Functionalities of one-dimensional dynamic ultramicropores in nickel(II) coordination polymers

Ni^II coordination polymers with a 4,4′-azobis(pyridine) (azpy) ligand, {[Ni₂(NCX)₄(azpy)₄]·G} _n (X = S, G (guest molecule) = MeOH (1·MeOH); X = S, G = EtOH (1·EtOH); X = S, G = H₂O (1·H₂O); X = S, G = no guest (1); X = Se, G = MeOH (2·MeOH); X = Se, G = H₂O (2·H₂O); X = Se, G = no guest (2)), have been synthesized and structurally characterized with their porosity. These compounds have one-dimensional periodic ultramicropores that contain the small guest molecules, H₂O, MeOH, or EtOH, whose hydroxy groups interact with the S or Se atoms of isothiocyanate or isoselenocyanate, respectively, via -S(Se)⋯HO- hydrogen bonds. Although the molecular dimensions of the MeOH guest are considerably larger than the window size of the ultramicropore, 1-MeOH and 2-MeOH easily release their guest molecules without decomposition of the framework to form 1 and 2 without any guest molecules. This shows that 1 and 2 have dynamic ultramicropores constructed from the interpenetrating framework. The guest desorption experiments using 1·MeOH and 1·EtOH reveal that the difference in the desorption behavior is due to van der Waals interactions that depend on the molecular shape of the guest molecule in the ultramicropores and/or an entrance blocking effect that depends on the minimum dimensions of the guest molecule for the pore windows. A marked difference in the N₂ and CH₄ adsorption isotherms was observed and is associated with the strength of the host-guest interaction.