Orbital Concept for Methane Activation

Theoretical thinking on methane C–H bond activation and hydroxylation by soluble and particulate methane monooxygenase (iron and copper enzyme species) and relatedmetal-oxo species such as FeO+ is developed. The tetrahedral Td structure ofmethane can be deformed into a C3v or D2d structure and bound at a coordinatively unsaturated metal-oxo site of a soluble methane monooxygenase model from extended Hückel calculations. Mechanistic aspects about methane hydroxylation by the bare transition-metal oxide ion FeO+ are analyzed by using density functional theory calculations. An important feature in the reaction is the spin crossover between the high-spin and low-spin potential energy surfaces in particular in the C–H activation process, the energy barrier of which is significantly decreased by the spin inversion. The hydroxylation mechanisms of soluble and particulate methane monooxygenase are considered. These mechanistic insights are reasonably extended to methane activation by metal-exchanged zeolites and IrO2 (110) surface.