A Coordinatively Linked Yb Metal–Organic Framework Demonstrates High Thermal Stability and Uncommon Gas-Adsorption Selectivity†

Porous metal–organic frameworks (MOFs), which have emerged as new zeolite analogues, have attracted considerable research interest in the past decade as, compared to traditional zeolites, they possess a high surface area, modifiable surface, and tunable pore size. These characteristics have led to an enormous application potential for MOFs in catalysis, gas storage, and adsorptive separation. One of the main concerns regarding porous MOFs is their limited thermal stability, which prevents them from competing with inorganic zeolites in practical applications. Most porous MOFs can only be heated up to 150–350 8C without losing their framework integrity. Interpenetration, which often arises from weak interactions, has been widely used to improve the thermal stability of porous MOFs, and interpenetrated porous MOFs that are stable up to 400 8C have been reported. Interpenetration increases the wall thickness and reduces the pore size of an MOF, both of which lead to enhanced thermal stability. If two interpenetrated frameworks can be linked through coordinative bonds, the thermal stability should be boosted still further (Scheme 1). Herein we report such a coordinatively linked, doubly interpenetrated Yb MOF with improved thermal stability (up to 500 8C) and uncommon gas-adsorption selectivity. We have previously reported a cobalt-based porous MOF with doubly interpenetrated, (8,3)-connected nets (PCN-9; PCN: porous coordination network). PCN-9 contains a square-planar Co4(m4-O) secondary building unit (SBU) where each Co center is five-coordinate with a coordination site open toward the channel. As a consequence of this interpenetration, PCN-9 is thermally stable up to 400 8C (by thermogravimetric analysis (TGA)). If the interpenetrated, (8,3)-connected nets can be linked at the open metal sites by a bridging ligand, the thermal stability of the resulting MOF should be still higher. A short bridge is the best candidate due to the proximity of the two nets, and we chose SO4 2 as the bridging ligand because it can chelate the two metal centers and stabilize the MOF further. In addition, it can be generated slowly under solvothermal conditions through decomposition of DMSO (dimethyl sulfoxide), thereby facilitating the formation of the coordinatively linked interpenetrated MOF. Initial attempts to use sulfates to bridge the doubly interpenetrated, (8,3)-connected nets in PCN-9 failed. There are two possible reasons for this failure: the limited coordination number (maximum of six) of the cobalt center and the need for additional counterions to balance the overall charge. The coordination number of the metal center can be increased by using Ln cations instead of Co and no additional counterions will be needed in this case to balance the overall charge. With these considerations in mind, a ytterbium MOF with coordinatively linked, doubly interpenetrated, (8,3)-connected nets (PCN-17) was synthesized. Studies of similar MOFs containing other lanthanides are currently underway and will be reported in due course elsewhere. PCN-17 is stable up to 480 8C and exhibits selective adsorption of H2 and O2 over N2 and CO. Crystals of PCN-17 were obtained upon heating a mixture of H3TATB (TATB= 4,4’,4’’-S-triazine-2,4,6-triyl tribenzoate) and ytterbium nitrate in DMSO at 145 8C for 72 hours. The formula of PCN-17 (Yb4(m4-H2O)(C24H12N3O6)8/3(SO4)2·3H2O·10DMSO) was determined by X-ray crystallography, elemental analysis, and thermogravimetric analysis (TGA). X-ray structural analysis revealed that PCN-17 crystallizes in the space group Im3m. As expected, it adopts a square-planar Yb4(m4H2O) SBU, with the m4-H2O molecule, which is probably disordered over two or more orientations (see below), residing at the center of a square of four Yb atoms (Figure 1a). The four Yb atoms in the SBU lie in the same plane and each coordinates to seven O atoms (four from four carboxylate groups of four different TATBs, two from the bridging sulfate generated in situ, and one from the m4H2O). The Yb···m4-H2O distance (2.70 D) indicates very weak Scheme 1. a) A single net. b) Two doubly interpenetrated nets. c) Interpenetrated nets linked by a coordinative bond. The vertical gold dotted line represents a p–p interaction; the blue solid line represents coordinative bonding.