Anion-induced structural transformation involving interpenetration control and luminescence switching

Abstract

Three metal–organic coordination compounds with different interpenetrating modes, [Cd(BCbpy)(BDC)]·3H₂O (1), [Cd₂(BCbpy)₂(BDC)Cl₂][Cd(BCbpy)₂(BDC)]·18H₂O (2), and [Cd(BCbpy)Cl₂]·3H₂O (3) (HBCbpyCl = 1-(4-carboxybenzyl)-4,4-bipyridinium chloride, H₂BDC = 1,4-benzenedicarboxylic acid), have been synthesized and structurally characterized. Compound 1 exhibits a 2D + 2D → 2D interpenetrating array with the coexistence of polyrotaxane and polycatenane structures. Compound 2 is an unprecedented example of a 2D + 2D + 1D → 2D interpenetrating structure which involves 2D + 2D → 2D parallel interpenetration and a 2D + 1D → 2D polyrotaxane. In the case of compound 3, the absence of the rod-like BDC ligand prevents threading through the ring-like units, no polyrotaxane structure is formed, and the compound features infinite 1D ring-containing chains arranged in parallel without any interpenetration. Interestingly, the structural transformation can be achieved through a solvent-mediated anion exchange process when compound 1 or 2 is exposed to the aqueous solution containing chloride ions. Upon progressive replacement of the BDC²⁻ ligands by chloride ions, the complicated network topology is simplified and undergoes an evolution from interpenetration to non-interpenetration, accompanied by a visually observed on–off–on fluorescence switching.