Y2 Ground State Research Articles

Induction of Cell Death by Ternary Copper(II) Complexes of l-Tyrosine and Diimines: Role of Coligands on DNA Binding and Cleavage and Anticancer Activity

The mononuclear mixed ligand copper(II) complexes of the type [Cu(L-tyr)(diimine)](ClO(4)), where tyr is L-tyrosine and diimine is 2,2'-bipyridine (bpy) (1), 1,10-phenanthroline (phen) (2), 5,6-dimethyl-1,10-phenanthroline (5,6-dmp) (3), and dipyrido[3,2-d:2',3'-f]quinoxaline (dpq) (4), have been isolated and characterized by analytical and spectral methods. In the X-ray crystal structure 3 Cu(II) possesses a distorted square pyramidal coordination geometry with the two nitrogen atoms of 5,6-dmp ligand and the amine nitrogen and carboxylate oxygen atoms of L-tyrosine located at the equatorial sites and the coordinated water molecule present in the apical position. The electronic absorption and electron paramagnetic resonance (EPR) spectral parameters reveal that the complexes retain their square-based geometries even in solution. All of the complexes display a ligand field band in the visible region (600-700 nm) in Tris-HCl/NaCl buffer (5:50 mM) at pH 7.2 and also axial EPR spectra in acetonitrile at 77 K with g(parallel) > g(perpendicular) indicating a d(x(2)-y(2)) ground state. The g(parallel) and A(parallel) values of 2.230 and (170-180) x 10(-4) cm(-1), respectively, conform to a square-based CuN(3)O coordination chromophore, which is consistent with the X-ray crystal structure of 3. The interaction of the complexes with calf thymus DNA (CT DNA) has been explored by using physical methods to propose modes of DNA binding of the complexes. Absorption (K(b)) and emission spectral studies and viscosity measurements indicate that 4 interacts with DNA more strongly than all of the other complexes through partial intercalation of the extended planar ring of dpq with DNA base stack. Interestingly, complex 3 exhibits a DNA binding affinity that is higher than that of 2, which suggests the involvement of 5,6-dimethyl groups on the phen ring in hydrophobic interaction with DNA surface. In contrast with the increase in relative viscosities of DNA bound to 2-4, the viscosity of DNA bound to 1 decreases, indicating the shortening of the DNA chain length by means of the formation of kinks or bends. All complexes exhibit effective DNA (pUC19 DNA) cleavage at 100 microM complex concentrations, and the order of DNA cleavage ability varies as 3 > 2 > 4 > 1. Interestingly, 3 exhibits a DNA cleavage rate constant that is higher than that of the other complexes only at 100 microM concentration, whereas 4 exhibits the highest cleavage rate constant at 80 microM complex concentration. The oxidative DNA cleavage follows the order 4 > 3 > 2 > 1. Mechanistic studies reveal that the DNA cleavage pathway involves hydroxyl radicals. Interestingly, only 4 displays efficient photonuclease activity upon irradiation with 365 nm light, which occurs through double-strand DNA breaks involving hydroxyl radicals. Furthermore, cytotoxicity studies on the nonsmall lung cancer (H-460) cell line show that the IC(50) values of 2-4 are more or less equal to cisplatin for the same cell line, indicating that they have the potential to act as very effective anticancer drugs in a time-dependent manner. The study of cytological changes reveals the higher induction of apoptosis and mitotic catastrophe for 4 and 3, respectively. The alkaline single-cell gel electrophoresis (comet assay), DNA laddering, and AO/EB and Hoechst 33258 staining assays have also been employed in finding the extent of DNA damage. Flow cytometry analysis shows an increase in the percentage of cells with apoptotic morphological features in the sub-G(0)/G(1) phase for 4, whereas it shows mitotic catastrophe for 3.

