Irreversible One-electron Research Articles

Bio-based hierarchical vertically aligned 2D ZnO nanostructures for ultra selective electrochemical sensing of p-Chloroaniline

Herein, we report synthesis of hierarchical 2D vertically aligned nanostructures (VANs) of ZnO onto the surface of FTO glass via bioinspired route for targeted electrocatalytic sensing of p-Chloroaniline (p-CA). The phytochemicals present in vegetal extract of Sechiun edule were employed to grow and form stable ZnO VANs onto FTO (ZnO-VANs(bio)/FTO). The morphological characterizations of ZnO-VANs(bio)/FTO showed the formation of vertical nanostructures (length ∼ 820 nm) with high density and good crystallinity. It exhibited an increased surface roughness 2.2-fold higher than seeded FTO (ZnO@seed/FTO). The ZnO-VANs(bio)/FTO@pH7 electrode showed a decrease in electron transfer resistance from 7.78 to 3.02 MΩ with 1.2-fold increase in effective surface area than FTO(bare). The oxidation of p-CA included an irreversible one-electron oxidation mechanism with formal potential (E0) of 0.978 V vs. Ag/AgCl and standard heterogeneous rate constant (k) of 0.737 s−1. ZnO-VANs(bio)/FTO@pH7 exhibited remarkable electrocatalytic sensing of p-CA with low limit of detection (0.021 µM) and excellent sensitivity (0.194 µA·µM−1·cm−2) between a linear window of 2–200 µM. The detection limit of fabricated sensor for p-CA sensing was about 2-fold lower than FTO(bare). Chlorobenzene, carbendazim, profenofos, nitrite, 4-nitrophenol, bisphenol a, and mercury could not interfere p-CA sensing, and only 4.7% reduction in peak current was observed upto 9th cycle. The ZnO-VANs(bio)/FTO@pH7 could efficiently sense p-CA in spiked river and residential water samples, comparable with HPLC (max 4.6% deviation). Therefore, ZnO VANs synthesized in a bio-based route is a potential electrocatalyst for binder-free sensor development with high stability and low detection limit for sensing pesticides and their intermediates.

Synthesis, spectral characterization and fluorescence study of functional [5]Helicene derivatives: Experimental and theoretical investigation

Two D–π–A structured [5]Helicene derivatives, labeled HL1 and HL2, were synthesized in a high overall yield through a short synthetic photochemical route using mild conditions. Structural analyses, photophysical and electrochemical properties and theoretical studies were investigated in a comparative manner. These compounds show deep green color in both solution and solid state and they are highly soluble in common organic solvents and even in non-polar solvents such as toluene and cyclohexane. The shapes of experimental and calculated absorption spectra of these helical scaffolds in a dichloromethane solution were found to be approximately the same, indicating that the alkoxy groups could efficiently prevent intermolecular contact and aggregation. These Helicene derivatives were found to emit a green light in the fluorescence process inducing a Stokes shift of 0.39–0.41 eV (3200–3303 cm−1). They exhibit an irreversible one-electron oxidation and one-electron reduction and their HOMO and LUMO energy levels were electrochemically estimated to be, respectively, -5.88 and -3.20 eV for HL1 and -5.76 and -3.26 eV for HL2. Complementary density functional theory (DFT) calculations have also been performed in order to rationalize the different behaviors in the target [5]Helicenes and to gain insight into their geometric, electronic and spectroscopic properties. The corresponding calculated results were found to be in good agreement with the experimental data.

On the Aqueous Chemistry of the UIV -DOTA Complex.

The 1,4,7,10-tetrazacyclodecane-1,4,7,10-tetraacetic acid (DOTA) aqueous complex of UIV with H2 O, OH- , and F- as axial ligands was studied by using UV/Vis spectrophotometry, ESI-MS, NMR spectroscopy, X-ray crystallography, and electrochemistry. The UIV -DOTA complex with either water or fluoride as axial ligands was found to be inert to oxidation by molecular oxygen, whereas the complex with hydroxide as an axial ligand slowly hydrolyzed and was oxidized by dioxygen to a diuranate precipitate. The combined data set acquired shows that, although axial substitution of fluoride and hydroxide ligands instead of water does not seem to significantly change the aqueous DOTA complex structure, it has an important effect on the electronic configuration of the complex. The UIV /UIII redox couple was found to be quasi-reversible for the complex with both axially bonded H2 O and hydroxide, but irreversible for the complex with axially bonded fluoride. Intriguingly, binding of the axial fluoride renders the irreversible one-electron UV /UIV oxidation of the [UIV (DOTA)(H2 O)] complex quasi-reversible, which suggests the formation of the short-lived pentavalent form of the complex, an aqueous non-uranyl chelated UV cation.

