title: Crystal transformation and host molecular motions in CO2 adsorption process of a metal benzoate pyrazine (M(II) = Rh, Cu).
abstract: For the purpose of investigating the correlation between host gas adsorption ability and structural flexibility, the combination of metal benzoate complexes [M(II)(2)(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 CO(2), H(2), N(2), O(2), and Ar gases at 195 K (CO(2)) 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 CO(2) 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 CO(2) inclusions for all hosts (1-3) revealed that large structural changes occurred through CO(2) 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 CO(2)/N(2) gas atmosphere. Solid-state (1)H and (2)H 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 degree-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 CO(2) 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 (1)H and (2)H NMR spectra, where the amount of adsorbed gas molecules was small for 3a at 0.1 MPa of CO(2).
authors.affiliation: Department of Nanosystem Science, Graduate School of Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama 236-0027, Japan. staka@yokohama-cu.ac.jp
authors.forename: Satoshi
authors.initials: S
authors.lastname: Takamizawa
authors.forename: Ei-ichi
authors.initials: E
authors.lastname: Nataka
authors.forename: Takamasa
authors.initials: T
authors.lastname: Akatsuka
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Miyake
authors.forename: Yoshiki
authors.initials: Y
authors.lastname: Kakizaki
authors.forename: Hirotoshi
authors.initials: H
authors.lastname: Takeuchi
authors.forename: Goro
authors.initials: G
authors.lastname: Maruta
authors.forename: Sadamu
authors.initials: S
authors.lastname: Takeda
mesh_terms: D000327:Adsorption; D001565:Benzoates / Q000737:chemistry*; D002245:Carbon Dioxide / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D017752:Greenhouse Effect; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D009038:Motion; D009942:Organometallic Compounds / Q000737:chemistry*; D011719:Pyrazines / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D013696:Temperature
chemical_list: D001565:Benzoates; D009942:Organometallic Compounds; D011719:Pyrazines; D002245:Carbon Dioxide; D003300:Copper; D012238:Rhodium