A Contrivance for a Dynamic Porous Framework:  Cooperative Guest Adsorption Based on Square Grids Connected by Amide−Amide Hydrogen Bonds

Flexible porous coordination polymers containing amide groups as a function origin have been synthesized and categorized as “Coordination Polymer with Amide Groups”. Bispyridyl ligands with a spacer of amide group afford two-dimensional (2-D) motifs with a deformed square grid, resulting in three-dimensional (3-D) frameworks of [Co(NO₃)₂(3-pna)₂]_n (1), [Co(Br)₂(3-pna)₂]_n (2), and {[Co(NCS)₂(4-peia)₂]·4Me₂CO}_n (3 ⊃ 4Me₂CO) (3-pna = N-3-pyridylnicotinamide, 4-peia = N-(2-pyridin-4-yl-ethyl)-isonicotinamide), where the 2-D motifs are bound by complementary hydrogen bond between the amide groups. In the case of the 3 ⊃ 4Me₂CO, the amide groups form a contrivance for a dynamic porous framework because of their relevant position and orientation in the mutual nearest neighboring motifs. Consequently, 3 ⊃ 4Me₂CO shows amorphous (nonporous)-to-crystal (porous) structural rearrangement in the Me₂CO adsorption and desorption process, where the framework of the 2-D motif is maintained. The adsorption isotherm has threshold pressure (P_th), a sort of gate pressure. The heat of Me₂CO adsorption (ΔH_ad = −25 kJ/mol) is obtained from the temperature dependence of threshold pressure (P_th), which is close to acetone vaporization enthalpy (ΔH_vap = 30.99 kJ/mol).