Theoretical Study of Dihydrogen Activation by a Trinuclear Ruthenium μ3-Imido Complex

Dihydrogen activation by a triruthenium mu(3)-imido complex, (CpRu)(3)(mu(3)-NH)(mu-H)(3) (1; Cp = eta(5)-C5H5), was theoretically investigated with the DFT method. Each of the three Ru centers can react with dihydrogen via a transition state, (CpRu)(3)(mu(3)-NH)(eta(2)-H-2)(mu-H)(3) (TS1), with a moderate energy barrier. This TS1 corresponds to a transition state for the approach of a dihydrogen molecule to one of the Ru centers. After TS1, the hydrogen hydrogen bond cleavage occurs without any other energy barrier to produce the pentahydrido intermediate, (CpRu)(3)(mu(3)-NH)(mu-H)(3)(H)(2) (INT1), without formation of a dihydrogen complex as an intermediate. In the hydrogen hydrogen cleavage, the d pi orbitals of three ruthenium centers overlap with the hydrogen hydrogen sigma* orbital to form the charge transfer from the ruthenium to the hydrogen hydrogen moiety. Natural population analysis indicates that all three ruthenium centers cooperatively participate in the changes in electronic structure during this process. INT1 further undergoes a nitrogen hydrogen bond formation, which occurs through drastic changes in the binding mode of the amide group and hydrides, to afford a mu-amido complex, (CpRu)(3)(mu-NH2)(mu-H)(4) (2). This process proceeds with a moderate activation barrier. Natural population analysis of this step reveals that one bridging hydride approaches the imido N atom, concomitantly changing from a hydride to a proton-like hydrogen atom with a simultaneous increase in the N atomic population. Fluxional positional changes of hydrides are crucial for the facile N-H bond formation, in which the Ru-3 core plays important roles.