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

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journal contribution
posted on 2010-03-24, 00:00 authored by Satoshi Takamizawa, Ei-ichi Nataka, Takamasa Akatsuka, Ryosuke Miyake, Yoshiki Kakizaki, Hirotoshi Takeuchi, Goro Maruta, Sadamu Takeda
For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [MII2(bza)4] (MII = 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 (13) 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.