p-, o-η4-Benzoquinone and the Related η6-Hydroquinone, η6-Catechol Complexes of Pentamethylcyclopentadienyliridium: Synthesis, Structures, and Reactivity

Treatment of hydroquinone with [Cp*Ir(solvent) 3 ][BF 4 ] 2 ( 1 ) in acidic medium afforded the stable π-bonded complex [Cp*Ir(η 6 -hydroquinone)][BF 4 ] 2 ( 2 ) in 93% yield. Complex 2 can be easily deprotonated by a base to give the related [Cp*Ir(η 5 -semiquinone)][BF 4 ] ( 3 ) and [Cp*Ir(η 4 - p -benzoquinone)] ( 4 ) compounds, identified by spectroscopic methods. Further, the X-ray molecular structure of p -benzoquinone iridium complex 4 is reported; such compounds are rare in the literature. Complex 4 reacts with an excess of HBF 4 ·Et 2 O to give the starting material 2 quantitatively. Interestingly, the chemical transformations from 2 to 4 and vice versa occur with facile reversible changes of the oxidation state from Ir(III) to Ir (I). On the other hand, the oxo−dienyl iridium complex [Cp*Ir(η 5 -2,6-dimethoxy-C 6 H 3 O)][BF 4 ] ( 5 ), identified by spectroscopic methods and X-ray analysis, reacts with NaOMe in methanol to give unexpectedly but reproducibly the substituted o -benzoquinone iridium compound [Cp*Ir{η 4 -(3-methoxy)- o- benzoquinone}] ( 6 ). The solution behavior and the reactivity of 6, as well as a rationale explaining its formation, is advanced.