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-Ph 2 PCH 2 PPh 2; X - = BF 4 -, CF 3 SO 3 - ). 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 CH 2 N 2, compound 4 reacts readily with diazomethane, yielding a mixture of uncharacterized products. Compounds 3 and 4 react with PMe 3 to yield [RhRu(PMe 3 )(CO) 3 (μ-CH) 2 (dppm) 2 ][X] ( 5 ), in which coordination of PMe 3 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.