Copper–dioxygen complex mediated C–H bond oxygenation: relevance for particulate methane monooxygenase (pMMO)

Abstract

SummaryParticulate methane monooxygenase (pMMO), an integral membrane protein found in methanotrophic bacteria, catalyzes the oxidation of methane to methanol. Expression and greater activity of the enzyme in the presence of copper ion suggest that pMMO is a cuprous metalloenzyme. Recent advances -especially the first crystal structures of pMMO -have energized the field, but the nature of the active site(s) and the mechanism of methane oxidation remain poorly understood -yet hotly contested. Herein the authors briefly review the current understanding of the pMMO metal sites, and discuss advances in small molecule Cu-O 2 chemistry that may contribute to an understanding of copper-ion mediated hydrocarbon oxidation chemistry.

A [Cu 2 O] 2+ core in Cu-ZSM-5, the active site in the oxidation of methane to methanol

“…The strong OOH bond of the delocalized radical [CuOOHOCu] 2ϩ species helps drive the reaction. The electronic structure of L-Cu II 2 O provides insight in the reaction mechanism and shows that upon approach of CH 4 , one of the SOMOs of the Cu 2 O core gains significant oxygen p-character oriented into the zeolite channel and toward the H-CH 3 bond. At the transition state this results in the formation of a cupric-oxyl intermediate that is highly activated for H atom abstraction and explains the reactivity of the bent mono--oxo dicupric core in Cu-ZSM-5 in the selective oxidation of methane into methanol.…”

“…5 B and C, filled triangle). This indicates that COH bond breaking is involved in the rate limiting step of the oxidation of CH 4 . Product analysis (by 1 H-NMR, Fig.…”

“…‡ In ZSM-5, the 10-membered rings of the lattice are the only sites with pore-sizes appropriate for access of O 2 , N 2 O, and CH 4, and within these rings, a Cu 2 O core could only bind two lattice Os to each Cu (28). We therefore initially modeled the site with a tetrahedral Al(OH) 4 (Fig. 1).…”

“…The disappearance of the 22,700 cm Ϫ1 absorption band as a function of temperature was used to evaluate the activation energy (E a ) for the reaction of oxygen-activated Cu-ZSM-5 with CH 4 (Fig. 5A).…”

“…Researchers have attempted to mimic MMO's activity synthetically by creating oxygen-activated iron and copper catalysts (4). Recently, an O 2 -activated zeolite Cu-ZSM-5 has been shown to convert methane to methanol at low temperatures (100°C) with selectivity Ͼ98% (5).…”