Porosity control in pillar-layered MOF architectures: hydrogen bonding in amino-functionalized 2-D layers

Abstract

Based on crystal engineering, three Zn/Cd-based metal–organic frameworks (MOFs) (FUM-153(Zn–H), FUM-167(Cd–H), and FUM-176(Cd–NH₂)) were successfully prepared from rigid O-donor/flexible N-donor linkers (H₂bdc (terephthalic acid) (L_(O)^H) or NH₂-H₂bdc (2-aminoterephthalic acid) (L_(O)^NH₂) and bpfb (N,N′-1,4-(phenylene) diisonicotinamide) (L_(N))) with Zn/Cd ions. Structurally, the major difference among the FUMs is the various behaviors of the DMF solvent in the lattice or the coordination environment of the structures. This study investigated the effect of hydrogen bonding of NH₂ substitution in 2-D layer linkers and its effect on the layer architecture. The results showed that hydrogen bonding indirectly affects the coordination of DMF solvent to the metal center and the porosity. To investigate the effect of the NH₂ substitution in the presence or absence of the DMF solvent in the FUMs, the Cambridge Structural Database (CSD) was surveyed for similar structures with the L_(O)^H and L_(O)^NH₂ linkers. Remarkably, the obtained data suggested that the existence of NH₂ substitution may reduce the probability of the coordination of DMF to the metal centers. Theoretical studies were carried out to explore the gas adsorption performance of the title FUMs. A better CH₄ adsorption compared to CO₂ for all FUMs was observed. It is worth noting that CH₄ and CO₂ gas adsorption in FUM-176(Cd–NH₂) is higher than that in FUM-153(Zn–H) and FUM-167(Cd–H). It can be deduced that the NH₂ substitution in 2-D layer linkers prevented the entering of DMF molecules into the framework (lattice or coordinated solvent). Consequently, more accessible pores are provided for gas molecules. The work demonstrates the role of linker functionality in the crystal structure, pore size, and gas adsorption properties.