Wings waving: coordinating solvent-induced structural diversity of new Cu(ii) flexible MOFs with crystal to crystal transformation and gas sorption capability

Abstract

A new semi-rigid 5-(bis(4-carboxybenzyl)amino)isophthalic acid (H₄L) is designed and synthesized to obtain novel flexible metal organic frameworks (MOFs). The tetracarboxylate ligand (L⁴⁻) links to Cu(II) paddle-wheel second build units (SBUs) under different solvent condition to afford two MOFs, [Cu₂(L)(S)₂]_n (S = DMF, 1 and H₂O, 2). Single crystal X-ray diffraction structural analyses reveal that the coordinating solvent molecules are DMF and H₂O in the SBUs for 1 and 2, respectively. The torsion angle between two methylene benzoate ring subunits of the ligand is 122.7° in 1 but 175.1° in 2. Three phenyl rings of the ligand are nonplanar and orient in different directions in 1 and 2. A reversible crystal to crystal transformation between 1 and 2 are investigated by exchanging the terminal ligated solvent molecule, in which the phenyl rings of the benzoate subunit act as “a pair of waving wings” accompanying the ligated solvent exchange. Time-dependent powder X-ray diffraction data confirms this reversible dynamic transformation. This hinge within the semi-rigid ligand is a built-in breathing mechanism and suggests a novel approach for general synthesis of breathing MOFs. A gas sorption study for 1 demonstrates 1 has the ability to selectively adsorb CO₂ over CH₄, H₂ and N₂.