Geminal Carbon–Hydrogen Bond Activation in Cumulenes Promoted by Adjacent Iridium/Ruthenium Centers

The tetracarbonyl complex [IrRu­(CO)4(dppm)2]­[BF4] (dppm = μ-Ph2PCH2PPh2) (1) reacts with allene and methylallene, resulting in double, geminal carbon–hydrogen bond activation accompanied by H migration, one each to the β- and γ-carbons of the allene, yielding the vinylidene-bridged complexes [IrRu­(CO)4(μ-CC­(H)­R)­(dppm)2]­[BF4] (R = CH3 (2), C2H5 (3)). Reaction of 1 with 1,1-dimethylallene results in the activation of the pair of geminal C–H bonds together with one of the methyl C–H bonds, yielding the vinylvinylidene-bridged product [IrRu­(CO)4(μ-CC­(H)­C­(CH3)CH2)­(dppm)2]­[BF4] (4), accompanied by H2 loss. The addition of 1,1-difluoroallene to compound 1 does not lead to C–H activation but instead forms the fluoroallene-bridged adduct [IrRu­(CO)4(μ-κ1:κ1-F2CC–CH2)­(dppm)2]­[BF4] (5), bound through the unsubstituted end of the allene and having the CH2 group bound to Ru. This species rearranges, over 24 h at ambient temperature, to its isomer (5a), having the CH2 end bound to Ir. Although ethylene and propylene fail to react with 1 at ambient temperature, they react with the tricarbonyl analogue [IrRu­(CO)3(dppm)2]­[BF4] (8) at −20 °C to yield the alkenyl-bridged hydrides [IrRu­(CO)3(H)­(μ-κ1:η2-C­(H)CHR)­(dppm)2]­[BF4] (R = H (6), CH3 (7)), by activation of a single C–H bond. Compound 8 reacts similarly with allene at −20 °C, yielding [IrRu­(H)­(CO)3(μ-κ1:η2-CHCCH2)­(dppm)2]­[BF4] (11). None of these alkenyl or allenyl hydride species give rise to a second C–H activation upon warming. However, warming 11 in the presence of CO does yield 2, together with decomposition products. Removal of a carbonyl from the vinylidene-bridged complexes [IrRu­(CO)4(μ-CC­(H)­R)­(dppm)2]­[BF4] (R = CH3 (2), R = H (9)) generates the alkynyl hydride complexes [IrRu­(H)­(CO)3(μ-κ1:η2-CCR)­(dppm)2]­[BF4] (R = CH3 (12), H (10)), both of which can be converted back to the respective vinylidenes by CO addition. On the basis of these observations, a mechanism is proposed for the transformations of allene and methylallene into the methyl- and ethylvinylidene-bridged products, noted above.