Two unprecedented 3D metal–organic polyrotaxane frameworks based on a new flexible tri(imidazole) ligand

Abstract

Two Cd(II)-containing coordination polymers based on a new tri(4-imidazolylphenyl)amine (Tipa) ligand and carboxylates, namely, [Cd(Tipa)(L1)]·2H₂O (1) and [Cd₃(Tipa)₂(L2)₂Cl₂]·4H₂O (2), where H₂L1 = 5-OH-1,3-benzenedicarboxylic acid and H₂L2 = 1,4-benzendicarboxylic acid, have been synthesized by varying the carboxylates under hydrothermal conditions. In 1, adjacent Cd(II) centers are linked by L1 anions to generate Cd-carboxylate chains, which are further extended by Tipa ligands to form a puckered double sheet. The most peculiar feature of 1 is the entangled fashion between the [Cd₂(Tipa)₂] loop and the L1 ligand rod. Every [Cd₂(Tipa)₂] loop of each layer is threaded by one L1 ligand rod of an adjacent layer. In this entangled mode, the 2D → 3D polyrotaxane framework was obtained, which represents an unusual 2D → 3D metal–organic polyrotaxane framework entangled in a highly rare parallel fashion. In 2, two identical 3D self-penetrating frameworks interpenetrate each other in a 2-fold mode. If we regard each Cd₂(Tipa)₂ window as a loop and each L2 ligand as a single rod, undoubtedly, the structure of 2 also can be described as a 3D metal–organic polyrotaxane framework. As far as we know, compound 2 is the first example of a 2-fold interpenetrating 3D framework with polyrotaxane features. The luminescent properties for the two compounds were also investigated.