Two 3D metal–organic frameworks of Cd(II): modulation of structures and porous properties based on linker functionalities

Abstract

Two new Cd(II) based metal–organic frameworks (MOFs), {[Cd(NH₂-bdc)(bpe)]·0.5EtOH}_n (1) and {[Cd(NO₂-bdc)(azbpy)]·4H₂O}_n (2) (NH₂-bdc = 2-amino terephthalic acid, bpe = 1,2-bis(4-pyridyl)ethane, NO₂-bdc = 2-nitro terephthalic acid, azbpy = 4,4′-azobipyridine), have been synthesized by a solvent diffusion technique and structurally characterized. Both the frameworks are constructed based on exo-bidentate pyridyl type linkers of similar length but different functionalities. Compound 1 has a 3D structure in which the –NH₂ functional group of NH₂-bdc is coordinated to Cd(II) and a 1D ultra-micropore accommodates ethanol guest molecules. The desolvated framework of 1 (1′) is rigid as realized from the PXRD patterns and shows a type-I CO₂ uptake profile with a reasonably high isosteric heat of adsorption value. Density functional theory (DFT) calculation shows that aromatic π electrons interact strongly with CO₂ and the binding energy is 33.4 kJ mol^-1. Compound 2 has a two-fold interpenetrated 3D porous framework structure where pendent –NO₂ groups of NO₂-bdc are aligned on the pore surface. The desolvated framework (2′) exhibits structural transformation and is nonporous to N₂. Smaller and gradual CO₂ uptake in 2′ can be attributed to the structural contraction. The solvent (H₂O, MeOH and EtOH) vapour adsorption studies suggest that the pore surface of 2′ is hydrophobic in nature.