A structural comparison between Co^II and Cu^II anilato-based ultramicroporous 3DMOFs

Introduction

Metal–Organic Frameworks (MOFs), porous materials self-assembled by metal ions and organic ligands, are attracting ever-growing interest in material chemistry. Their high porosity, tunable pore size and large surface area make MOFs promising candidates to separate CO₂. Ultramicroporosity (pore size < 0,7 nm) and the presence of nitrogen atoms are crucial requirements in the design of MOFs for CO₂ uptake._{. Based on this,} by combining 3 6-N-ditriazolyl-2,5-dihydroxy-1,4-benzoquinone (trz₂An), as the organic linker, with Co^II or Cu^II, two new ultramicroporous MOFs, formulated as [Co(trz₂An)]n·3H₂O (1) and[Cu(trz₂An)]·(H₂O)_2.5(2), have been obtained.

Material and Methods

MOFs 1 and 2 were synthesized by optimizing the synthetic procedure reported in literature. A solution of CoCl₂·6H₂O or CuCl₂·2H₂O was slowly added to a mixture of trz₂An, NaOH and water and heated in autoclave at 130 °C for 48 hours. The rectangular crystals, suitable for a single X-ray diffraction study, were washed three times using an acid aqueous solution in order to solubilize and remove the Co(OH)₂ and Cu(OH)₂ obtained during the reaction.

Results

In 1, Co^II ions are equatorially coordinated to four oxygen atoms of two bis(bidentate) trz₂An ligands. The distorted octahedral coordination sphere of Co^II ions is completed with two nitrogen atoms from the N4 atoms of the 1,2,4-triazole substituted pendant rings of trz₂An ligands. Two voids of 90.4 ų were found in the unit cell of the crystals, giving a void volume of 23.5%. MOF 2 is characterized by cubic cavities with a volume void of 28 % due to the coordination to the N atom at the 4-position of the triazole ring, which induces an alternate orientation of Cu-anilate chains.

Conclusions

The same trz₂An linker has been employed to obtain, in combination with Co^II and Cu^II, two robust, isomorphous and ultramicroporous MOFs, suitable for CO₂uptake and separation.