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A PDIII sulfate dimer initiates rapid methane monofunctionalization by H atom abstraction

Abstract

An electrogenerated PdIII2 species in fuming sulfuric acid is competent for rapid and concurrent methane monohydroxylation to methyl bisulfate (CH3OSO3H) and methane sulfonation to methanesulfonic acid (CH3SO3H). In situ NMR at 50 °C is used to track the selective transformation of methane to CH3OSO3H and CH3SO3H at high conversions. Integrating a set of kinetic and computational studies, the mechanism of methane monofunctionalization by PdIII2 is examined. Experimental rate laws and common kinetic isotope effects for CH3OSO3H and CH3SO3H formation suggest that both transformations proceed via a common rate-limiting C−H activation step. Introduction of O2 or Pd2II,III suppresses CH3SO3H generation, indicating a radical chain sequence. Although the metal−metal bonded PdIII2 complex is a net two-electron oxidant, our aggregate kinetic data point to a mechanistic model that features rate-limiting H atom abstraction by the PdIII2 complex to generate a methyl radical intermediate. The CH3• intermediate then recombines with Pd2II,III to furnish a CH3PdIII2 intermediate that reductively eliminates CH3OSO3H. Alternatively, the CH3• intermediate can enter a chain reaction with SO3 to generate CH3SO3H. DFT computations support the radical-based C−H activation by PdIII2 and delineate H atom abstraction pathways with computed reaction barriers and kinetic isotope effects (KIEs) that are consistent with experimental data. These mechanistic investigations challenge the paradigm of electrophilic C−H activation and highlight H atom abstraction as a potent pathway for selective methane C−H oxidative functionalization at high reaction rates.

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