Mixed-valent bis(µ-oxo)CuIICuIII in C-H bond activation – COMEBAC

Methane is a greenhouse gas that remains in the atmosphere for approximately 9-15 years. It is over 20 times more effective in greenhouse effect than carbon dioxide over a 100-year period. Methane is also a primary constituent of natural gas and an important energy source. To reduce its greenhouse gas effect and to increase its potential as a petroleum alternative for fuels and in the petrochemical industry, its transformation into a liquid form such as methanol is of current interest in chemistry. Currently, industrial methanol production is accomplished by the steam reforming of methane, which requires high temperatures and pressures. Therefore, alternative processes such as the selective direct oxidation of methane to methanol are of considerable interest. However, methane has the strongest C-H bond of any hydrocarbon (104 kcal/mol), thus its selective oxidation to methanol without further oxidation is extremely challenging.
In nature, methane monooxygenases (MMO) accomplish the direct conversion of methane into methanol at ambient temperature and atmospheric pressure allowing the harnessing of methane as an energy source and for the synthesis of the molecules required for life. MMO exists as soluble and particulate forms. The soluble enzyme (sMMO) contains a (mu-oxo)FeIIFeII active site which reacts with dioxygen to produce a bis(mu-oxo)FeIVFeIV as active species in methane oxidation. Inspired by sMMO active site, chemists have attempted to understand or reproduce the reactivity of sMMO by studying small metal complexes as active site models. Thanks to this approach, several structural and functional models for the sMMO active site, have been reported providing a better understanding of sMMO functioning and mechanism as well as the development of promising catalysts for oxidation. Compared to sMMO, the knowledge on the particulate form (pMMO) is recent and pMMO has been the subject of a controversy about the metal content and the structure of its active site. Nevertheless, the most recent results propose a dinuclear copper center, which reacts with dioxygen to produce a Cu2/O2 as active species in methane oxidation such as (mu-eta2:eta2-peroxo)CuIICuII or bis(mu-oxo)CuIIICuIII. However, DFT calculations suggest that a 1-electron reduced mixed-valent bis(mu-oxo)CuIICuIII species has greater oxidizing power to cleave the methane C-H bond rather than either the symmetric (mu-eta2:eta2-peroxo)CuIICuII or bis(mu-oxo)CuIIICuIII.
This proposal aims at designing new copper-based catalysts efficient for alkane oxidation. The design of theses new catalysts comes from the inspiration of the structure and the functioning of the copper-containing pMMO. A mixed-valent bis(mu-oxo)CuIICuIII species has been recently proposed to occur as highly reactive intermediate during the hydroxylation of methane into methanol. The first part of COMEBAC will be devoted to the synthesis of ligands able to stabilize a mixed-valent bis(mu-oxo)CuIICuIII species in order to characterize it by spectro-electrochemistry. The second step will concern the study of its reactivity toward organic substrates (alkanes, alkenes …). Electrochemical studies coupled with theoretical calculations, as an electronic mapping, will be involved in all aspects of the project, towards the conception of the best ligands for the stabilization of the mixed-valent (mu-oxo)CuIICuIII species and optimization of the reactivity in alkane oxidation. COMEBAC is a basic research project the results of which can be reasonably considered to impact the field of hydrocarbons remediation or that of aliphatic alcohols production, and even more generally in oxidase/oxygenase mimicry.