Electrochemical and electrocatalytic properties of meso-triphenylcorrole and its complexes with Mn(III), Co(III), Cu(III), and Zn(II)

The method of cyclic voltammetry (CV) was used to compare electrochemical and electrocatalytic properties of meso-triphenylcorrole [H₃(ms-Ph)₃Cor] and also its complexes with Mn(III), Co(III), Cu(III), and Zn(II) in 0.1M KOH. Metal-localized redox transitions are detected in the complexes of Mn (III ?? IV) and (III ?? II), Co (III ?? II), Cu (III ?? II). It is shown that the manganese complex features most effective catalytic properties in the reaction of molecular oxygen electroreduction.     

Solvent and salt effects on the redox behaviour of trisacetylacetonato manganese(III)

The polarographic and voltammetric behaviour of trisacetylacetonato manganese(III) [Mn(acac)₃] has been studied in methanol, ethanol, tetrahydrofurane, butyrolactone, propylenecarbonate, N,N-dimethylformamide, acetonitrile, nitromethane, N-methylpyrrolidone(2), 1,2-dichloroethane, dichloromethane, dimethylsulfoxide and acetic acid, Mn(acac)₃ was found to undergo a reversible one-electron reduction to Mn(acac)₃^? in most of the solvents mentioned. A further reduction at very negative potentials has been also observed in several solvents. The oxidation of Mn(acac)₃ to Mn(acac)₃⁺ has been studied by cyclovoltammetry in dichloromethane, nitromethane, acetonitrile, propylenecarbonate, N-methylpyrrolidinone(2), N,N-dimethylformamide and dimethylsulfoxide. The polarographic behaviour of NaMn(acac)₃ and Mn(acac)₂ has been investigated in the seven solvents listed above as well as in methanol. The half-wave potentials and the peak potentials referred to bisbiphenylchromium(I)/bisbiphenylchromium(0) as a reference redox system were found to vary with the nature of the solvent. Conductivity studies of Mn(acac)₃ and NaMn(acac)₃ have been carried out in these solvents. U.v.-visible and near i.r. spectra have been recorded of Mn(acac)₃, NaMn(acac)₃, Mn(acac)₂ and Na(acac) in the solvents mentioned. It has further been observed that the half-wave potentials for the polarographic reduction of Mn(acac)₃ shifted to more positive values by the addition of alkali metal ions and to more negative values by the addition of halide ions. The interactions of the solvent with Mn(acac)₃ and the variation of redox potentials with both the solvent and the added electrolytes will be discussed.     

Thermochemistry of Solution of Fe(III) and Mn(III) Complexes with Natural Porphyrins

The enthalpies of solution of Fe(III), Mn(III) complexes with porphyrins of the chlorophyllgroup (chlorophyll ligand, pheophorbid, chlorin e ₆) and protoporphyrin in various organic solvents at298.15 K were determined calorimetrically. The influence of the structural features of porphyrin molecules and nature of the solvent on the enthalpy characteristics of porphyrin-solvent interaction was discussed.     

Simultaneous determination of soluble manganese(III), manganese(II) and total manganese in natural (pore)waters

A new spectrophotometric protocol was developed for the simultaneous determination of soluble Mn(III), Mn(II) and total Mn [sum of soluble Mn(III) and Mn(II)] in sediment porewaters using a water soluble meso-substituted porphyrin [α,β,γ,δ-tetrakis(4-carboxyphenyl)porphine (T(4-CP)P)]. A simple kinetic rate model is used to quantify soluble Mn(II), Mn(III) and total Mn concentrations during a metal substitution reaction. Under optimized conditions, the method accurately determines soluble Mn(II) and Mn(III) within a concentration range of 100 nM-10 μM. The detection limit of total soluble Mn is 50 nM. Using this method, soluble Mn(II) and Mn(III) concentrations were determined in standard solutions within 0.4-2% of the known values and agreed closely with results of inductively coupled plasma mass spectrometric and voltammetric analyses. The procedure was successfully applied to determine soluble Mn(II), Mn(III) and total Mn in sediment porewaters of the Lower St. Lawrence Estuary. Mn(III) represented up to 85% of the total soluble Mn pool in surface sediments.     

