Proton Order-Disorder Phenomena in a Hydrogen-Bonded Rhodium- eta super(5)-Semiquinone Complex: A Possible Dielectric Response Mechanism

A newly synthesized one-dimensional (1D) hydrogen-bonded (H-bonded) rhodium(II)- eta super(5)-semiquinone complex, [Cp*Rh( eta super(5)-p-HSQ- Me sub(4))]PF sub(6) ([1]PF sub(6); Cp*=1,2,3,4,5-pentamethylcyclopentadienyl; HSQ=semiquinone) exhibits a paraelectric-antiferroelectric second-order phase transition at 237.1K. 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 PF sub(6) super(-) ion. The relative permittivity epsilon sub(b) along the H-bonded chains reaches relatively high values (ca., 130) in the RT phase. The temperature dependence of super(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 super(-4)-10 super(-6)s in the temperature range of 240-270K. DFT calculations predict that the protonation/deprotonation of [1] super(+) leads to interesting hapticity changes of the semiquinone ligand accompanied by reduction/oxidation by the pi -bonded rhodium fragment, producing the stable eta super(6)-hydroquinone complex, [Cp*Rh super(3+)( eta super(6)-p-H sub(2)Q-Me sub(4))] super(2+) ([2] super(2+)), and eta super(4)-benzoquinone complex, [Cp*Rh super(+)( eta super(4)-p-BQ-Me sub(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] super(2+) and [3], which would be generated in the H-bonded chain. Proton dynamics: A one-dimensional (1D) hydrogen-bonded (H-bonded) rhodium(II)- eta super(5)-semiquinone complex exhibits a paraelectric-antiferroelectric second-order phase transition at 237.1K as a result of proton transfer dynamics in the strong H-bonds (see figure). A possible dielectric response mechanism in the room-temperature (RT) phase is discussed on the basis of the rapid migration of the protonic solitons.