Hydrogen-Bonded Porous Coordination Polymers:  Structural Transformation, Sorption Properties, and Particle Size from Kinetic Studies

Three new coordination polymers, [CoCl₂(4-pmna)₂]_n (1), {[Co(NCS)₂(4-pmna)₂]·2Me₂CO}_n (2 ⊃ 2Me₂CO), and {[Co(4-pmna)₂(H₂O)₂](NO₃)₂·2CH₃OH}_n (3 ⊃ 2H₂O·2MeOH) (4-pmna = N-(pyridin-4-ylmethyl)nicotinamide), have been synthesized and characterized using single-crystal X-ray diffraction. The cobalt(II) atoms are bridged by 4-pmna ligands in all three compounds to form double-stranded one-dimensional “repeated rhomboid-type” chains with rectangular-shaped cavities. In 1, each chain slips and obstructs the neighboring cavities so that there are no guest-incorporated pores. Both 2 ⊃ 2Me₂CO and 3 ⊃ 2H₂O·2MeOH do not have such a staggered arrangement and have pores that can be filled with a guest molecule. Compound 3 ⊃ 2H₂O·2MeOH traps guest molecules with multiple hydrogen bonds and shows a reversible structural rearrangement during adsorption and desorption. The new crystalline compound, 3, is stabilized by forming hydrogen bonds with the amide moieties of the 4-pmna ligands and was characterized using infrared spectroscopy. The clathration enthalpy of the reaction 3 + 2H₂O(l) + 2MeOH(l) ⇄ 3 ⊃ 2H₂O·2MeOH (≈35 kJ/mol) was estimated from differential scanning calorimetry data by considering the vaporization enthalpies of H₂O and MeOH. The desorption process of 3 ⊃ 2H₂O·2MeOH → 3 follows a single zero-order reaction mechanism under isothermal conditions. The activation energy of ca. 100 kJ/mol was obtained by plotting the logarithm of the reaction time for the same reacted fraction versus the reciprocal of the temperature. Moreover, the distribution of the one-dimensional channels in 3 ⊃ 2H₂O·2MeOH was estimated using the observation that the reaction rate is directly proportional to the total sectional area.