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) (<b>2</b>), 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] (<b>3</b>/<b>3a</b>), 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 <b>2</b>, as the triflate salt (<b>2-CF</b>_<b>3</b><b>SO</b>_<b>3</b>), with methylallene also yields two isomers, [IrOs­(CO)₃(μ-η³:κ¹-C­(CHCH₃)­(CH₂)₂)­(dppm)₂]­[CF₃SO₃] (<b>4</b>/<b>4a</b>); 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 <b>2-CF</b>_<b>3</b><b>SO</b>_<b>3</b> 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] (<b>5</b>), 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] (<b>6</b>), to generate analogous products, [IrOs­(CO)₃(μ-η³:κ¹-CH₂CCHR)­(dppm)₂]­[BF₄] (R = H (<b>7</b>), CH₃ (<b>8</b>), respectively). Reaction of <b>6-CF</b>_<b>3</b><b>SO</b>_<b>3</b> with 1,1-dimethylallene yields [IrOs­(CO)₄(μ-CC­(H)­C­(CH₃)CH₂)­(dppm)₂]­[CF₃SO₃] (<b>9</b>), 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 <b>6-CF</b>_<b>3</b><b>SO</b>_<b>3</b> yields [IrOs­(CO)₄(μ-κ¹:κ¹-F₂C<i>C</i><i>C</i>H₂)­(dppm)₂][CF₃SO₃] (<b>10</b>), 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