Engineering Mono- vs Dinuclear Iron(II)-Active Sites to Steer Liquid Oxygenate Selectivity in Methane Oxidation Pathways Using O2.

The selective oxidation of methane to liquid oxygenates using O2 under mild conditions remains a grand challenge due to the chemical inertness of methane and the competing thermodynamically favored overoxidation. We report the development of well-defined mono- and dinuclear bipyridyl Fe(II) hydroxyl species within a porous metal-organic framework [MOF-253(Al)], which enable methane oxidation by O2 via distinct oxidation pathways with exceptionally high product selectivity. The MOF-supported dinuclear Fe(II) catalyst (MOF-253-Fe2) having a [(bpy)FeII(μ2-OH)(OH)]2 active site achieves 100% liquid selectivity toward acetic acid with a productivity of 2757 mmolCH3CO2H molFe-1 h-1 at 160 °C. In comparison, the MOF-supported mononuclear (bpy)FeII(OH)2 catalyst (MOF-253-Fe) exhibits a remarkably high methanol productivity of 6060 mmolCH3OH molFe-1 h-1 with 96% selectivity in the aqueous phase at 95 °C. Spectroscopic and mechanistic studies suggested that the confinement and stabilization of [(bpy)FeII(μ2-OH)(OH)]2 active site in the MOF-253-Fe2 framework facilitate redox cooperativity and multielectron transfer, enabling sequential O2 activation, methane C-H bond activation, followed by C-C bond coupling with in situ-generated CO to yield acetic acid through the formation of [(bpy)FeIII(μ2-OH)(OH)2]2 catalytic intermediate. In contrast, the MOF-253-Fe catalyst lacks such synergy but efficiently promotes methane C-H hydroxylation to yield methanol. This work underscores the importance of active site engineering in governing product selectivity and provides valuable insights into the design of advanced catalysts for selective methane functionalization.