Hydrogen-Bonded Porous Coordination Polymers: Structural Transformation, Sorption Properties, and Particle Size from Kinetic Studies
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posted on 2006-12-20, 00:00 authored by Kazuhiro Uemura, Kazuya Saito, Susumu Kitagawa, Hidetoshi KitaThree new coordination polymers, [CoCl<sub>2</sub>(4-pmna)<sub>2</sub>]<i><sub>n</sub></i> (<b>1</b>), {[Co(NCS)<sub>2</sub>(4-pmna)<sub>2</sub>]·2Me<sub>2</sub>CO}<i><sub>n</sub></i> (<b>2
</b>⊃<b> 2Me</b><b><sub>2</sub></b><b>CO</b>), and {[Co(4-pmna)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>](NO<sub>3</sub>)<sub>2</sub>·2CH<sub>3</sub>OH}<i><sub>n</sub></i> (<b>3 </b>⊃ <b>2H</b><b><sub>2</sub></b><b>O</b>·<b>2MeOH</b>) (4-pmna = <i>N</i>-(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 <b>1</b>, each chain slips
and obstructs the neighboring cavities so that there are no guest-incorporated pores. Both <b>2 </b>⊃<b> 2Me</b><b><sub>2</sub></b><b>CO</b>
and <b>3 </b>⊃ <b>2H</b><b><sub>2</sub></b><b>O</b>·<b>2MeOH</b> do not have such a staggered arrangement and have pores that can be filled with
a guest molecule. Compound <b>3 </b>⊃<b> 2H</b><b><sub>2</sub></b><b>O</b>·<b>2MeOH</b> traps guest molecules with multiple hydrogen bonds and
shows a reversible structural rearrangement during adsorption and desorption. The new crystalline
compound, <b>3</b>, 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 <b>3</b> + 2H<sub>2</sub>O(l) +
2MeOH(l) ⇄ <b>3 </b>⊃<b> 2H</b><b><sub>2</sub></b><b>O</b>·<b>2MeOH</b> (≈35 kJ/mol) was estimated from differential scanning calorimetry data by
considering the vaporization enthalpies of H<sub>2</sub>O and MeOH. The desorption process of <b>3 </b>⊃<b> 2H</b><b><sub>2</sub></b><b>O</b>·<b>2MeOH</b>
→ <b>3</b> 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 <b>3
</b>⊃<b> 2H</b><b><sub>2</sub></b><b>O</b>·<b>2MeOH</b> was estimated using the observation that the reaction rate is directly proportional to the
total sectional area.