Spectroscopic characterization and biological activity of metal complexes with an ONO trifunctionalized hydrazone ligand

A series of Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), La(III), Ru(III), Hf(IV), Zr(IV) and U(VI) complexes of phenylamino acetoacetylacetone hydrazone have been synthesized and characterized by elemental analyses, IR, UV–Vis, magnetic moments, conductances, thermal analyses (DTA and TGA) and ESR measurements. The IR data show that the ligand is neutral bidentate, monobasic bidentate, monobasic tridentate or dibasic tridentate towards the metal ion. Molar conductances in DMF indicate that the complexes are non-electrolytes. The ESR spectra of solid [(L)(HL)Cu2(NO3)(H2O)]·1/2H2O (10) and [(H2L)Cu(Cl)2(H2O)2] (11) show axial spectra with g∥ > g⊥ > 2.0023 indicating d(x² − y²) ground state with significant covalent bond character. However, [(HL)2Mn2(Cl)2(H2O)4·H2O (13) shows an isotropic spectrum, indicating manganese(II) to be octahedral. Antibacterial and antifungal tests of the ligand and some of its metal complexes revealed that the complexes are more potent bactericides and fungicides than the ligand.

Reaction of a Copper−Dioxygen Complex with Nitrogen Monoxide (•NO) Leads to a Copper(II)−Peroxynitrite Species

A discrete peroxynitrite-copper(II) complex, [(TMG3tren)CuII(-OONO)]+ (3), has been generated in solution (ESI-MS, m/z = 565.15; tetragonal EPR) by reacting *NO(g) with superoxo complex [(TMG3tren)CuII(O2*-)]+ (2). Complex 3 undergoes a thermal transformation to give CuII-nitrite complex [(TMG3tren)CuII(-ONO)]+ (4) (X-ray) along with ca. 0.5 molar equiv dioxygen. A DFT calculation derived structure with cyclic bidentate k2-O,O'-OONO bound peroxynitrite moiety and dx2-y2 ground state is proposed. Experiments using 18O2 suggest that the adjacent peroxo oxygen atoms in 3 are derived from molecular oxygen. Further, 18O2 containing 3 undergoes O-O bond cleavage to form singly 18-O-labeled 4. The results suggest the viability of biological CuI/O2/(*NO) peroxynitrite formation and chemistry, that is, not coming from free superoxide plus *NO reaction.

Oxidation of tetranuclear copper(I) complexes [(Pip)nCuX]4; n=1 or 2, Pip=Piperidine and X=Cl or Br, with two equivalents of tetrachloro-1,2-benzoquinone (TClBQ) and sequentional oxidation with one equivalent of TClBQ followed by dioxygen in aprotic media: Models for intermediates in catalytic catechol–copper(II) systems

Copper(I) halides react quantitatively with piperidine in dioxygen-free CH2Cl2 or PhNO2 to form tetranuclear copper(I) complexes [(pip)nCuX]4; n=1 or 2 and X=Cl or Br. These complexes are very soluble in CH2Cl2 and PhNO2 and completely reduce tetrachloro1,2-benzoquinone (TClBQ) and O2 to the 3,4,5,6-tetrachlorocatecholato ligand (Cat) and the oxo form. The stable solid complexes [(pip)nCuX]4Cat2 and [(pip)nCuX]4CatO are closely related to the intermediate, bridging μ-catecholato complex, which is formed during the mono-oxygenation and oxidation of PhOH by oxytyrosinase. Cryoscopic and analytical data for these complexes indicate that [(pip)nCuX]4Cat2 and [(pip)nCuX]4CatO are discrete tetranuclear species. Electronic transition spectra in the near i.r. with high molecular absorptivity are diagnostic for a tetranuclear ‘Cu4X4’ core structure. The electronic transitions are more likely to be due to charge transfer between a minimum of three halo ligands to the copper(II) center. The room temperature e.p.r. spectra of [(pip)nCuX]4Cat2 and [(pip)nCuX]4CatO in CH2Cl2 are isotropic with four hyperfine lines. The room temperature solid state e.p.r. spectra of these complexes show an axial spectra with dx2−y2 ground state, suggesting a square pyramidal arrangement around copper(II) centers in all n = 1 complexes and an elongated tetragonal octahedral arrangement around copper(II) centers in all n = 2 complexes. Cyclic voltammetry measurements show that they are more likely to be irreversible and show slight quasi-reversibility when X=Br and n = 2. Constant potential electrolysis indicate that the number of electrons consumed are equal to four which will be due to the reduction of four copper(II) species to copper(I).

