Copper(II)-hydroperoxo species are often detected as key intermediates in metalloenzymes and biomimetic compounds containing copper. However, the only reactivity has previously been observed for the copper(II)-hydroperoxo complexes is electrophilic, occurring through O-O bond cleavage. Here we report that a mononuclear end-on copper(II)-hydroperoxo complex, which has been successfully characterized by various physicochemical methods including UV-vis, rRaman, CSI-MS and EPR, is a reactive oxidant that utilizes a nucleophilic mechanism. In addition, DFT calculations fully support the electronic structure of this complex as a copper(II)-hydroperoxo complex with trigonal bipyramidal coordination geometry. A positive Hammett ρ value (2.0(3)) is observed in the reaction of copper(II)-hydroperoxo complex with para-substituted acyl chlorides, which clearly indicates nucleophilic character for the copper(II)-hydroperoxo complex. The copper(II)-hydroperoxo complex is an especially reactive oxidant in aldehyde deformylation with 2-PPA and CCA relative to the other metal-bound reactive oxygen species reported so far. The observation of nucleophilic reactivity for a copper(II)-hydroperoxo species expands the known chemistry of metal-reactive oxygen species.

Copper(ii)-alkylperoxo adducts, [Cu(CHDAP)(OOR)] (CHDAP = N,N'-dicyclohexyl-2,11-diaza[3,3](2,6)pyridinophane; R = C(CH)Ph and Bu), were prepared and characterized using various physicochemical methods. These are the first synthetic Cu(ii)-alkylperoxo complexes that can perform aldehyde deformylation (i.e., nucleophilic reactivity) under the stoichiometric reaction conditions, which was confirmed by kinetic studies.

The importance of redox-inactive metal ions in modulating the reactivity of redox-active biological systems is a subject of great current interest. In this work, the effect of redox-inactive metal ions (M 3+ = Sc 3+ , Y 3+ , Yb 3+ , La 3+ ) on the nucleophilic reactivity of a mononuclear ligand-based alkylperoxocopper(II) complex, [Cu( i Pr 2 -tren-C(CH 3 ) 2 O 2 )] + (1), was examined. 1 was prepared by the addition of hydrogen peroxide and triethylamine to the solution of [Cu( i Pr 3 -tren(2) in methanol (CH 3 OH) at 30 °C. 1 was characterized using density functional theory (DFT) calculations and spectroscopic methods such as UV−vis, resonance Raman (rR), and electron paramagnetic resonance (EPR). DFT calculations support the electronic structure of 1 with an intermediate geometry between the trigonal-bipyramidal and square-pyramidal geometries, which is consistent with the observed EPR signal exhibiting a signal with g ⊥ = 2.03 (A ⊥ = 16 G) and g || = 2.19 (A || = 158 G). The Cu−O bond stretching frequency of 1 was observed at 507 cm −1 for 16 O 2 species (486 cm −1 for 18 O 2 species), and its O−O vibrational energy was determined to be 799 cm −1 for 16 O 2 species (759 cm −1 for 18 O 2 species) by rR spectroscopy. The reactivity of 1 was investigated in oxidative nucleophilic reactions. The positive slope of the Hammett plot (ρ = 2.3(1)) with para-substituted benzaldehydes and the reactivity order with 1°-, 2°-, and 3°-CHO demonstrate well the nucleophilic character of this copper(II) ligand-based alkylperoxo complex. The Lewis acidity of M 3+ improves the oxidizing ability of 1. The modulated reactivity of 1 with M 3+ was revealed to be an opposite trend of the Lewis acidity of M 3+ in aldehyde deformylation.

Soluble Mn(III)–L complexes appear to constitute a substantial portion of manganese (Mn) in many environments and serve as critical high-potential species for biogeochemical processes. However, the inherent reactivity and lability of these complexes—the same chemical characteristics that make them uniquely important in biogeochemistry—also make them incredibly difficult to measure. Here we present experimental results demonstrating the limits of common analytical methods used to quantify these complexes. The leucoberbelin-blue method is extremely useful for detecting many high-valent Mn species, but it is incompatible with the subset of Mn(III) complexes that rapidly decompose under low-pH conditions—a methodological requirement for the assay. The Cd-porphyrin method works well for measuring Mn(II) species, but it does not work for measuring Mn(III) species, because additional chemistry occurs that is inconsistent with the proposed reaction mechanism. In both cases, the behavior of Mn(III) species in these methods ultimately stems from inter- and intramolecular redox chemistry that curtails the use of these approaches as a reflection of ligand-binding strength. With growing appreciation for the importance of high-valent Mn species and their cycling in the environment, these results underscore the need for additional method development to enable quantifying such species rapidly and accurately in nature.

This paper overviews the final remarks lecture delivered (by K. D. K.) at the end of this bioinorganic chemistry Faraday Discussion, held online for a worldwide audience from January 31 – February 3, 2022.