Chiral or achiral: four isomeric Cd(ii) coordination polymers based on phenylenediacrylate ligands

Abstract

Four isomeric coordination polymers of formula [Cd(L)(H₂O)₂]_n (L = ppda and mpda) were synthesized from two isomeric dicarboxylic ligands, p-phenylenediacrylic acid (H₂ppda) and m-phenylenediacrylic acid (H₂mpda). With the linear ppda ligand, compounds 1p and 2p crystallizes in centrosymmetric space groups, whereas the V-shaped mpda ligand induces spontaneous resolution to generate chiral crystals of 1m and 2m crystallized in chiral groups. Compounds 1p and 1m contain zigzag and helical [Cd(L)]_n coordination chains, respectively, and show almost identical 2D chiral hydrogen-bonded networks formed via O–H⋯O interactions between [Cd(COO)₂(H₂O)₂] units. In 1p, the centrosymmetric ppda ligand dictates an achiral 3D lattice. In 1m, however, the acentric mpda linker and the hydrogen bonds collaborate to give a homochiral 3D network. Compounds 2p and 2m are both 3D coordination frameworks, built of similar μ-O_carboxylate bridged helical chains and reinforced by similar 2D hydrogen-bonded networks. Ppda interlinks the helical chains in 2p in a heterochiral fashion while mpda serves as chiral discriminative linkers between the chains in 2m to give a 3D homochiral network. These compounds show enhanced ligand-centered luminescence.