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.