Structural and Chemical Effects of the PBu Bridge at Unsaturated Dimolybdenum Complexes Having Hydride and Hydrocarbyl Ligands.

A high-yield synthetic route for the preparation of the unsaturated anion [MoCp(μ-PBu)(μ-CO)] (2) was implemented, via two-electron reduction of the chloride complex [MoCp(μ-Cl)(μ-PBu)(CO)] (1). Reaction of 2 with [NH][PF] led to the formation of the 30-electron complex [MoCp(H)(μ-PBu)(CO)] (3), in which the hydride ligand adopts an uncommon terminal disposition. DFT analysis of the electronic structure of 3 gave support to the presence of a M≡M triple bond in this complex following from a σδδ configuration, a view also supported by the high electron density accumulated at the corresponding Mo-Mo bond critical point. In contrast, reactions of 2 with IMe or ClCHPh gave the alkyl-bridged complexes [MoCp(μ-κ:η-CHR)(μ-PBu)(CO)] (R = H (4a), Ph (4b)), which in solution display agostic Mo-H-C interactions. Decarbonylation of 4a took place rapidly under photochemical conditions to give the 30-electron complex [MoCp(μ-κ:η-CH)(μ-PBu)(μ-CO)] (7), with a stronger agostic coordination of its methyl ligand. In contrast, irradiation of 4b led to the formation of the benzylidyne derivative [MoCp(μ-CPh)(μ-PBu)(μ-CO)] (9), following from fast decarbonylation and dehydrogenation of the bridging benzyl ligand. Low-temperature photochemistry allowed for the NMR characterization of an intermediate preceding the hydrogen elimination, identified as the carbene hydride [MoCp(H)(μ-CHPh)(μ-PBu)(CO)] (10), a product which evolves slowly by H elimination to the benzylidyne derivative. Analogous dehydrogenation of the methyl ligand in 7 could be accomplished upon moderate heating, to yield the corresponding methylidyne derivative [MoCp(μ-CH)(μ-PBu)(μ-CO)] (9). A complete reaction mechanism accounting for these photochemical reactions was elaborated, based on the reaction intermediates identified experimentally and on extensive DFT calculations. Surprisingly, for both systems the C-H bond activation steps are relatively easy thermal processes occurring with modest activation energies after photochemical ejection of CO, with a rate-determining step involving the formation of agostic carbenes requiring also a strong structural reorganization of the central MoPC rings of these molecules.