Proton Order–Disorder Phenomena in a Hydrogen‐Bonded Rhodium–η 5 ‐Semiquinone Complex: A Possible Dielectric Response Mechanism

A newly synthesized one‐dimensional (1D) hydrogen‐bonded (H‐bonded) rhodium(II)–η 5 ‐semiquinone complex, [Cp*Rh(η 5 ‐ p ‐HSQ‐Me 4 )]PF 6 ([ 1 ]PF 6 ; 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 PF 6 − 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 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 ‐H 2 Q‐Me 4 )] 2+ ([ 2 ] 2+ ), and η 4 ‐benzoquinone complex, [Cp*Rh + (η 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.