Solvent, anion, and structural effects on the redox potentials and UV-visible spectral properties of mononuclear manganese corroles

Solvent Effects on the Phosphorescence of Gold(III) Complexes Chelated by β-Multisubstituted Corroles

A set of gold corrole complexes containing four different β-substituent groups (Br/I/CF₃), namely, 4Br-Au, 4I-Au, and 4CF₃-Au, were investigated; all showed room temperature phosphorescence. The phosphorescence quantum yields of the corroles were determined using tetraphenylporphyrin as a reference: Φ_ph (4I-Au, 0.75%) > Φ_ph (4Br-Au, 0.64%) > Φ_ph (4CF₃-Au, 0.38%). 4CF₃-Au exhibited near-IR emission (858 nm, aerobic); absorbance intensity for the Q-band was higher than that for the Soret band. Complex 4I-Au showed a longer phosphorescence lifetime (82 μs) compared to those of 4Br-Au (53 μs) and 4CF₃-Au (28 μs; N₂, tol). Thermally activated delayed fluorescence (TADF) emission of 4I/Br-Au complexes was observed: stronger emission intensity correlated with increasing temperature. Good negative correlations for 4I/Br-Au were observed between the Soret band absorption energy and the solvent polarizability: excited states of 4I/Br-Au are more polar than their ground states. TD-DFT calculations revealed very fast intersystem crossing (ISC) rate constants, 2.20 × 10¹² s^-1 (4CF₃-Au) > 1.96 × 10¹¹ s^-1 (4Br-Au) > 1.15 × 10¹¹ s^-1 (4I-Au), and importantly, the reverse intersystem crossing (rISC) rate constants are determined as 1.68 × 10⁷ s^-1 (4I-Au) > 2.40 × 10³ s^-1 (4Br-Au) ≫ 8.09 × 10^-8 s^-1 (4CF₃-Au). The exceptionally low rISC rate constant of 4CF₃-Au is attributed to its more steric and deformed structure bearing a larger energy gap between the S₁ and T₁ states.

Solvent and Anion Effects on the Electrochemistry of Manganese Dipyrrin-Bisphenols

A series of "N₂O₂-type" manganese dipyrrin-bisphenols (DPP), formulated as (Ar)DPPMn, where Ar = pentafluorophenyl (F₅Ph), phenyl (Ph), or mesityl (Mes), were electrochemically and spectroscopically characterized in nonaqueous media with and without added anions in the form of tetrabutylammonium salts (TBAX where X = ClO₄^-, PF₆^-, BF₄^-, F^-, Cl^-, OH^-, or CN^-). Two major one-electron reductions are observed under most solution conditions, the first of which is assigned as a Mn^III/II process and the second as electron addition to the π-ring system as confirmed by spectroelectrochemistry. Each Mn^III complex also exhibits one or two one-electron oxidations, the exact number depending upon the positive potential limit of the electrochemical solvent. The two oxidations are separated by 580-590 mV in CH₃CN containing 0.1 M TBAPF₆ and are assigned as π-ring-centered electron transfers to stepwise form a (Ar)DPPMn^III π-cation radical and dication under these solution conditions. Comparisons are made between redox properties of (Ar)DPPMn and manganese(III) porphyrins, corroles, and corrolazines each of which contains an innocent trianionic complexing ligand. The redox behavior and spectroscopic properties of [(Ar)DPPMn]ⁿ where n = 0, -1, or +1 are also compared to that of other structurally related [(Ar)DPPM]ⁿ complexes under similar solution conditions where M = Co^II, Cu^II, B^III, or Au^III.

Electrocatalytic Reduction of CO2 to Acetic Acid by a Molecular Manganese Corrole Complex

The controlled electrochemical reduction of carbon dioxide to value added chemicals is an important strategy in terms of renewable energy technologies. Therefore, the development of efficient and stable catalysts in an aqueous environment is of great importance. In this context, we focused on synthesizing and studying a molecular Mn^III‐corrole complex, which is modified on the three meso‐positions with polyethylene glycol moieties for direct and selective production of acetic acid from CO₂. Electrochemical reduction of Mn^III leads to an electroactive Mn^II species, which binds CO₂ and stabilizes the reduced intermediates. This catalyst allows to electrochemically reduce CO₂ to acetic acid in a moderate acidic aqueous medium (pH 6) with a selectivity of 63 % and a turn over frequency (TOF) of 8.25 h⁻¹, when immobilized on a carbon paper (CP) electrode. In terms of high selectivity towards acetate, we propose the formation and reduction of an oxalate type intermediate, stabilized at the Mn^III‐corrole center.

