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.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.