Anomalous Enhancement of Proton Conductivity for Water Molecular Clusters Stabilized in Interstitial Spaces of Porous Molecular Crystals. Issue 42 (3rd September 2014)

• Record Type: Journal Article
• Authors: Tadokoro, Makoto; Ohhata, Yuki; Shimazaki, Yuriko; Ishimaru, Shin'ichi; Yamada, Teppei; Nagao, Yuki; Sugaya, Tomoaki; Isoda, Kyosuke; Suzuki, Yuta; Kitagawa, Hiroshi; Matsui, Hiroshi
• Is Part Of: Chemistry. Volume 20:Issue 42(2014)
• Page Start: 13698
• Page End: 13709
• Publication Date: 2014-09-03
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201402900
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>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<sup>III</sup>(H<sub>2</sub>bim)<sub>3</sub>]<sup>3+</sup> (H<sub>2</sub>bim: 2, 2′‐biimidazole) and TATC<sup>3−</sup> (1, 3, 5‐ tricarboxyl‐2, 4, 6‐triazinate); the O atoms in the COO<sup>−</sup> groups of TATC<sup>3−</sup> in the porous outer wall are strongly hydrogen bonded with H<sub>2</sub>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 H<sub>2</sub>O molecules as a novel <italic>C</italic><sub>2</sub>‐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 H<sub>2</sub>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<sup>−5</sup> S cm<sup>−1</sup> 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<sup>−5</sup> S cm<sup>−1</sup> at 274 K due to a structural transition upon approaching this temperature that elongates the unit cell along the <italic>b</italic>‐axis. The proton‐transfer route can be predicted in WMCs, as O(4) of an H<sub>2</sub>O molecule at the center of an SCTC shows a motion that rotates the dipole in the <italic>b</italic>‐axis direction, but not the <italic>c</italic>‐axis; the thermal ellipsoids of O(4) based on anisotropic temperature factors obtained by X‐ray crystallography reflect a structural fluctuation along the <italic>b</italic>‐axis direction induced by [Co<sup>III</sup>(H<sub>2</sub>bim)<sub>3</sub>]<sup>3+</sup>.</p> </abstract>