TOWARDS PARTIAL OXIDATION OF METHANE TO METHANOL: OXIDATION OF MONOMETHYL MII-CH3 (M=Pt, Pd) COMPLEXES WITH O2 IN WATER
Vedernikov, Andrei N
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C H Functionalization of methane, catalyzed by PtII compounds with H2PtCl6 as stoichiometric oxidant, has been reported by Shilov et al. in the 1970's. Since then a number of attempts have been made to utilize atmospheric oxygen instead of expensive H2PtCl6. The key to a success is to achieve fast and selective PtIIMe - to - PtIVMe oxidation with O2. Previously our group has reported di(2-pyridyl)methanesulfonate (dpms) ligand - enabled aerobic oxidation of PtIIMe to produce PtIVMe intermediates and methanol. In this work, factors affecting the rate and selectivity of aerobic oxidation of aqueous (dpms)MIIMe(OH2) complexes (M = Pt, Pd) are studied in detail with special attention paid to the effect of additives and the solution pH. We found that oxidation of (dpms)PtIIMe(OH2) is fastest at pH 8.0 and formation of a Pt-to-Pt methyl group transfer product, a C1 - symmetric (dpms)PtIVMe2(OH) complex, occurs at pH > 10. The latter becomes the major product at pH 14. Results of a kinetics study, isotopic labeling experiments and DFT calculations (collaboration with Prof. W.A. Goddard) are reported and the mechanism of the oxidation reactions is discussed. Compared to (dpms)PtIIMe(OH2), (dpms)PtIIMe(OAc) complex is less reactive towards O2, whereas (dpms)PtIIMe(I)- complex reacts at a faster rate than (dpms)PtIIMe(OH2) at pH 6.5; chloro- and bromo-analogs are unreactive. Reactivity of PdII complexes containing the same auxiliary dpms ligand is more diverse compared to the PtII analogs. For example, neutral (dpms)PtIIPh(DMSO) is inert towards O2, while (dpms)PdIIMe(SMe2) undergoes aerobic functionalization to form methanol, among other products, already at room temperature. Oxidation of (dpms)PdIIMe(X) , X = I, OH with O2 in water results in formation of methanol and ethane under milder conditions compared to (dpms)PtIIMe(OH2). Palladium complexes have been submitted to oxidation with I2 and peroxides in aqueous solution, kinetics studies, and model reacions. Results show that when O2 is used as the oxidant, photochemical oxidation leads to both ethane and methanol in high combined yield under ambient light and temperature. Reaction selectivity towards MeOH can be modulated by adjusting the reaction pH. The mechanism of these oxidation reactions is proposed, and is different from the mechanism of oxidation of analogous Pt complexes.