Synthesis, Structure, and DNA Binding Studies of Copper(II) Complexes of Terpyridine Derivatives

Abstract Two copper(II) complexes, Cu(ttpy)22+1 and Cu(Brphtpy)22+2, were synthesized and characterized for their DNA binding and cleaving ability. The crystal structure of complex 1 reveals an axially compressed six-coordinate geometry around Cu(II). In spite of having an axially compressed geometry, the polycrystalline EPR spectrum of complex 1 at 298 K shows g|| > g⊥, indicating a dx2−y2 ground state for the Cu(II) complex. Both complexes 1 and 2 bind to CT DNA intercalatively with moderate binding strength (Kb = (8.4 ± 0.2) × 103 and (10.9 ± 0.2) × 104 M−1 (1 M = mol dm−3) respectively). Both of these complexes cleave plasmid DNA efficiently in the presence of peroxide.

Reactivity of the new potentially Binucleating Ligand, 2-(Acetichydrazido-N-Methylidene-α-Naphthol)-Benzothiazol, towards Manganese(II), Nickel(II), Cobalt(II), Copper(II) and Zinc(II) salts

A new series of manganese(II), nickel(II), cobalt(II), copper(II) and zinc(II) complexes of 2-(acetichydrazido-N-methylidene-α-naphthol)-benzothiazol (H2L) was prepared and characterized by elemental analysis, IR, UV–VIS, magnetic susceptibility, thermogravimetric analysis (TGA), molar conductance and ESR measurements. Molar conductances of the complexes in DMF are commensurate with their nonelectrolytic nature. IR spectral data show that the ligand can be bidentate and/or tridentate. ESR spectra of the solid copper(II) Complexes 8 and 9 show a d x2 − y2 ground state with covalent character. The fungicidal activities of the ligand and its metal complexes against Aspergillus niger and Fusarium oxysporum are discussed.

Spectroscopic characterization of chromium (IV, V, VI) in Cr:Li2MSiO4 (M=Mg,Zn)

To understand the exceptionally long fluorescence lifetime of CrIV in Li2MSiO4 (M=Mg,Zn), several spectroscopic investigations are performed on Cr:Li2MSiO4 (M=Mg,Zn) crystals. X-ray absorption near-edge structure investigations attest that chromium is localized in tetrahedral sites and that CrVI is the major species while CrIV is the minor one in both compounds. Electron paramagnetic resonance studies confirm the occurrence of CrV in elongated tetrahedral environment (dx2−y2 ground state), corresponding probably to the silicon site, and suggest the presence of several charge compensation schemes. Fluorescence and fluorescence dynamics of CrIV:Li2MSiO4 are reported. The very long CrIV excited state lifetime previously observed on powders samples is confirmed for the crystals (117 μs at room temperature, 305 μs at 30 K for Cr:Li2MgSiO4) and is explained by combined contributions to the emission from both E1 level and the lowest component of the T23 level, in thermal equilibrium. Contrary to the situation found in most CrIV activated compounds, the E1 level lies below the T23 one.

Synthesis and Characterization of New Azopyrimidine Complexes

New azo compounds containing barbituric acid, thiobarbituric acid or thiouracil, together with their complexes, were prepared. All the complexes are tetrahedral. Axial ESR spectra were observed for the complexes with a dx2� y2 ground state with small g|| values indicating strong interaction between the ligands and their metal ions. Conductivity measurements proved semiconducting properties for the ligands and their complexes. Energies of activation (Ea) and order of the thermal reactions (n) were calculated by DTA.

Spectroscopic and Theoretical Studies of Mononuclear Copper(II) Alkyl- and Hydroperoxo Complexes: Electronic Structure Contributions to Reactivity

Spectroscopic studies combined with calculations are used to describe the electronic structure and vibrational properties of mononuclear four-coordinate end-on alkylperoxo and hydroperoxo Cu(II) complexes. EPR defines a Cu x2−y2 ground state with ∼62% Cu character. From absorption, MCD, and resonance Raman spectroscopies, the main bonding interaction between the alkyl(hydro)peroxide and Cu(II) is found to involve the π-donation of the alkyl(hydro)peroxide π*v into the Cu x2−y2 orbital, which dominates the observed spectroscopic features, producing an intense absorption band at ∼16 600 cm-1 (∼600 nm). On the basis of the vibrational frequencies, isotope shifts, and normal coordinate analyses, the dominant vibrations of the alkyl(hydro)peroxo complexes are assigned and the Cu−O and O−O force constants are determined. The observed strong Cu−O bond and the large alkyl(hydro)peroxide-to-Cu(II) charge donation are ascribed to the low coordination number of Cu and the distorted Td ligand field. The observed strong O−O bond mainly derives from polarization by the alkylcarbon/proton. The unoccupied peroxide σ* orbital is also greatly stabilized in energy, and the complexes are activated for electrophilic attack. Experimentally calibrated density functional calculations, coupled with frontier molecular orbital theory, are employed to obtain insight into the reactivity of these model complexes. Mechanisms of electrophilic attack, O−O bond cleavage, and H atom abstraction are evaluated, and their relevance to dopamine β-monooxygenase and peptidylglycine α-hydroxylating monooxygenase reactivities is considered.

