Synthesis And Properties Of Functionalized Calcium-phosphonate Monoester Frameworks

Compared with metal-carboxylate coordination polymers,metal-organophosphonates have stronger coordination bonds and better thermal and chemical stability,thus showing more advantages and prospects in the fields of ion exchange,catalysis,proton conduction,biomedicine,and magnetism,etc.Phosphonate monoester（PME）group maintains monoanionic and bidentate oxygen donor set as similar to a carboxylic group,which in principle may show similar charge and coordination mode as carboxylic acid ligands.The monoester functional group of PME-based coordination polymer can not only change the inherent dense coordination tendency of metal phosphonates but also can be used as a unique functionalization method to achieve regulation and modification of the phosphonate skeleton.However,reported PME coordination polymers are mainly based on PME ligands with non-polar alkyl groups,and the types of ester groups and their regulation on properties of phosphonate skeletons need to be expanded and further studied.Based on this,we report in this thesis the design and synthesis of two phosphonate monoester coordination polymers Ca-L₁（H₂L₁=1,4-benzenediphosphonic acid bis（monomethoxyethyl ester））and Ca-L₂（H₂L₂=1,4-benzenediphosphonic acid bis（monoethyl ester））and their hydrophilicity-hydrophobicity,water stability,ion conductivity as well as”top-down”nanosheet exfoliation.The main contents are as follows:（1）The phosphonate monoester ligands Na₂L₁ and H₂L₂ functionalized with methoxyethyl and ethyl tethers were designed and synthesized,and two calcium-PME coordination polymers,Ca-L₁,and Ca-L₂ were constructed through solvothermal reactions.Single crystal X-ray diffraction study shows that Ca-L₁ adopts a layered structure,in which one-dimensional zig-zag（Ca O₈）_n inorganic chains and phosphonate ligands assemble into a two-dimensional coordination layer and monoester functional groups extend away from both sides of the layer.The coordination layers are stacked by van der Waals interaction to form a three-dimensional supramolecular solid.The structure of Ca-L₂ is determined through structural modeling,indexing,and Le Bail refinement.The main difference between the two is that the different PME functional groups and interlayer spacings.The coordination number of calcium atoms,coordination mode of phosphonate groups,and cell parameters are consistent or close between the two.（2）We explored the hydrophilicity-hydrophobicity and water stability of Ca-L₁ and Ca-L₂ materials.Ca-L₁ based on methoxyethyl phosphonate monoester is hydrophilic（CA=28°）and dissolves quickly in liquid water;on the other hand,Ca-L₂ based on ethyl phosphonate monoester is hydrophobic（CA=108°）,and the main part can maintain a stable structure after immersing in liquid water for 72 hours with the morphology unchanged.It shows that different monoester functional groups have significant effects on the hydrophilicity-hydrophobicity and water stability of the materials.In addition,the unique layered structure also helps to maximize the effect of the ester group on the hydrophobicity and water stability of the phosphonate skeleton.（3）The lithium-ion conductivities of Ca-L₁ and Ca-L₂-derived solid-state electrolytes at room and elevated temperatures were studied.Electrochemical impedance spectroscopy results show that compared with the ethyl functional group,the methoxyethyl functional group can reduce the migration energy barrier of Li⁺in the crystalline grains.Therefore,Ca-L₁ materials have higher ionic conductivities and lower Arrhenius activation energies than Ca-L₂ materials.（4）Ca-L₁ and Ca-L₂ bulk crystalline materials were successfully peeled off into stable single-layer and several-layer nanosheets by“top-down”ultrasonic peeling.AFM study shows that the thickness of the thinnest Ca-L₁ and Ca-L₂ nanosheets is 1.2 nm and 1.0 nm,respectively,close to theoretical monolayer thickness.This is the first research report on phosphonate monoester framework nanosheets.PXRD and IR confirmed that the two nanosheets maintain the same structure as the bulk solid.TEM,SEM,and DLS further revealed that the two nanosheets exhibited typical thin-layer morphological features such as size dispersion,larger lateral size,and high aspect ratio.