Reversible phase transformations upon water uptake/removal in crystalline material of a bipodal N-donor ligand and evaluation of the stability of the hydrates formed

Abstract

Isolation of the anhydrous form of 2,7-bis(imidazol-1-ylmethyl)-naphthalene (L, 1) as well as three of its hydrates, in particular the hemihydrate (L·0.5H₂O, 2), monohydrate (L·H₂O, 3) and dihydrate (L·2H₂O, 4), allowed to gain insight into the mechanism of the hydration process. SCXRD studies revealed that in the hemihydrate, the water molecules interact with only one N-donor site originating from the ligand and serve as a bridge between two ligand molecules, whereas in the monohydrate, the water molecules interact with both N-donor sites available in the bipodal ligand, leading to the formation of binary supramolecular 1D chains. In the dihydrate, the water molecules are forming 1D chains by themselves and simultaneously interact with the ligand to yield 2D supramolecular layers. The lattice energies as well as the stabilisation energies of the hydrates indicate a clear relationship between the stability of the structures and the hydration level. Supportive studies on bulk material, such as temperature dependent PXRD, as well as TGA were also performed. PXRD indicated a reversible transition between anhydrate formed upon heating and dihydrate formed after immersion of the sample in water. Water adsorption/desorption isotherms further confirmed that the crystalline material undergoes transformations induced by water molecules.