Beyond the Triple Bond: Unlocking Dinitrogen Activation with Tai-lored 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. Us-ing 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). Notably, two superbasic phosphines also form high-energy hypervalent complexes with N2, albeit at significantly higher energies. The superbasic nature and potential for hypervalency of these phosphines lead to substantial N₂ activation in bis-phosphine-N₂ complexes, where N₂ is "sand-wiched" 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 transfers lead 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 trans-formation suggest its potential feasibility under optimized conditions. This theoretical exploration highlights the promise of super-base phosphines as a strategy for metal-free N₂ activation, opening doors for the development of more efficient and sustainable ni-trogen fixation and utilization methods.