Methylene-Bridged Heterobinuclear Complexes of Iridium and Ruthenium:  Models for Bimetallic Fischer−Tropsch Catalysts

The heterobinuclear complexes [IrRu(CO) 3 (μ-H)(dppm) 2 ] ( 1 ) and [IrRuH(CO) 3 (μ-CO)(dppm) 2 ] ( 2 ) are prepared from the reactions of [PPN][HRu(CO) 4 ] with [IrCl(dppm) 2 ] and [Ir(CO)(dppm) 2 ][Cl], respectively (PPN = (Ph 3 P) 2 N; dppm = Ph 2 PCH 2 PPh 2 ). Protonation of both monohydride species yields the dihydride [IrRu(CO) 3 (μ-H) 2 (dppm) 2 ][BF 4 ] ( 3 ), which under an atmosphere of carbon monoxide gives [IrRu(CO) 4 (dppm) 2 ][BF 4 ] ( 4 ). The methylene-bridged complex [IrRu(CO) 3 (μ-CH 2 )(μ-CO)(dppm) 2 ][BF 4 ] ( 5 ) is obtained by the reaction of compound 4 with diazomethane at ambient temperature. Although 5 does not react further with diazomethane under these conditions, carbonyl abstraction using trimethylamine oxide in the presence of CH 2 N 2 yields the methylene-bridged ethylene adduct [IrRu(C 2 H 4 )(CO) 3 (μ-CH 2 )(dppm) 2 ][BF 4 ] ( 6 ). Labeling studies indicate that the majority of the 13 C-labeled methylene group of 5 remains in the bridging site, with approximately 10% of the label incorporated into the ethylene formed. Compound 6 can also be prepared from 5 and ethylene in the presence of Me 3 NO. The compounds [IrRuL(CO) 3 (μ-CH 2 )(dppm) 2 ][BF 4 ] (L = NCMe, PMe 3, CH 2 CHCN) can also be prepared from 5 in the presence of Me 3 NO or by ethylene displacement from 6 . Although the PMe 3 adduct has this group bound to Ir, as for the ethylene ligand in 6, the acrylonitrile and acetonitrile groups are bound to Ru. Furthermore, the acrylonitrile ligand is N-bound through the cyano group, analogous to the acetonitrile ligand. The structures of [IrRuH(CO) 3 (μ-CO)(dppm) 2 ] ( 2 ), [IrRu(CO) 3 (μ-CH 2 )(μ-CO)(dppm) 2 ][BF 4 ] ( 5 ), and [IrRu(PMe 3 )(CO) 3 (μ-CH 2 )(dppm) 2 ][BF 4 ] ( 7 ) have been determined by X-ray methods. Compounds 2 and 7 have comparable edge-shared bioctahedral structures in which the geometries at the different metals in each structure are similar. The bridging carbonyl in 2 is replaced by a methylene group in 7, and the Ir-bound hydride is replaced by PMe 3 . Compound 5 has bridging CO and CH 2 groups on opposite faces of the IrRuP 4 framework with two terminal carbonyls on Ru and one on Ir.

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