η⁵-Semiquinone Complexes and the Related η⁴-Benzoquinone of (Pentamethylcyclopentadienyl)rhodium and -iridium: Synthesis, Structures, Hydrogen Bonding, and Electrochemical Behavior

Abstract

Treatment of hydroquinone with [Cp*M(solvent)3][OTf]2 (M = Rh, Ir) in acetone afforded the π-bonded complexes {[Cp*M(η5-semiquinone)][OTf]}n (M = Rh (1a), M = Ir (1b)) in 95% yield. The 1H NMR spectra of 1a,b recorded in CD3OD indicate strong hydrogen bonding in solution. The crystal structures of 1a and 1b were determined and exhibit strong intermolecular hydrogen bonding, forming organometallic polymers in which the integrity of the system is maintained by hydrogen bonding between metal−semiquinone subunits. Deprotonation of 1a produced the related η4-quinone complex [Cp*Rh(η4-quinone)] (2a), which was fully characterized and its X-ray molecular structure determined. Furthermore, the electrochemical behavior of these η4-quinone π-complexes [Cp*M(η4-quinone)] (M = Rh (2a), M = Ir (2b)) was investigated and compared to that of the well-known couple quinone/hydroquinone. The latter are important species in chemistry and biology; their biological action is often linked to their electron-transfer rates and redox behavior.