Alkyne/Methylene Coupling at Adjacent Iridium/Osmium Centers: Facile Carbon−Carbon and Carbon−Oxygen Bond Formation

The methylene-bridged complex [IrOs(CO)4(μ-CH2)(dppm)2][CF3SO3] (dppm = μ-Ph2PCH2PPh2) (3) can be synthesized by the addition of diazomethane to [IrOs(CO)5(dppm)2][CF3SO3] (1) or [IrOs(CO)4(dppm)2][CF3SO3] (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-κ1:κ1-C(CO2CH3)C(CO2CH3)CH2)(dppm)2][CF3SO3] (5) and [IrOs(CO)3(μ-κ1:κ1-C(CO2CH3)C(CO2CH3)CH2)(dppm)2][CF3SO3] (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)3(μ-CH2)(dppm)2][CF3SO3] (4) yielding [IrOs(CO)3(μ-κ1:κ1-C(CF3)C(CF3)CH2)(dppm)2][CF3SO3] (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-κ1:κ1-CH2(CF3)CC(CF3)CH2)(dppm)2][CF3SO3] (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-κ1:κ1:κ1-CH2OC(OCH3)CC(CO2CH3)CH)(dppm)2][CF3SO3] (8). Attempts to insert a methylene unit into the iridium−carbon bond of the alkyne-bridged complexes [IrOs(CO)3(μ-κ1:κ1-RCCR)(dppm)2][CF3SO3] (R = CO2Me (12), CF3 (13)) yields the C3-bridged complex [IrOs(CO)3(μ-κ1:κ1-CH2(CF3)CCCF3)(dppm)2][CF3SO3] (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.