Reactivity of Nickel Cations Liganded by Acetylacetone and Benzoylacetone with N₂: Organic Ligand Effects

N₂ adsorption on metal sites under mild conditions and its dependence on the chemical environment are crucial for understanding the fundamental mechanism and designing an efficient catalyst. Here, electrospray ionization mass spectrometry (ESI-MS) experiments reveal distinct differences in N₂ binding capabilities among nickel complexes: both [(acetylacetonato)Ni]⁺ and [(benzoylacetonato)Ni]⁺ gas-phase cations exhibit high N₂ adsorption efficiency, while the bare Ni⁺ and [(benzoylacetone)Ni(acetonitrile)]⁺ cations show negligible reactivity toward N₂. Density functional theory calculations reveal that organic ligands modulate the nickel oxidation state from +1 in the isolated Ni⁺ to +2 in organometallic complexes, thereby enhancing the electrophilicity of the metal center and promoting N₂ chemisorption. The stabilization of N₂ adsorption products is further driven by (1) the formation of stable coordination bonds and (2) increased vibrational degrees of freedom, which facilitate thermal energy redistribution. In addition, steric hindrance from the acetonitrile ligand significantly suppresses the N₂ adsorption. Frontier orbital analysis confirms the σ-donation from the lone pairs of N₂ to unoccupied Ni-d orbitals. This work advances the understanding of the synergistic effects between organic ligands and transition metals in N₂ adsorption and activation.