Transition-metal complexes containing terminal alkylidene groups (L n MdCRR 0 ) have displayed a rich chemistry in carbonÀ carbon bond formation, including cyclopropanations, 1 olefin metathesis, 2 and the oligomerization of alkynes. 3 Bimetallic alkylidene complexes, in which the alkylidene moiety bridges two metal centers, have been much less studied and are expected to be less reactive than their unsaturated counterparts, owing to the formal saturation of these bridging units. Nevertheless, bridging alkylidene units have also shown interesting reactivity. 5 In particular, the prototypical alkylidene unit, the methylene (ÀCH 2 À) group, has been proposed to play a key role as a surface-bound species in the sequential carbonÀcarbon bond formation that occurs in the FischerÀTropsch (FT) process. 6 In FT chemistry the surface-bound methylene groups are presumably bound in a bridging arrangement between a pair of adjacent metals and as such, the chemistry of this bridging group should parallel that observed in methylene-bridged binuclear complexes. 5a,c,6f,7 Consequently, these well-defined methylene-bridged complexes can serve as useful models for heterogeneous FT chemistry, in keeping with the surface-cluster analogy initially proposed by Muetterties. 8 As part of a study, the long-term goal of which is to determine the roles of the different metals in mixed-metal FT catalysts, 9 we have investigated the chemistry of methylene-bridged complexes involving the Rh/Ru, 7g,h,10 Ir/Ru, 7i Rh/Os, 7e,f,10 and Ir/Os 11 metal combinations. It is our contention that to fully understand the roles of the different metals and metal combinations in this chemistry, a careful comparison of different, but related, combinations of metals is necessary. The Rh/Os system was found to be particularly reactive, coupling up to four methylene units to generate either allyl and methyl or butanediyl fragments at previous study the metal centers, 7e,f mimicking aspects of FT reactivity. In this previous study we proposed that the key (although unobserved) intermediates in the methylene-coupling transformations were hydrocarbyl-bridged units and that the stepwise addition of methylene groups occurred by insertion into the RhÀC bond of these bridging fragments. In order to learn more about the reactivity of these bridging hydrocarbyl groups and the roles played by the different metals in the growth of the carbonÀ carbon chain, we have investigated the stepwise coupling of methylene groups with a variety of unsaturated substrates. 5c,7a,7d,7h,10À12In the study described herein we focus our investigation on methylene-bridged complexes involving the Ir/Ru metal combination and their coupling with cumulenes. By implementing a third-row, group 9 metal, we sought to utilize the greater ABSTRACT: The methylene-bridged complex [IrRu(CO) 4 (μ-CH 2 )-(dppm) 2 ][BF 4 ] (1; dppm = μ-Ph 2 PCH 2 PPh 2 ) reacts with 1,1-dimethylallene and 1,2-butadiene, resulting in coupling of the cumulene and the bridging methylene group, yielding the complexes [IrRu(CO...

