Unsymmetrically Bridged Methyl Groups as Intermediates in the Transformation of Bridging Methylene to Bridging Acetyl Groups: Ligand Migrations and Migratory Insertions in Mixed Iridium/Ruthenium Complexes

Protonation of the methylene-bridged, tetracarbonyl species [IrRu(CO)4(μ-CH2)(dppm)2][X] (X = CF3SO3, BF4) (1) at −90 °C yields the methyl-bridged product [IrRu(CO)4(μ-CH3)(dppm)2][X]2 (X = CF3SO3, BF4) (3), in which the methyl group is carbon-bound to Ir while engaged in an agostic interaction with Ru. Compound 3 is unusual in that exchange of the terminal and agostic protons of the methyl group is slow on the NMR time scale at −90 °C, allowing for the direct observation of both sets of protons and measurement of their respective C−H coupling constants at this temperature. Protonation of [IrRu(PMe3)(CO)3(μ-CH2)(dppm)2][CF3SO3] also yields an unsymmetrically bridged methyl complex, [IrRu(PMe3)(CO)3(μ-CH3)(dppm)2][CF3SO3]2 (7), analogous to 3, and again 1H NMR spectroscopy at temperatures below −60 °C shows separate resonances for the agostic and the terminal hydrogens of the methyl group. Both compounds display very low 1 J CH values (65 Hz (3); 72 Hz (7)) for the agostic interactions, consistent with substantial weakening of the C−H bond. Warming a solution of 3 to −80 °C affords a new species, [IrRu(CH3)(CO)4(dppm)2][X]2 (4), in which the methyl group has migrated from the bridging site to a terminal position on Ir. Further warming of the triflate salt of 4 to ambient temperature results in disproportionation to give an unstable tricarbonyl, [IrRu(CH3)(CF3SO3)(CO)3(dppm)2][CF3SO3] (6), that subsequently decomposes, and the pentacarbonyl species [IrRu(CH3)(CO)5(dppm)2][CF3SO3]2 (5). Compound 5 can be obtained as the sole product upon warming a solution of 4 under an atmosphere of CO. Stirring a solution of 5 for two days gives the migratory-insertion product, [IrRu(CO)4(μ-C(CH3)O)(dppm)2][X]2 (8), in which the acetyl group is carbon-bound to Ir and oxygen-bound to Ru. One carbonyl ligand can be removed from 8 by reflux in CH2Cl2 to give [IrRu(CO)3(μ-C(CH3)O)(dppm)2][BF4]2 (9), in the case of 8-BF 4 , or [IrRu(CF3SO3)(CO)3(μ-C(CH3)O)(dppm)2][CF3SO3] (10), in the case of 8-CF 3 SO 3 , with retention of the bridging acetyl group. Refluxing 8-CF 3 SO 3 in THF removes two carbonyl ligands to give the dicarbonyl species [IrRu(CF3SO3)(CO)2(μ-C(CH3)O)(dppm)2][CF3SO3] (11). Reaction of 9 or 10 with H2 yields the monohydride species [IrRuH(CO)3(μ-C(CH3)O)(dppm)2][X]2 (12; X = BF4, CF3SO3) via heterolytic cleavage of dihydrogen, while reaction of 11 with H2 at −90 °C yields the dihydrogen adduct [IrRu(H2)(CO)2(μ-C(CH3)O)(dppm)2][CF3SO3] (13), which upon warming abov