Synthesis,structure And Properties Of Coordination Polymers Based On 4,4',4"-nitrilotribenzoic Acid And 3,3',5,5'-azobenzenetetracarboxylic Acid Ligands

Metal-organic frameworks?MOF?,also known as porous coordination polymer?PCP?,is a kind of crystalline porous materials with infinite periodic network structure.They can exhibit intriguing topological structures as well as have potential applications in gas adsorption/separation,heterogeneous catalysis,proton conduction,nonlinear optics and magnetism.The carboxylate ligands with diverse coordination modes are extensively studied for construction of MOFs.Herein the 4,4',4''-nitrilotribenzoic acid?H₃-TCA?and 3,3',5,5'-azobenzenetetracarboxylic acid?H₄-ABTC?ligands are selected for construction of MOFs.By virtue of the flexibility of the two ligands,they can exhibit various configurations and adopt suitable configuration in a certain structure,leading to compounds with diverse structures.Reactions of the two carboxylate ligands with Co?II?,Tb?III?and Dy?III?ions under solvothermal conditions yielded ten novel coordination polymers with the formula as following:?1?{[Co₈??₃-OH?₄?TCA?₄?H₂O?₄]}·6DMF·2.5H₂O}_n?2?{[Co₃?TCA?₂?MeNH?₂]·2DMF·4H₂O}_n?3?{[Tb?TCA??H₂O?₂]·2CH₃CH₂OH·1.5H₂O}_n?4?[Ln?TCA?]_n?Ln=Tb?3?and Dy?4???5?{[Ln₂?H-ABTC?₂?DMF?₄]·?DMF?₂·?H₂O?}_n?Ln=Tb?6?and Dy?7???6?{[Ln?H-ABTC??H₂O?₄]·?H₂O?₃}_n?Ln=Tb?8?and Dy?9???7?{?Me₂NH₂?₃[Tb₃?ABTC?₃?H₂O??DMF?]·6DMF·2.5H₂O}_n?10??H₃-TCA:4,4',4''-nitrilotribenzoicacid,H₄-ABTC:3,3',5,5'-azobenzenetetracarboxylic acid,DMF:N,N'-dimethylformamide?This paper is divided into the following four parts:The first chapter is the preface,which mainly introduces the concept of metal-organic framework,the synthesis methods of metal-organic framework,the influence of reaction conditions on structures and properties of the metal-organic frameworks.Five compounds are described in chapter two.In compound 1,four tridentate hydroxyl groups link eight Co?II?ions to give a octanuclear[Co₈??₃-OH?₄]secondary building unit.Each[Co₈??₃-OH?₄]unit is surrounded by twelve TCA^3?ligands,which bridge the[Co₈??₃-OH?₄]units to form a three-dimensional?3D?framework with one-dimensional?1D?channels.Compound 2 was obtained by altering the ratio of the starting materials for synthesis of compound 1.Compound 2 has a two-dimensional?2D?layered structure.In compound 2,three Co?II?ions are connected by the carboxylate groups of TCA^3?ligands to give a linear trinuclear Co₃ unit wherein each terminated Co?II?ion is coordinated by a capped dimethylamine molecule derived from hydrosis of DMF solvent.Each trinuclear Co₃ unit is linked by six TCA^3?ligands to generate a 2D structure.The dimethylamine molecules occupying the coordinated sites of the two terminate Co?II?ions of the linear Co₃ units lead to the formation of 2D layer,but not 3D structure.The 2D layers are packed in an off-side fashion resulting in a nonporous structure of compound 2.Therefore,diverse structures can be obtained by varying of the reaction conditions.Such preparation approach was also used for synthesis of compounds 3-5.In compound 3,the Tb?III?ions are bridged by carboxylate groups of the TCA^3?ligands to form a 1D chain.Each TCA^3?ligand links three 1D Tb-carboxylate chains to give a 3D framework featuring 1D channels with solvents filled in.Compound 4 was obtained by altering of the reaction condition for synthesis of compound 3.Compound5 has an isostructual with compound 4.The 1D Tb-carboxylate chains are connected by the TCA^3?ligands to give the 3D structure of 4,which is similar to that of structure3.It shold be noted that compound 3 is a porous structure with the solvent occupying the