Mononuclear Ni(II) Complexes with a S3O Coordination Sphere Based on a Tripodal Cysteine-Rich Ligand: pH Tuning of the Superoxide Dismutase Activity.

The superoxide dismutase (SOD) activity of mononuclear NiII complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including 1H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S3O coordination, with the three thiolates of the three cysteine moieties from L3S coordinated to the NiII ion, together with either a water molecule at physiological pH, as [NiL3S(OH2)]-, or a hydroxo ion in more basic conditions, as [NiL3S(OH)]2-. The 1H NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irreversible one-electron oxidation process assigned to the NiII/NiIII redox system with Epa = 0.48 and 0.31 V versus SCE for NiL3S(OH2) and NiL3S(OH), respectively. The SOD activity of both complexes has been tested. On the basis of the xanthine oxidase assay, an IC50 of about 1 μM has been measured at pH 7.4, where NiL3S(OH2) is mainly present (93% of the NiII species), while the IC50 is larger than 100 μM at pH 9.6, where NiL3S(OH) is the major species (92% of the NiII species). Interestingly, only NiL3S(OH2) displays SOD activity, suggesting that the presence of a labile ligand is required. The SOD activity has been also evaluated under catalytic conditions at pH 7.75, where the ratio between NiL3S(OH2)/ NiL3S(OH) is about (86:14), and a rate constant, kcat = 1.8 × 105 M-1 s-1, has been measured. NiL3S(OH2) is thus the first low-molecular weight, synthetic, bioinspired Ni complex that displays catalytic SOD activity in water at physiological pH, although it does not contain any N-donor ligand in its first coordination sphere, as in the NiSOD. Overall, the data show that a key structural feature is the presence of a labile ligand in the coordination sphere of the NiII ion.

Biocompatibility, in Vitro Antiproliferative, and in Silico EGFR/VEGFR2 Studies of Heteroleptic Metal(II) Complexes of Thiosemicarbazones and Naproxen.

Eight heteroleptic nickel(II) and copper(II) complexes of the type [M(L1-4)(nap)2] (1-8), where L1-4 = 2-(1-(4-substitutedphenyl)ethylidene)hydrazinecarbothioamide, nap = naproxen, and M = Ni(II) or Cu(II), have been synthesized and characterized. UV-vis and EPR spectral studies showed distorted octahedral geometry around metal(II) ions. The cyclic voltammogram of complexes 1-8 displayed an irreversible one-electron transfer process in the cathodic region (Epc = -0.66 to -1.43 V), and nickel(II) complexes 1-4 displayed an irreversible one-electron oxidation process in the anodic region (Epa = 0.75 to 1.10 V). The obtained magnetic moment values (1.82-1.93 μB) for copper(II) complexes 5-8 indicate distortion from octahedral geometry, which is further supported by EPR studies. The geometry of the complexes is retained in both solid and solution phases as evidenced from UV-vis and EPR studies. All the complexes showed stability for almost 72 h in biologically relevant solutions. The reducing ability of the copper(II) complexes in the presence of ascorbic acid was analyzed by UV-vis and cyclic voltammetry techniques, which indicates the reduction of the copper(II) to a copper(I) center, and possible interaction within the cells. An in vitro antiproliferative study revealed the nontoxic nature of complexes to normal human dermal fibroblast (NHDF) up to a concentration of 100 ng/mL. The antiproliferative activity of the complexes was tested against three cancerous (human breast adenocarcinoma (MCF-7), hepatoma (HepG2), and lung (A549)) cell lines using MTT reduction assay, which showed enhanced activity for complexes 4 and 8 containing the hydrophobic substituent. Apoptotic and cellular uptake studies showed that complex 8 is readily taken up by HepG2 cell lines and induces ROS-mediated mitochondrial and caspase-dependent apoptosis. In silico studies indicated hydrogen bonding, hydrophobic, and π-pair (π-π, π-σ, and π-cation) interactions between the complexes and EGFR/VEGFR2 kinase receptors.

