2D coordination polymers of cadmium(II) and zinc(II) derived from <i>N</i>,<i>N</i>′-bis(glycinyl)pyromellitic diimide: microwave-assisted synthesis, structures, spectroscopic properties and influence of metal-ion size

Two new two-dimensional (2D) coordination polymers (CPs), namely, poly[diaqua[μ₄-2,2'-(1,3,5,7-tetraoxo-1,2,3,5,6,7-hexahydropyrrolo[3,4-f]isoindole-2,6-diyl)diacetato-κ⁴O:O':O'':O''']cadmium(II)], [Cd(C₁₄H₆N₂O₈)(H₂O)₂]_n (1), and poly[[tetraaqua[μ₄-2,2'-(1,3,5,7-tetraoxo-1,2,3,5,6,7-hexahydropyrrolo[3,4-f]isoindole-2,6-diyl)diacetato-κ⁴O:O':O'':O'''][μ₂-2,2'-(1,3,5,7-tetraoxo-1,2,3,5,6,7-hexahydropyrrolo[3,4-f]isoindole-2,6-diyl)diacetato-κ²O:O']dizinc(II)] dihydrate], {[Zn(C₁₄H₆N₂O₈(H₂O)₂]·H₂O}_n (2), have been synthesized by the microwave-irradiated reaction of Cd(CH₃COO)₂·2H₂O and Zn(CH₃COO)₂·2H₂O, respectively, with N,N'-bis(glycinyl)pyromellitic diimide {BGPD, namely, 2,2'-(1,3,5,7-tetraoxo-1,2,3,5,6,7-hexahydropyrrolo[3,4-f]isoindole-2,6-diyl)diacetic acid, H₂L}. In the crystal structure of 1, the Cd^II ion is six-coordinated by four carboxylate O atoms from four symmetry-related L^2- dianions and two coordinated water molecules, furnishing an octahedral coordination geometry. The bridging L^2- dianion links four symmetry-related Cd^II cations into a 2D layer-like structure with a 3,4-connected bex topology. In the crystal structure of 2, the Zn^II ion is five-coordinated by three carboxylate O atoms from three different L^2- dianions and two coordination water molecules, furnishing a trigonal bipyramidal coordination geometry. Two crystallographically independent ligands serve as μ₄- and μ₂-bridges, respectively, to connect the Zn^II ions, thereby forming a 2D layer with a 3,3-connected hcb topology. Crystal structure analysis reveals the presence of n→π* interactions between two carbonyl groups of the pyromellitic diimide moieties in 1 and 2. CP 1 exhibits an enhanced fluorescence emission compared with free H₂L. The framework of 2 decomposes from 720 K, indicating its high thermal stability. A comparative analysis of a series of structures based on the BGPD ligand indicates that the metal-ion size has a great influence on the connection modes of the metal ions due to different steric effects, which, in turn, affects the structures of the SBUs (secondary building units) and frameworks.