Electron-Electrophile Coupled Dinitrogen Reduction in a Cerium-Meta-Tetraphenolate System: A Computational Study

Abstract

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-t Bu-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 (K 0 ) 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 threeelectron-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 K 0 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.