Methylene-bridged heterobinuclear complexes of iridium and ruthenium: Models for bimetallic Fischer-Tropsch catalysts

The heterobinuclear complexes [IrRu(CO)(3)(mu -H)(dppm)(2)] (1) and [IrRuH(CO)(3)(mu -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 = (Ph3P)(2)N; dppm = Ph2PCH2PPh2). Protonation of both monohydride species yields the dihydride [IrRu(CO)(3)(mu -H)(2)(dppm)(2)][BF4] (3), which under an atmosphere of carbon monoxide gives [IrRu(CO)(4)(dppm)(2)][BF4] (4). The methylene-bridged complex [IrRu(CO)(3)(mu -CH2)(mu -CO)(dppm)(2)][BF4] (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 CH2N2 yields the methylene-bridged ethylene adduct [IrRu(C2H4)(CO)(3)(mu -CH2)(dppm)(2)][BF4] (6). Labeling studies indicate that the majority of the C-13-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 Me3NO. The compounds [IrRuL(CO)(3)(mu -CH2)(dppm)(2)] [BF4] (L = NCMe, PMe3, CH2CHCN) can also be prepared from 5 in the presence of Me3NO or by ethylene displacement from 6. Although the PMe3 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)(mu -CO)(dppm)(2)] (2), [IrRu(CO)(3)(mu -CH2)(mu -CO)(dppm)(2)][BF4] (5), and [IrRu(PMe3)(CO)(3)(mu -CH2)(dppm)(2)][BF4] (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 PMe3. Compound 5 has bridging CO and CH2 groups on opposite faces of the IrRuP4 framework with two terminal carbonyls on Ru and one on Ir.