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)-eta(5)-semiquinone complex, [Cp*Rh(eta(5)-p-HSQ-Me-4)]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 epsilon(b)' along the H-bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of C-13 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 pi-bonded rhodium fragment, producing the stable eta(6)-hydroquinone complex, [Cp*Rh3+(eta(6)-p-H(2)Q-Me-4)](2+) ([2](2+)), and eta(4)-benzoquinone complex, [Cp*Rh+(eta(4)-p-BQ-Me-4)] ([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.