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.