Conformation Preference of a Flexible Cyclohexanetetracarboxylate Ligand in Three New Metal-Organic Frameworks: Structures, Magnetic and Luminescent Properties

Three novel complexes of formula [Co-5(OH)(2)(cht)(2)(H2O)(10)](n) center dot 2nH(2)O (1) (H(4)cht = cyclohexane-1,2,4,5-tetracarboxylic acid), [Zn-2(cht)(H2O)(3)](n) center dot nH(2)O (2), and [Cd-2(cht)(H2O)(5)](n) center dot 2nH(2)O (3) were synthesized by a hydrothermal reaction of cyclohexane-1,2,4,5-tetracarboxylic acid in the presence of sodium hydroxide with Co(NO3)(2) center dot 6H(2)O, Zn(NO3)(2) center dot 6H(2)O, and Cd(NO3)(2) center dot 4H(2)O, respectively. These complexes were obtained by controlling the molar ratios of starting materials and the pH values of reaction mixtures with an initial pH of 5.87 for 1, 5.50 for 2, and 5.57 for 3, respectively. The single-crystal X-ray investigations reveal that the cht ligands change their conformations exclusively to the a,e,e,a form in all three complexes. Complex 1 features a two-dimensional (2D) metal-organic framework (MOF) with an axis surrounded by two zigzag structures, a rectangular channel surrounded by two a-carboqate-metal-a-carboxlate and two e-carboxylate-metal-e-carboxylate chains, and two smaller distorted square-pyramidal channels surrounded by hydroxyl oxygen, cyclohexane, and its two a-carboxylates. Each cht ligand connects five cobalt atoms and each cobalt atom is six-coordinated with carboxylate-oxygen, hydroxyl, and water molecules. Complex 2 possesses a three-dimensional (3D) MOF structure that consists of the cht ligands and two types of cobalt; one is four-coordinated with carboxylate-oxygens and the other is six-coordinated with carboxylate-oxygens and water molecules. Each cht ligand is bonded to seven zinc atoms. Complex 3 is also a 2D MOF structure constructed by seven-coordinated cadmium atoms and the cht ligands. The e-carboxylates of two parallel cht ligands are connected to a type of cadmium atoms to form 1D chains, and these chains are further connected through a-carboxylates of the ligands and another type of cadmium atoms to give the 2D structure. Extensive hydrogen bonding interactions involving carboxylate-oxygen atoms and crystallization water molecules afford 3D supramolecular networks in complexes 1 and 2. Magnetic susceptibility measurement of 1 confirmed a six-coordinated high-spin cobalt(II) ion and the presence of magnetic exchange coupling among the three hydroxyl-oxygen bridged cobalt ions. Fluorescent spectra of complexes 2 and 3 show strong fluorescent emissions in the blue region.