Proton Order–Disorder Phenomena in a Hydrogen‐Bonded Rhodium–η5‐Semiquinone Complex: A Possible Dielectric Response Mechanism |
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Authors: | Dr Minoru Mitsumi Kazunari Ezaki Yuuki Komatsu Prof?Dr Koshiro Toriumi Dr Tatsuya Miyatou Prof?Dr Motohiro Mizuno Nobuaki Azuma Prof?Dr Yuji Miyazaki Prof?Dr Motohiro Nakano Prof?Dr Yasutaka Kitagawa Dr Takayasu Hanashima Dr Ryoji Kiyanagi Dr Takashi Ohhara Prof?Dr Kazuhiro Nakasuji |
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Institution: | 1. 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);2. Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920‐1192 (Japan);3. Research Center for Structural Thermodynamics, Graduate School of Science, Osaka University, 1‐1?Machikaneyama, Toyonaka, Osaka 560‐0043 (Japan);4. Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, 1‐3?Machikaneyama, Toyonaka, Osaka 560‐8531 (Japan);5. Research Center for Neutron Science and Technology, CROSS Tokai, Ibaraki 319‐1106 (Japan);6. J‐PARC center, Japan Atomic Energy Agency, Tokai, Ibaraki 319‐1195 (Japan);7. School of Materials Science, Fukui University of Technology, 3‐6?Gakuen, Fukui 910‐8505 (Japan) |
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Abstract: | A newly synthesized one‐dimensional (1D) hydrogen‐bonded (H‐bonded) rhodium(II)–η5‐semiquinone complex, Cp*Rh(η5‐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 13C 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 η6‐hydroquinone complex, Cp*Rh3+(η6‐p‐H2Q‐Me4)]2+ ( 2 ]2+), and η4‐benzoquinone complex, Cp*Rh+(η4‐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. |
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Keywords: | density functional calculations hydrogen bonding NMR spectroscopy proton transport quinones rhodium |
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