Alkyne/Methylene Coupling Reactions at Adjacent Rh/Os Centers:  Stepwise Transformations from C₁- through C₄-Bridged Species

Reaction of the methylene-bridged complex [RhOs(CO)₃(μ-CH₂)(dppm)₂][CF₃SO₃] (dppm = μ-Ph₂PCH₂PPh₂) with dimethyl acetylenedicarboxylate or hexafluorobutyne results in alkyne insertion into the Rh−CH₂ bond, yielding [RhOs(CO)₃(μ-η¹:η¹-RCC(R)CH₂)(dppm)₂][CF₃SO₃] (R = CO₂Me (5), CF₃ (6)). Carbonyl removal from each of these species results in a Rh-bound carbonyl being replaced by a triflate ion. Reaction of 5 with diazomethane results in CH₂ insertion into the Rh−C(R) bond with accompanying C−H activation of the original osmium-bound CH₂ group, yielding [RhOs(CO)₃(μ-η¹:η¹-CH₂C(R)C(R)CH)(μ-H)(dppm)₂][CF₃SO₃] (R = CO₂Me). Reaction of the alkyne-bridged species [RhOs(CO)₃(μ-η¹:η¹-RCCR)(dppm)₂][CF₃SO₃] (R = CO₂Me) with diazomethane at −78 °C yields [RhOs(CO)₃(μ-η¹:η¹-CH₂C(R)CR)(dppm)₂][CF₃SO₃] (12), which upon warming to ambient temperature, in the absence of diazomethane, undergoes alkyne deinsertion to yield [RhOs(CO)₃(η²-RC⋮CR)(μ-CH₂)(dppm)₂][CF₃SO₃] (13). This species and the inserted product (12) appear to be in equilibrium, so reaction of 13 with diazomethane proceeds via 12 to give [RhOs(CO)₃(η¹:η¹-CH₂CH₂C(R)CR)(dppm)₂][CF₃SO₂] (14), in which the hydrocarbyl fragment is chelating to Os and interacting with Rh via an agostic interaction with one of the Os-bound CH₂ hydrogens. Upon standing for weeks, or more rapidly upon refluxing, compound 14 transforms into an isomer (15) in which the C₄ unit has migrated from sites on Os adjacent to Rh to sites on Os remote from Rh. An X-ray structure determination of compound 15 as the BF₄^- salt confirms the geometry of this final species. Reaction of the hexafluorobutyne-bridged species [RhOs(CO)₃(μ-η¹:η¹-RCCR)(dppm)₂][CF₃SO₃] (R = CF₃) with diazomethane does not give a product analogous to that described for dimethyl acetylenedicarboxylate, but yields the isomer 6. These carbon−carbon bond forming processes are discussed.