2D coordination polymers of cadmium(II) and zinc(II) derived from N,N'-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 CdII ion is six-coordinated by four carboxylate O atoms from four symmetry-related L2- dianions and two coordinated water molecules, furnishing an octahedral coordination geometry. The bridging L2- dianion links four symmetry-related CdII cations into a 2D layer-like structure with a 3,4-connected bex topology. In the crystal structure of 2, the ZnII ion is five-coordinated by three carboxylate O atoms from three different L2- 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 ZnII 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.

Coordination polymers of CdII and PbII derived from bipyridine-glycoluril: influence of metal-ion size and counter-ions.

Two new one-dimensional (1D) coordination polymers (CPs), namely catena-poly[[[aquacadmium(II)]-bis(µ-4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]phenanthroline-6,13-dione)] bis(perchlorate) dihydrate], {[Cd(C14H10N6O2)2(H2O)](ClO4)2·2H2O}n or {[Cd(BPG)2(H2O)](ClO4)2·2H2O}n, 1, and catena-poly[[lead(II)-bis(µ-4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]phenanthroline-6,13-dione)] bis(perchlorate) dihydrate], {[Pb(C14H10N6O2)2](ClO4)2·2H2O}n or {[Pb(BPG)2](ClO4)2·2H2O}n, 2, have been synthesized using bipyridine-glycoluril (BPG; systematic name: 4b,5,7,7a-tetrahydro-4b,7a-epiminomethanoimino-6H-imidazo[4,5-f][1,10]phenanthroline-6,13-dione), a urea-fused tecton, in a mixed-solvent system. The CdII ion in 1 is heptacoordinated and the PbII ion in 2 is hexacoordinated, with the CdII ion adopting a pentagonal bipyramidal geometry and the PbII ion adopting a distorted octahedral geometry. Both CPs form infinite linear chain structures which are hydrogen bonded to each other leading to the formation of three-dimensional supramolecular network structures. Topological analysis of CPs 1 and 2 reveals that the structures exhibit 1D chain-like arrangements in an AB-AB sequence and shows platonic uniform 2-connected uninodal topologies. Furthermore, a comparative analysis of a series of structures based on the BPG ligand indicates that the size of the metal ion and the types of counter-ions used have a great influence on the resulting frameworks and properties.