Synthesis, Structural Characterization, and Magnetic Properties of a New Series of Coordination Polymers: Importance of Steric Hindrance at the Coordination Sphere

Abstract

Six cobalt(II) containing coordination polymers {Co(hfipbb)(L1)0.5}n (1), {Co(hfipbb)(L2)0.5}n (2), {Co(oba)(L1)0.5}n (3), {Co(oba)(L2)}n·nH2O (4), {Co(1,2-pda)(L1)0.5}n (5), and [Co(1,2-pda)(L2)(H2O)]n·nH2O (6), that are formed from two positional isomeric bis(pyridyl) ligands with a long flexible spacer 1,4-bis(2-pyridylaminomethyl)benzene (L1) and 1,4-bis(3-pyridylaminomethyl)benzene (L2) and three different bent carboxylic acids 4,4′-(hexa-fluoroisopropylidene)bis-(benzoicacid) (H2hfipbb), 4,4′-oxybenzoic acid (H2oba), and 1,2-phenylenediaceticacid (1,2-H2pda), have been synthesized under hydrothermal conditions. Compounds 1–6 are characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) and elemental analysis. In the crystal structures of compounds 1, 3, and 5, two-dimensional (2D) metal-carboxylic acid layers, composed of dicobalt tetracarboxylate paddle-wheel clusters, are formed whereby these layers are pillared by the secondary ligand L1 in a typical trans–trans–trans conformation to result in a three-dimensional (3D) layered-pillared structure. However, in the crystals of compounds 2, 4, and 6 with secondary ligand L2, it does not favor the formation of paddle-wheels resulting in three completely different coordination polymers. The geometry of the carboxylic acid influences the formation of 2D metal acid layers in the compounds 1, 3, and 5 to form interpenetrated helical double layers to single layers. In compound 2, the secondary ligand L2 diagonally connects the 3D metal acid framework in a regular trans–trans–trans conformation. In compound 4, the ligand L2 exists in cis–cis–trans conformation to form [Co2L22] loops (metallo-macrocycles) which are connected by the oba2– ligand to form polyrotaxane-like 2D polymers. In compound 6, ligand L2 exists in an unusual cis–trans–cis conformation to allow the pda2– in a rare cis conformation to form one-dimensional (1D) ladders. The conformations of the pyridyl ligands L1 and L2 have been explained based on the torsion angle measurement. The steric hindrance created by the isomeric flexible pyridyl ligands at the metal coordination sphere plays an important role in the modulation of the conformation of the secondary ligand that drives the self-assembly of the coordination polymers. Finally, temperature-dependent magnetic susceptibility studies for the compounds 1–5 have been described.