Organometallic phthalazin-1(2H)-ones: Electrochemistry and advantage of solvothermal synthesis

o-Ferrocenylcarbonylbenzoic acid (η5-C5H5)Fe(η5-C5H4C(O)C6H4COOH) (I) and o-cymantrenylcarbonylbenzoic acid (II) were obtained from ferrocene or (η5-C5H5)Mn(CO)3, respectively, by the Friedel–Crafts reaction with phthalic anhydride. Methyl o-ferrocenylcarbonylbenzoate (η5-C5H5)Fe(η5-C5H4C(O)C6H4COOMe (III) and methyl o-cymantrenylcarbonylbenzoate (CO)3Mn(η5-C5H4C(O)C6H4COOMe) (IV) were synthesized from I and II, respectively using dimethyl carbonate (DMC) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to promote the methylation reaction. 4-Ferrocenylphthalazin-1(2H)-one (V) and 4-cymantrenylphthalazin-1(2H)-one (VI) were obtained by the interaction of hydrazine hydrate with I and II, respectively, by the prolonged reflux or convenient solvothermal synthesis. Cyclic voltammetry measurements showed that compounds III and V undergo reversible one-electron oxidation, localized presumably at the ferrocene unit. For III the irreversible one-electron oxidation is apparently associated with the oxidation of the benzoate fragment. All new compounds were characterized by spectroscopic methods and the molecular structures of II and III were determined by X-ray diffraction analysis.

Synthesis and redox chemistry of Pd(ii) complexes of a pincer verdazyl ligand.

A series of palladium(ii) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL (L = Cl, CH3CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(ii) complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH3CN, 12: L = Cl) or its closed-shell monoanionic charge state (10: L = CH3CN, 9: L = Cl). The charge state of the ligand was determined using X-ray crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically interconverted between radical ligand (6, 12) and reduced, closed-shell anion (9, 10) using decamethylferrocene as the reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.

Homoleptic copper(II) complexes of an osazone. Reversal of the coordination ability of hexafluorophosphate and perchlorate anions

Using the 1:1 condensate of benzil and 2-hydrazinopyridine as the ligand LH (H: dissociable proton), three copper(II) complexes of the type [Cu(L)(LH)]ClO4 (1), Cu(L)(LH)PF6 (2) and CuL2 (3) are synthesized. From the X-ray crystal structures, it is found that 1 is distorted square pyramidal, whereas complex 2 in distorted octahedral via coordination of PF6− anion to the metal with a Cu(II)F bond distance of 2.722(3)Å. Although ClO4− is more coordinating than PF6−, a reversal of their coordinating ability is observed here. The CuN bond trans to CuF is somewhat longer (2.304Å). Complex 3 is a distorted octahedral homoleptic neutral mer complex. In 3 two axial CuN bond lengths are shorter (1.967Å and 1.978Å) than the equatorial one giving rise to a compressed octahedron. EPR spectrum of 1 is isotropic (g=2.057), but g|| (2.138)>g⊥ (2.027) in 2 showing Jahn-Teller elongation. On the other hand, in 3, g||<g⊥ (1.986, 2.112), which is another type of manifestation of the Jahn-Teller effect. In the density functional theory calculations, the SOMO of 2 shows a (dx2−y2)1 ground state in contrary to 3 where (dz2)1 is the ground state. Thus experimental and theoretical results are in one-to-one-correspondence. In cyclic voltammetry at an Au electrode whose surface is modified by thiocyanate, 3 displays an irreversible one-electron oxidation with an anodic peak potential of 0.98V vs normal hydrogen electrode.

Metal-Free Peralkylation of the closo-Hexaborate Anion

The synthesis of fully alkylated closo-hexaborate dianions is reported. The reaction of [NBu4][B6H6Hfac], benzyl bromide, and triethylamine under microwave heating conditions affords persubstituted [NBu4][B6(CH2Ar)6Hfac] (Ar = C6H5, 4–Br-C6H4), which have been isolated and characterized by NMR spectroscopy, mass spectrometry, single-crystal X-ray diffraction, and other spectroscopic techniques. Electrochemical studies of these clusters reveal an irreversible one-electron oxidation, likely indicating degradative cage rupture. The observed metal-free alkylation is proposed to proceed as a consequence of the pronounced nucleophilic character of the hexaborate anion. This work represents the first example of a perfunctionalized hexaborate cluster featuring B–C bonds.

A Chiral Hemiporphyrazine Derivative: Synthesis and Chiroptical Properties.