Electrochemistry of [(TMpyP)M(II)]4+ (X-)4 (X- = Cl- or BPh4-) and [(TMpyP)M(III)Cl]4+ (Cl-)4 in N,N-dimethylformamide where M is one of 15 different metal ions

The electrochemistry of 16 different water-soluble porphyrins of the type [(TMpyP)M(II)]4+ (X-)4 or [(TMpyP)M(III)Cl]4+ (Cl-)4 is reported in nonaqueous media where TMpyP is the dianion of meso-tetrakis(N-methylpyridiniumyl)porphyrin and X- = Cl- or BPh4-. These studies were carried out to examine the effect of the metal ion and porphyrin counterion (X-) on the electrochemical properties of the TMpyP complexes with a special emphasis being given to the overall number of electrons added and the number of electrode processes upon reduction. All of the investigated compounds with electroinactive central metal ions undergo an overall addition of six electrons. This occurs for most compounds via three two-electron-transfer steps, but more than three processes are observed for porphyrins having metal ions with a low electronegativity (e.g., Cd(II)). The first reduction of each porphyrin having an M(II) ion or an electroinactive M(III) ion yields a porphyrin dianion which is characterized by an intense band located close to 800 nm, and this reversible reduction is followed by further reductions of the 1-methyl-4-pyridyl groups at more negative potentials. Four of the compounds with electroactive central metal ions, [(TMpyP)M(III)Cl]4+(Cl-)4 (M = Co, Fe, Mn, or Au), undergo an additional reversible M(III)/M(II) process prior to reactions involving the porphyrin pi-ring system and the 1-methyl-4-pyridyl substituents.     

Organic acid solutions in the chromatography of inorganic ions VIII. Cation-exchange of Mn(II), Cd(II), Co(II), Ni(II), Zn(II), Cu(II), Fe(III), Sc(III), Y(III), Eu(III), Dy(III), Ho(III), Yb(III), Ti(IV) and Nb(V) in malate media

Summary The cation-exchange behaviour of Mn(II), Cd(II), Co(II), Ni(II), Zn(II), Cu(II), Fe(III), Sc(III), Y(III), Eu(III), Dy(III), Ho(III), Yb(III), Ti(IV) and Nb(V) in malate media at various concentrations and pH, was studied with Dowex 50 WX8 resin (200–400 mesh) in the ammonium form. Separation of Fe(III)/Cu(II), Fe(III)/Cu(II)/Zn(II), Fe(III)/Co(II)/Mn(II), Cu(II)/Ni(II)/Mn(II), Fe(III)/Cu(II)/Co(II)/Mn(II), Fe(III)/Cu(II)/Ni(II)/Cd(II), Yb(III)/Eu(III), Sc(III)/Y(III),Sc(III)/Yb(III)/Dy(III) and Nb(V)/Yb(III)/Ho(III) has been achieved, among others.This work was supported by C.N.R. of Italy.     

Cyano-bridged Mn(III)-M(III) single-chain magnets with M(III)=Co(III), Fe(III), Mn(III), and Cr(III)

