Direct O2 Activation by Ligand-Constrained Pnictogen Complexes: Contrasting Mechanisms and OAT Reactivity across the P, Sb, and Bi Triad

Abstract

Electronic structure-reactivity relationships are fundamental to advancing the redox chemistry of main-group elements. Herein, we investigate a series of planarized C2v pnictogen complexes (Pn = P, Sb, Bi) to correlate their electronic structures with reactivity trends across the pnictogen group. Through single-crystal X-ray diffraction, UV-vis spectroscopy, electrochemical measurements, and density functional theory (DFT) calculations, we demonstrate a systematic reduction in HOMO-LUMO gap progressing from phosphorus to bismuth, accompanied by enhanced stabilization of the nucleophilic lone pair. Oxygen atom transfer (OAT) reactivity, probed using triphenylpnictines (PnPh3, Pn = P, Sb, Bi) as mechanistic and thermodynamic reporters, reveals distinctive oxidation pathways. The phosphorus complex (1) undergoes a four-electron oxidation to yield a dioxophosphorane species, whereas the heavier congeners (2 and 3) generate monooxygenated Pn=O products exclusively. This mechanistic divergence is attributed to pronounced orbital contraction in heavier pnictogens, which stabilizes the lone pair and attenuates its participation in multielectron redox processes. Computational and experimental analyses corroborate these trends, with Hammett studies and pKa calculations of peroxo intermediates revealing enhanced nucleophilicity of the oxygen center with increasing pnictogen atomic weight, thereby influencing the OAT mechanism. These findings provide fundamental insights into periodic trends governing oxidation chemistry and demonstrate how strategic ligand design can modulate pnictogen-based multielectron reactivity. The broader implications extend to small-molecule activation and catalysis, offering a predictive framework for designing advanced redox-active main-group systems.