The synthesis of an optically active hemiporphyrazine with chiral binaphthyl substituents (1) is reported, providing the first example of the incorporation of an intrinsically chiral moiety into the macrocyclic core of a hemiporphyrazine analogue. A negative circular dichroism (CD) signal is observed in the 325-450 nm region of the CD spectrum of (S,S)-1, while mainly positive bands are observed in the 220-325 nm region. Mirror symmetry is observed across the entire wavelength range of the CD spectra of (R,R)-1 and (S,S)-1. An irreversible one-electron oxidation wave with an onset potential at 1.07 V is observed by cyclic voltammetry, along with a reversible one-electron reduction wave at -0.85 V. Density functional calculations reproduce the experimentally observed data and trends, and provide further insight into the nature of the electronic transitions.

Tetranuclear dioxomolybdenum (VI) cluster anion, hydrogen bond interaction with 1,2-di(4-pyridyl)ethylene

Tetranuclear dioxomolybdenum cluster complex (TMC) [C12H11N2+]2[Mo4O10(OCH3)62−]·CH3OH has been synthesized by the reaction of MoO2(acac)2 with 1,2-di(4-pyridyl)ethylene. The cluster core is composed of four molybdenum atoms arranged in the rhombus shape bridged by two oxo ligands, one bridging O, and three methanol molecules. The TMC [C12H11N2+]2[Mo4O10(OCH3)62−]·CH3OH is comprised of two moieties of pyridyl cation [C12H11N2+]2 (1,2-di(4-pyridyl)ethylene) and cluster anion [Mo4O10(OCH3)62−]. The cluster is stabilized by two intermolecular interactions (hydrogen bond and π–π stacking interactions). The 1D chain is formed as a result of the hydrogen interaction between (1, 2-di(4-pyridyl)ethylene) and [Mo4O10(OCH3)62−]. Thermogravimetric analysis clearly indicated the thermal stability of the cluster. Electrochemical measurement showed two irreversible reduction processes at scan rates (−1 to −2 V) and an irreversible one-electron oxidation at +0.79 V. Optical absorption measurement shows that the fundamental absorption edge obeys Tauc’s relation for the allowed non-direct transition. Optical band gap (Eg) values equal 2.4 eV.

Irreversible One-Electron Reduction of Dioxygen in Ionic Liquid Containing Olefinic Substituents

Irreversible One-Electron Reduction of Dioxygen in Ionic Liquid Containing Olefinic Substituents

Electrochemical behavior of some ethylenedioxycoumarins: Cathodic dimerization

Electrochemical behavior of recently synthesized 7,8-ethylenedioxy-4-methyl-2(H)-1-benzopyran-2-one, (1), 7,8-ethylenedioxy-2(H)-1-benzopyran-2-one, (2), and 6,7-ethylenedioxy-4-methyl-2(H)-1-benzopyran-2-one, (3) was studied using cyclic voltammetry at a hanging mercury drop electrode and controlled-potential coulometric techniques. The results indicated that an irreversible one-electron cathodic peak was observed for each compound in the potential range from 0.0 to −2.0V. The coulometric experiments were performed in a low yield and the products were identified by FT-IR spectrometry, 1H NMR and mass spectrometry. The coulometric results indicated a dimeric product for each compound. A mechanism for the electrodimerization pathway was proposed.

Assembly of heterobimetallic Ni(II)-Ln(III) (Ln(III) = Dy(III), Tb(III), Gd(III), Ho(III), Er(III), Y(III)) complexes using a ferrocene ligand: slow relaxation of the magnetization in Dy(III), Tb(III) and Ho(III) analogues.

A family of dinuclear 3d-4f heterobimetallic complexes [LNi(H2O)(μ-OAc)Ln(NO3)2]·CH3CN; {Ln = Dy(III) (1), Tb(III) (2), Ho(III) (3), Gd(III) (4), Er(III) (5), Y(III) (6)} have been synthesized by utilizing a ferrocene-based, dual compartmental ligand H2L. 1-6 are isostructural and crystallize in the triclinic (P1) space group. In these complexes Ni(II) is present in the inner coordination sphere of the dianionic [L](2-) ligand; Ln(III) is encapsulated in the outer coordination pocket. Ni(II) shows a 2N, 4O coordination environment in a distorted octahedral geometry, while the Ln(III) ion possesses a 9O coordination environment in a distorted tricapped trigonal prismatic geometry. ESI-MS studies suggest that the structural integrity of 1-6 is retained in solution. Electrochemical studies reveal that these complexes show a reversible one-electron response typical of the ferrocene motif along with an irreversible one-electron oxidation involving the Ni(II)/Ni(III) couple. Magnetic studies revealed the presence of ferromagnetic exchange coupling between Ni(II) and Ln(III) centers as shown by the increase of χMT value upon cooling below 50 K for compounds 1, 2, 4 and 5. Further, dynamic magnetic susceptibility measurements (1-3) confirm the absence of an out-of-phase (χ'') signal at zero dc fields. However, when these measurements were carried out at 1000 Oe dc field the χ'' signal was observed, although maxima could not be detected up to 2 K.

