and iv) host-guest chemistry on crystal-to-crystal transformation.Chapter 2 focuses on the crystal transformation system of ZnII complexes. Three ligands, namely, 3-pyrazinyl-5-(4-pyridyl)-1,2,4-triazole (L1), 3-(2-pyridyl)-5-(4-pyridyl)-1,2,4-triazole (L2) and 3-(2-pyridyl)-5-(4-pyridyl)-pyrazole (L3), are prepared, and six new ZnII complexes, namely, [Zn(L1)2] n (1), {[Zn(L1)2]·CH3OH}n (2), [Zn(L1)2(H2O)] (3), [Zn(L2)2]n (4), [Zn(L2)2(H2O)] (5) and [Zn(L3)2] n (6), based on these three ligands are synthesized under solvothermal conditions. Of the six complexes, 1, 4 and 6 are isomorphic with 3D frameworks in R-3 space group; 3 and 5 are isomorphic with discrete structure; 2 is a 2D framework. The crystal transformations occur among 1, 2 and 3 upon experimental controls. The transformation of 1 to 2 is made by heating the crystals of 1 in methanol/glycol reflux vapor, and when the crystals of 2 are heating in vacuum, 1 can be obtained. The crystals of 2 are exposed in the air at room temperature, spontaneously changing into 3, while one can transform 3 to 2 by solvothermal reaction in the methanol. When heating the crystals of 1 in water reflux vapor, 3 can be obtained; however, 3 can not transform to 1. In comparison, the crystals of 4 can spontaneously change into 5 by exposing them in the air at room temperature, but the transformation is irreversible, and complex 6 shows thermal stability and chemical stability. The above-mentioned transformations are characterized by several physical measurements such as thermogravimetric analysis (TGA), crystal X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), temperature-dependent X-ray powder diffraction (T-PXRD), etc. The interpretation of these transformations is also attempted by considering thermodynamic aspects.Chapter 3 focuses on the crystal transformation system of CuII complexes. Six new CuII complexes, namely, [Cu(L1)2(H2O)] (7), [Cu(L1)2(NH3)2]·6H2O (8), [Cu(L1)2(NH3)2]·4H2O (8'), [Cu(L1)2(NHCH3)2]·2CH3CH2OH (9) , [Cu(L1)2(NH2CH3)2] (9') , [Cu(L1)2(H2O)2] (10), are synthesized through crystal transformation procedures. All the six complexes are with 0D structures, of which 8-10 are six-coordinated with octahedral configuration and only 7 is five-coordinated with square-pyramid configuration. In this system, 7 is used as a starting material and is reacted with ammonia or methylamine to prepare 8, 8', 9 and 9', which are all considered as intermediate products. Moreover, 8 and 8'can react with methylamine to produce 9', and 9 and 9'can react with ammonia to produce 8. All these intermediate products can be exposed in the air at room temperature to spontaneously change into 10, and the reversible transformation of 10 to 8 in ammonia vapor or 10 to 9'in methylamine vapor can be realized. In complex 7-10, the typical Jahn-Teller effect in CuII ions, which adopt pseudo-octahedral coordination, is observed. Furthermore, the crystal transformation in this system undergoes ligand substituent reactions, accompanying with Jahn-Teller switching phenomenon. The above-mentioned transformations are characterized by several physical measurements such as thermogravimetric analysis (TGA), crystal X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), etc. The interpretation of these transformations is also attempted by invoking ligand field splitting energy differences.Chapter 4 is the conclusion of this thesis.
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