Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O2

Runzi Li; Firoz Shah Tuglak Khan; Shabnam Hematian

doi:10.3390/molecules27031000

Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O₂

Molecules. 2022 Feb 1;27(3):1000. doi: 10.3390/molecules27031000.

Authors

Runzi Li¹, Firoz Shah Tuglak Khan¹, Shabnam Hematian¹

Affiliation

¹ Department of Chemistry and Biochemistry, University of North Carolina at Greensboro, Greensboro, NC 27412, USA.

Abstract

The oxidation of transition metals such as manganese and copper by dioxygen (O₂) is of great interest to chemists and biochemists for fundamental and practical reasons. In this report, the O₂ 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(O₂^•-)]⁺ (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-(O₂^2-)-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-(O₂^2-)-Mn^IV(TPP)-(O₂^2-)-Cu^II(tmpa)]²⁺ (7; λ_max = 420 nm and δ_pyrrolic = -44.90 ppm). The final decomposition product of the (TPP)Mn/Cu/O₂ chemistry in MeTHF is [(TPP)Mn^III(MeTHF)₂]⁺ (6), whose X-ray structure is also presented and compared to literature analogs.

Keywords: copper chelates; dioxygen chemistry; manganese porphyrins; nuclear magnetic resonance spectroscopy; variable-temperature absorption spectroscopy.