Novel Supramolecular Isomerism in Coordination Polymer Synthesis from Unsymmetrical Bridging Ligands: Solvent Influence on the Ligand Placement Orientation and Final Network Structure

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Abstract
The assembly of Co(NCS)2 with 1-methyl-1′-(3-pyridyl)-2-(4-pyridyl)ethene (L1) exhibits a novel supramolecular isomerism of [Co(L1)2(NCS)2]∞ caused by different placement orientation of L1 around metal centers. The reaction in MeOH/H2O and EtOH/H2O resulted in a double chain structure of 1, and that in EtOH/CH3NO2 led to an open framework structure of 2. The reaction in MeOH/CH3NO2 solvent system concomitantly afforded 1 and 2. The assemblies of 1-(3-pyridyl)-2-(4-pyrimidyl)ethene (L2) with Co(NCS)2 created the water-coordinated complexes of Co(L2)2(H2O)2(NCS)2 (3 and 4), an MeOH coordinated complex of Co(L2)2(H2O)2(NCS)2 (5), and an open framework coordination polymer of [Co(L1)2(NCS)2]∞ (6) depending on the reaction solvent system. From these observations, it is suggested that in the formation of 1, the solvent-coordinated intermediate species would be generated first and its trans coordination configuration should define the placement orientation of L1 in the resulting polymer of 1. On the other hand, it is presumed that the solvent-coordinated intermediate would not be produced during the formation of 2 due to the weaker coordination ability of EtOH and CH3NO2 molecules. The open framework coordination polymers of 2 and 6 are converted in the solid state into the isomeric coordination polymer of 1 and hydrogen bonded network structure of 3, respectively.

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