Proton Order–Disorder Phenomena in a Hydrogen‐Bonded Rhodium–η⁵‐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.

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex

“…Proton–electron coupling in solid-state materials, 1,5-dihalo-2,6-naphthoquinhydrones, has also been demonstrated . More recently, several solid-state molecular compounds exhibiting proton–electron coupling have been reported. − However, although spin and charge are two important characteristics of electrons, no previous study has succeeded in demonstrating proton-transfer-coupled spin-transition (PCST) behavior in a crystalline molecular material. The successful coupling of proton transfer and spin transition will lead to the realization of various coupling properties.…”

1-Methyl-1,4-cyclohexadiene as a Traceless Reducing Agent for the Synthesis of Catechols and Hydroquinones

“…Durohydroquinone (4l). 52,53 It was obtained as 40 equiv trifluoroacetic acid, 24 h, eluent: hexane/ethyl acetate 85:15, 7.5 mg, yield = 25%. 1 H NMR (CDCl 3 , 300 MHz): δ 4.47 (br s, 2OH), 2.17 (s, 12H).…”

Observation of proton-transfer-coupled spin transition by single-crystal neutron-diffraction measurement

“…2 Numerous studies have investigated the proton dynamics phenomena in crystalline compounds, including proton transfer, 3 proton order/disorder, 4 proton migration, 5 and proton tautomerism. 6 In particular, recent studies on proton dynamics have encompassed couplings with other dynamical phenomena such as electron transfer or electricstate changes, 7 spin transition, 8,9 and structural deformations induced by external stimuli. 10 The insights afforded by such investigations into crystal proton dynamics have been actively applied in the design of functional materials, such as proton conductors, 11 ferro/antiferroelectric compounds, 12 luminescent materials, 13 and switchable materials that exhibit changes in their physical properties response to external stimuli.…”