Photooxidation of Methane to Methanol by Perrhenate in Water under Ambient Conditions

The activation and functionalization of alkanes remains to be a field of much activity [1]. This is not only of academic interest, but practical aspects of industrial and commercial applications play an important role. In particular, the facile conversion of methane to methanol could provide a basis for the “methanol economy” [2] with novel utilizations in organic chemistry. The application of such systems for the chemical storage of solar energy and its simple manipulation and transportation is a very attractive aspect. Unfortunately, methane is characterized by a quite low reactivity owing to its high C–H bond energy which amounts to 104 kcal. Accordingly, high activation energies are required to facilitate functionalization. However, under these conditions it is difficult to control any reaction and to achieve selectivity. In particular, oxidations can frequently not be stopped when the requested product is formed, and over-oxidation may take place terminating at the thermodynamic sink, e. g. carbon dioxide. For that reason it is quite intriguing that Nature developed the enzyme methane-monooxygenase which accomplishes the conversion of methane to methanol under ambient conditions. Nevertheless, much progress has been achieved in this research field by the investigation of the interaction of alkanes including methane with transition metal centers as an important branch of organometallic chemistry [3 – 6]. In this context it is quite surprising that photochemical studies are still rare in this area. Although irradiation has been used to generate active metal complexes [3 – 6] the photoconversion of alkanes themselves is largely unexplored. The light energy should not only promote photochemical reactions in general but in particular could supply the requested activation energy. Selectivity may be preserved since the photochemistry can occur under ambient conditions. These considerations initiated our present study. The thermal oxidation of alkanes by OsO4 in aqueous solution has been examined before [7, 8], but it proceeds only at higher temperatures, and overoxidation can apparently not be avoided. For an initial attempt we selected this system but studied the photochemistry under ambient conditions. This investigation was also facilitated by taking into account the results of previous work with OsO4 as an efficient photooxidant [9]. However, owing to the detrimental properties of OsO4 it was later replaced by the isoelectronic anion ReO4 which promotes a rather smooth photoconversion of methane to methanol.