Impact of ligand functionalization on the structure of strontium MOFs prepared in a deep eutectic solvent

Abstract

The 1 : 2 choline chloride : 2-imidazolidinone (e-urea) deep eutectic solvent (DES) was employed for the ionothermal synthesis of strontium-based metal–organic frameworks (Sr-MOFs) using eight dicarboxylic acid-based ligands with varying relative orientations of the two coordinating moieties. With the four linear ligands based on the 2,5-difunctionalized terephthalic acid backbone, the nature of the atom/group at these sites (hydrogen, bromine, trifluoromethyl or hydroxyl) was shown to impact the nature of the secondary building unit (SBU) of the three-dimensional MOFs isolated and characterized by single-crystal X-ray diffraction. Modifying the exocyclic bond angle of the dicarboxylate linker was found to affect both the SBU and dimensionality of the coordination polymer. While the thiophene-based ligand afforded a two-dimensional arrangement, a three-dimensional organization was observed with the furan-based system and a one-dimensional coordination polymer with the isophthalic derivative. Reticulation of the latter system into a three-dimensional MOF was successfully undertaken by the use of the 3,3′,5,5′-azobenzenetetracarboxylic acid ligand comprising two bridged isophthalic acid units, highlighting the robustness of the SBU, in this case. The eight Sr-MOFs obtained feature e-urea molecules bound to the metal cation via coordination of the carbonyl unit assisted by hydrogen bonding of the neighbouring NH groups. These coordinated solvent molecules were found to occupy the pores and could unfortunately not be removed via thermal activation towards exploitation of the potential porosity. The optical properties of the Sr-MOFs were characterized by absorption and emission spectroscopy. The Sr-MOF based on the 2,5-dihydroxyterephthalic acid ligand was determined to be luminescent (λ_em = 575 nm) with a 26% quantum yield in the crystalline state, upon excitation at 360 nm.