Crystal Transformation and Host Molecular Motions in CO₂ Adsorption Process of a Metal Benzoate Pyrazine (M^II = Rh, Cu)

For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [M^II₂(bza)₄] (M^II = Rh (a), Cu (b); bza = benzoate) and pyrazine derivatives (pyz = pyrazine (1), 2-mpyz = 2-methylpyrazine (2), 2,3-dmpyz = 2,3-dimethylpyrazine (3)) yields a series of one-dimensionally assembled complexes. The study of the adsorption properties of this series was examined for CO₂, H₂, N₂, O₂, and Ar gases at 195 K (CO₂) or at 77 K (all others). The adsorption manners of these complexes are similar for each gas, while the pressure at which adsorption started or rapidly grew increased with a rise in the number of methyl groups in the case of adsorbable gases. The maximum amount of adsorption was a positive integer, e.g., 3 molecules per M₂ unit for 1 and 2 and 2 molecules per M₂ unit for 3 in the case of CO₂ adsorption for all complexes at 0.1 MPa of adsorbable gases. Structural transformation was observed accompanying gas adsorption. This transformation was observed when the adsorption amount reached 1 molecule per M₂ unit, suggesting a correlation of the adsorption amount and dynamic adsorption behavior. Single-crystal X-ray analyses of nonincluded crystals and CO₂ inclusions for all hosts (1−3) revealed that large structural changes occurred through CO₂ adsorption to increase the inner space for adsorption gases, depending on the substituents on the pyrazine ring. These facts were confirmed as a transition by DSC measurements using a mixed CO₂/N₂ gas atmosphere. Solid-state ¹H and ²H NMR studies of the crystalline sample of 1a and its partially deuterated samples of 1a′ (deuterated phenyl group) and 1a′′ (deuterated pyrazine) revealed rapid 180°-flip motions of the aromatic rings of the host skeletons, which form the walls of the channels. These “rotating” motions would help the diffusion of CO₂ molecules through a narrow channel at relatively low pressure. Indeed, the motions of phenyl groups and methyl-substituted pyrazine moieties of phenyl deuterated 3a were confirmed to be very slow by solid-state ¹H and ²H NMR spectra, where the amount of adsorbed gas molecules was small for 3a at 0.1 MPa of CO₂.