A functional model for pMMO (particulate methane monooxygenase): Hydroxylation of alkanes with H2O2 catalyzed by β-diketiminatocopper(II) complexes

The reaction of copper(II) complexes supported by a series of β-diketiminate ligands (R1,R2L, [(Dipp)N-C(R2)–C(R1)–C(R2)–N(Dipp)]−, Dipp = 2,6-diisopropylphenyl; see Chart 2) and H2O2 has been examined spectroscopically at a low temperature. The β-diketiminatocopper(II) complexes with R2 = H (no substituent on the β-carbon) provided a copper–oxygen intermediate that exhibited the same spectroscopic features as those of the bis(μ-oxo)dicopper(III) complex generated by the reaction of corresponding β-diketiminatocopper(I) complex and O2. On the other hand, the β-diketiminatocopper(II) complexes with methyl substituent on the β-carbon (R2 = Me) did not produce such an intermediate in the same reaction. The β-diketiminatocopper(II) complexes carrying an electron-withdrawing substituent on the α-carbon (R1 = NO2 or CN) but no β-substituent (R2 = H) exhibited a high catalytic activity in the oxygenation reaction of alkanes with H2O2. Mechanism of the catalytic oxygenation reaction as well as the substituent effects of the ligands on the copper(II)-H2O2 reactivity is discussed.