Effect of redox potential on manganese-mediated benzylalcohol and sulfide oxidation

Tridentate 1,3-bis($2^{\prime}$-Ar-imino)isoindolinato manganese(II) complexes were found to efficiently catalyze the mild oxidation of organic sulfides to sulfoxides and benzyl alcohols to benzaldehydes with mCPBA and PhIO, but they proved almost ineffective by the use of $t$BuOOH and H2O2 at room temperature. The effect of electron-withdrawing and electron-donating substituents on the substrates (thioanisole and benzylalcohol), and the redox properties of the metal center by varying the aryl groups on the bis-iminoisoindoline were investigated, and showed a significant impact on the catalytic C–H oxidation and sulfoxidation reactions. Based on these results, including the linear correlations between the oxidation reactivity of the catalysts and $\mathrm{Mn}^{\mathrm{III}}/\mathrm{Mn}^{\mathrm{II}}$ redox potentials, the Hammett correlation with $\rho = -0.27$ for 4R-PhSMe and $\rho = -0.27$ for 4R-PhCH2OH, electrophilic oxomanganese(IV) intermediate has been suggested as key oxidant. Furthermore, the small negative slope $({-}0.5)$ from the $\log k_{\mathrm{rel}}$ versus $E^{\mathrm{o}}_{\mathrm{ox}}$ for 4R-PhSMe gives clear evidence for the direct oxygen atom transfer (OAT) mechanism instead of electron transfer (ET) mechanism between the $\mathrm{Mn}^{\mathrm{IV}}\mathrm{O}$ and PhSMe

Effect of redox potential on manganese-mediated benzylalcohol and sulfide oxidation

Tridentate 1,3-bis($2^{\prime}$-Ar-imino)isoindolinato manganese(II) complexes were found to efficiently catalyze the mild oxidation of organic sulfides to sulfoxides and benzyl alcohols to benzaldehydes with mCPBA and PhIO, but they proved almost ineffective by the use of $t$BuOOH and H2O2 at room temperature. The effect of electron-withdrawing and electron-donating substituents on the substrates (thioanisole and benzylalcohol), and the redox properties of the metal center by varying the aryl groups on the bis-iminoisoindoline were investigated, and showed a significant impact on the catalytic C–H oxidation and sulfoxidation reactions. Based on these results, including the linear correlations between the oxidation reactivity of the catalysts and $\mathrm{Mn}^{\mathrm{III}}/\mathrm{Mn}^{\mathrm{II}}$ redox potentials, the Hammett correlation with $\rho = -0.27$ for 4R-PhSMe and $\rho = -0.27$ for 4R-PhCH2OH, electrophilic oxomanganese(IV) intermediate has been suggested as key oxidant. Furthermore, the small negative slope $({-}0.5)$ from the $\log k_{\mathrm{rel}}$ versus $E^{\mathrm{o}}_{\mathrm{ox}}$ for 4R-PhSMe gives clear evidence for the direct oxygen atom transfer (OAT) mechanism instead of electron transfer (ET) mechanism between the $\mathrm{Mn}^{\mathrm{IV}}\mathrm{O}$ and PhSMe

Bleach catalysis in aqueous medium by iron(III)-isoindoline complexes and hydrogen peroxide

Hydrogen peroxide and peroxymonocarbonate anion-based bleach reactions are important for many applications such as paper bleach, waste water treatment and laundry. Nonheme iron(III) complexes, $[\mathrm{Fe}^{\mathrm{III}}(\mathrm{L}^{1-4})\mathrm{Cl}_{2}]$ with the 1,3-bis($2^{\prime}$-Ar-imino)isoindolines ligands ($\mathrm{HL}^{n}$, $n=$1–4, Ar $=$ pyridyl, thiazolyl, benzimidazolyl and N-methylbenzimidazolyl, respectively) have been shown to catalyze the oxidative degradation of morin as a soluble model of a bleachable stain by $\mathrm{H}_{2}\mathrm{O}_{2}$ in buffered aqueous solution. In these experiments the bleaching activity of the catalysts was significantly influenced by the Lewis acidity and redox properties of the metal centers, and showed a linear correlation with the $\mathrm{Fe}^{\mathrm{III}}/\mathrm{Fe}^{\mathrm{II}}$ redox potentials (in the range of 197–415 mV) controlled by the modification of the electron donor properties of the ligand introducing various aryl groups on the bis-iminoisoindoline moiety. A similar trend but with low yields was observed for the disproportionation of $\mathrm{H}_{2}\mathrm{O}_{2}$ (catalase-like reaction) which is a major side reaction of catalytic bleach with transition metal complexes. The effect of bicarbonate ions might be explained by the reduction of Fe(III) ions and/or the formation of peroxymonocarbonate monoanion, which is a much stronger oxidant and could increase the formation of the catalytically active high-valent oxoiron species