A series of isostructural cyano-bridged Mn(III)(h.s.)-M(III)(l.s.) alternating chains, [Mn(III)(5-TMAMsalen)M(III)(CN)(6)]?4H(2)O (5-TMAMsalen(2-)=N,N'-ethylenebis(5-trimethylammoniomethylsalicylideneiminate), Mn(III)(h.s.)=high-spin Mn(III), M(III)(l.s.)=low-spin Co(III), Mn-Co; Fe(III), Mn-Fe; Mn(III), Mn-Mn; Cr(III), Mn-Cr) was synthesized by assembling [Mn(III)(5-TMAMsalen)](3+) and [M(III)(CN)(6)](3-). The chains present in the four compounds, which crystallize in the monoclinic space group C2/c, are composed of an [-Mn(III)-NC-M(III)-CN-] repeating motif, for which the -NC-M(III)-CN- motif is provided by the [M(III)(CN)(6)](3-) moiety adopting a trans bridging mode between [Mn(III)(5-TMAMsalen)](3+) cations. The Mn(III) and M(III) ions occupy special crystallographic positions: a C(2) axis and an inversion center, respectively, forming a highly symmetrical chain with only one kind of cyano bridge. The Jahn-Teller axis of the Mn(III)(h.s.) ion is perpendicular to the N(2)O(2) plane formed by the 5-TMAMsalen tetradentate ligand. These Jahn-Teller axes are all perfectly aligned along the unique chain direction without a bending angle, although the chains are corrugated with an Mn-N(axis) -C angle of about 144°. In the crystal structures, the chains are well separated with the nearest inter-chain M???M distance being relatively large at 9?? due to steric hindrance of the bulky trimethylammoniomethyl groups of the 5-TMAMsalen ligand. The magnetic properties of these compounds have been thoroughly studied. Mn-Fe and Mn-Mn display intra-chain ferromagnetic interactions, whereas Mn-Cr is characterized by an antiferromagnetic exchange that induces a ferrimagnetic spin arrangement along the chain. Detailed analyses of both static and dynamic magnetic properties have demonstrated without ambiguity the single-chain magnet (SCM) behavior of these three systems, whereas Mn-Co is merely paramagnetic with S(Mn)=2 and D/k(B)=-5.3?K (D being a zero-field splitting parameter). At low temperatures, the Mn-M compounds with M=Fe, Mn, and Cr display remarkably large M versus H hysteresis loops for applied magnetic fields along the easy magnetic direction that corresponds to the chain direction. The temperature dependence of the associated relaxation time for this series of compounds systematically exhibits a crossover between two Arrhenius laws corresponding to infinite-chain and finite-chain regimes for the SCM behavior. These isostructural hetero-spin SCMs offer a unique series of alternating [-Mn-NC-M-CN-] chains, enabling physicists to test theoretical SCM models between the Ising and Heisenberg limits.     

A single tripodal ligand stabilizing three different oxidation states (II, III, and IV) of manganese

A series of mononuclear Mn(II), Mn(III), and Mn(IV) complexes was prepared using a single tripodal ligand (H(3)L). Addition of a cation (NH(4)(+), K(+), Na(+)) to [Mn(III)L] showed a pronounced effect on the redox potentials. Different variants of Jahn-Teller distortion, axial elongation and compression, were observed in the Mn(III) complexes.     

Spectroscopic approach in characterization of chromium(III), manganese(II), iron(III) and copper(II) complexes with a nitrogen donor tetradentate, 14-membered azamacrocyclic ligand

The complexes of Cr(III), Mn(II), Fe(III) and Cu(II) were synthesized with the macrocyclic ligand i.e. 2,3,9,10-tetraketo-1,4,8,11-tetraazacyclotetradecane. The ligand was prepared by the [2 + 2] condensation reaction of diethyloxalate and 1,3-diamino propane. These complexes were found to have the general composition M(L)X3 and M'(L)X2 [where M = Mn(II) and Cu(II), M' = Cr(III) and Fe(III), L = ligand (N4) and X = Cl-, NO3-, 1/2SO4(2-) and [CH3COO-]. The ligand and its transition metal complexes were characterized by the elemental analyses, molar conductance, magnetic susceptibility, mass, IR, electronic, and EPR spectral studies. On the basis of IR, electronic and EPR spectral studies an octahedral geometry has been assigned for Cr(III), Mn(II) and Fe(III) and a tetragonal geometry for Cu(II) complexes.     

Structure and properties of new mixed-valent [Mn(III)2Mn(IV)3Ln(III)5O5] complexes (Ln(III) = Tm(III), Lu(III), and Yb(III))

By using 2'-hydroxyacetophenoxime, a new family of complexes with an [Mn(III)(2)Mn(IV)(3)Ln(5)O(5)] core was obtained with Ln = Tm (1), Lu (2), and Yb (3). Heterometallic Mn/Tm and Mn/Lu combinations have had no precedence so far. Studies of the magnetic properties indicate the presence of intracomplex antiferromagnetic interactions in 1 and 3, as well as a dominating ferromagnetic interaction between Mn(III) and Mn(IV) spins in 2, leading to an S(T) = 5/2 ground state.     

