Coordination Microenvironment-Driven Frontier Orbital Modulation for Dinitrogen Capture in Acetylphenol-Ligated...

Elucidating how coordination microenvironments dictate intrinsic molecular activation is essential for the rational design of efficient catalysts. By combining reaction mass spectrometry and density functional theory calculations, we demonstrate a coordination microenvironment-driven frontier orbital modulation mechanism that governs dinitrogen (N₂) capture in mononuclear cobalt cations ([Co(L)]⁺). Experiments reveal that oxygen-coordinated [Co(Hap)]⁺ and [Co(Dhap)]⁺ (Hap = 2-acetylphenol, Dhap = 2-acetyl-4-hydroxyphenol) exhibit prominent room-temperature N₂-capture activity. In sharp contrast, the sulfur-coordinated analogue [Co(Map)]⁺ (Map = 2-acetylthiophenol) is unexpectedly inert. Frontier molecular orbital analysis reveals that an inversion of unoccupied orbital energy levels fundamentally causes this reactivity divergence. While O-coordinated complexes feature symmetry-matched vacant metal lowest unoccupied molecular orbital (LUMO), the S-ligated analogue's LUMO anomalously shifts to the ligand π* antibonding orbital. Natural bond orbital analysis indicates that strong electron donation from sulfur elevates the metal-centered empty orbitals, disrupting the critical energy and symmetry matching required for N₂ fixation. This work establishes that ligand-controlled frontier orbital modulation serves as a decisive factor in N₂ capture, providing fundamental physicochemical guidelines for the rational design of N₂-fixation catalytic sites.