BSr ₃ Superalkali as a Promising Catalyst for Ambient Nitrogen Reduction: A Pathway toward Efficient Ammonia Synthesis

The transformation of the earth-abundant dinitrogen molecule (N2) into ammonia (NH3) under mild conditions remains a fundamental challenge in chemistry. The BSr3 superatom designed in our previous work (Wiszowska N, Rogoża N, Sikorska C, Phys. Chem. Chem. Phys. (2025), 27: 23468–23486) has been used as a catalyst for the conversion of molecular nitrogen into ammonia. Owing to its low ionization energy (4.033 eV), the BSr3 transfers electron density to adsorbed N2, partially populating antibonding π* orbitals, lowering the triple N≡N bond order and initiating N–N bond elongation. Subsequent proton-coupled electron-transfer steps further elongate the N–N distance in a stepwise manner, establishing a correlation between structural distortion and the progressive weakening of the N≡N bond, ultimately facilitating bond cleavage. The catalytic conversion of N2 to NH3 catalyzed by BSr3 proceeds via an associative distal pathway, with initial hydrogenation at the nitrogen atom coordinated to strontium atoms (NSr), followed by release of the first NH3 molecule upon N–N bond rupture and subsequent hydrogenation of the second nitrogen (NB, bound to a boron atom of the BSr3 cluster). The release of the NH3 molecule is the potential-determining step, with a corresponding free energy uphill of 0.28 eV. The preceding exergonic hydrogenation steps help offset the energy cost of NH3 desorption, ensuring overall thermodynamic favorability. These findings position the BSr3 superalkali cluster as a promising noble-metal-free catalyst for sustainable ammonia synthesis under ambient conditions.