title: Proton Order-Disorder Phenomena in a Hydrogen-Bonded Rhodium-η(5)-Semiquinone Complex: A Possible Dielectric Response Mechanism.
abstract: A newly synthesized one-dimensional (1D) hydrogen-bonded (H-bonded) rhodium(II)-η(5)-semiquinone complex, [Cp*Rh(η(5)-p-HSQ-Me4)]PF6 ([1]PF6; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; HSQ = semiquinone) exhibits a paraelectric-antiferroelectric second-order phase transition at 237.1 K. Neutron and X-ray crystal structure analyses reveal that the H-bonded proton is disordered over two sites in the room-temperature (RT) phase. The phase transition would arise from this proton disorder together with rotation or libration of the Cp* ring and PF6(-) ion. The relative permittivity εb' along the H-bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of (13)C CP/MAS NMR spectra demonstrates that the proton is dynamically disordered in the RT phase and that the proton exchange has already occurred in the low-temperature (LT) phase. Rate constants for the proton exchange are estimated to be 10(-4)-10(-6) s in the temperature range of 240-270 K. DFT calculations predict that the protonation/deprotonation of [1](+) leads to interesting hapticity changes of the semiquinone ligand accompanied by reduction/oxidation by the π-bonded rhodium fragment, producing the stable η(6)-hydroquinone complex, [Cp*Rh(3+)(η(6)-p-H2Q-Me4)](2+) ([2](2+)), and η(4)-benzoquinone complex, [Cp*Rh(+)(η(4)-p-BQ-Me4)] ([3]), respectively. Possible mechanisms leading to the dielectric response are discussed on the basis of the migration of the protonic solitons comprising of [2](2+) and [3], which would be generated in the H-bonded chain.
authors.affiliation: Department of Material Science, Graduate School of Material Science, University of Hyogo and, Research Center for New Functional Materials, Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan). mitsumi@sci.u-hyogo.ac.jp.
authors.forename: Minoru
authors.initials: M
authors.lastname: Mitsumi
authors.affiliation: Department of Material Science, Graduate School of Material Science, University of Hyogo and, Research Center for New Functional Materials, Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan).
authors.forename: Kazunari
authors.initials: K
authors.lastname: Ezaki
authors.affiliation: Department of Material Science, Graduate School of Material Science, University of Hyogo and, Research Center for New Functional Materials, Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan).
authors.forename: Yuuki
authors.initials: Y
authors.lastname: Komatsu
authors.affiliation: Department of Material Science, Graduate School of Material Science, University of Hyogo and, Research Center for New Functional Materials, Graduate School of Material Science, University of Hyogo, 3-2-1 Kouto, Kamigori-cho, Ako-gun, Hyogo 678-1297 (Japan).
authors.forename: Koshiro
authors.initials: K
authors.lastname: Toriumi
authors.affiliation: Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192 (Japan).
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Miyatou
authors.affiliation: Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192 (Japan). mizuno@se.kanazawa-u.ac.jp.
authors.forename: Motohiro
authors.initials: M
authors.lastname: Mizuno
authors.affiliation: Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan).
authors.forename: Nobuaki
authors.initials: N
authors.lastname: Azuma
authors.affiliation: Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan). miyazaki@chem.sci.osaka-u.ac.jp.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Miyazaki
authors.affiliation: Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan).
authors.forename: Motohiro
authors.initials: M
authors.lastname: Nakano
authors.affiliation: Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan). kitagawa@cheng.es.osaka-u.ac.jp.
authors.forename: Yasutaka
authors.initials: Y
authors.lastname: Kitagawa
authors.affiliation: Research Center for Neutron Science and Technology, CROSS Tokai, Ibaraki 319-1106 (Japan).
authors.forename: Takayasu
authors.initials: T
authors.lastname: Hanashima
authors.affiliation: J-PARC center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195 (Japan).
authors.forename: Ryoji
authors.initials: R
authors.lastname: Kiyanagi
authors.affiliation: J-PARC center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195 (Japan). takashi.ohhara@j-parc.jp.
authors.forename: Takashi
authors.initials: T
authors.lastname: Ohhara
authors.affiliation: School of Materials Science, Fukui University of Technology, 3-6 Gakuen, Fukui 910-8505 (Japan).
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Nakasuji
keywords: NMR spectroscopy; density functional calculations; hydrogen bonding; proton transport; quinones; rhodium