Three Dimensional Porous Organic Polymers: Metal Coordination Guidance Synthesis And Their Properties

Covalent organic frameworks(COFs),a novel class of crystalline porous organic polymers with highly ordered framework structures,have attracted wide interest in adsorption,separation,catalysis,and optoelectronics.The construction of three-dimensional(3D)COFs is still a huge challenge due to the limited types of organic building blocks and available topology networks.In this topic,we aimed to develop a new strategy based on octahedral metal complex to construct 3D COFs with novel topological networks,and focus on the adsorption and photocatalytic reactions properties of these 3D COFs.The main contents are as follows:1.Design and synthesis of octahedral metal complex building unit:A C₃ symmetric octahedral complex Na₂Ti(2,3-DHTA)₃ with six aldehyde reaction nodes was designed and synthesized based on the coordination strategy of organic ligand2,3-dihydroxyterephthalaldehyde(2,3-DHTA)and titanium oxide acetylacetonate.The structures of Na₂Ti(2,3-DHTA)₃ was studied by FT-IR,NMR and ESI-MS.X-ray single-crystal structure determination revealed that Ti(IV)center coordinated with six oxygen atoms from the three 2,3-DHTA ligands,forming an octahedral Ti O₆ coordination geometry,with an average Ti-O bond length of 1.966(?).The six aldehyde groups of Na₂Ti(2,3-DHTA)₃were found to be oriented in six different directions with C₃ symmetry.The complex Na₂Ti(2,3-DHTA)₃ can be used as a new building block through metal coordination guidance to design new 3D COFs,which will build a good foundation for the directional synthesis of3D COFs.2.Synthesis and photocatalytic Meerwein addication properties of 3D covalent organic framework with soc topology:The first 3D anionic titanium-based COF(Ti-COF-1)with the edge transitive(6,4)-connected soc topology was constructed by using octahedral Ti(IV)complex Na₂Ti(2,3-DHTA)₃ and Py TTA as building units through metal coordination guidance.The formation of C=N bonds in Ti-COF-1 was revealed by FT-IR and ¹³C CP-MAS solid state NMR.Powder X-ray diffraction(PXRD)showed that Ti-COF-1 has good crystallicity.SEM and TEM analysis showed that Ti-COF-1 is consisted of uniform cubes.Structure simulated analysis showed that Ti-COF-1 has a soc topology network,which contains cage-like cavities and three dimensional open channels.Nitrogen isothermal adsorption/desorption analysis of Ti-COF-1 showed a high BET surface area of 1000.4 m²g^-1 and the pore size of 1.55 nm.Ti-COF-1 showed the excellent cationic dyes adsorption performance,and the saturated adsorption capacity of Rh B was up to 361.2 mg·g^-1.Moreover,Ti-COF-1 showed a broad absorption band in UV spectrum with the optical energy gap of1.86 e V,and exhibited high photocatalytic activity and cycling stability towards Meerwein addition reactions.Ti-COF-1 with soc topology demonstrates an attractive strategy for designing 3D functional COFs and the application of photocatalysis.3.Synthesis and photocatalytic heteroarenes arylation properties of 3D covalent organic framework with spn topology:A 3D anionic framework Ti-COF-2 with spn topology was constructed via aldehyde amine condensation of octahedral Ti(IV)complex Na₂Ti(2,3-DHTA)₃ and TAPT.FT-IR and ¹³C NMR spectrum demonstrated the formation of Ti-COF-2.PXRD and simulated structure analysis confirmed that Ti-COF-2 has a crystal structure with cage-like cavities and three dimensional channels through spn topology.SEM and TEM analysis showed that Ti-COF-2 exhibits uniform petal-like morphology,good crystallinity and uniform nanopores.N₂ adsorption-desorption measurements offered a high BET surface area of 381.3 m² g^-1 with the 2.4 nm pore size.Ti-COF-2 showed the excellent cationic dyes adsorption performance,and the saturated adsorption capacity of Rh B was up to364.0 mg·g^-1.In addition,Ti-COF-2 exhibits the wide UV absorption with an energy gap of about 1.93 e V,which makes it show high photocatalytic activity and cyclic stability in the C-H arylation of heteroarenes.We believe that the construction strategy of Ti-COF-2 will extend to enrich the structure and functionality of 3D frameworks.