ETA(4)-QUINONE, ETA(5)-SEMIQUINONE AND ETA(6)-HYDROQUINONE COMPLEXES OF PENTAMETHYLCYCLOPENTADIENYL IRIDIUM - X-RAY MOLECULAR-STRUCTURE OF [CP-ASTERISK-IR(ETA(4)-1,4 BENZOQUINONE)]

Treatment of hydroquinone with [Cp Ir(solvent)(3)][BF4](2) (1) in aci dic medium afforded the stable pi-bonded complex [Cp Ir(eta(6)-hydroq uinone)][BF4](2) (2) in high yield. Complex 2 can be easily deprotonat ed by a base to give the related [Cp Ir(eta(5)-semiquinone)][BF4] (3) and [Cp Ir(eta(4)-quinone)] (4) compounds identified by spectroscopi c methods. Complex 4 reacts with an excess of HBF4 . Et2O to give the starting material 2 quantitatively. Interestingly, these chemical tran sformations occur with facile reversible changes of the oxidation stat e from Ir(III) to Ir(I). Further the X-ray molecular structure of 4 is reported. (C) 1998 Elsevier Science S.A.