Preparation And Properties Of Chiral Tricarboxylic Acid Metal Complexes

The abundant natural enantiomers of organic molecules,such as amino acids,lactic acid nature,tartaric acid,should have served to be ideal origins for the formation of chiral metal organic frameworks(HMOFs)since they are cheap,non-toxic and readily available.However,due to their flexible structures and small size,it is difficult to develop multidimensional HMOFs from these natural products.In order to overcome these shortcomings,we designed and synthesized a pair of novel chiral tridentate carboxylic acid ligands(R,R)-H3L and(S,S)-H3L by the reaction of chiral lactic acid and rigid 3,5-dihydroxybenzoic acid.9 pairs of HMOFs,namely,{[Ln2(L)2(H2O)3]·4H2O}n {Ln=Pr,Nd}、{[Ln2(L)2(H2O)2]·4H2O}n{Ln=Sm,Eu,Gd,Tb,Dy,Ho,Er},were prepared by hydrothermal reactions based on rare earth elements(Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Er).The crystal structures of the HMOFs were determined by X-ray single crystal diffraction.The results show that they are all crystallized in the chiral space group with two different structures.Complexes 1-4 crystalized in P21212 space group,demonstrating a 3D porous topological structure with a point symbol of(43·62·8)(45·65)(48·67)(49·65·8)(49·6),whereas complexes 5-18 with P21 space group exhibited a 3D binuclear topological structure with a point symbol of(4·62)(44·62)(48·611·82).Such structural difference could be ascribed to the Lanthanide contraction,which results in the structure changing from binuclear units to binuclear clusters along with the atom arrangement changes in the chiral spiral chains.Moreover,the complexes were further characterized by elemental analysis,IR,UV,X-ray powder diffraction and thermogravimetric analysis.Circular dichroism(CD)confirmed the presence of the homochiral structures.The fluorescence investigations of Nd,Sm,Eu,Tb complexes show that Complexes 3 and 4 exhibited near-infrared emission of Nd,and complexes 5,7 and 11 demonstrated characteristic orange,red and green light emission.And Sm and Tb complexes owned relatively high fluorescence lifetime.Complexes 1,3,5,7,9 and 11 demonstrated certain SHG activities.The luminescence of the complex 11 could be quenched by nitrobenzaldehyde and the metal cation,which may be used for molecular sensing and recognizing.