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

Two Cd(II)-containing coordination polymers based on a new tri(4-imidazolylphenyl) amine (Tipa) ligand and carboxylates, namely, [Cd(Tipa)(L1)]center dot 2H(2)O (1) and [Cd-3(Tipa)(2)(L2)(2)Cl-2]center dot 4H(2)O (2), where H(2)L1 =5-OH-1,3-benzenedicarboxylic acid and H(2)L2 = 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 [Cd-2(Tipa)(2)] loop and the L1 ligand rod. Every [Cd-2(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 Cd-2(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.