Unusual Ligand Transformations Initiated by dppm Deprotonation in Methylene-Bridged Rh/Os Complexes

The reaction of [RhOs(CO)3(μ-CH2)(dppm)2][CF3SO3] (dppm = μ-Ph2PCH2PPh2) with 1,3,4,5-tetramethylimidazol-2-ylidene (IMe4) results in competing substitution of the Rh-bound carbonyl by IMe4 and dppm deprotonation by IMe4 to give the two products [RhOs(IMe4)(CO)2(μ-CH2)(dppm)2][CF3SO3] and [RhOs(CO)3(μ-CH2)(μ-κ1:η2-dppm-H)(dppm)] [3; dppm-H = bis(diphenylphosphino)methanide], respectively. In the latter product, the dppm-H group is P-bound to Os while bound to Rh by the other PPh2 group and the adjacent methanide C. The reaction of the tetracarbonyl species [RhOs(CO)4(μ-CH2)(dppm)2][CF3SO3] with IMe4 results in the exclusive deprotonation of a dppm ligand to give [RhOs(CO)4(μ-CH2)(μ-κ1:κ1-dppm-H)(dppm)] (4) in which dppm-H is P-bound to both metals. Both deprotonated products are cleanly prepared by the reaction of their respective precursors with potassium bis(trimethylsilyl)amide. Reversible conversion of the μ-κ1:η2-dppm-H complex to the μ-κ1:κ1-dppm-H complex is achieved by the addition or removal of CO, respectively. In the absence of CO, compound 3 slowly converts in solution to [RhOs(CO)3(μ-κ1:κ1:κ1-Ph2PCHPPh2CH2)(dppm)] (5) as a result of dissociation of the Rh-bound PPh2 moiety of the dppm-H group and its attack at the bridging CH2 group. Compound 4 is also unstable, yielding the ketenyl- and ketenylidene/hydride tautomers [RhOs(CO)3(μ-κ1:η2-CHCO)(dppm)2] (6a) and [RhOs(H)(CO)3(μ-κ1:κ1-CCO)(dppm)2] (6b), initiated by proton transfer from μ-CH2 to dppm-H. Slow conversion of these tautomers to a pair of isomers of [RhOs(H)(CO)3(μ-κ1:κ1:κ1-Ph2PCH(COCH)PPh2)(dppm)] (7a and 7b) subsequently occurs in which proton transfer from a dppm group to the ketenylidene fragment gives rise to coupling of the resulting dppm-H methanide C and the ketenyl unit. Attempts to couple the ketenyl- or ketenylidene-bridged fragments in 6a/6b with dimethyl acetylenedicarboxylate (DMAD) yield [RhOs(κ1-CHCO)(CO)3(μ-DMAD)(dppm)2], in which the ketenyl group is terminally bound to Os.