Insertion, coupling and elimination processes in the reactions of the unsaturated alkyl-bridged complexes [Mo2(η5-C5H5)2(μ-CH2R)(μ-PCy2)(CO)2] (R = H, Ph) with isocyanides and secondary phosphines

Abstract

The methyl-bridged complex [Mo₂Cp₂(μ-Me)(μ-PCy₂)(CO)₂] (Cp = η⁵-C₅H₅) reacted with stoichiometric amounts of CN^tBu at 243 K to give the C,O:C,O-bridged acyl complex [Mo₂Cp₂{μ-C,O:C,O-C(O)Me}(μ-PCy₂)(CN^tBu)(CO)], which at room temperature slowly rearranges into its iminoacyl-bridged isomer [Mo₂Cp₂(μ-C,N:C,N-MeCN^tBu)(μ-PCy₂)(CO)₂]. In contrast, the C:O-bridged acyl complex [Mo₂Cp₂{μ-C:O-C(O)Me}(μ-PCy₂)(CN^tBu)(CO)] was the major product obtained when the above reaction was carried out at room temperature. Density Functional Theory (DFT) was used to find the most likely structures of all these isomers, of which the iminoacyl complex was the absolute minimum. In contrast to the above reactions, up to three molecules of the ligand added rapidly to the methyl complex when using the aryl isocyanides CNR (R = o-C₆H₄Me, p-C₆H₄OMe), triggering the coupling between the methyl ligand and one of the cyclopentadienyl groups to give the corresponding methylcyclopentadiene derivatives [Mo₂Cp(η⁴-C₅H₅Me)(μ-PCy₂)(CNR)₃(CO)]. Carbonylation of the latter complex (R = o-C₆H₄Me) induced the displacement of the η⁴-bound ligand, but also gave small yields of the hydride derivative [Mo₂Cp(η⁵-C₅H₄Me)(μ-H)(μ-PCy₂){CN(o-C₆H₄Me)}(CO)₃] (Mo–Mo = 3.2467(5) Å), the latter resulting from a C–H cleavage in the methylcyclopentadiene ligand. The reaction of the title complexes with phosphines HPR^′₂ (R′ = Et, Ph) gave two major products: the corresponding aldehyde complexes [Mo₂Cp₂(μ-PCy₂)(μ-PR^′₂){η²-C(O)HR}(CO)] (Mo–Mo = 2.8288(5) Å when R = CH₂Ph and R′ = Et) and the dicarbonyl complexes [Mo₂Cp₂(μ-PCy₂)(μ-PR^′₂)(CO)₂], these following from alternative reductive elimination processes, from hydrogen and either acyl or alkyl ligands, respectively.