Methylene-Bridged Complexes of Rhodium/Ruthenium as Models for Bimetallic Fischer−Tropsch Catalysts: Comparisons with the Rh/Os and Ir/Ru Analogues

The cationic heterobinuclear complex [RhRu(CO)3(μ-H)2(dppm)2][X] (1) is prepared by
protonation of the monohydride species [RhRu(CO)3(μ-H)(dppm)2] (dppm = μ-Ph2PCH2PPh2;
X- = BF4-, CF3SO3-). Treatment of 1 with carbon monoxide affords the tetracarbonyl complex
ion [RhRu(CO)4(dppm)2][X] (2), which yields the methylene-bridged product [RhRu(CO)4(μ-CH)2(dppm)2][X] (3), upon treatment with excess diazomethane at −78 °C. Reaction of 3
with trimethylamine-N-oxide results in CO removal, affording the methylene-bridged
tricarbonyl species [RhRu(CO)3(μ-CH)2(dppm)2][X] (4). Although compound 3 is unreactive
toward further treatment with CH2N2, compound 4 reacts readily with diazomethane,
yielding a mixture of uncharacterized products. Compounds 3 and 4 react with PMe3 to
yield [RhRu(PMe3)(CO)3(μ-CH)2(dppm)2][X] (5), in which coordination of PMe3 at the
unsaturated Rh center occurs. The structures of compounds 2, 3, and 5 have been determined
by X-ray methods. Comparisons of the reactivity of compound 3 with that of the analogous
Ir/Ru and Rh/Os complexes toward diazomethane are presented.