Methylene-to-Acetyl Conversion in Heterobinuclear Rh/Os Complexes:  Models for Oxygenate Formation by Bimetallic Fischer−Tropsch Catalysts

Abstract

Protonation of the methylene-bridged complex [RhOs(CO) 3 (μ-CH 2 )(μ-CO)(dppm) 2 ][CF 3 SO 3 ] with triflic acid at −80 °C yields the methyl complex [RhOs(CO) 4 (μ-CH 3 )(dppm) 2 ][CF 3 SO 3 ] 2 ( 1 ) in which the methyl group is primarily bound to Os while involved in an agostic interaction with Rh. Warming this species to −20 °C results in methyl migration to a terminal site on Rh, yielding [RhOs(CH 3 )(CO) 2 (μ-CO) 2 (dppm) 2 ][CF 3 SO 3 ] 2 ( 2 ), and subsequent warming to ambient temperature results in migratory insertion to yield the acetyl-bridged [RhOs(CF 3 SO 3 )(CO) 2 (μ-C(CH 3 )O)(μ-CO)(dppm) 2 ][CF 3 SO 3 ] ( 3 ). Replacement of the coordinated triflate anion in 3 by CO and PMe 3 occurs at low temperature to give [RhOs(L)(CO) 2 (μ-C(CH 3 )O)(μ-CO)(dppm) 2 ][CF 3 SO 3 ] 2 (L = CO ( 4 ), PMe 3 ( 5a )). Warming the PMe 3 adduct results in transformation of the acetyl group, which is C-bound to Rh in 5a, to a group that is C-bound to Os in 5b . One carbonyl in 3 is labile and is readily lost, yielding [RhOs(CF 3 SO 3 )(CO) 2 (μ-C(CH 3 )O)(dppm) 2 ][CF 3 SO 3 ] ( 6 ). Attempts to obtain the above methyl or acetyl species by reaction of the hydride-bridged species [RhOs(X)(CO) 3 (μ-H)(μ-CO)(dppm) 2 ][X] (X = CF 3 SO 3 ( 7a ), BF 4 ( 7b )) with diazomethane were unsuccessful.