Design, structural diversity and properties of novel zwitterionic metal–organic frameworks

Abstract

Seven new zwitterionic metal–organic frameworks (ZW MOFs) of compositions {[Cd(L1)(OH₂)]·2H₂O}_n (1), {[Mn(L1)(OH₂)₂]·H₂O}_n (2), {[Cu(HL1)₂(OH₂)₃]·9H₂O}_n (3), {[Mn₂(L2)₂(OH₂)₄]·3H₂O}_n (4), [Co(L2)(OH₂)₄]·H₂O (5), [Ni(L2)(OH₂)₃]_n (6), and {[Cd(L2)(OH₂)₃]·4H₂O}_n (7), where H₃L1Br = 3-carboxy-1-(3,5-dicarboxybenzyl)pyridinium bromide and H₃L2Br = 4-carboxy-1-(3,5-dicarboxybenzyl)pyridinium bromide, have been synthesized under hydrothermal conditions. We demonstrate that the diversity of these crystal structures suggests that the tridentate and flexible nature of ZW ligands L1 and L2 make them excellent candidates for the synthesis of new ZW MOFs. A multi-charged anionic nature is a common feature of L1 and L2, and therefore, allows the rational design of ZW MOFs without the presence of additional counterions for charge compensation. All materials were structurally characterized by single-crystal X-ray diffraction and further characterized by elemental analyses, infrared spectroscopy (IR), powder X-ray diffraction (PXRD), thermogravimetric analyses (TGA), differential scanning calorimetry (DSC) and adsorption measurements. Most interestingly, permanent porosity could be observed for 1, originated from 4 Å channel pores and confirmed by methanol adsorption experiments, which yielded an uptake of 7.43 wt% at 25 °C; and respectively, anhydrates of 1, 2, 4 and 6 can be rehydrated upon exposure to ambient air, as evidenced by TGA and PXRD measurements. In addition, we report an in-depth CSD analysis of selected structural parameters, coordination modes and topologies exhibited by MOFs based on ZW ligands L1 and L2 along with the regio-isomeric analogue L3, where H₃L3Br = N-(4-carboxybenzyl)-(3,5-dicarboxyl)pyridinium bromide.