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

Abstract

The tetracarbonyl complex [IrRu(CO) 4 (dppm) 2 ][BF 4 ] (dppm = μ-Ph 2 PCH 2 PPh 2 ) (1) reacts with allene and methylallene, resulting in double, geminal carbon−hydrogen bond activation accompanied by H migration, one each to the βand γ-carbons of the allene, yielding the vinylidene-bridged complexes [IrRu(CO) 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 4), accompanied by H 2 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-κ 5), bound through the unsubstituted end of the allene and having the CH 2 group bound to Ru. This species rearranges, over 24 h at ambient temperature, to its isomer (5a), having the CH 2 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 ][BF 4 ] (8) at −20 °C to yield the alkenyl-bridged hydrides [IrRu(CO) 3 (H)(μ-κ 1 :η 2 -C(H)CHR)(dppm) 2 ][BF 4 ] (R = H (6), CH 3 (7)), by activation of a single C−H bond. Compound 8 reacts similarly with allene at −20 °C, yielding [IrRu(H)(CO) 3 (μ-κ 1 :η 2 -CH CCH 2 )(dppm) 2 ][BF 4 ] (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 ][BF 4 ] (R = CH 3 (2), R = H (9)) generates the alkynyl hydride complexes [IrRu(H)(CO) 3 (μ-κ 1 :η 2 -CCR)(dppm) 2 ][BF 4 ] (R = CH 3 ( 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.

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

“…However, attempts to generate 9 by reaction of 6 with this substrate does not occur in the presence of 100 equiv of the substrate over several days, even when carried out at slightly elevated temperatures. The Ir/Ru analogue also did not react with this substrate …”

“…In the 1 H NMR spectrum, the dppm CH 2 protons appear at δ 4.79 and 4.46, while the CH 2 protons of the fluoroallene ligand appear as a triplet at δ 1.55, coupling to the 31 P nuclei at δ −12.0 ( 3 J HP = 11.8 Hz). The observation of one allene CH 2 resonance is consistent with binding through this end of the cumulene, as is typically observed. ,,, The upfield chemical shift of the CH 2 protons also suggests that they are adjacent to Os and not Ir, the latter of which characteristically resonate downfield at ca. δ 4.…”

“…Compound 6-CF 3 SO 3 also reacts with excess 1,1-dimethylallene at ambient temperature over 12 h, in this case, giving a very different product, [IrOs­(CO) 4 (μ-CC­(H)­C­(CH 3 )CH 2 )­(dppm) 2 ]­[CF 3 SO 3 ] ( 9 ), having a bridging 3-methyl-1,3-butadienylidene fragment, as shown in Scheme , resulting from the activation of the two geminal C–H bonds of the cumulene, and one C–H bond of a methyl group, with the concomitant evolution of H 2 . This reactivity is reminiscent of that observed in previous work with the related [IrRu­(CO) 4 (dppm) 2 ] + system …”

“…The products obtained in the reactions of the methylene-bridged compound [IrOs­(CO) 3 (μ-CH 2 )­(dppm) 2 ] + ( 2 ) and the tetracarbonyl complex [IrOs­(CO) 4 (dppm) 2 ] + ( 6 ) with a number of allenes, as described in this paper, display a surprising structural diversity in comparison to those of the other closely related group 8/group 9 metal combinations previously studied in our group (Rh/Ru, Rh/Os, and Ir/Ru , ). Although the Rh-based systems display significant differences in reactivity compared with the Ir/Ru system, the current Ir/Os system demonstrates essentially all of the diverse reactions observed in the previous three metal combinations, depending on both the allene and the metal complex investigated.…”

“…The addition of allene to the analogous methylene-bridged Ir/Ru complex also resulted in β-H elimination and reductive elimination to generate 1,3-butadiene, while the addition of substituted allenes yielded iridacycle products, presumably through an intermediate, such as F , in which migration of the Ru-bound end of the bridging hydrocarbyl unit to Ir occurred, a probable consequence of strain within the C 3 -bridged unit in F , and favored by the stronger Ir–C bonds. Surprisingly, the addition of a second equivalent of cumulene to the chelated complexes resulted in multiple C–H bond activation reactions, which were also observed upon addition of allenes to the related tetracarbonyl complex, [IrRu­(CO) 4 (dppm) 2 ] + . The propensity for Ir to undergo facile C–H bond activation in this Ir/Ru system led us to investigate the related Ir/Os complexes in order to determine the influence of the adjacent Os center.…”