Ins and Outs of HCl

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Science  28 Jul 2006:
Vol. 313, Issue 5786, pp. 411
DOI: 10.1126/science.313.5786.411d

The capacity of porous solids, such as zeolites and metal organic frameworks, to adsorb gases reversibly has been studied extensively for storage and catalysis applications. In contrast, reversible gas incorporation that alters the covalent bonding structure of a solid is a much rarer phenomenon. Mínguez Espallargas et al. present a compound that stores and releases HCl through a complex sequence of bond cleavage and formation steps.

In the initial structure, two 3-halopyridinium cations [in which the halogen atom (X) can be either Cl or Br] form an ionic crystal through N-H hydrogen bonding to the chloride ligands of a [CuCl4] dianion. If the crystals are ground and left open to air, the yellow powders turn blue after a few hours for the chloro species and a few days for the bromo species, indicative of a change in Cu coordination geometry from distorted tetrahedral to square planar. The transformation is accelerated if the vial is immersed in a silver nitrate solution that can capture gaseous HCl through precipitation of AgCl. Powder x-ray diffraction confirms HCl elimination via cleavage of N-H and Cu-Cl bonds, leading to N-Cu coordination and rearrangement of the ionic chains into a two-dimensional network held together by Cu-Cl⋯X-C bonds. Reexposure of the solids to HCl vapor recovers the original ionic compounds. — PDS

J. Am. Chem. Soc. 128, 10.1021/ja0625733 (2006).

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