Establishing Microporosity in Open Metal−Organic Frameworks: Gas Sorption Isotherms for Zn(BDC) (BDC = 1,4-Benzenedicarboxylate)

• DOI: 10.1021/ja981669x
• Publication Date: 2002-07-26T05:21:53Z
• Authors: Hailian Li; Mohamed Eddaoudi; Thomas L. Groy; O. M. Yaghi
• Keywords: 01 natural sciences; 0104 chemical sciences

Construction of microporous metal-organic frameworks by copolymerization of organic molecules with metal ions has received widespread attention in recent years, with significant strides made toward the development of their synthetic and structural design chemistry.1 Cognizant of the fact that access to the pores and understanding the inclusion chemistry of these materials are essential to their ultimate utility, we prepared rigid frameworks that maintain their structural integrity and porosity during anion-exchange and guest sorption from solution and in the absence of guests.2-4 Although gas sorption isotherm measurements are often used to confirm and study microporosity in crystalline zeolites and related molecular sieves,5 such studies have not been established in the chemistry of open metal-organic frameworks6 thus leaving unanswered vital questions regarding the existence of permanent porosity in this class of materials. Herein, we present the synthesis, structural characterization, and gas sorption isotherm measurements for the Zn(BDC) microporous framework of crystalline Zn(BDC)‚(DMF)(H2O) (BDC ) 1,4benzenedicarboxylate and DMF ) N,N′-dimethylformamide). Slow vapor diffusion at room temperature of triethylamine (0.05 mL) and toluene (5 mL) into a DMF solution (2 mL) containing a mixture of Zn(NO3)2‚6H2O (0.073 g, 0.246 mmol) and the acid form of BDC (0.040 g, 0.241 mmol) diluted with toluene (8 mL) yields colorless prism-shaped crystals that were formulated as Zn(BDC)‚(DMF)(H2O). X-ray single-crystal analysis8 on a sample obtained from the reaction product revealed an extended open-framework structure composed of the building unit shown in Figure 1. A total of four carboxylate units of different, but symmetrically equivalent, BDC building blocks are bonded to two zinc atoms in a di-monodentate fashion. Each zinc is also linked to a terminal water ligand to form an overall arrangement that is reminiscent of the carboxylate bridged M-M bonded molecular complexes. Although the Zn-Zn distance (Zn1-Zn1A ) 2.940 (3) A) is indicative of some M-M interaction, it does not represent an actual bond.9 The structure extends into the (011) crystallographic plane by having identical Zn-Zn units linked to remaining carboxylate functionalities of BDC to yield 2-D microporous layers. These layers are held together along the a axis by hydrogen-bonding interactions between water ligands of one layer and carboxylate oxygens of an adjacent layer as illustrated in a. Stacking of the layers in the