Non-framework Coordination Polymers With Tunable Bimodal Porosities Based on Inter-connected Metal-organic Polyhedra

Abstract

One discerns coordination polymers (CPs) and metalorganic frameworks (MOFs) because the former provides a more comprehensive concept for network solids based on metal ions and multitopic organic ligands, while the latter implies the presence of a crystal lattice in the structure. MOF materials with three-dimensional backbones and welldefined network structures possess many advantages compared to their non-framework analogues, such as homogeneous and reproducible material properties, and atomlevel information on the structures. However, the labile nature of some coordination bonds, such as those between first-row transition elements and carboxylate ligands may lead to a gradual or even complete degradation of the scaffold under ambient conditions. For most MOFs, therefore, the sustainability of the crystal lattice is a prerequisite to realize the fascinating potentials of these materials, including hydrogen storage or carbon capture and sequestration. Consequently, it is desirable to have a new type of materials that possess the advantages of porous MOFs but without the constraint of an ordered crystal lattice. Approaches taken in this direction include polymerizations using rigid building blocks such as phthalocyanins, liquid-crystal templation and the derivation of disordered structures using rapid precipitation methods. We provide a potential alternative in another perspective, and hereby report some examples of non-framework CPs that have intrinsic porosities tunable between microporous and mesoporous regimes. The idea is based on the realization that some of the well-known metal-organic polyhedral cages are equipped with solvent-coordinated sites. Cu2 paddlewheels and bent dicarboxylates are prototypical building blocks for such cases. Thus we decided to take advantage of the solvent sites of paddlewheel units to crosslink polyhedral cage molecules into a network. The polymeric materials would possess a mesoporosity depending on the size of the cage and linker molecules, as well as an intrinsic microporosity originating from the inner cage voids (Scheme 1). We used 2,7-naphthalenedicarboxylate and Cu to build a macromolecule with 12 Cu2 paddlewheel units, [Cu24(2,7ndc)24(DMF)10(H2O)14] (1). The structure of the cuboctahedral cage molecule was determined by X-ray crystallography on weakly diffracting single-crystals using synchrotron X-rays from Pohang Accelerator Laboratory, Korea (Figure 1). The approximately spherical molecule measures about 3.3 nm, and the inner-cage void can host an imaginary sphere with the diameter of 1.8 nm. Such metal-organic cuboctahedra were first reported by Yaghi and Zaworotko, and continue to be a subject of active research. In the packing structure, a given cuboctahedral cage