Construction Of Extremely Large Cages In Metal-Organic Frameworks

Sculpturing three-dimensional(3D)space with specific size and geometry at molecular level is a key theme in both reticular chemistry and supramolecular chemistry,typically manifested in the form of cages.These cages,existed as either repeating units in extended structures,such as Metal-Organic Frameworks(MOFs),or individual molecules like molecular cage,provide unique environment to interact with guest inside,making them useful in the uptake,seperation and conversion of gases.Expanding the cage size into mesopore regime(2-50 nm)allows for the enclosure of larger guests,such as inorganic clusters,nanoparticles,drugs,proteins and nucleic acids.This greatly enriches the chemistry within the molecularly defined 3D space.However,as the cage size increases,the difficulty rises exponentially for the synthesis as well as the structure characterization.So far,the internal diameter of 3D molecularly defined artificial spherical cages,reported both in MOFs and as individual molecular cages,hit the limit around 6 nm.Pushing this size boundary forward is likely to extend our perception on the assembling principles of molecules,and open the access to larger guests for the exploration of new applications.In this thesis,we reported construction of extremely large 3D cages in MOFs and verification of internal space,design and synthesis of a series of mesoporous MOF crystals with recorded cage size and unit cell volume,pushing this size boundary forward.The main contents and innovations are as follows:(1)Structure design and simulation.In this work,the expansion efficiency was selected as evaluation criteria,by comparing expansion efficiency of different types of polyhedrons,yys and liu polyhedron with high expansion efficiency are selected as the basis to obtain larger cages.Because the yys and liu co-exist in the MOF-919 structure,so the MOF-919 was selected as parent MOFs,by elongation the organic linkers,to design and simulate the expanded structures.The MOF-929 structure with cage size of6.9 and 8.5 nm and the MOF-939 structure with cage size of 9.3 and 11.4 nm were successfully simulated.(2)Synthesis and characterization of organic linkers and MOFs.Four organic linkers varied in functional groups with length of 0.85 nm,and one organic linker with the lengths 1.3 nm,were successfully synthesized and applied to the synthesis of MOF-929 and MOF-939,respectively.10 mesoporous MOFs with extremely large 3D cage were obtained.A series of diffraction techniques were also used for the preliminary characterization of the structures,including small-angle X-ray scattering technique(SAXS),synchrotron powder X-ray diffraction technique(PXRD)and selected area electron diffraction technique,to confirm that the simulated structures were successfully prepared in experiment.The permanent porosity and pore size verified by nitrogen adsorption analysis,and the basic information of these structure were characterized using infrared spectroscopy,elemental analysis,X-ray photoelectron spectroscopy(XPS),and scanning electron microscopy(SEM).(3)Structure resolution and visualization.Firstly,we used diffraction techniques to reveal the structure in reciprocal space.The structure of MOF-919-Sc was successfully resolved using synchrotron single crystal X-ray diffraction,and the structures of MOF-929 and MOF-939 were resolved by combining various powder refinement techniques,including using Powley refinement to determine the unit cell parameters and space groups,using Le Bail refinement to generate electron density maps to verify their frameworks of MOFs and using Rietveld refinement to determine the precise atomic positions,bond lengths and bond angles.In addition,single crystal of MOF-929 was analyzed using three-dimensional electron diffraction(3D ED)techniques to verify the accuracy of the structural resolution.Finally,these cage structure in the MOF-929 structure was visualized in real space along different orientations using transmission electron microscopy techniques,such as HRTEM and i DPC-STEM technique,to study the position and spatial arrangement of these 3D cage in the MOFs.(4)Exploration the limit of constructing molecularly defined 3D space and verification of internal space.By systemically analysis of the areal density of chemical bonds in the construction of 3D space using molecules,we found that the average area supported by per chemical bond is the critical factor for the pore size expansion,and the polyhedron with high expansion efficiency can improve the utilization of chemical bonds and are suitable for the preparation of larger cages.In addition,we used nucleic acid molecules as probes to verify the internal space of these cages,and the results showed that MOF-929 and MOF-939 can efficiently extract total RNA and plasmid molecules from aqueous solutions,demonstrating that the 3D cages with specific geometry and large cage size are well suited for loading of biomolecules.