Bleach catalysis in aqueous medium by iron(III)-isoindoline complexes and hydrogen peroxide

Hydrogen peroxide and peroxymonocarbonate anion-based bleach reactions are important for many applications such as paper bleach, waste water treatment and laundry. Nonheme iron(III) complexes, $[\mathrm{Fe}^{\mathrm{III}}(\mathrm{L}^{1-4})\mathrm{Cl}_{2}]$ with the 1,3-bis($2^{\prime}$-Ar-imino)isoindolines ligands ($\mathrm{HL}^{n}$, $n=$1–4, Ar $=$ pyridyl, thiazolyl, benzimidazolyl and N-methylbenzimidazolyl, respectively) have been shown to catalyze the oxidative degradation of morin as a soluble model of a bleachable stain by $\mathrm{H}_{2}\mathrm{O}_{2}$ in buffered aqueous solution. In these experiments the bleaching activity of the catalysts was significantly influenced by the Lewis acidity and redox properties of the metal centers, and showed a linear correlation with the $\mathrm{Fe}^{\mathrm{III}}/\mathrm{Fe}^{\mathrm{II}}$ redox potentials (in the range of 197–415 mV) controlled by the modification of the electron donor properties of the ligand introducing various aryl groups on the bis-iminoisoindoline moiety. A similar trend but with low yields was observed for the disproportionation of $\mathrm{H}_{2}\mathrm{O}_{2}$ (catalase-like reaction) which is a major side reaction of catalytic bleach with transition metal complexes. The effect of bicarbonate ions might be explained by the reduction of Fe(III) ions and/or the formation of peroxymonocarbonate monoanion, which is a much stronger oxidant and could increase the formation of the catalytically active high-valent oxoiron species

Effect of Redox Potential on Diiron-Mediated Disproportionation of Hydrogen Peroxide

Heme and nonheme dimanganese catalases are widely distributed in living organisms to participate in antioxidant defenses that protect biological systems from oxidative stress. The key step in these processes is the disproportionation of H2O2 to O2 and water, which can be interpreted via two different mechanisms, namely via the formation of high-valent oxoiron(IV) and peroxodimanganese(III) or diiron(III) intermediates. In order to better understand the mechanism of this important process, we have chosen such synthetic model compounds that can be used to map the nature of the catalytically active species and the factors influencing their activities. Our previously reported μ-1,2-peroxo-diiron(III)-containing biomimics are good candidates, as both proposed reactive intermediates (FeIVO and FeIII2(μ-O2)) can be derived from them. Based on this, we have investigated and compared five heterobidentate-ligand-containing model systems including the previously reported and fully characterized [FeII(L1−4)3]2+ (L1 = 2-(2′-pyridyl)-1H-benzimidazole, L2 = 2-(2′-pyridyl)-N-methyl-benzimidazole, L3 = 2-(4-thiazolyl)-1H-benzimidazole and L4 = 2-(4′-methyl-2′-pyridyl)-1H-benzimidazole) and the novel [FeII(L5)3]2+ (L5 = 2-(1H-1,2,4-triazol-3-yl)-pyridine) precursor complexes with their spectroscopically characterized μ-1,2-peroxo-diiron(III) intermediates. Based on the reaction kinetic measurements and previous computational studies, it can be said that the disproportionation reaction of H2O2 can be interpreted through the formation of an electrophilic oxoiron(IV) intermediate that can be derived from the homolysis of the O–O bond of the forming μ-1,2-peroxo-diiron(III) complexes. We also found that the disproportionation rate of the H2O2 shows a linear correlation with the FeIII/FeII redox potential (in the range of 804 mV-1039 mV vs. SCE) of the catalysts controlled by the modification of the ligand environment. Furthermore, it is important to note that the two most active catalysts with L3 and L5 ligands have a high-spin electronic configuration