Homochiral Coordination Polymers Based on Amino Acid-Functionalized Isophthalic Acid: Synthesis, Structure Determination, and Optical Properties

Abstract

Three interesting homochiral metal–organic frameworks (HMOFs), namely, [Zn8(L)4(OH)4(H2O)2]n (1), {[Cd2(HL)2(H2O)4]·6H2O}n (2), and {[Pb2(HL)2]·CH3OH·2.5H2O}n (3), have been hydro/solvothermally synthesized through the reaction of the designed chiral ligand (S)-5-(((1-carboxyethyl)amino)methyl)isophthalic acid (H3L) with corresponding metal sources. Crystallographic analysis indicates that the triply deprotonated H3L ligands can adopt anionic (L)3– and zwitterionic (HL)2– forms, various coordination modes, and versatile hydrogen-bonding connections to construct interesting HMOFs with unique architectures. Complexes 1 and 2 are two-dimensional (2D) coordination polymers, but their structural motifs are very different. In 1, anionic (L)3– ligands bridge tetrametallic Zn-clusters to form a 2D layer with (3,6)-connected kgd net. In 2, zwitterionic (HL)2– ligands link CdII ions to generate two types of independent wave-like layers of 63 topology, and the two independent layers are further connected to form the unique double-layered homochiral framework through interlayered hydrogen-bonding interactions. The 2D frameworks of 1 and 2 are further extended into their three-dimensional (3D) supramolecular structures through complicated interlayered hydrogen-bonding interactions. Coordination polymer 3 is a 3D interpenetrating porous helicate of (62·12)(6·122) topology. Very interestingly, the metal–organic frameworks of 2 and 3 possess two positive charge centers, respectively, from metal ions and zwitterionic (HL)2– ligand and thus are intriguing HMOFs incorporating Brönsted acid with Lewis acid for potential applications. Furthermore, the thermal stabilities and solid-state optical properties, including CD spectra, and nonlinear optical and luminescent properties of these complexes were also carried out. More importantly, it was found that the coordination mode, hydrogen-bonding site, and charge of H3L ligand can be adjusted through the protonation of its amino group, which perhaps provides a pathway to design and develop novel HMOF materials based on this type of amino acid-functionalized polycarboxylate chiral ligands.