Stable mononuclear rhodium(II) polypyridyl complexes: synthesis, spectroscopic and structural characterisation

The reaction of RhCl3·3H2O and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol–water (1∶1) resulted in the hydrolysis of tptz to bis(2-pyridylcarbonyl)amide anion (bpca) and afforded a mixture of RhIII and RhII complexes which were separated and characterised as [RhIII(bpca)2][PF6] and [RhII(bpca)2]·3H2O 1. Similarly the reaction of tptz and Rh(tpy)Cl3 (tpy = 2,2′∶6′,2″-terpyridyl) yielded mixed ligand complexes [RhIII(bpca)(tpy)]2+ and [RhIi(bpca)(tpy)]Cl·8H2O 2. The molecular structures of complexes 1 and 2 have been established by single-crystal X-ray analysis. In complex 1 two bpca moieties are co-ordinated to RhII with nitrogen as donor atoms in a mutually perpendicular fashion. In complex 2 bpca and tpy are bound to the metal ion in a similar fashion to that found in 1. The axial Rh–N bond distances in 1 and 2 are significantly shorter compared to equatorial Rh–N bond distances, indicating an axially compressed octahedral geometry of the metal ion. Complexes 1 and 2 exhibit absorption bands in the 545–600 nm region whereas their RhIII analogues do not show any band in this region. Electrochemical studies of 1 and 2 revealed a metal based reduction (RhII→RhI) at –1.13 and –0.72 V, respectively, followed by two ligand-based redox couples. EPR studies of 1 and 2 in acetonitrile at 77 K show g|| > g⊥ ≈ ge indicating a dx2 – y2 ground state and a compressed octahedral geometry for the metal ion, consistent with the crystal structures.

An e.s.r. study of copper(II) complexes of N-hydroxyalkylsalicylideneimines

X-band e.s.r. spectra of copper(II) complexes of empirical formula Cu(Sal.NCH2CH2O)X (where X= H2O, py or Cl) and [Cu(Sal.NCH2CH2CH2O)]2 (Sal.N= OC6H4CHN) in solid and solution states at room temperature and at 77K are reported. In frozen solution at 77K the spectra show axial symmetry with a dx2−y2 ground state. The in-plane π-bonds show a moderate degree of covalence, while the out-of-plane π-bonds possess ionic character. The orbital populations of these complexes are 70.9, 81.3, 93.3 and 82.4% respectively, indicating a dx2−y2 ground state.

Crystal and molecular structure, and electronic properties of hexakis(imidazole)copper(II) formate

The crystal and molecular structure of [Cu(Im)6] (HCOO)2 (Im = imidazole) has been determined by X-ray diffraction methods. The compound is built of centrosymmetric Cu(Im) inf6 sup2− cations and non-coordinated HCO2− anions, linked through H-bonds giving a layered structure in the ac plane. The coordination polyhedron around the Cu atoms can be described as a rhombically-distorted octahedron (CuN2N′2N′'2 chromophores). Both electronic and e.p.r. spectra are indicative of an essentially dx2− y2 ground state for the CuII ions. No exchange coupling has been detected down to 4.2 K by means of magnetic susceptibility measurements. This feature is discussed on the basis of the structural data.

