Synthesis, structural characterization, and catalytic ammonia oxidation activity of a Mn(II) Complex: Experimental and DFT-based mechanistic insights

Ammonia, recognized for its high hydrogen density and ease of transport, stands out as a highly promising molecule for hydrogen storage and on-demand release, positioning it as a cornerstone for advancing clean energy technologies through efficient decompositions. However, conventional thermal cracking of ammonia remain highly energy-intensive. In this context, molecular complexes present a sustainable alternative for the homogeneous oxidation of ammonia to dinitrogen under room temperature. Herein, this report presents a Mn^II-complex (1) containing a tetradentate N,N,N,N-donor ligand, N,N′-(ethane-1,2-diyl)bis(1-(pyridin-2-yl)methanimine), which effectively catalyzes ammonia oxidation at room temperature. Utilizing a triarylamonium oxidant alongside a 2,4,6-collidine base, the catalyst achieves the turn over number (TON) up to 12.73 (ie 12.73 equivalents of N₂ per Mn center). The Mn^II-complex was characterized (FTIR, UV-vis, XPS, single‑crystal X‑ray diffraction) and further examined by DFT. Mechanistic insights into catalytic ammonia oxidation were obtained through a series of DFT calculations, which reveal that N–N bond formation occurs via nucleophilic attack of NH₃ followed by sequential proton- and electron-transfer steps. These results demonstrate the promise of the Mn(II) complex for sustainable ammonia oxidation and elucidate the mechanism of N–N bond formation under ambient conditions.