High-frequency and -field EPR investigation of a manganese(III) N-confused porphyrin complex, [Mn(NCTPP)(py)2

We report the first high-frequency and -field electron paramagnetic resonance (HFEPR) study of a Mn(III) N-confused porphyrin (NCP) complex (NCP is also known as inverted porphyrin or 2-aza-21-carbaporphyrin). We have found a striking variation in the electronic properties of the S = 2 Mn(III) ion coordinated by NCP compared to other Mn(III) porphyrinoid complexes. Thus, inversion of a single pyrrole ring greatly changes the equatorial ligand field exerted and leads to large magnitudes of both the axial and rhombic zero-field splitting [respectively, D = -3.084(3) cm(-1), E = -0.608(3) cm(-1)], which are unprecedented in other Mn(III) porphyrinoids.     

Visible and near-infrared intense luminescence from water-soluble lanthanide [Tb(III), Eu(III), Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), Er(III)] complexes

The synthesis of a new ligand (1) containing a single phenanthroline (phen) chromophore and a flexibly connected diethylenetriamine tetracarboxylic acid unit (DTTA) as a lanthanide (Ln) coordination site is reported [1 is 4-[(9-methyl-1,10-phenantrol-2-yl)methyl]-1,4,7-triazaheptane-1,1,7,7-tetraacetic acid]. From 1, an extended series of water-soluble Ln.1 complexes was obtained, where Ln is Eu(III), Tb(III), Gd(III), Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), and Er(III). The stoichiometry for the association was found 1:1, with an association constant K(A) > or = 10(7) s(-1) as determined by employing luminescence spectroscopy. The luminescence and photophysical properties of the series of lanthanide complexes were investigated in both H2O and D2O solutions. High efficiencies for the sensitized emission, phi(se), in air-equilibrated water were observed for the Ln.1 complexes of Eu(III) and Tb(III) in the visible region (phi(se) = 0.24 and 0.15, respectively) and of Sm(III), Dy(III), Pr(III), Ho(III), Yb(III), Nd(III), and Er(III) in the vis and/or near-infrared region [phi(se) = 2.5 x 10(-3), 5 x 10(-4), 3 x 10(-5), 2 x 10(-5), 2 x 10(-4), 4 x 10(-5), and (in D2O) 4 x 10(-5), respectively]. For Eu.1 and Tb.1, luminescence data for water and deuterated water allowed us to estimate that no solvent molecules (q) are bound to the ion centers (q = 0). Luminescence quenching by oxygen was investigated in selected cases.     

New class of potent catalysts of O2.-dismutation. Mn(III) ortho-methoxyethylpyridyl- and di-ortho-methoxyethylimidazolylporphyrins

Three new Mn(III) porphyrin catalysts of O2.-dismutation (superoxide dismutase mimics), bearing ether oxygen atoms within their side chains, were synthesized and characterized: Mn(III) 5,10,15,20-tetrakis[N-(2-methoxyethyl)pyridinium-2-yl]porphyrin (MnTMOE-2-PyP(5+)), Mn(III)5,10,15,20-tetrakis[N-methyl-N'-(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTM,MOE-2-ImP(5+)) and Mn(III) 5,10,15,20-tetrakis[N,N'-di(2-methoxyethyl)imidazolium-2-yl]porphyrin (MnTDMOE-2-ImP(5+)). Their catalytic rate constants for O2.-dismutation (disproportionation) and the related metal-centered redox potentials vs. NHE are: log k(cat)= 8.04 (E(1/2)=+251 mV) for MnTMOE-2-PyP(5+), log k(cat)= 7.98 (E(1/2)=+356 mV) for MnTM,MOE-2-ImP(5+) and log k(cat)= 7.59 (E(1/2)=+365 mV) for MnTDMOE-2-ImP(5+). The new porphyrins were compared to the previously described SOD mimics Mn(III) 5,10,15,20-tetrakis(N-ethylpyridinium-2-yl)porphyrin (MnTE-2-PyP(5+)), Mn(III) 5,10,15,20-tetrakis(N-n-butylpyridinium-2-yl)porphyrin (MnTnBu-2-PyP(5+)) and Mn(III) 5,10,15,20-tetrakis(N,N'-diethylimidazolium-2-yl)porphyrin (MnTDE-2-ImP(5+)). MnTMOE-2-PyP(5+) has side chains of the same length and the same E(1/2), as MnTnBu-2-PyP(5+)(k(cat)= 7.25, E(1/2)=+ 254 mV), yet it is 6-fold more potent a catalyst of O2.-dismutation , presumably due to the presence of the ether oxygen. The log k(cat)vs. E(1/2) relationship for all Mn porphyrin-based SOD mimics thus far studied is discussed. None of the new compounds were toxic to Escherichia coli in the concentration range studied (up to 30 microM), and protected SOD-deficient E. coli in a concentration-dependent manner. At 3 microM levels, the MnTDMOE-2-ImP(5+), bearing an oxygen atom within each of the eight side chains, was the most effective and offered much higher protection than MnTE-2-PyP(5+), while MnTDE-2-ImP(5+) was of very low efficacy.     

