title: Anomalous enhancement of proton conductivity for water molecular clusters stabilized in interstitial spaces of porous molecular crystals.
abstract: In an investigation into the proton conductivity of crystallized water clusters confined within low-dimensional nanoporous materials, we have found that water-stable nanoporous crystals are formed by complementary hydrogen bonding between [Co(III) (H2 bim)3 ](3+) (H2 bim: 2,2'-biimidazole) and TATC(3-) (1,3,5- tricarboxyl-2,4,6-triazinate); the O atoms in the -COO(-) groups of TATC(3-) in the porous outer wall are strongly hydrogen bonded with H2 O, forming two types of WMCs (water molecular clusters): a spirocyclic tetramer chain (SCTC) that forms infinite open 1D channels, and an isolated cyclic tetramer (ICT) present in the void space. The ICT is constructed from four H2 O molecules as a novel C2 -type WMC, which are hydrogen bonded with four-, three-, and two-coordination spheres, respectively. The largest structural fluctuation is observed at elevated temperatures from the two-coordinated H2 O molecules, which begin to rapidly and isotropically fluctuate on heating. This behavior can be rationalized by a simple model for the elucidation of pre-melting phenomena, similar to those in ice surfaces as the temperature increases. Moreover, high proton conductivity of SCTCs (ca. 10(-5) S cm(-1) at 300 K with an activation energy of 0.30 eV) through a proton-hole mechanism was observed for pellet samples using the alternating impedance method. The proton conductivity exhibits a slight enhancement of about 0.1×10(-5) S cm(-1) at 274 K due to a structural transition upon approaching this temperature that elongates the unit cell along the b-axis. The proton-transfer route can be predicted in WMCs, as O(4) of an H2 O molecule at the center of an SCTC shows a motion that rotates the dipole in the b-axis direction, but not the c-axis; the thermal ellipsoids of O(4) based on anisotropic temperature factors obtained by X-ray crystallography reflect a structural fluctuation along the b-axis direction induced by [Co(III) (H2 bim)3 ](3+) .
authors.affiliation: Department of Chemistry, Faculty of Science, Tokyo University of Science, Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601 (Japan), Fax: (+81) 3 3620 3858. tadokoro@rs.kagu.tus.ac.jp.
authors.forename: Makoto
authors.initials: M
authors.lastname: Tadokoro
authors.forename: Yuki
authors.initials: Y
authors.lastname: Ohata
authors.forename: Yuki
authors.initials: Y
authors.lastname: Ohhata
authors.forename: Yuriko
authors.initials: Y
authors.lastname: Shimazaki
authors.forename: Shin'ichi
authors.initials: S
authors.lastname: Ishimaru
authors.forename: Teppei
authors.initials: T
authors.lastname: Yamada
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nagao
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Sugaya
authors.forename: Kyosuke
authors.initials: K
authors.lastname: Isoda
authors.forename: Yuta
authors.initials: Y
authors.lastname: Suzuki
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kitagawa
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Matsui
keywords: hydrogen bonds; phase transitions; proton conductivity; supramolecules; water