Experimental and Theoretical Investigation of Intrinsic Pyridine Isomer Effects on Physical Property Tuning of Metallo Supramolecular Polymers Bearing Multiple Pyridine Ligands

Abstract

Coordinative polymers have gained great attention over the last two decades because of the potential versatile modification and functionalization of their physical properties. The pyridine isomer effects on metallo-supramolecular materials, where metal salts are blended with the polymers bearing pyridine coordinative ligands, have been frequently argued based only on steric effects. Here the pyridine isomer effects are investigated by both experimental and computational approaches. In the experimental approach, ZnCl2 was blended with amorphous polyesters uniformly bearing either 4-vinylpyridine (4VP)- or 2-vinylpyridine (2VP)-type ligands at the side groups. In addition, a polyester bearing both ligand types was also used to prepare blends with ZnCl2, which enabled deeper understanding of the pyridine isomer effects on the pyridine–metal coordination. Experimental characterizations by Fourier transform infrared spectroscropy, differential scanning calorimetry, and rheological measurements first revealed that the coordination of ZnCl2 occurs more predominantly on 4VP ligands. Such pyridine isomer effects on the coordination are further investigated by density functional theory computation for complexes of ZnCl2 with side chain models of 4VP or 2VP. On the basis of the obtained complex structures and stabilities, ZnCl2-complexation with two 4VP units was energetically more favored than for those with two 2VP units or 4VP/2VP mixtures. The coordination preference of the 4VP to ZnCl2 was also confirmed by the stabilization of the interaction between the natural bond orbitals of the Zn-pyridyl N pair. Therefore, the theoretical approach can be applied to demonstrate the experimentally found unique isomeric effects of pyridine on metallo-supramolecular matrix, which will bring important insights into the practical application of coordinative polymers.