Unexpected 4-Fold [2 + 2] Interpenetration and Polycatenation Behaviors in Porous Luminescent Zinc Metal–Organic Frameworks Constructed from Flexible 3,5-Bis(4-pyridylmethoxy)benzoate Ligand

Abstract

Six zinc metal–organic frameworks, namely, [Zn(bpmb)(HCOO)]n·[Zn(bpmb)(OH)]n·4nH2O (1), [Zn(bpmb)(HCOO)]n (2), [Zn2(bpmb)2(fum)]n·9nH2O (3), [Zn3(bpmb)2(d-cam)2]n (4), [Zn2(bpmb)(1,3,5-btc)]n·nDMA (5), and [Zn(bpmb)(1,2,4,5-H2btec)]n·4nH2O (6) (Hbpmb = 3,5-bis(4-pyridylmethoxy)benzoic acid; H2fum = fumaric acid; d-H2cam = d-camphoric acid; 1,3,5-H3btc = 1,3,5-benzenetricarboxylic acid; 1,2,4,5-H4btec = 1,2,4,5-benzenetetracarboxylic acid; DMA = N,N-dimethylacetamide), were synthesized under solvothermal conditions. Their structures were characterized by EA, IR, PXRD, TG, and single-crystal XRD. Complex 1 is constructed of two independent neutral 3D MOFs with large 1D nanosized square channels, which are further entangled by each other to give an unexpected 4-fold [2 + 2] interpenetrating architecture. Complex 2 is a 3D MOF constructed from tetradentate bpmb and bidentate formate ligands, which is an isomer of the [Zn(bpmb)(HCOO)]n piece in 1. Complex 3 displays a 2D double-layered network that possesses 1D nanosized pores. Each 2D double layer is further interlocked with two neighboring ones to form an interesting 3D polycatenation array. Complex 4 possesses a 2D network constructed from a trinuclear [Zn3(COO)6N4] cluster and Zn–cam double-helical chain. Complex 5 is a 3D porous MOF constructed from tridentate bpmb and hexadentate 1,3,5-btc ligands. Complex 6 exhibits a 2-fold interpenetrating 2D wavelike (4,4) network constructed from bidentate bpmb and 1,2,4,5-H2btec ligands. The bpmb ligand adopts four types of coordination modes in 1–6. The secondary carboxylate ligands have obvious effects on their coordination architectures. Tetrahedral-, octahedral-, and trigonal-bipyramidal geometries around Zn(II) ions are observed. A general solvothermal method is proposed for preparing carboxylate complexes in DMF−H2O solution without any basic additive. In the preparation of 3, maleic acid is converted to a fumarate ligand. The thermal stabilities of six complexes and N2 adsorption behaviors of 1 and 3 are investigated. All the Zn(II) complexes emit blue luminescence originating from ligand-centered emission.