Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O(2)

The oxidation of transition metals such as manganese and copper by dioxygen (O(2)) is of great interest to chemists and biochemists for fundamental and practical reasons. In this report, the O(2) reactivities of 1:1 and 1:2 mixtures of [(TPP)Mn(II)] (1; TPP: Tetraphenylporphyrin) and [(tmpa)Cu(I)(MeCN)](+) (2; TMPA: Tris(2-pyridylmethyl)amine) in 2-methyltetrahydrofuran (MeTHF) are described. Variable-temperature (−110 °C to room temperature) absorption spectroscopic measurements support that, at low temperature, oxygenation of the (TPP)Mn/Cu mixtures leads to rapid formation of a cupric superoxo intermediate, [(tmpa)Cu(II)(O(2)(•–))](+) (3), independent of the presence of the manganese porphyrin complex (1). Complex 3 subsequently reacts with 1 to form a heterobinuclear μ-peroxo species, [(tmpa)Cu(II)–(O(2)(2–))–Mn(III)(TPP)](+) (4; λ(max) = 443 nm), which thermally converts to a μ-oxo complex, [(tmpa)Cu(II)–O–Mn(III)(TPP)](+) (5; λ(max) = 434 and 466 nm), confirmed by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy. In the 1:2 (TPP)Mn/Cu mixture, 4 is subsequently attacked by a second equivalent of 3, giving a bis-μ-peroxo species, i.e., [(tmpa)Cu(II)−(O(2)(2−))−Mn(IV)(TPP)−(O(2)(2−))−Cu(II)(tmpa)](2+) (7; λ(max) = 420 nm and δ(pyrrolic) = −44.90 ppm). The final decomposition product of the (TPP)Mn/Cu/O(2) chemistry in MeTHF is [(TPP)Mn(III)(MeTHF)(2)](+) (6), whose X-ray structure is also presented and compared to literature analogs.