Beyond the triple bond: unlocking dinitrogen activation with tailored superbase phosphines

Abstract

Activating atmospheric dinitrogen (N₂), a molecule with a remarkably strong triple bond, remains a major challenge in chemistry. This theoretical study explores the potential of superbase phosphines, specifically those decorated with imidazolin-2-imine ((ImN)₃P) and imidazolin-2-methylidene ((ImCH)₃P) to facilitate N₂ activation and subsequent hydrazine (H₂NNH₂) formation. Using density functional theory (DFT) at the M06L/6-311++G(d,p) level, we investigated the interactions between these phosphines and N₂. Mono-phosphine–N₂ complexes exhibit weak, noncovalent interactions (−0.6 to −7.1 kcal mol⁻¹). Notably, two superbasic phosphines also form high-energy hypervalent complexes with N₂, albeit at significantly higher energies. The superbasic nature and potential for the hypervalency of these phosphines lead to substantial N₂ activation in bis-phosphine–N₂ complexes, where N₂ is “sandwiched” between two phosphine moieties through hypervalent P–N bonds. Among the phosphines studied, only (ImN)₃P forms an exothermic sandwich complex with N₂, stabilized by hydrogen bonding between the ImN substituents and the central N₂ molecule. A two-step, exothermic hydrogen transfer pathway from (ImN)₃P to N₂ results in the formation of a bis-phosphine–diimine (HNNH) sandwich complex. Subsequent hydrogen transfer leads to the formation of a bis-phosphine–hydrazine (H₂NNH₂) complex, a process that, although endothermic, exhibits surmountable activation barriers. The relatively low energy requirements for this overall transformation suggest its potential feasibility under the optimized conditions. This theoretical exploration highlights the promise of superbase phosphines as a strategy for metal-free N₂ activation, opening doors for the development of more efficient and sustainable nitrogen fixation and utilization methods.