Studies on Some Bivalent Metal Complexes of 2-Amino-benzophenone Salicylaldazine

Abstract 2-Aminobenzophenonesalicylaldazine (Habs) reacts with metal(II) acetate and yields the complexes M(abs)2(H2O)2 [M = Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)]. The complexes were characterized by elemental analyses, molar conductance, molecular weight, magnetic susceptibility, electronic, ESR, IR and NMR spectral data. The electrical conductivity shows the non-ionic nature of the complexes. The magnetic moment and electronic spectra suggest octahedral stereochemistry for the cobalt(II) and nickel(II) complexes. ESR data suggest tetragonal distortion in the copper(II) complex having dx 2 - y2 ground state. Infrared spectra suggest bonding through the phenoxy oxygen and azomethine nitrogen in the Co(II), Ni(II) and Cu(II) complexes and only the phenoxy oxygen in the Mn(II), Zn(II) and Cd(II) complexes which is also supported by NMR spectral studies.

ESR Study of Paramagnetic Nickel Species Obtained by Reduction and Oxidation of Bis(1,3,5-triphenylformazanato)nickel(II)

Abstract Diamagnetic bis(1,3,5-triphenylformazanato)nickel(II) was reduced or oxidized by chemical and electrochemical methods. The paramagnetic Ni(I) and Ni(III) complexes thus obtained were studied by means of ESR spectroscopy. Ni(I) complexes formed by alkali metal reductions showed remarkable changes in their g values, depending on the kinds of alkali metal used. The change of g-values, which related to the ionic radii of the alkali metal ions, was well interpreted by the axial coordination effect due to the alkali metal ions. The electrochemical formation of the Ni(I) complex showed a solvent-dependency; the nickel(II) ion was reduced in DMF solution, but not in acetonitrile. The g-components were 2.030 for g// and 2.001 for g⊥. The relation between the two g-components gave a dx2−y2 ground state for the Ni(I) ion in the complex. Their anisotropy was very small. The Ni(II) ion oxidized by both chemical and electrochemical methods showed a dz2 ground state. The three g-components were 2.056, 2.039, and 2.000 in DMF, which are similar to those obtained from the other oxidation methods.

Synthesis and spectral studies of novel copper(II) chelates of sulphur-containing oximes

Copper complexes of benzil-α-monoxime-2-iminoethanethiol (BMETH2), benzil-α-monoxime-2-iminobenzenethiol (BMBTH2), benzil-α-monoximethiosemicarbazone (BMTH2) and benzil-α-monoximesemicarbazone (BMSH2) were synthesized and characterized by physicochemical and spectroscopic measurements. The molar conductivities show that all of the complexes are nonelectrolytes. The i.r. spectra suggest that the ligands are tetradentate in all cases, whilst the electronic spectra suggest planar geometries for all the complexes. The calculated e.s.r. parameters show an axial dx2 - y2 ground state and suggest coordination through sulphur, in agreement with the i.r. results. In all of these complexes, the =N—;O group acts as a bridging function to give dimeric structures.

EPR and Spectral Studies of a Molecular and Crystal Structure of Cu (3,5-dimethylpyridine)3(NO3)2

X-band single crystal EPR, IR and UV-VIS data are used to deduce a molecular and crystal structure of a new compound with a chemical formula Cu(3,5-dimethylpyridine)3(NO3)2. EPR shows that four magnetically inequivalent molecules exist in the crystal unit cell of C4„ symmetry. IR spectra suggested that both monodentate and bidentate NO3-groups exist in the crystal. A most probable Cu(II) complex geometry with d x2_ y2 ground state is a square pyramid with three nitrogens from 3,5-dimethylpyridine molecules and one oxygen from monodentate nitrato group in a basal plane and an oxygen from NO3-group in apical position. There exists a chemical contact between complexes as it is indicated by the exchange interaction with |J|< 1 cm . It suggests that square pyramid complexes are arranged in polymeric chains with semi-coordination of bidentate NO3-group to the adjacent Cu(II) ion. An extended analysis of an exchange averaging effect between EPR lines from two and four Cu(II) sites is presented with equations relating molecular and crystal g-factors. PACS numbers: 33.20.Ea, 61.66.—f, 76.30.Da

Some Electronic Properties of Sr1-xAxCuO2+δwhere A=La, K, and Ca

The electronic properties of Sr1_xAxCu02+δ (A = La, K, Ca) were examined. The crystal structure of the synthesized compounds was identified as orthorhombic with some admixture of tetragonal phase. For Sr 1 _x Lax Cu02+ δ the insulator—metal like transition was observed with increasing La content. The dx2_ y2 ground state of Cu ions was deduced from electron spin resonance measurements. The electronic state of Cu ions and their surrounding local symmetry was also found to be La and oxygen content dependent. PACS numbers: 71.30.+h, 72.15.—v, 76.30.Fc