New Voltammetry-Based Analytical Method for Indirect Determination of Procymidone in Brazilian Apples

Procymidone, a potentially carcinogenic and mutagenic pesticide, can contribute to environmental and human contamination when applied to apple crops. In this work, we propose a reliable and sensitive method to determine procymidone in Brazilian apples. The method involves differential pulse (DPV) and square-wave voltammetry (SWV) techniques on a glassy carbon electrode. In a supporting electrolyte solution of 0.5 mol L−1 NaOH, procymidone undergoes an irreversible one-electron oxidation at +1.42 V by cyclic voltammetric vs. Ag|AgCl, KCl 3 M reference electrode. The proposed DPV and SWV methods have a good linear response in the 8.00–20.0 mg L−1 range, with limits of detection (LOD) of 0.678 and 0.228 mg L−1, respectively, in the absence of the matrix. We obtained improved LOD (0.097 mg L−1) in the presence of apple matrix and the supporting electrolyte solution. We used three commercial apple samples to evaluate recovery, and we achieved recovery percentages ranging from 94.6 to 110 % for procymidone determinations. We also tested the proposed voltammetric method for reproducibility, repeatability, and potential interferents, and the results were satisfactory for electroanalytical purposes.

Template Synthesis of Polyaza Macrocyclic Copper(II) and Nickel(II) Complexes: Spectral Characterization and Antimicrobial Studies

The template synthesis of copper(II) and nickel(II) complexes derived from 2,6-diformyl-4-methylphenol with diethylenetriamine or 1,2-bis(3-aminopropylamino)ethane produce the 12-membered N3O and 17-membered N4O macrocyclic complexes, respectively. The geometry of the complexes has been determined with the help of electronic and EPR spectroscopic values and found to be five coordinated square pyramidal and, six coordinated distorted tetragonal for 12-membered and 17-membered macrocyclic complexes, respectively. Electrochemical studies of the mononuclear N3O and N4O copper(II) complexes show one irreversible oneelectron reduction wave at Epc = –1.35 and –1.15 V respectively, and the corresponding nickel(II) complexes show irreversible one-electron reduction wave at Epc = –1.25 and –1.22 V, respectively. The nickel(II) complexes show irreversible one-electron oxidation wave at Epa = +0.84 and +0.82 V, respectively. All the complexes were evaluated for in vitro antimicrobial activity against the human pathogenic bacteria and fungi.

Redox coupled-spin crossover in cobalt β-diketonate complexes: Structural, electrochemical and computational studies

Structural, electrochemical, spectroelectrochemical, magnetic and spectroscopic studies are reported for the octahedral cobalt β-diketonate complexes, [Co(β-diketonate)2(N–N)] {β-diketonate=2,2,6,6-tetramethylheptane-3,5-dionate (tmhd); N–N=1,10-phenanthroline (phen) 1, 2,2′-bipyridine (2,2′-bpy) 2 and dimethylaminoethylamine (dmae) 3; β-diketonate=1,3-diphenylpropane-1,3-dionate (dbm); N–N=phen 4, 2,2′-bpy 5, dmae 6}. X-ray crystallographic studies of the redox pair [Co(tmhd)2(2,2′-bipy)]0/+2/2+ show a shortening of the Co-ligand bond lengths by between 0.18 and 0.22Å upon oxidation and a significantly more regular octahedral geometry around the cobalt in the cation consistent with spin crossover in addition to a change in oxidation state. Cyclic voltammetry of 1–6 reveals an irreversible one-electron oxidation to CoIII with large peak separations between the oxidation and reduction peaks, indicative of redox coupled-spin crossover (RCSCO); i.e. [Co(β-diketonate)2(N–N)] (S=3/2)↔[Co(β-diketonate)2(N–N)]++e− (S=0). Moreover, the complexes represent rare examples of RCSCO species with a CoO4N2 coordination sphere. The tmhd complexes are more easily oxidized than the respective dbm analogues with the oxidation peak potentials in the order bipy<phen<dmae. Oxidation of 1–6 with AgBF4 yields the corresponding CoIII cations, [Co(β-diketonate)2(N–N)]BF41+–6+ which has been confirmed by 1H NMR spectroscopy. Spectroelectrochemistry of the redox pairs [Co(β-diketonate)2(N–N)]0/+ is consistent with the isolated compounds being identical to the species formed at the electrode. Theoretical studies reveal that the SOMO is essentially metal d-orbital and β-diketonate based, consistent with the strong effect of the β-diketonate ligand on the oxidation potential. In addition, there are substantial changes in the relative stabilities of the various spin states compared with [Co(tacn)2]2+/3+ such that the high spin states become more accessible. The above results are consistent with a square scheme mechanism.

