The mixed-metal complex, [RhOs(CO)(4)(dppm)(2)][BF(4)] (1; dppm = micro-Ph(2)PCH(2)PPh(2)) reacts with diazomethane to yield a number of products resulting from methylene incorporation into the bimetallic core. At -80 degrees C the reaction between 1 and CH(2)N(2) yields the methylene-bridged [RhOs(CO)(3)(micro-CH(2))(micro-CO)(dppm)(2)][BF(4)] (2), which reacts further at ambient temperature to give the allyl methyl species, [RhOs(eta(1)-C(3)H(5))(CH(3))(CO)(3)(dppm)(2)][BF(4)] (4). At intermediate temperatures compounds 1 and 2 react with diazomethane to yield the butanediyl complex [RhOs(C(4)H(8))(CO)(3)(dppm)(2)][BF(4)] (3) by the incorporation and coupling of four methylene units. Compound 2 is proposed to be an intermediate in the formation of 3 and 4 from 1 and on the basis of labeling studies a mechanism has been proposed in which sequential insertions of diazomethane-generated methylene fragments into the Rh-C bond of bridging hydrocarbyl fragments occur. Reaction of the tricarbonyl species, [RhOs(CO)(3)(micro-CH(2))(dppm)(2)][BF(4)] with diazomethane over a range of temperatures generates the ethylene complex [RhOs(eta(2)-C(2)H(4))(CO)(3)(dppm)(2)][BF(4)] (7a), but no further incorporation of methylene groups is observed. This observation suggests that carbonyl loss in the formation of the above allyl and butanediyl species only occurs after incorporation of the third methylene fragment. Attempts to generate C(2)-bridged species by the reaction of 1 with ethylene gave no reaction, however, in the presence of trimethylamine oxide the ethylene adducts [RhOs(eta(2)-C(2)H(4))(CO)(3)(dppm)(2)][BF(4)] (7b; an isomer of 7a) and [RhOs(eta(2)-C(2)H(4))(2)(CO)(2)(dppm)(2)][BF(4)] (8) were obtained. The relationship of the above products to the selective coupling of methylene groups, and the roles of the different metals are discussed.

Protonation of the methylene-bridged complex [RhOs(CO)3(μ-CH2)(μ-CO)(dppm)2][CF3SO3] with triflic acid at −80 °C yields the methyl complex [RhOs(CO)4(μ-CH3)(dppm)2][CF3SO3]2 (1) in which the methyl group is primarily bound to Os while involved in an agostic interaction with Rh. Warming this species to −20 °C results in methyl migration to a terminal site on Rh, yielding [RhOs(CH3)(CO)2(μ-CO)2(dppm)2][CF3SO3]2 (2), and subsequent warming to ambient temperature results in migratory insertion to yield the acetyl-bridged [RhOs(CF3SO3)(CO)2(μ-C(CH3)O)(μ-CO)(dppm)2][CF3SO3] (3). Replacement of the coordinated triflate anion in 3 by CO and PMe3 occurs at low temperature to give [RhOs(L)(CO)2(μ-C(CH3)O)(μ-CO)(dppm)2][CF3SO3]2 (L = CO (4), PMe3 (5a)). Warming the PMe3 adduct results in transformation of the acetyl group, which is C-bound to Rh in 5a, to a group that is C-bound to Os in 5b. One carbonyl in 3 is labile and is readily lost, yielding [RhOs(CF3SO3)(CO)2(μ-C(CH3)O)(dppm)2][CF3SO3] (6). Attempts to obtain the above methyl or acetyl species by reaction of the hydride-bridged species [RhOs(X)(CO)3(μ-H)(μ-CO)(dppm)2][X] (X = CF3SO3 (7a), BF4 (7b)) with diazomethane were unsuccessful.

The heterobinuclear complexes [IrRu(CO)3(μ-H)(dppm)2] (1) and [IrRuH(CO)3(μ-CO)(dppm)2] (2) are prepared from the reactions of [PPN][HRu(CO)4] with [IrCl(dppm)2] and [Ir(CO)(dppm)2][Cl], respectively (PPN = (Ph3P)2N; dppm = Ph2PCH2PPh2). Protonation of both monohydride species yields the dihydride [IrRu(CO)3(μ-H)2(dppm)2][BF4] (3), which under an atmosphere of carbon monoxide gives [IrRu(CO)4(dppm)2][BF4] (4). The methylene-bridged complex [IrRu(CO)3(μ-CH2)(μ-CO)(dppm)2][BF4] (5) is obtained by the reaction of compound 4 with diazomethane at ambient temperature. Although 5 does not react further with diazomethane under these conditions, carbonyl abstraction using trimethylamine oxide in the presence of CH2N2 yields the methylene-bridged ethylene adduct [IrRu(C2H4)(CO)3(μ-CH2)(dppm)2][BF4] (6). Labeling studies indicate that the majority of the 13C-labeled methylene group of 5 remains in the bridging site, with approximately 10% of the label incorporated into the ethylene formed. Compound 6 can also be prepared from 5 and ethylene in the presence of Me3NO. The compounds [IrRuL(CO)3(μ-CH2)(dppm)2][BF4] (L = NCMe, PMe3, CH2CHCN) can also be prepared from 5 in the presence of Me3NO or by ethylene displacement from 6. Although the PMe3 adduct has this group bound to Ir, as for the ethylene ligand in 6, the acrylonitrile and acetonitrile groups are bound to Ru. Furthermore, the acrylonitrile ligand is N-bound through the cyano group, analogous to the acetonitrile ligand. The structures of [IrRuH(CO)3(μ-CO)(dppm)2] (2), [IrRu(CO)3(μ-CH2)(μ-CO)(dppm)2][BF4] (5), and [IrRu(PMe3)(CO)3(μ-CH2)(dppm)2][BF4] (7) have been determined by X-ray methods. Compounds 2 and 7 have comparable edge-shared bioctahedral structures in which the geometries at the different metals in each structure are similar. The bridging carbonyl in 2 is replaced by a methylene group in 7, and the Ir-bound hydride is replaced by PMe3. Compound 5 has bridging CO and CH2 groups on opposite faces of the IrRuP4 framework with two terminal carbonyls on Ru and one on Ir.

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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...