Diverse Coordination Modes and Transformations of Allenes at Adjacent Iridium/Osmium Centers

The methylene-bridged complex, [IrOs­(CO)3(μ-CH2)­(dppm)2]­[BF4] (dppm = μ-Ph2PCH2PPh2) (2), reacts with
allene, resulting in C–C bond formation, to yield an equilibrium
mix of two isomers of [IrOs­(CO)3(μ-η3:κ1-C­(CH2)3)­(dppm)2]­[BF4] (3/3a), in which the
hapticity of the trimethylenemethane ligand with respect to the two
metals, as well as the carbonyl ligand arrangement, is different in
each isomer. Reaction of 2, as the triflate salt (2-CF3SO3), with methylallene also yields two isomers, [IrOs­(CO)3(μ-η3:κ1-C­(CHCH3)­(CH2)2)­(dppm)2]­[CF3SO3] (4/4a); however, in this
case, the binding mode of the substituted trimethylenemethane moiety
is the same in each isomer and differs only in the position of the
methyl group on the allylic moiety. The addition of 1,1-dimethylallene
to 2-CF3SO3 results in loss of 4-methyl-1,3-pentadiene and subsequent
reaction of the remaining “[IrOs­(CO)3(dppm)2]+” species with excess 1,1-dimethylallene
to give [IrOs­(CO)3(μ-η3:κ1-CH2CCMe2)­(dppm)2]­[CF3SO3] (5), in which the dimethylallene
moiety is κ1-bound to Os through the central carbon
and η3-bound to Ir. Both allene and methylallene
react with the tetracarbonyl complex, [IrOs­(CO)4(dppm)2]­[BF4] (6), to generate analogous
products, [IrOs­(CO)3(μ-η3:κ1-CH2CCHR)­(dppm)2]­[BF4] (R
= H (7), CH3 (8), respectively).
Reaction of 6-CF3SO3 with 1,1-dimethylallene yields [IrOs­(CO)4(μ-CC­(H)­C­(CH3)CH2)­(dppm)2]­[CF3SO3] (9), the result of activation of the geminal C–H bonds of the
unsubstituted end of the allene, and additional activation of a methyl
C–H bond. The addition of 1,1-difluoroallene to 6-CF3SO3 yields [IrOs­(CO)4(μ-κ1:κ1-F2CCCH2)­(dppm)2][CF3SO3] (10), in which this cumulene bridges both metal
centers through the central carbon and the CH2 end of the
substrate. These reactivities are compared to those of related Ir2, Rh/Ru, Rh/Os, and Ir/Ru complexes.