Theoretical Study of Imide Formation in Nitrogen Fixation Catalyzed by Molybdenum Complex Bearing PCP-Type Pincer Ligand with Metallocenes

Homogeneous catalysts using a mononuclear molybdenum nitride (Mo≡N) complex bearing PCP-type pincer ligands allow nitrogen fixation under very mild conditions. The catalytic cycle involves three hydrogenation processes yielding an Mo-ammine complex [MoI(NH₃)(PCP)] from the Mo-nitride complex [MoI(N)(PCP)]. We primarily focused on the first hydrogenation step, forming an Mo-imide complex [MoI(NH)(PCP)] since previous experimental and theoretical studies suggest that imide formation is the rate-limiting step in the catalytic cycle. The choice of protonating agent and reductant strongly influences the catalytic reactivity in imide formation. In this computational quantum chemical study, 2,4,6-collidinium (ColH⁺) was employed as the protonation agent, while metallocenes Cp₂M^II and decamethylmetallocenes Cp*₂M^II (M = V, Cr, Mn, Fe, Co, and Ni) were employed as reductants. The reaction of ColH⁺ with the metallocenes yields protonated metallocenes, where a cyclopentadienyl ring of the metallocenes is protonated. Protonated Cp*₂Cr^II and Cp*₂Co^II are potential proton-coupled electron transfer (PCET) mediators to facilitate the imide formation of [MoI(N)(PCP)] with low activation free energies. The concerted reaction mechanism was compared with the stepwise reaction, where ColH⁺ directly protonates [MoI(N)(PCP)], followed by reduction with the decamethylmetallocenes. Furthermore, we analyzed how proton transfer and electron transfer are concerted in the reaction of the PCET mediators with [MoI(N)(PCP)] by tracing electronic states along the reaction coordinates.