Polarographic and voltammetric studies of tetrabutylammonium hexacyanoferrate(III) and tetrabutylammonium hexacyanomanganate(III) in non-aqueous solvents

Tetrabutylammonium hexacyanomanganate(III) [(bu₄N)₃Mn(CN)₆] has been studied by polarography and cyclic voltammetry in formamide, methanol, ethanol, N-methylformamide, dichloromethane, dimethylsulfoxide, acetonitrile, N,N-dimethylformamide, propylenecarbonate, butyrolactone, N-methylthiopyrrolidone(2), N,N-dimethylthioformamide, 1.2-dichloroethane, N-methylpyrrolidone(2) and tetramethylenesulfone. Similar studies have been carried out for tetrabutylammonium hexacyanoferrate(III) [(bu₄N)₃ Fe(CN)₆] in the solvents formamide, N-methylformamide, dichloromethane, butyrolactone, N-methylthiopyrrolidone(2) and tetramethylenesulfone. The half-wave potentials of the reductions (bu₄N)₃Mn(CN)₆ to (bu₄N)₄Mn(CN)₆ and (bu₄N)₃Fe(CN)₆ to (bu₄N)₄Fe(CN)₆ versus bisbiphenylchromium(I)/bisbiphenylchromium(0) as a reference redox system were found to vary with the nature of the solvent. Comparison with data previously obtained for the respective tetraethylammonium salts of hexacyanoferrate(III) and hexacyanomanganate(III) have shown that the redox behaviour of these compounds is influenced by both the solvent and the tetraalkylammonium ions. Correlations exist between the half-wave potentials of (bu₄N)₃Mn(CN)₆ and (bu₄N)₃Fe(CN)₆ and both the acceptor number of the solvents and the free enthalpies of transfer of the chloride ion. The results are discussed in the concept of donor-acceptor interactions.     

Unusual multi-step sequential Au(III)/Au(II) processes of gold(III) quinoxalinoporphyrins in acidic non-aqueous media

The electrochemistry of gold(III) mono- and bis-quinoxalinoporphyrins was examined in CH(2)Cl(2) or PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) before and after the addition of trifluoroacetic acid to solution. The investigated porphyrins are represented as Au(PQ)PF(6) and Au(QPQ)PF(6), where P is the dianion of the 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and Q is a quinoxaline group fused to a β,β'-pyrrolic position of the porphyrin macrocycle; in Au(QPQ)PF(6) there is a linear arrangement where the quinoxalines are fused to pyrrolic positions that are opposite each other. The porphyrin without the fused quinoxaline groups, Au(P)PF(6), was also investigated under the same solution conditions. In the absence of acid, all three gold(III) porphyrins undergo a single reversible Au(III)/Au(II) process leading to the formation of a Au(II) porphyrin which can be further reduced at more negative potentials to give stepwise the Au(II) porphyrin π-anion radical and dianion, respectively. However, in the presence of acid, the initial Au(III)/Au(II) processes of Au(PQ)PF(6) and Au(QPQ)PF(6) are followed by an internal electron transfer and protonation to regenerate new Au(III) porphyrins assigned as Au(III)(PQH)(+) and Au(III)(QPQH)(+). Both protonated gold(III) quinoxalinoporphyrins then undergo a second Au(III)/Au(II) process at more negative potentials. The electrogenerated monoprotonated monoquinoxalinoporphyrin, Au(II)(PQH), is then further reduced to its π-anion radical and dianion forms, but this is not the case for the monoprotonated bis-quinoxalinoporphyrin, Au(II)(QPQH), which accepts a second proton and is rapidly converted to Au(III)(HQPQH)(+) before undergoing a third Au(III)/Au(II) process to produce Au(II)(HQPQH) as a final product. Thus, Au(P)PF(6) undergoes one metal-centered reduction while Au(PQ)PF(6) and Au(QPQ)PF(6) exhibit two and three Au(III)/Au(II) processes, respectively. These unusual multistep sequential Au(III)/Au(II) processes were monitored by thin-layer spectroelectrochemistry and a reduction/oxidation mechanism for Au(PQ)PF(6) and Au(QPQ)PF(6) in acidic media is proposed.     