A tris-(manganese(iii))corrole–porphyrin–corrole triad: synthesis, characterization and catalytic epoxidation

A homotrimetallic manganese(iii) corrole–porphyrin–corrole triad has been synthesized and structurally characterized.

Dioxygen Activation and O-O Bond Formation Reactions by Manganese Corroles

Activation of dioxygen (O2) in enzymatic and biomimetic reactions has been intensively investigated over the past several decades. More recently, O-O bond formation, which is the reverse of the O2-activation reaction, has been the focus of current research. Herein, we report the O2-activation and O-O bond formation reactions by manganese corrole complexes. In the O2-activation reaction, Mn(V)-oxo and Mn(IV)-peroxo intermediates were formed when Mn(III) corroles were exposed to O2 in the presence of base (e.g., OH-) and hydrogen atom (H atom) donor (e.g., THF or cyclic olefins); the O2-activation reaction did not occur in the absence of base and H atom donor. Moreover, formation of the Mn(V)-oxo and Mn(IV)-peroxo species was dependent on the amounts of base present in the reaction solution. The role of the base was proposed to lower the oxidation potential of the Mn(III) corroles, thereby facilitating the binding of O2 and forming a Mn(IV)-superoxo species. The putative Mn(IV)-superoxo species was then converted to the corresponding Mn(IV)-hydroperoxo species by abstracting a H atom from H atom donor, followed by the O-O bond cleavage of the putative Mn(IV)-hydroperoxo species to form a Mn(V)-oxo species. We have also shown that addition of hydroxide ion to the Mn(V)-oxo species afforded the Mn(IV)-peroxo species via O-O bond formation and the resulting Mn(IV)-peroxo species reverted to the Mn(V)-oxo species upon addition of proton, indicating that the O-O bond formation and cleavage reactions between the Mn(V)-oxo and Mn(IV)-peroxo complexes are reversible. The present study reports the first example of using the same manganese complex in both O2-activation and O-O bond formation reactions.

CO2 Fixation with Epoxides under Mild Conditions with a Cooperative Metal Corrole/Quaternary Ammonium Salt Catalyst System

The cooperative catalytic activity of several metal corrole complexes in combination with tetrabutyl‐ammonium bromide (TBAB) has been investigated for the reaction of epoxides with CO₂ leading to cyclic carbonates. It was found that the use of just 0.05 mol % of a manganese(III)corrole with 2 mol % TBAB exhibits excellent catalytic activity under an atmosphere of CO₂.

Spectroelectrochemical characterization of meso triaryl-substituted Mn(IV), Mn(III) and Mn(II) corroles. Effect of solvent and oxidation state on UV-visible spectra and redox potentials in nonaqueous media

Two series of substituted manganese triarylcorroles were synthesized and characterized as to their electrochemical and spectroelectrochemical properties in CH 2 Cl 2, CH 3 CN and pyridine. The investigated compounds are represented as ( YPh )3 CorMn III and ( YPh )3 CorMn IV Cl , where Cor is a trianion of the corrole and Y is a Cl , F , H or CH 3 para-substituent on the three phenyl rings of the macrocycle. Each neutral Mn(III) corrole exists as a four-coordinate complex in CH 2 Cl 2 and CH 3 CN and as a five-coordinate species in pyridine. ( YPh )3 CorMn III undergoes two oxidations to stepwise generate a Mn(IV) corrole and a Mn(IV) π-cation radical. It also undergoes one reduction to generate a Mn(II) corrole in CH 2 Cl 2 or CH 3 CN . In contrast, the reduction of ( YPh )3 CorMn III leads to a Mn(III) corrole π-anion radical in pyridine. One oxidation is observed for ( YPh )3 CorMn IV Cl in CH 2 Cl 2 and CH 3 CN to generate a Mn(IV) corrole π-cation radical while Mn(III) and Mn(II) corroles are stepwise formed after reduction of the same compound. The second reduction of ( YPh )3 CorMn IV Cl in pyridine gives a Mn(III) π-anion radical as opposed to a Mn(II) corrole with an unreduced π-ring system. The neutral, reduced and oxidized forms of each corrole were characterized by electrochemistry and UV-visible spectroelectrochemistry and comparisons are made between the UV-visible spectra and redox potentials of the compounds in different central metal oxidation states. An overall reduction/oxidation mechanism in the three solvents is proposed.

Catalytic antioxidant therapy by metallodrugs: lessons from metallocorroles

This article provides a perspective on the utility of metal-based catalytic antioxidants for disease prevention or treatment, with focus on their mode of action and its dependence (DCA) or independence (ICA) on the involvement of cofactors.

Alkynyl corroles: synthesis by Sonogashira coupling reaction and the physicochemical properties

Alkynyl corroles were synthesized from iodidphenyl corrole precursor by using Sonogashira coupling reaction with or without the presence of copper(I) iodide co-catalyst. The alkynyl group on corrole macrocycle has a significant effect on the photophysical and electrochemical properties of free-base and metal corrole derivatives.

