PNNL

Abstract

An electrogenerated PdIII 2 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 methane transformation exclusively to CH3OSO3H and CH3SO3H at high conversions. Integrating a set of kinetic and computational studies, the mechanism of methane monofunctionalization by PdIII 2 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 PdII,III 2 suppresses CH3SO3H generation, indicating a radical chain sequence. Although the metal-metal bonded PdIII 2 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 PdIII 2 complex to generate a methyl radical intermediate. The CH3 • intermediate then recombines with PdII,III 2 to furnish a CH3PdIII 2 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 PdIII 2 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.