Synthesis And Property Of MOFs Based On C₂ Low Symmetry Multicaboxylate Ligands

Metal–organic frameworks（MOFs）are kinds of novel porous hybrid materials consisted of inorganic second building units（SBUs）and organic SBUs,which have shown great potential applications in many fields.Undoubtedly,the properties of MOFs are greatly influenced by the structures,which can be regulated with the variety of inorganic or organic SBUs.Owing to the in suit synthesis method which is often employed to construct inorganic SBUs,the shapes and connectivities are difficult to be regulated directionally.Therefore,organic SBUs with different shapes,sizes,symmetry,functional groups and connectivities are often applied to assemble functional MOFs.Because of the flexible coordination modes between carboxyl groups and metal ions,carboxylate ligands,especially with high symmetry（such as linear and rectangle carboxylates）,are often selected.However,the coordination modes between ligands with high symmetry and metal ions are often unitary,which are prone to form predictable topologies,such as pcu,fcu,ftw,shp and so on.Nowadays,carboxylate ligands with reduced-symmetry are often employed to construct novel structures,in addition,three main strategies are applied to form the reduced-symmetry,（1）partial replacement;（2）partial truncation and（3）partial extension or carboxylate addition/rearrangement.Reduced-symmetry ligands with diverse pairs of chemically inequivalent carboxylates are tend to enhance the coordination flexibility of carboxylates and metal ions,which is beneficial for the formation of inorganic SBUs with various connectivity.Unprecedented topologies as well as diverse pore environments with differernt sizes and active sites are inclined to be formed in the structure.The regulation of the structures by reducing the symmetry of ligands may realize the enhancement on properties for gas adsorption and separation,catalysis and so on.Based on the linker desymmetrization strategy,two reported ligands,H₄CPTTA（5′-（4-carboxyphenyl）-[1,1′:3′,1″-terphenyl]-3,4″,5-tricarboxylic acid）and H₅CPTPTA（5′-（4-carboxyphenyl）-[1,1′:3′,1″-terphenyl]-3,3″,5,5″-tetracarboxylic acid）,which derived from the rearrangement of carboxyl groups between two extended ligands with C₃-symmetry:BHB and BTB（BHB=3,3′,3″,5,5′,5″-benzene-1,3,5-triylhexabenzoic acid,BTB=1,3,5-benzenetribenzoate）were selected.Two kinds of chemically inequivalent carboxylates in ligands are tend to form multiple coordination modes with metal ions and construct MOFs with different structure features.Nine novel MOFs were constructed with H₄CPTTA and H₅CPTPTA by regulating the coordionation modes of inorganic SBUs,modulating active sites and changing interpenetration degrees.Owing to various features of different compounds,gas adsorption and separation,detection of explosives,the conversion of CO₂ were investigated.The results are following three parts:（1）By regulating the coordionation modes of inorganic SBUs,three novel Zn-MOFs with different inorganic SBUs were constructed,and their properties were investigated owing to different structural features.Compound 1 was considered as a new（4,4,6）-connected topology.[Zn₃（COO）₄（H₂O）₄]and[Zn₃（COO）₆（H₂O）₂]clusters existed in the structure,and the compound showed excellent gas adsorption performance and separation capacity for C₃H₈/CH₄ mixtures.In compound 2,1H-[1,2,3]triazole（HTZ）was introduced and[Zn₄（TZ）₃]clusters were formed.Furthermore,H₄CPTTA was selected to construct a novel two-fold interpenetrated anionic framework Zn-MOF with hea topology.Owing to the large void volume and low-polar surfaces of the anionic framework,compound 2 showed high performance of C₂H₆/C₂H₄ selectivity.To enhance the separation performance for C₂H₆/C₂H₄,by utilizing post-synthetic ion-exchange strategy,compound 2-Li was successfully obtained with the exchange of dimethyl amine ions in the channels to Li⁺ions.With more exposed aromatic surfaces,the selectivity of C₂H₆/C₂H₄ increased apparently.The second carboxylate ligand,3,3′,5,5′-biphenyltetracarboxylate（H₄BPTC）was selected and introduced to the reaction system,a novel MOF,compound 3 with[Zn₃（μ3-OH）（COO）₄（COOH）₂（H₂O）₂]clusters was synthesized.It possessed a new（4,4,6）-connected topology and exhibited good performance of explosives detection.（2）By modulating active sites,two novel MOFs with different active sites,compounds 4 and 5,were constructed,respectively.Compound 4 with flu topology was successfully constructed with Zn²⁺and H₄CPTTA.The uncoordinated carboxylic acid sites（regarded as Br（?）nsted acid sites,BAS）from[Zn₃]clusters afforded a good conversion in CO₂-epoxide catalytic cycloaddition processes.In order to enhance the catalytic effect,compound 5 was assembled by a reduced-symmetry H₅CPTPTA,leading to three types of channels with various functional sites.Furthermore,the smaller channel exists as OMS and BAS,whereas the larger one only exists as OMS.The synergistic catalysis of OMS and BAS makes compound 5 exhibit excellent CO₂cycloaddition efficiency towards propylene oxide.（3）By changing interpenetration degrees,compounds 6-9 with different interpenetration degrees and dia topology were constructed.In compounds 6 and 7,two kinds of distorted and unsaturated inorganic SBUs were formed owing to the reduced-symmetry of H₄CPTTA.Although the stability of the interpenetrated compound 7 was improved compared to compound 6,the structure still collapsed partially after removing the guest molecules.According to the hard/soft acid/base（HSAB）principle,high-valent metal ions,Bi³⁺and In³⁺were selected to construct compounds 8 and 9,respectively.As expected,compounds 8 and 9 exhibited saturated metal ions and the formation of the two-fold interpenetrated structure further enhance the stability,which could remain stable in water and acid and alkali aqueous solution.In addition,the different interpenetration degree of the single net between compounds 8 and 9 formed different apertures,which led to the diverse performances of“gate-opening”towards gas molecules.