Studies on some salicylaldehyde Schiff base derivatives and their complexes with Cr(III), Mn(II), Fe(III), Ni(II) and Cu(II)

The formation constants of some transition metal ions Cr(III), Mn(II), Fe(III), Ni(II) and Cu(II) binary complexes containing Schiff bases resulting from condensation of salicylaldehyde with aniline (I), 2-aminopyridine (II), 4-aminopyridine (III) and 2-aminopyrimidine (IV) were determined pH-metrically in ethanolic medium (80%, v/v). The formation constants were determined for all binary complexes. The important infrared (IR) spectral bands corresponding to the active groups in the four ligands and the solid complexes under investigation were studied. The solid complexes have been synthesized and studied by thermogravimetric analysis. The thermal dehydration and decomposition of these complexes were studied kinetically using the integral method applying the Coats-Redfern equation. It was found that the thermal decomposition of the complexes follow second order kinetics. The thermodynamic parameters of the decomposition are also reported. The electronic absorption spectra of the investigated ligands were carried out to determine the pK(a) values spectrophotometrically.     

Chloromethylthio-Substituted Chromium(III), Cobalt(III), and Aluminum(III) Acetylacetonates

SCl-Substituted chromium(III), cobalt(III), and aluminum(III) acetylacetonates react with carbenes similarly to organylsulfenyl chlorides, with insertion of the carbenes into the S-Cl bond. Reaction with diazomethane gave chloromethylthio-substituted complexes whose structure was proved by IR, ¹H NMR, and mass spectroscopy. The thermal oxidative degradation of the complexes was studied. Chlorination of SCH₃-substituted acetylacetonates does not yield such complexes.     

A study on the solution and gas-phase chemistry of Mn(III) and Fe(III) tetraarylporphyrin complexes by fast-atom bombardment mass spectrometry.: 2: Synthesis and characterization of molecular complexes

The chemical behavior of Fe(III) and Mn(III) tetraarylporphorin (TAP) complexes with N-alkylimidazoles and other suitable ligands was studied by direct reaction in the fast-atom bombardment matrix and in the gas phase. The coordination reaction occurs at the metal center and yields molecular adducts of porphyrin/ligand (PL) and PL₂ stoichiometry. Coordinative competition between free and covalently linked ligands can be used to probe the conformation of “tailed” Mn(III)-TAP.     

Zn(II), Ni(II), Cu(II), and Fe(III) complexes of potentially bimetalating tris(pyridine- and imidazole-appended) picket-fence naphthylporphyrins with benzyl ether spacers: implications for cytochrome c oxidase active-site modeling

Two new unsymmetrical picket-fence naphthylporphyrin ligands, 1 and 2, and several of their metalated porphyrinato complexes have been synthesized as precursor model compounds for the binuclear (Fe/Cu) cytochrome c oxidase (CcO) active site. 1 and 2 have a naphthylporphyrin superstructure that has been specifically incorporated to confer long-term configurational stability to the atropisomeric products. The two picket-fence porphyrin ligands also bear covalently linked, axially offset tris(heterocycle) coordination sites for a copper ion, much like that found in the native enzyme. Monometallic porphyrin complexes [M = Zn(II), Ni(II), Cu(II), and Fe(III)] of the pyridine-appended ligand 1 have been prepared and spectroscopically and magnetically characterized. An unusual monomeric iron(III) hydroxo porphyrin complex was isolated upon workup of the compound formed under ferrous sulfate/acetic acid reflux conditions. There is general difficulty in forming binuclear complexes of 1, which is attributed to the conformational flexibility of the benzyl ether type picket spacers. The potential of ligands such as 1 and 2 for future CcO active-site modeling studies is considered.