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

For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [M-2(II)(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, H-2, N-2, O-2, 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 M-2 unit for 1 and 2 and 2 molecules per M-2 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 M-2 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/N-2 gas atmosphere. Solid-state H-1 and H-2 NMR studies of the crystalline sample of la and its partially deuterated samples of 1a' (deuterated phenyl group) and 1a '' (deuterated pyrazine) revealed rapid 180 degrees-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 H-1 and H-2 NMR spectra, where the amount of adsorbed gas molecules was small for 3a at 0.1 MPa Of CO2.