Conformation control of a flexible 1,4-phenylenediacetate ligand in coordination complexes: a rigidity-modulated strategy

Abstract

Five coordination complexes based on the flexible ligand 1,4-phenylenediacetic acid (H₂pda), formulated as Zn(trans-pda) (1a), Mn(H₂O)₂(trans-pda) (1b), [M(cis-pda)(4,4′-bipy)]·H₂O (M = Zn (2a), Co (2b), Cd (2c); H₂pda = 1,4-phenylenediacetic acid; 4,4′-bipy = 4,4′-bipyridine), are reported. The crystal structures of the as-synthesized complexes were determined by single-crystal X-ray diffraction analyses. Complexes 1a and 1b have similar three-dimensional (3-D) network structures built by tetrahedral Zn nodes (for 1a) or octahedral Mn nodes (for 1b) and trans-pda²⁻ bridges. Complexes 2a, 2b, and 2c possess similar two-dimensional (2-D) layered structures constructed from interconnecting metal-cis-pda²⁻ double chains by 4,4′-bipy ligands. It is interesting that conformation of the flexible pda²⁻ ligand is modulated in presence of the rigid 4,4′-bipy ligand. The trans-pda²⁻ ligand was typically isolated in a single pda²⁻ system whereas the cis-pda²⁻ ligand was observed in a mixed pda²⁻/4,4′-bipy system. Furthermore, the dimensionalities of the complexes vary from 3-D (1a and 1b) to 2-D (2a, 2b, and 2c) when auxiliary 4,4′-bipy ligands were introduced. The photoluminescent studies on solid-state samples of complexes 1a, 2a and 2c indicate that the complexes exhibit intense fluorescent emissions at room temperature.