Selective Photocatalytic Hydroxylation of Hydrocarbons

The low-temperature conversion of hydrocarbons into more reactive intermediates, e.g. alcohols, is a great challenge for catalysis. In contrast nature has already proved that reactions of this type can be performed under ambient conditions using enzymes such as methane monooxygenases that are based on iron or copper ions in the active site. Studies on selective oxidations, using low molecular weight iron or copper complexes as model compounds for these enzymes thus far mainly have been performed as thermal processes. In contrast, in our collaborative project, we now want to incorporate photochemical key-steps for the following reasons: a) Excited state molecules are much more powerful oxidants (as well as reductants) than the corresponding ground state species. Thus, a peroxo metal complex for example, that will not oxidize a certain hydrocarbon under ambient conditions, might well be able to perform this reaction upon irradiation. b) It could be possible to photochemically generate new "oxygen complexes" as reactive species that are otherwise very difficult to obtain through a "normal" chemical reaction. c) The electron distribution and spin characteristics of metal-oxo fragments and related reactive intermediates may be influenced and controlled by light absorption. Therefore, we plan to study reactions such as the reaction of metal ozonides to form a mononuclear metal-oxo complex releasing dioxygen if activated by light. Such complexes are regarded as extremely powerful oxidants and could be used for selective oxidations in situ. Furthermore, it is well known that several peroxo complexes are light sensitive. Again, investigations will be performed to find out, if such a peroxide can be directly transformed into a metal oxo complex by photoinduced processes. Methane and other hydrocarbons will be reacted with different "oxygen complexes", e.g. dinuclear copper peroxo species, dioxygen adducts or metal oxo compounds under exposure to mono- and polychromatic light.

The low-temperature conversion of hydrocarbons such as methane (natural gas) into more reactive intermediates is a great challenge for catalysis. While it is easy to completely oxidize (combust) methane to form carbon dioxide and water, it is very difficult to convert this relatively inert hydrocarbon selectively into methanol under mild conditions. In contrast to efforts in the lab, nature has already proved that this reaction can be performed under ambient conditions. Methane monooxygenase enzymes, either based on iron or copper ions in the active site of the protein, are here acting as the catalysts. Therefore, the effort of a large number of research groups is trying to model these enzymes using low-molecular weight iron or copper complexes. During these investigations, different types of reactive oxygen complexes have been identified as intermediates, and some of these reactive species could be characterized in great detail. It is surprising, however, that studies on selective oxidations using such compounds so far only were performed as thermal processes. In the present project, for the first time also the incorporation of photochemical key-steps was investigated. This strategy was guided by the following facts: Excited state molecules are much more powerful oxidants than the corresponding ground state species, which should strongly facilitate the process It should be possible to photochemically generate new reactive oxygen containing metal complexes as intermediates, which otherwise would be hard to synthesize The electron distribution and spin-characteristics of metal-oxo fragments may be significantly influenced and controlled by light absorption Following these basic ideas, the FWF-project I31-N17 Selective Photocatalytic Hydroxylation of Hydrocarbons such as Methane using Air as the Oxidant was launched to systematically explore the reactivity of mono- and dinuclear copper systems and several related transition metal complexes in the presence of non-activated hydrocarbon substrates. Irradiation of selected complexes was carried out at low temperatures and under ambient conditions with visible light and the formation of oxidation products was investigated. A comparison of selectivity, reactivity and stability of different new alkane oxidation catalysts was also carried out. Furthermore the possibility of switching the copper(I) and copper(II) redox states by light-absorption and the coupling of oxygen activating copper compounds with other metal complexes acting as photosensitizers was introduced as an alternative strategy to trigger bio-inspired photochemical redox chemistry under mild reaction conditions in homogeneous solution.