Iridium-containing cumulenes: how to prepare and how to use

This microreview summarizes the work on iridium-containing cumulenes of the general compositions trans-[IrX{C(C)nRR′}(PiPr3)2] with n=1, 2, 3 and 4, which recently has been carried out in our laboratory. It is shown that all the parent compounds with X=Cl and an IrCC, IrCCC, IrCCCC or IrCCCCC chain can be prepared using [IrH2Cl(PiPr3)2] as the starting material. Independent on the length of the chain, the iridiacumulenes are highly reactive to both nucleophiles and electrophiles. For n=1 and 3, the corresponding chloro complexes can be converted to organometallic derivatives trans-[IrR″{=C(C)nRR′}(PiPr3)2], which in the presence of CO smoothly undergoe migratory insertion reactions. Upon treatment with CO, the azido compounds trans-[Ir(N3){C(C)nRR′}(PiPr3)2] (n=1, 2 and 3) react similarly. A remarkable feature is that the hydroxoiridium(I) derivatives trans-[Ir(OH)(CCRR′)(PiPr3)2] and trans-[Ir(OH)(CCCRR′)(PiPr3)2] behave as Broensted bases and react with HX (X=OPh, F, CCR) by elimination of water and the formation of new iridiacumulenes containing an IrX bond. While the reactions of the chloro complexes trans-[IrCl(CCCRR′)(PiPr3)2] and trans-[IrCl(CCCCRR′)(PiPr3)2] with HCl proceed by oxidative addition to afford neutral iridium(III) compounds, the iridium allenylidenes react with CF3CO2H in polar solvents to generate cationic iridium carbynes trans-[IrCl(CCHCRR′)(PiPr3)2]X, being the first representatives of this type.