https://doi.org/10.1039/d4dt02703e
Abstract
Activating atmospheric dinitrogen (N2), a molecule with a remarkably strong triple bond, remains a major challenge inchemistry. This theoretical study explores the potential of superbase phosphines, specifically those decorated withimidazolin-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 interactionsbetween these phosphines and N2. Mono-phosphine-N2 complexes exhibit weak, noncovalent interactions (-0.6 to -7.1kcal/mol). Notably, two superbasic phosphines also form high-energy hypervalent complexes with N2, albeit at significantlyhigher energies. The superbasic nature and potential for hypervalency of these phosphines lead to substantial N2 activationin 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 hydrogenbonding between the ImN- substituents and the central N2 molecule. A two-step, exothermic hydrogen transfer pathwayfrom (ImN)3P to N2 results in the formation of a bis-phosphine-diimine (HNNH) sandwich complex. Subsequent hydrogentransfers 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 transformation suggest itspotential feasibility under optimized conditions. This theoretical exploration highlights the promise of superbase phosphinesas a strategy for metal-free N2 activation, opening doors for the development of more efficient and sustainable nitrogenfixation and utilization methods.
