Catalase vs Peroxidase Activity of a Manganese(II) Compound: Identification of a Mn(III)−(μ-O)₂−Mn(IV) Reaction Intermediate by Electrospray Ionization Mass Spectrometry and Electron Paramagnetic Resonance Spectroscopy

Abstract

Herein, we report reactivity studies of the mononuclear water-soluble complex [Mn(II)(HPClNOL)(η 1 -NO 3 )(η 2 -NO 3 )] 1, where HPClNOL ) 1-(bis-pyridin-2-ylmethyl-amino)-3-chloropropan-2-ol, toward peroxides (H 2 O 2 and tertbutylhydroperoxide). Both the catalase (in aqueous solution) and peroxidase (in CH 3 CN) activities of 1 were evaluated using a range of techniques including electronic absorption spectroscopy, volumetry (kinetic studies), pH monitoring during H 2 O 2 disproportionation, electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry in the positive ion mode [ESI(+)-MS], and gas chromatography (GC). Electrochemical studies showed that 1 can be oxidized to Mn(III) and Mn(IV). The catalase-like activity of 1 was evaluated with and without pH control. The results show that the pH decreases when the reaction is performed in unbuffered media. Furthermore, the activity of 1 is greater in buffered than in unbuffered media, demonstrating that pH influences the activity of 1 toward H 2 O 2 . . The peroxidase activity of 1 was also evaluated by monitoring cyclohexane oxidation, using H 2 O 2 or tert-butylhydroperoxide as the terminal oxidants. Low yields (<7%) were obtained for H 2 O 2 , probably because it competes with 1 for the catalase-like activity. In contrast, using tert-butylhydroperoxide, up to 29% of cyclohexane conversion was obtained. A mechanistic model for the catalase activity of 1 that incorporates the observed lag phase in O 2 production, the pH variation, and the formation of a Mn(III)-(µ-O) 2 -Mn(IV) intermediate is proposed.

Iron, copper, and manganese complexes with in vitro superoxide dismutase and/or catalase activities that keep Saccharomyces cerevisiae cells alive under severe oxidative stress

“…Therefore, a major strategy for the development of effective antioxidants has focused on coordination compounds containing a redox-active metal center [18][19][20][21][22][23]38,[51][52][53] .…”

“…The SOD activity of the complexes was evaluated employing a modification of the nitrobluetetrazolium (NBT) method 35 The catalase (CAT) activity was determined from the evaluation of the volume of oxygen produced by the reaction of the complexes with H 2 O 2 as described previously 38 .…”

The application of electrospray ionization mass spectrometry to homogeneous catalysis

“…The use of ESI-MS to complement more traditional reactivity studies is highlighted in a report on manganese-based peroxidase and catalase activity which appeared in 2009. Based on ESI-MS experiments and supported by UV-Vis and EPR experiments, the binuclear manganese intermediate [(PClNOL)Mn III -(µ-O) 2 -Mn IV (PClNOL)] + (Figure 1 H, m/z 722) was identified and proposed to be responsible for the catalase-like activity of the manganese(II) compound [Mn II (HPClNOL)(η 1 -NO 3 )(η 2 -NO 3 )] HPClNOL = N((CH 2 )C 5 H 4 N) 2 (CH 2 (CH)OH(CH 2 )Cl)[42].…”

Direct Selective Oxidative Functionalization of C–H Bonds with H2O2: Mn-Aminopyridine Complexes Challenge the Dominance of Non-Heme Fe Catalysts

“…Moderate efficiencies were reported (up to 62 turnovers), with cyclohexyl hydroperoxide as the major product [75]. The cyclohexanol/cyclohexanone ratios were in the range 1.3-1.4, indicating a sizeable contribution of a radical oxidation pathway.…”

“…Fernandes and co-workers reported the water-soluble manganese(II) complex with ligand 9 (Figure 4), of the type [(9)Mn(η 1 -NO 3 )(η 2 -NO 3 )], which catalyzed the oxidation of cyclohexane with H 2 O 2 (at 50 • C) in CH 3 CN or in tBuOH [75]. Moderate efficiencies were reported (up to 62 turnovers), with cyclohexyl hydroperoxide as the major product [75].…”