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

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.