PREPARATION AND CHARACTERIZATION OF NICOTINAMIDE COMPLEXES OF COPPER(II) HALOGENO-CARBOXYLATES

Abstract The synthesis and characterization of the complexes CuX2(nia)2 (where X=ClCH2COO−, Cl2CHCOO−, Cl3CCOO−, CH3CHClCOO− or CH3CCl2COO−, and nia=nicotinamide) is reported. Characterization of the compounds was based on elemental analysis, electronic and EPR spectra, and magnetic susceptibility measurements over a temperature range. Available evidence supports a pseudo−octahedral environment around copper(II). The magnetic properties for the compounds indicate the presence of weak antiferromagnetic interactions. EPR spectra of finely ground polycrystalline samples are consistent with an essentially dx2−y2 ground state.

Copper(II) complexes with anti-inflammatory drugs as ligands. Solution behaviour and electrochemistry of mono- and bi-nuclear complexes

The copper complexes of 1-methyl-5-(p-toluoyl)-1H-pyrrole-2-acetic acid (tolmetin, HL1), α-methyl-4-(2-methylpropyl)benzeneacetic acid (ibuprofen, HL2) and 6-methoxy-α-methylnaphthalene-2-acetic acid (naproxen, HL3) common anti-inflammatory drugs, were prepared and characterized. The available evidence supports a dimeric structure for the dimethyl sulfoxide adducts, [Cu2L4(dmso)2], and monomeric for the pyridine (py) analogues, [CuL(py)2(H2O)]. The solution behaviour of the dimers in dimethylformamide (dmf) shows that the complexes are converted into monomeric compounds by addition of more than 25% of pyridine. The cyclic voltammograms of the dimers of dmf suggest that the complexes retain the dimeric structure in solution. The EPR spectra of the monomers as CH2Cl2–dmf (4 : 1) glasses confirm the monomeric structure, having a dx2–y2 ground state. Paramagnetic 1H NMR studies have shown that the H2O molecule of the monomeric form remains co-ordinated to CuII in CD3Cl, (CD3)2CO or CD3CN. The crystal structure of [CuL32(py)2(H2O)] has been determined: space group P21, a= 16.112(3), b= 5.8692(9), c= 18.130(3)A, β= 75.818(7)°. In this complex the copper atom is five-co-ordinate. The atoms N(1) and N(2) of the two pyridine molecules and one carboxylate oxygen atom from each of the two ligands form the basal plane of a square pyramid with a water molecule at its apex.

Synthesis and characterisation of sulfur-rich manganese(III) and vanadium(IV) complexes containing dithioacid ligands

The chemistry of the bidentate (S,S′)– donor ligand 2-aminocyclopent-1-ene-1-carbodithioic acid (Hacda) and its N-alkylated derivatives [R = Et (Heacda), Prn(Hpacda), or Bun(Hbacda)] with vanadium(IV) and manganese(III) has been studied. The vanadium(IV) complexes [V(aacda)4]1a–1d are all eight-co-ordinated non-oxo species containing a VS8 chromophore. Their ESR spectra in frozen solution (140 K) exhibit axial anisotropy with a 16-line 51V hyperfine pattern. The observed order in spin-Hamiltonian parameters (g‖≈ 1.932, g⊥≈ 1.985, A‖≈ 175 × 10–4 cm–1 and A⊥≈ 74 × 10–4 cm–1) indicates a dx2–y2 ground state with a dodecahedral (D2d) structure. Electrochemical studies of 1a–1d show a quasi-reversible one-electron VIV–VIII reduction at ≈–0.7 V vs. saturated calomel electrode (SCE). The manganese(III) complexes [Mn(aacda)3]2a–2c are all high-spin species (µeff 5.01–5.10) and exhibit two ligand-to-metal charge-transfer bands at ≈ 610 and 530 nm. Their cyclic voltammograms reveal the presence of two quasi-Nernstian couples: E½(ox)= 0.1 V and E½(red)=–0.33 V vs. SCE due to MnIV–MnIII and MnIII–MnII electron transfers respectively.