Syntheses,Structures And Dielectric Properties Of Ionic Co-crystals Based On Rare Earth Nitrate And Imidazole Derivatives

The study of molecular motion in solid state can not only help us understand the relationship between the dynamic structural changes of microscopic molecules units and their macroscopic physical properties,but also guide the design of related functional molecule materials.Generally,molecules or ions within crystals only show small-amplitude restricted vibrations near the equilibrium position owing to their confinement in a highly close-packing environment.To enable large-amplitude molecular/ionic motions in crystalline solids,two basic approaches that aim at creating motion-free confined space,i.e.,engineering porous crystals with permanent porosity or non-porous crystals with loose and incompact packing,have been widely carried out over the past few decades.Following the latter approach,the realization of large-amplitude confined ionic motions such as swinging,flipping,rotating or hopping,have been intensively explored within a large number of ionic co-crystals consisting of discrete cation-anion groups bound together by electrostatic attraction.Up to now,most of these reported zero-dimensional ionic crystals are the combinations of various-size near-spherical anion and cation,whether with rigid or flexible skeletons,and a few are the associations of compass-shaped charge-transfer complexes.Nevertheless,the simultaneous combinations of near-spherical and compass-shaped ion-pair are barely explored.From the perspective of crystal engineering,it is often difficult to achieve an effective stacking/assembly with different-shape rigid molecules as the secondary building blocks.However,to some extent,such a characteristic is just what we desired,which may be utilized as a useful strategy for promoting the local molecular motion in solid state.As a tentative research work to address the above-mentioned issue,we selected a medium-sized near-spherical anionic group and a compass-shaped aromatic ammonium as two distinct but rigid building blocks,and our research yielded four new ionic co-crystals.Their crystal structure and microscopic molecular motion were studied in detail by means of variable-temperature single-crystal X-ray diffraction,differential scanning calorimetry and variable temperature/frequency dielectric measurements.The main research contents are as follows:（1）Ionic co-crystals（HMImz）₃[La（NO₃）₆]（1）and（HMImz）₃[Nd（NO₃）₆]（2）were synthesized by the reaction of lanthanum nitrate and neodymium nitrate with N-methylimidazole nitrate（abbreviated as（HMImz）NO₃）,respectively.In these two isomorphic compounds,the medium-sized near-spherical[La（NO₃）₆]^3-anionic groups and the compass-shaped[HMImz]⁺cations are stacked mainly through some weak hydrogen bonds and the non-covalent Coulomb interaction as well as van der Waals interaction,to form a zero-dimensional supramolecular compound.Through characterizations,we found that the different confined spaces of three coexisting（HMImz）⁺ammoniums are fine-tuned over a very broad temperature range,and one of these（HMImz）⁺ammoniums displayed dynamically disordered due to local incompact stacking.Interestingly,such a unique effect can be utilized to modulate the dipolar polarization across a quite wide temperature.（2）Ionicco-crystals（HMImz）₂[Eu（NO₃）₅（H₂O）]（3）and（HMImz）₄[Er（NO₃）₅]（NO₃）₂（4）were synthesized by the reaction of europium nitrate and erbium nitrate with N-methylimidazole nitrate,respectively.These two zero-dimensional supramolecular compounds have different components and structures,both of which are assembled by weak intermolecular forces such as Coulomb interaction.Due to the poor thermal stability,only the structures of these two compounds are mainly described in this thesis.（3）On the basis of compound 1,the molecular solid solution of compound 1 can be successfully obtained by replacing the isomeric N-methylimidazolium with 4-methylimidazolium which is almost the same in shape and size but has a large difference in electric dipole.Further analysis and test show that the dipolar polarization property can be tuned effectively by changing the proportion of cationic component in the molecular solid solution.