Electrochemistry and spectroelectrochemistry of bismanganese biscorroles dyads

Three manganese biscorrole dyads were synthesized, physicochemically characterized and investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each dyad contained the same two corroles linked in a face-to-face arrangement via one of the three different linking groups, 9,9-dimethylxanthene, anthracene or diphenylether, the exact nature of which determined the distance and possible interaction between the two metallomacrocycles. The initial compounds contained Mn ( III ) in their air stable form and were shown to exhibit two major redox processes, one being a Mn (III)/ Mn (IV) conversion and the other being either Mn ( III )/ Mn ( II ) or reduction at the conjugated macrocycle to give a Mn ( III ) corrole π-anion radical when the solvent was pyridine. The potentials and reversibility of each electron transfer reaction were shown to depend upon the solvent (pyridine, CH2Cl2 , or PhCN ), type of spacer separating the two macrocycles and/or the presence or absence of axial ligation. The site of each electron transfer was assigned on the basis of spectroscopic and electrochemical data and by comparison with reactions and properties of the monocorrole ( Mes2PhCor)Mn which was characterized in a previous publication and also examined in the current study under the same solution conditions as the newly investigated dyads. Some electrode reactions of the dyads were followed by coupled chemical reactions and these were also elucidated in the present study.

The formation of the split EPR signal from the S(3) state of Photosystem II does not involve primary charge separation

Metalloradical EPR signals have been found in intact Photosystem II at cryogenic temperatures. They reflect the light-driven formation of the tyrosine Z radical (Y(Z)) in magnetic interaction with the CaMn(4) cluster in a particular S state. These so-called split EPR signals, induced at cryogenic temperatures, provide means to study the otherwise transient Y(Z) and to probe the S states with EPR spectroscopy. In the S(0) and S(1) states, the respective split signals are induced by illumination of the sample in the visible light range only. In the S(3) state the split EPR signal is induced irrespective of illumination wavelength within the entire 415-900nm range (visible and near-IR region) [Su, J. H., Havelius, K. G. V., Ho, F. M., Han, G., Mamedov, F., and Styring, S. (2007) Biochemistry 46, 10703-10712]. An important question is whether a single mechanism can explain the induction of the Split S(3) signal across the entire wavelength range or whether wavelength-dependent mechanisms are required. In this paper we confirm that the Y(Z) radical formation in the S(1) state, reflected in the Split S(1) signal, is driven by P680-centered charge separation. The situation in the S(3) state is different. In Photosystem II centers with pre-reduced quinone A (Q(A)), where the P680-centered charge separation is blocked, the Split S(3) EPR signal could still be induced in the majority of the Photosystem II centers using both visible and NIR (830nm) light. This shows that P680-centered charge separation is not involved. The amount of oxidized electron donors and reduced electron acceptors (Q(A)(-)) was well correlated after visible light illumination at cryogenic temperatures in the S(1) state. This was not the case in the S(3) state, where the Split S(3) EPR signal was formed in the majority of the centers in a pathway other than P680-centered charge separation. Instead, we propose that one mechanism exists over the entire wavelength interval to drive the formation of the Split S(3) signal. The origin for this, probably involving excitation of one of the Mn ions in the CaMn(4) cluster in Photosystem II, is discussed.

Superoxide dismutase mimics: chemistry, pharmacology, and therapeutic potential

Oxidative stress has become widely viewed as an underlying condition in a number of diseases, such as ischemia-reperfusion disorders, central nervous system disorders, cardiovascular conditions, cancer, and diabetes. Thus, natural and synthetic antioxidants have been actively sought. Superoxide dismutase is a first line of defense against oxidative stress under physiological and pathological conditions. Therefore, the development of therapeutics aimed at mimicking superoxide dismutase was a natural maneuver. Metalloporphyrins, as well as Mn cyclic polyamines, Mn salen derivatives and nitroxides were all originally developed as SOD mimics. The same thermodynamic and electrostatic properties that make them potent SOD mimics may allow them to reduce other reactive species such as peroxynitrite, peroxynitrite-derived CO(3)(*-), peroxyl radical, and less efficiently H(2)O(2). By doing so SOD mimics can decrease both primary and secondary oxidative events, the latter arising from the inhibition of cellular transcriptional activity. To better judge the therapeutic potential and the advantage of one over the other type of compound, comparative studies of different classes of drugs in the same cellular and/or animal models are needed. We here provide a comprehensive overview of the chemical properties and some in vivo effects observed with various classes of compounds with a special emphasis on porphyrin-based compounds.