Electron-electrophile coupled dinitrogen reduction in a cerium-meta-tetraphenolate system: a computational study

The use of lanthanide complexes for catalytic dinitrogen reduction is a new development in homogeneous catalysis. Density functional theory calculations on our recently reported cerium phenolate catalyst [K₂Ce₂(sol)₄(mTP)₂] (mTP = {(OC₆H₂-2-^tBu-4-Me)₂CH}₂-1,3-C₆H₄; sol = OMe₂ here; THF in the experiment) have been undertaken to elucidate the reduction, activation and silylation steps at the bound dinitrogen molecule, in the presence of the reductant, potassium metal (K⁰) and the electrophile Me₃SiCl (TMSCl). Out of the total of six electron reductions required to cleave the N₂, the first two-electron reduction step was found to be highly disfavoured unless potassium cations (K⁺) are included, upon which the step is rendered strongly exergonic; N-Si bond formation at the two-electron stage is predicted to be unfavourable. The three-electron-reduced N₂-adduct is found to be at the reductive activation limit in the absence of added electrophiles, which can form N-element bonds and lower the overall charge. Added electron density beyond three-electron reduction no longer localises on N₂, preventing formal N₂^4- formation. A pathway in which both K⁰ and Me₃SiCl work in concert was modelled, and six sequential reduction-silylation steps were calculated, showing how the N-N bond is cleaved after the third reduction, eventually releasing two equivalents of N(SiMe₃)₃, and regenerating the starting complex with the highest barrier of any step being 22 kcal mol^-1. We establish alkali metal coordination and coupled electron-electrophile transfer as key factors in the design of rare-earth-mediated dinitrogen functionalisation.