New Acyclic Schiff-Base Nickel(II) Complexes and their Electrochemical, Kinetic, and Antimicrobial Studies

Five new nickel(II) complexes were synthesized. All the complexes were characterized by elemental and spectral analysis. Electronic spectra of the complexes show d–d transition in the range of 500–800 nm. Electrochemical studies of the complexes show an irreversible one-electron reduction process around –0.65 to –0.91 V and an irreversible one-electron oxidation process around 0.87 to 1.00 V. The reduction and oxidation potential of nickel(II) complexes shifts toward cathodic and anodic directions, respectively, upon increasing the chain length. The catalytic activity of the nickel(II) complexes on the hydrolysis of 4-nitrophenylphosphate was determined. All the nickel(II) complexes were screened for antibacterial activity.

Chemistry and reactivity of dinuclear manganese oxamate complexes: Aerobic catechol oxidation catalyzed by high-valent bis(oxo)-bridged dimanganese(IV) complexes with a homologous series of binucleating 4,5-disubstituted- o-phenylenedioxamate ligands

The high-valent bis(oxo)-bridged dimanganese(IV) complexes with the series of binucleating 4,5-X 2- o-phenylenebis(oxamate) ligands (opbaX 2; X = H, Cl, Me) ( 1a– c) have been synthesized and characterized structurally, spectroscopically and magnetically. Complexes 1a– c possess unique Mn 2(μ-O) 2 core structures with two o-phenylenediamidate type additional bridges which lead to exceptionally short Mn–Mn distances (2.63–2.65 Å) and fairly bent Mn–O–Mn angles (94.1°–94.6°). The cyclovoltammograms of 1a– c in acetonitrile (25 °C, 0.1 M Bu 4NPF 6) show an irreversible one-electron oxidation peak at moderately high anodic potentials ( E ap = 0.50–0.85 V versus SCE), while no reductions are observed in the potential range studied (down to −2.0 V versus SCE). These dinuclear manganese oxamate complexes are excellent catalysts for the aerobic oxidation of 3,5-di- tert-butylcatechol to the corresponding o-quinone in acetonitrile at 25 °C. The order of increasing catecholase activity ( k obs) with the electron donor character of the ligand substituents as 1b (X = Cl) < 1a (X = H) < 1c (X = Me) correlates with Hammett σ + values ( ρ = −0.95). A mechanism involving initial activation of the catechol substrate by coordination to the dimetal center and subsequent oxidation to quinone by O 2 is proposed, which is consistent with the observed saturation kinetics.

Fiber-Optic Infrared Spectroelectrochemical Studies of Six-Coordinate Manganese Nitrosyl Porphyrins in Nonaqueous Media

The redox behavior of the six-coordinate (por)Mn(NO)(1-MeIm) (por = tetraphenylporphyrin dianion (TPP), tetratolylporphyrin dianion (TTP), or tetra-p-methoxyphenylporphyrin dianion (T(p-OMe)PP)) complexes were examined by cyclic voltammetry at room temperature and at -78 degrees C in two nonaqueous solvents (CH2Cl2 and THF) at a Pt disk electrode. In CH2Cl2 at room temperature, the compounds undergo four oxidations and two reductions within the solvent limit; in THF, the compounds undergo one oxidation and three reductions. In both solvents, the first oxidation represents a chemically irreversible one-electron process involving the rapid loss of nitric oxide. The oxidation occurs at the MnNO site as judged from bulk electrolysis, UV-vis spectroscopy at room temperature, and IR spectroelectrochemistry at room temperature and at -78 degrees C. The second oxidation, accessible in CH2Cl2, is also chemically irreversible and occurs at the porphyrin ring; the third and the fourth oxidations are, on the other hand, chemically reversible but also occur at the porphyrin ring. The first reduction is chemically irreversible in CH2Cl2, occurs at the porphyrin ring, and is followed by loss of NO. In THF, the first reduction is chemically reversible and is followed by reversible loss of NO.