Metal-Organic Frameworks Constructed By Amino Acid-Functionalized Aromatic Polycarboxylic Acids: Synthesis And Properties

Metal-organic frameworks?MOFs?are a new type of inorganic-organic hybrid crystalline multifunctional materials.They are composed of metal nodes?metal ions or metal clusters?and linkers?organic ligands?.Owing to their high specific surface area and porosity,unsaturated metal active sites and unique optical and electrical properties,MOFs materials have huge potential applications in many areas,such as gas separation and storage,optical devices,catalysis,identification and resolution of enantiomers,magnetic materials,drug transmission,and so on.In the construction of coordination polymers with novel structures and expected functions,it is extremely important to select suitable metal ions and organic ligands.In this paper,based on a strategy of using natural chiral alanine or phenylalanine to modify aromatic polycarboxylic acid,two purely chiral rigid aromatic polycarboxylic acid ligands were synthesized.Using these ligands to assemble with metal ions,a series of purely chiral metal-organic framework?HMOFs?materials were constructed.The structures and related properties of resulting MOFs were determined.The main content of this paper is divided into the following two parts:In the first part,two new enantiopure aromatic polycarboxylic ligands,?S?-5-???1-carboxy-2-phenylethyl?amino?methyl?isophthalic acid?H₃L¹?and?S?-4'-???1-carboxyethyl?amino?methyl?-[1,1'-biphenyl]-3,5-dicarboxylic acid?H₃L²?,were synthesized by grafting L-phenylalanine and L-alanine to 5-methylisophthalic acid and 4'-methylbiphenyl-3,5-phthalic acid,respectively.By adjusting raction conditions,such as solvent types and their proportions,reaction temperature and time,metal salt types,and so on,six homochiral 2-3D coordination polymers and two centrallysymmetriccomplexes,namely,[Pb?HL¹?]_n?1?,{[Cu₂?HL¹?₂?bpe?]?H₂O}_n?6?,[Co₂?da?₂?H₂O?₆][Co?H₂O?₆]?4H₂O?7?and 4,4?-bipyridine,bpe=1,2-di-4-pyridylethene,da=5,5'-??dimethylammonio?bis?methylene??diisophthalate)were hydro/solvothermally Mn²⁺with the selected ligands.By means of X-ray single crystal diffraction?XRD?,infrared spectroscopy?IR?,thermal analysis?TGA?,X-ray powder diffraction?PXRD?,elemental analysis?EA?,fluorescence?FL?,circular dichroism?CD?and other experimental test methods,the structures and properties of the synthesized complexes were studied in detail.Crystallographic analysis indicated that the chiral aromatic carboxylic ligands H₃L¹ and H₃L²,which were synthesized by grafting flexible amino acid groups on rigid aromatic rings,not only had multiple coordination modes and existence forms?such as negative divalent zwitterion,divalent anion,and trivalent anion?in the construction of HMOFs,but also was easy to form single or double 2₁ helical chain substructures which could be further connected into the 2-3D helical structures 1-6 with different topological architectures.The chirality of these complexes was further confirmed by CD spectra.Consequently,when the chiral ligands H₃L¹ and H₃L² assembled with metal ions,the inherent chirality of the ligands was not only transferred to the synthesized complexes 1-6,but also induced to produce new spiral chirality,forming purely chiral metal-organic helical frameworks with various structures.In particular,through the assembly of ligand H₃L¹ and Cu^IIwith purposeful addition of auxiliary bridges bipy or bpe,two heterogeneous isostructural 3D homochiral porous coordination polymers 5 and 6 were synthesized,which suggested that the sizes of effective pores were successfully controlled by changing the length of the auxiliary bridges without changing the structures of metal-organic frameworks.Zero-dimensional metallocycle 7 and 1D coordination polymer 8 were synthesized by solvothermal reactions of chiral ligands H₃L¹ and metal ions Co^II or Mn^II in a mixture sovlent of water and DMF.Surprisingly,under this synthetic condition,the chiral ligand H₃L¹ reacted in situ to produce an achiral aromatic tetracharboxylate ligand da bound by dimethylamine to the two5-methylisophthalates.The results of fluorescent and second-order nonlinear optical property studies showed that complexes 1-4 have strong fluorescent emissions and complexes 2-4have obvious second-order nonlinear optical activities.In addition,the fluorescence recognition of common solvent molecules and metal cations was studied by using 2as fluorescent probe.It was found that the solvents,such as DMF?N,N-dimethylformamide?,DMAc?N,N-dimethylacetamide?and acetone,and as well as metal ion Fe³⁺had strong fluorescence quenching effects on the fluorescence of complex 2,indicating that complex 2 has potential applications in the visual detectionIn the second part,two isostructural Ln-HMOFs,namely[Eu?HL¹?Cl]_n?9?and[Tb?HL¹?Cl]_n?10?,were obtained through the assemblies of ligands with rare earth 5-connected 3D helical HMOFs composed of interesting coaxial triple helical chains.Ligand H₃L¹ not only transfered its inherent chirality to the synthesized Ln-HMOFs,but also induced the formation of novel helical structures.The purity and chirality of complexes 9 and 10 were further confirmed by PXRD and CD spectra.Photoluminescence investigation showed that ligand H₃L¹ have good sensitization to the luminescence of Eu^IIIand Tb^III,and complexes 9 and 10 emit strong characteristic emissions of the central ions Eu^IIIand Tb^III upon photoexcitation,respectively.The results of second-order nonlinear optical property studies displayed that 9 and 10 have good SHG effect and perhaps have potential application in nonlinear optical materials.