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-CH 2 )(dppm) 2 ][X] (X = CF 3 SO 3, BF 4 ) ( 1 ) at −90 °C yields the methyl-bridged product [IrRu(CO) 4 (μ-CH 3 )(dppm) 2 ][X] 2 (X = CF 3 SO 3, BF 4 ) ( 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(PMe 3 )(CO) 3 (μ-CH 2 )(dppm) 2 ][CF 3 SO 3 ] also yields an unsymmetrically bridged methyl complex, [IrRu(PMe 3 )(CO) 3 (μ-CH 3 )(dppm) 2 ][CF 3 SO 3 ] 2 ( 7 ), analogous to 3, and again 1 H 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(CH 3 )(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(CH 3 )(CF 3 SO 3 )(CO) 3 (dppm) 2 ][CF 3 SO 3 ] ( 6 ), that subsequently decomposes, and the pentacarbonyl species [IrRu(CH 3 )(CO) 5 (dppm) 2 ][CF 3 SO 3 ] 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(CH 3 )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 CH 2 Cl 2 to give [IrRu(CO) 3 (μ-C(CH 3 )O)(dppm) 2 ][BF 4 ] 2 ( 9 ), in the case of 8-BF 4, or [IrRu(CF 3 SO 3 )(CO) 3 (μ-C(CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] ( 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(CF 3 SO 3 )(CO) 2 (μ-C(CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] ( 11 ). Reaction of 9 or 10 with H 2 yields the monohydride species [IrRuH(CO) 3 (μ-C(CH 3 )O)(dppm) 2 ][X] 2 ( 12; X = BF 4, CF 3 SO 3 ) via heterolytic cleavage of dihydrogen, while reaction of 11 with H 2 at −90 °C yields the dihydrogen adduct [IrRu(H 2 )(CO) 2 (μ-C(CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] ( 13 ), which upon warming above −60 °C yields the dihydride [IrRu(H) 2 (CF 3 SO 3 )(CO) 2 (μ-C(CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] ( 14 ). Although compound 12 yields acetaldehyde upon reaction with CO, compound 14 gives rise to triflate ion displacement by CO, but no acetaldehyde is formed.

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