Electrochemical dinitrogen to ammonia reduction at a nickel(ii) site: an easy access to an air-stable catalyst

Abstract

The electrochemical nitrogen reduction reaction (NRR) for producing NH₃ (ammonia) is a suitable alternative to the Haber–Bosch process due to its eco-friendly and non-polluting nature. Herein, we report a homogeneous Ni-based tetranuclear solvated oligomer [(L-Me)₄Ni₄(EtOH)₄] as a precatalyst for the NRR to NH₃ in THF (tetrahydrofuran) medium. The monomeric tri-coordinated Ni(II) ion chelated by a spencer type O₂N hard-donor dianionic ligand (Me-L²⁻) is the first of its type for electrochemical N₂ (dinitrogen) fixation. The factors that control the facile N₂ binding at the Ni atom of the catalyst have not been much studied before by energy decomposition analysis-natural orbital for chemical valence (EDA-NOCV) analysis. Coordinating solvents energetically prefer to bind at the single Ni(II) site of the monomeric active catalyst (L-Me)Ni(II), and due to the σ-donation (Ni(II) ← N₂) ability of N₂, while under electrochemical conditions, the Ni(II) centre of (L-Me)Ni(II) prefers to strongly bind N₂via desired π-backdonation (Ni(II) → N₂). The Ni(II) state of this molecule was studied by EPR (electron paramagnetic resonance). The proposed solution dynamics involving oligomers binding with the N₂ molecule and electrochemical N₂-bonded intermediates have been characterised by electrospray ionisation mass spectrometry (ESI-MS). Due to the favourable electron affinity of (L-Me)Ni(II)(N₂), it is suitable to undergo protonation at the terminal N atom of the bonded N₂. The lone pair of electrons of O atoms of the L-Me ligand stabilises the N–H unit via intramolecular H-bond formation. Our detailed density functional theory (DFT) calculations showed that all the steps leading to the reduction of N₂ to NH₃via the addition of electrons and protons are favourable.