Proton Order-Disorder Phenomena in a Hydrogen-Bonded Rhodium-η⁵-Semiquinone Complex: A Possible Dielectric Response Mechanism

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 ε_b′ 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, [CpRh³⁺(η⁶-p-H₂Q-Me₄)]²⁺ ([2]²⁺), and η⁴-benzoquinone complex, [CpRh⁺(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.