Ionothermal Synthesis,Structural Characterization And Properties Of P-xylylenediphosphonate Coordination Polymers

Metal phosphonates as significant organic-inorganic hybrid materials have been extensively pursued as gas adsorption separation,ion-exchange materials,catalysts,proton conductors and so forth.Phosphonate group can provide more modes of coordination and myriad different arrangements that structures can take,then forming stronger bonds compared with most compounds based on the carboxylic acid ligand,that is of significance to design and prepare metal phosphonates with rich structures and high thermal stability.Most of metalphosphonates,to date,were synthesized by using hydro/solvothermal method,and the solvent used are often crucial to final structures.Ionothermal synthesis has been developed by using ionic liquids or deep eutectic solvents as the solvents and potential templates,showing unique advantages to prepare novel inorganic porous crystalline solids.Deep eutectic mixtures are generally composed of quaternary ammonium salts and hydrogen-bond donors,its species and structure are very rich and dynamic and its price is lower.Ionic liquids with rich species and disignable properties provide a new approach to synthesize novel metal phosphonates.In this paper,two deep eutectic mixtures?oxalic acid/quaternary ammonium salts and urea/quaternary ammonium salts?were selected as the solvent and source of template or the structure-directing agent,the p-xylylenebisphosphonate ligands?H4xbp?as the ligand coordinated with metals zinc or copper.As a result,seven novel crystalline metal diphosphonates were successfully ionothermally synthesized by adjusting the physical and chemical properties of the system and optimizing the synthetic conditions,and their structures were determined by single-crystal diffraction data.Powder XRD,CHN element analysis,FT-IR,TG-DSC and SEM were also illustrated characterize these samples.The main contents and conclusions are as follows:1.In the deep eutectic solvent of oxalic acid and tetramethylammonium chloride,the effect of adding a small amount of water and NaOH on the reaction results was systematically studied,and four novel crystalline zinc diphosphonates,Zn₂?xbp??I?,Zn₂?H₂O?₂?xbp??II?,Na2Zn3?H₂O?₂?xbp?₂?III?and NaZn?H₂xbp?0.5?xbp?0.5·1.5H₂O?IV?were ionothermally synthesized from tetraethyl-p-xylylenebisphosphonate?Texbp?and Zn?OAc?₂.They had a similar characteristic of inorganic layers pillared by organic parts of ligands on the whole.The introduction of sodium ions led to two Na-containing zinc diphosphonates?III and IV?,wherein the sodium ions adopt in NaO4 tetrahedra or NaO5 distorted hexahedra in the inorganic layers.Moreover,the phase transition behavior of compound IV to compound II was systematically studied.Powder XRD patterns and SEM images of the isolated samples were collected to detect the phase-transformation process,and the concentration change of Na+ ions in solution with different soaking time was measured by ICP method.The phase-transition behavior from compound IV to II occurs after soaking in water at room temperature,experiencing the process of bond cleavage,Na+ ions dissociation,and self-assembly between Zn₂+ ions and ligands.2.The compound Cu₂?H₂O?₂?xbp??V?and an unknown compound VI were obtained from tetraethyl-p-xylylenebisphosphonate?Texbp?and Cu₂₊ in the deep eutectic solvent of oxalic acid/choline chloride.The compound Cu₂?H₂O?₂?xbp??V?has similar three-dimensional structure to the reported compound in the literature.However,the template or structure-directing role of the quaternary ammonium salt cation did not play in the final structures.3.In the deep eutectic solvent of urea/protonated organic amines,two novel crystalline zinc diphosphonates-compound VII and compound?CH₃NH₃?₄Zn₂?xbp?₂·H₂O?VIII?were obtained from tetraethyl-p-xylylenebis phosphonate?Texbp?and Zn₂+.Because the size of the compound VII is too small,its structure is to be solved.But the results of CHN elemental analysis,IR and TG-DSC analysis showed that NH4+ derived from urea as the structure-directing agent.The compound?CH₃NH₃?₄Zn₂?xbp?₂·H₂O?VIII?have a special three dimensional open framework with rhombic channels.It is found that reducing the coordination number of the phosphonate group just like dicarboxylate linkers in MOFs.The organic cations [CH₃NH₃]+ played a structure-directing role and make the compound being neutral and stabilize framework structure.