A dynamically entangled coordination polymer: synthesis, structure, luminescence, single-crystal-to-single-crystal reversible guest inclusion and structural transformation

Abstract

A Zn^II coordination polymer {[Zn₂(cpa)₂(bpy)]·3H₂O}_n (1) (cpa^2- = 4-(methoxycarbonyl)benzoate and bpy = 4,4'-bipyridine) has been synthesized under solvothermal condition and structurally characterized. This coordination polymer has nanotubular threefold entangled (2D → 3D) structure with embedded water molecules; the water molecules can be partially exchanged in reversible single-crystal-to-single-crystal (SC-SC) fashion by different solvent molecules like methanol, ethanol and acetone giving rise to {[Zn₂(cpa)₂(bpy)] · (0.5MeOH) · (2.5H₂O)}_n (2), {[Zn₂(cpa)₂(bpy)]·(0.5EtOH)·(0.5H₂O)}n (3) and {[Zn₂(cpa)₂(bpy)]·(0.5Me₂CO)·(H₂O)}_n (4). Inclusion of EtOH or MeOH leaves the size of the voids in the framework unaltered. Inclusion of acetone, however, is accompanied by shrinking of the voids in the framework. Heating of 1 at 100 °C under vacuum for 4 h affords the de-solvated compound, {Zn₂(cpa)₂(bpy)}_n (1'). Single-crystal X-ray structure of 1' shows sliding of the individual nanotubular components expanding the overall framework. Thus, the coordination polymer exhibits dynamic motion of the molecular components in SC-SC fashion. All compounds were further characterized via IR spectroscopy, PXRD, elemental and TGA analysis. When 1 is placed in benzene at 100 ° C for 2 days, compound {[Zn₂(cpa)₂(bpy)]·(2.5H₂O)}_n (5) is formed in a SC-SC fashion where coordination number of Zn^II ion increases from four to five. Compound 1 also exhibits reversible guest-dependent photoluminescence properties.