volumn of the unit cell of 40.2%,while compound 4 is nonporous.The Tb?III?ions are eight-coordinated in compound 3 and 4.The Tb?III?ion in compound 3 is coordinated to six carboxylate oxygen atoms from five TCA^3?ligands and two coordinated water oxygen atoms,while Tb?III?ion binds to eight carboxylate oxygen atoms from seven TCA^3?ligands.As a result,the 1D Tb-carboxylate chains in compound 4 is more compact than those in compound 3.Such a result leads to a porous structure 3 and a nonporous structure 4.As described above,compound 1 has a 3D framework with 1D channels.The voids of the channels is occupied by solvent molecules,which accounts for 58.4%volumn of the unit cell.The gas adsorption/separation properties of compound 1 are investigated in detail.The permanent porosity of compound 1 is confirmed by the N₂ adsorption isotherm at 77 K,which give a BET surface area of 1195 m³/g.The adsorption data of the C₂H₂,C₂H₄ and C₂H₆ showed that compound 1 adsorb C₂H₆ than other two gases,which is much different from that of other MOFs.Due to the C₂H₂ having the largest polarizability,the material will exhibit the largest uptake for C₂H₂ molecules and the smallest uptake for C₂H₆.Thus compound 1 exhibits a reverse adsorption behavior for C₂H₆ and C₂H₂ gases,which is rarely observed for MOFs.The ideal adsorbed solution theory calculation?IAST?was conducted on the equimolar C₂H₆/C₂H₂ mixture,which gives the adsorption selectivity of 0.78.Moreover,the C₃H₄,C₃H₆ and C₃H₈ adsorption data were also collected.Compound 1 shows a highest uptake for C₃H₄ and the smallest uptake for C₃H₈.The adsorption selectivities for equimolar C₃H₄/C₃H₆ and C₃H₄/C₃H₈mixtures are 2.6 and 1.8,respectively.Thus an effective C₃H₄/C₃H₆ and C₃H₄/C₃H₈separation is achieved for compound 1.It is still a challenge for separation of C₃H₄/C₃H₆ mixture due to their have similar molecular sizes.The present compound 2is demonstrated as an excellent candidate for separation of C₃H₄/C₃H₆ mixture.Compound 3 has a 1D channels that expected to show porous.However,attempts to remove of the guest solvents from the channels are unsuccessful.The activation of the compound 3 needed to be further studied.For the nonporous compound 4,its luminescent properties are studied.Compound 4 shows the characteristic Tb?III?emission.The chapter three describes five compounds based on the 3,3',5,5'-azobenzenetetracarboxylic acid?H₄-ABTC?ligand and lanthanide ions.Compound 6and 7 are isomorphous.The structure 6 is described representatively.The uncoordinated carboxylate group in 3,3',5,5'-azobenzenetetracarboxylic ligand in compound 6 and 7should be protonated based on the charge-balance.Firstly,two Tb?III?ions are linked by carboxylate groups to form a dinuclear[Tb₂?COO?₂]secondary building unit,which is further linked by the H-ABTC^3-ligands to give a 3D framework with 1D channels.DMF and water solvents fill the 1D channels.Compound 8 was obtained by changing the reaction condition for synthesis of compound 6.Compound 9 is isomorphous with compound 8.Similar to those of compound 6 and 7,the uncoordinated carboxylate group in 3,3',5,5'-azobenzenetetracarboxylic ligand in compound 8 and 9 should be protonated.In compound 8 and 9,the lanthanide ions are linked by the H-ABTC^3-ligands to form a 1D double chain.Compound 10 is based on two different metal nodes.The 3D structure is formed by the single Tb?III?nodes and dinuclear Tb₂ nodes linking by ABTC^4-ligands.Research efforts to remove of the solvent molecules from the pores of compound6,9 and 10 are not successful.So the activation of samples of these compounds should be further investigated.Chapter four is the summary of this paper.