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

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, [Cu-2(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 degrees in 1 but 175.1 degrees 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, H-2 and N-2.