Reversible Gas Uptake by a Nonporous Crystalline Solid Involving Multiple Changes in Covalent Bonding

Abstract

Hydrogen chloride gas (HCl) is absorbed (and reversibly released) by a nonporous crystalline solid, [CuCl₂(3-Clpy)₂] (3-Clpy = 3-chloropyridine), under ambient conditions leading to conversion from the blue coordination compound to the yellow salt (3-ClpyH)₂[CuCl₄]. These reactions require substantial motions within the crystalline solid including a change in the copper coordination environment from square planar to tetrahedral. This process also involves cleavage of the covalent bond of the gaseous molecules (H−Cl) and of coordination bonds of the molecular solid compound (Cu−N) and formation of N−H and Cu−Cl bonds. These reactions are not a single-crystal-to-single-crystal transformation; thus, the crystal structure determinations have been performed using X-ray powder diffraction. Importantly, we demonstrate that these reactions proceed in the absence of solvent or water vapor, ruling out the possibility of a water-assisted (microscopic recrystallization) mechanism, which is remarkable given all the structural changes needed for the process to take place. Gas-phase FTIR spectroscopy has permitted us to establish that this process is actually a solid−gas equilibrium, and time-resolved X-ray powder diffraction (both in situ and ex situ) has been used for the study of possible intermediates as well as the kinetics of the reaction.