Bridging Methylene to Bridging Acyl Conversion in Heterobinuclear Rh/Ru Complexes: Models for Adjacent-Metal Involvement in Bimetallic Catalysts

Protonation of the methylene-bridged complex [RhRu(CO) 4 (μ-CH 2 )(dppm) 2 ][CF 3 SO 3 ] ( 1 ) at −80 °C yields an asymmetrically bridged methyl complex in which the methyl ligand is σ-bound to Ru while being involved in an agostic interaction with Rh. Warming to −40 °C results in migration of the methyl group to a terminal site on Rh, and further warming to 0 °C results in migratory insertion to give the acetyl-bridged complex [RhRu(OSO 2 CF 3 )(CO) 2 (μ-C(CH 3 )O)(μ-CO)(dppm) 2 ][CF 3 SO 3 ] ( 4 ), in which the acetyl carbon is bound to Rh and its oxygen is bound to Ru. At ambient temperature one carbonyl is lost from 4 to give [RhRu(OSO 2 CF 3 )(CO) 2 (μ-C(CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] ( 5 ). Carbon-13 NMR spectroscopy and the X-ray structures of both acetyl-bridged products suggest significant carbene character in the acyl carbon. Attempts to obtain methyl and subsequently acetyl species by methylene insertion into metal−hydride bonds of the appropriate precursors [RhRu(X)(CO) 4 (μ-H)(dppm) 2 ][X] (X - = CF 3 SO 3 -, BF 4 - ) failed, with these hydride species being unreactive to CH 2 N 2 . Similarly, reaction of 1 with HCl yielded only the methyl complex [RhRu(CH 3 )(CO) 3 (μ-Cl)(dppm) 2 ][CF 3 SO 3 ], which did not undergo migratory insertion. Protonation of the methylene-bridged PMe 3 adduct [RhRu(PMe 3 )(CO) 3 (μ-CH 2 )(dppm) 2 ][CF 3 SO 3 ] yields [Rh Ru (OSO 2 CF 3 )(CO) 2 (μ- C (CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] in which the binding of the acetyl group is reversed, being C-bound to Ru and O-bound to Rh. This species undergoes facile rearrangement to 5 at ambient temperature.