Structural and Chemical Effects of the P^tBu₂ Bridge at Unsaturated Dimolybdenum Complexes Having Hydride and...

A high-yield synthetic route for the preparation of the unsaturated anion [Mo₂Cp₂(μ-P^tBu₂)(μ-CO)₂]^- (2) was implemented, via two-electron reduction of the chloride complex [Mo₂Cp₂(μ-Cl)(μ-P^tBu₂)(CO)₂] (1). Reaction of 2 with [NH₄][PF₆] led to the formation of the 30-electron complex [Mo₂Cp₂(H)(μ-P^tBu₂)(CO)₂] (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 σ²δ²δ² 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 ClCH₂Ph gave the alkyl-bridged complexes [Mo₂Cp₂(μ-κ¹:η²-CH₂R)(μ-P^tBu₂)(CO)₂] (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 [Mo₂Cp₂(μ-κ¹:η²-CH₃)(μ-P^tBu₂)(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 [Mo₂Cp₂(μ-CPh)(μ-P^tBu₂)(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 [Mo₂Cp₂(H)(μ-CHPh)(μ-P^tBu₂)(CO)] (10), a product which evolves slowly by H₂ 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 [Mo₂Cp₂(μ-CH)(μ-P^tBu₂)(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 Mo₂PC rings of these molecules.