The methylene-bridged complex [IrOs(CO) 4 ( μ-CH 2 )(dppm) 2 ][CF 3 SO 3 ] (dppm = μ-Ph 2 PCH 2 PPh 2 ) (3) can be synthesized by the addition of diazomethane to [IrOs(CO) 5 (dppm) 2 ][CF 3 SO 3 ] (1) or [IrOs(CO) 4 -(dppm) 2 ][CF 3 SO 3 ] (2). Reaction of 3 with dimethyl acetylenedicarboxylate (DMAD) leads to the insertion of the alkyne into the iridium-carbon bond, yielding both [IrOs(CO) 4 (μ-κ 1 :κ 1 -C(CO 2 CH 3 )dC(CO 2 CH 3 )-CH 2 )(dppm) 2 ][CF 3 SO 3 ] (5) and [IrOs(CO) 3 (μ-κ 1 :κ 1 -C(CO 2 CH 3 )dC(CO 2 CH 3 )CH 2 )(dppm) 2 ]-[CF 3 SO 3 ] (6), each of which can be obtained as the exclusive product under either CO or an Ar purge, respectively. Hexafluorobutyne (HFB) fails to react with 3, but reacts with [IrOs (CO) (7). Reaction of 7 with diazomethane results in the insertion of a second methylene unit into the iridium-carbon bond, yielding [IrOs(CO) 3 (μ-κ 1 :κ 1 -CH 2 (CF 3 )CdC(CF 3 )CH 2 )(dppm) 2 ][CF 3 SO 3 ] (9), which can be characterized by NMR spectroscopy only at low temperatures owing to deinsertion of the iridium-bound methylene group at ambient temperature. Compound 6 also reacts with diazomethane but in this case results in the formation of a new carbon-oxygen bond between the newly introduced methylene unit and a carbonyl oxygen of the inserted DMAD fragment. This bond formation is accompanied by carbon-hydrogen bond activation of the original osmium-bound methylene group, yielding [IrOs(CO) 3 (μ-H)(μ-κ 1 :κ 1 :κ 1 -CH 2 OC(OCH 3 )dCC(CO 2 CH 3 )dCH)(dppm) 2 ][CF 3 SO 3 ] (8). Attempts to insert a methylene unit into the iridium-carbon bond of the alkyne-bridged complexes [IrOs(CO) 3 (μ-κ 1 :κ 1 -RCdCR)(dppm) 2 ][CF 3 SO 3 ] (R = CO 2 Me (12), CF 3 (13)) yields the C 3 -bridged complex [IrOs(CO) 3 (μ-κ 1 :κ 1 -CH 2 (CF 3 )CdCCF 3 )(dppm) 2 ][CF 3 SO 3 ] (14) in the case of 13, but no further methylene incorporation is observed. Compound 12 reacts with diazomethane to give a number of unidentified products under a variety of conditions. The reactivities of the aforementioned complexes are compared to that of related late metal combinations.

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.

The methylene-bridged complex, [IrOs(CO) 3 (μ-CH 2 )(dppm) 2 ][BF 4 ] (dppm = μ-Ph 2 PCH 2 PPh 2 ) (2), reacts with allene, resulting in C−C bond formation, to yield an equilibrium mix of two isomers of [IrOs(CO) 3 (μ-η 3 :κ 1 -C(CH 2 ) 3 )(dppm) 2 ][BF 4 ] (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-CF 3 SO 3 ), with methylallene also yields two isomers, [IrOs(CO) 3 (μ-η 3 :κ 1 -C(CHCH 3 )(CH 2 ) 2 )(dppm) 2 ][CF 3 SO 3 ] (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-CF 3 SO 3 results in loss of 4-methyl-1,3pentadiene and subsequent reaction of the remaining "[IrOs(CO) 3 (dppm) 2 ] + " species with excess 1,1-dimethylallene to give [IrOs(CO) 3 (μ-η 3 :κ 1 -CH 2 CCMe 2 )(dppm) 2 ][CF 3 SO 3 ] (5), in which the dimethylallene moiety is κ 1 -bound to Os through the central carbon and η 3 -bound to Ir. Both allene and methylallene react with the tetracarbonyl complex, [IrOs(CO) 4 (dppm) 2 ]-[BF 4 ] (6), to generate analogous products, [IrOs(CO) 3 (μ-η 3 :κ 1 -CH 2 CCHR)(dppm) 2 ][BF 4 ] (R = H (7), CH 3 (8), respectively). Reaction of 6-CF 3 SO 3 with 1,1-dimethylallene yields [IrOs(CO) 4 (μ-CC(H)C(CH 3 )CH 2 )(dppm) 2 ][CF 3 SO 3 ] (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-CF 3 SO 3 yields [IrOs(CO) 4 (μ-κ 1 :κ 1 -F 2 CCCH 2 )(dppm) 2 ][CF 3 SO 3 ] (10), in which this cumulene bridges both metal centers through the central carbon and the CH 2 end of the substrate. These reactivities are compared to those of related Ir 2 , Rh/Ru, Rh/Os, and Ir/Ru complexes.

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