Synthesis, characterization and superoxide dismutase activity of a biomimetic Mn(III) complex covalently anchored to mesoporous silica.

A mononuclear Mn(III) complex of a clickable ligand, [Mn(hbpapn)(H2O)2]ClO4·4.5H2O, where H2hbpapn = 1,3-bis[(2-hydroxybenzyl)(propargyl)amino]propane, has been prepared and fully characterized. The complex catalyzes the dismutation of superoxide employing a Mn(III)/Mn(IV) redox cycle, with catalytic rate constant of 3.9 × 106 M-1 s-1 determined through the nitro blue tetrazolium photoreduction inhibition assay, in aqueous medium of pH 7.8. The alkyne function of the ligand was used for the covalent attachment of the catalyst to azide modified mesoporous silicas with different texture and morphology, through click chemistry. In these materials the catalyst is essentially linked to the inner pore walls, isolated and protected from the external medium. The hybrid materials can be recycled, and retain or improve the superoxide dismutase activity of the free catalyst with the pore size of the solid matrix playing a role on the activity of the catalyst.

Effect of coordination dissymmetry on the catalytic activity of manganese catalase mimics.

Two mixed-valence Mn(II)Mn(III) complexes, [Mn2L1(OAc)2(H2O)]BPh4·2.5H2O and [Mn2L2(OAc)2]·4H2O, obtained with unsymmetrical N4O2-hexadentate L1(2-) (H2L1 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N-(2-hydroxybenzyl)benzylaminomethyl)-4-methylphenol) and N4O3-heptadentate L2(3-) (NaH2L2 = 2-(N,N-bis(2-(pyridylmethyl)aminomethyl)-6-(N'-(2-hydroxybenzyl)(carboxymethyl)aminomethyl)-4-methylphenol sodium salt) ligands, have been prepared and characterized. Both complexes share a µ-phenolate-bis(µ-acetate)Mn(II)Mn(III) core and N3O3-coordination sphere around the Mn(II) ion, but differ in the donor groups surrounding Mn(III) (NO4(solvent) and NO5). In non-protic solvents, these two complexes are able to disproportionate at least 3600 equiv. of H2O2 without significant decomposition, with first-order dependence on catalyst and saturation kinetics on [H2O2]. Spectroscopic monitoring of the reaction mixtures revealed the two complexes disproportionate H2O2 employing a different redox cycle, with retention of dinuclearity. The higher catalytic efficiency of [Mn2L2(OAc)2] was rationalized in terms of the larger labilizing effect of the heptadentate ligand that favors the acetate-shift and the replacement of the non-coordinating benzyl arm of L1 by a carboxylate arm in L2 which facilitates the formation of the catalyst-H2O2 adduct, placing [Mn2L2(OAc)2] as the most efficient among the phenolate-bridged diMn catalysts based on the kcat/KM criterion.

Preparation, characterization and activity of CuZn and Cu2 superoxide dismutase mimics encapsulated in mesoporous silica.

Encapsulation of three superoxide dismutase (SOD) functional mimics, [CuZn(dien)2(µ-Im)(ClO4)2]ClO4 (1), [Cu2(dien)2(µ-Im)(ClO4)2]ClO4 (2) (Im = imidazolate, dien = diethylenetriamine), and [CuZn(salpn)Cl2] (3) (H2salpn = 1,3-bis(salicylideneamino)propane) in mesoporous MCM-41 silica afforded three hybrid catalysts 1@MCM-41, 2@MCM-41 and 3@MCM-41. Spectroscopic and magnetic analyses of these materials confirmed the metal centers of the complexes keep the coordination sphere after insertion into the MCM-41 silica matrix. For the imidazolate-bridged complexes the silica channels restraint the relative orientation of the two metal ions. While 3@MCM-41 shows SOD activity significantly lower than the host-free complex, insertion of the imidazolate-bridged CuZn or Cu2 complexes by ion exchange onto mesoporous MCM-41 silica affords durable and recoverable supported catalysts with much better SOD activity than the free complexes. For confined imidazolate-bridged complexes, 1@MCM-41 and 2@MCM-41, the small pore size of the silica matrix improves the SOD activity more than a host with larger pores. This high SOD activity is attributed to the close-fitting of the complexes into the nanochannels of MCM-41 silica that favors the Cu active site and HImZn(or Cu) group stay in close proximity during catalysis.

Insights into Second-Sphere Effects on Redox Potentials, Spectroscopic Properties, and Superoxide Dismutase Activity of Manganese Complexes with Schiff-Base Ligands.

Six Mn-Schiff base complexes, [Mn(X-salpn)]0/+ (salpn = 1,3-bis(sal-ic-ylidenamino)propane, X = H [1], 5-Cl [2], 2,5-F2 [3], 3,5-Cl2 [4], 5-NO2 [5], 3,5-(NO2)2 [6]), were synthesized and characterized in solution, and second-sphere effects on their electrochemical and spectroscopic properties were analyzed. The six complexes catalyze the dismutation of superoxide with catalytic rate constants in the range 0.65 to 1.54 × 106 M-1 s-1 obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutases assay, in aqueous medium of pH 7.8. In solution, these compounds possess two labile solvent molecules in the axial positions favoring coordination of the highly nucleophilic O2 •- to the metal center. Even complex 5, [Mn(5-(NO2)salpn) (OAc) (H2O)], with an axial acetate in the solid state, behaves as a 1:1 electrolyte in methanolic solution. Electron paramagnetic resonance and UV-vis monitoring of the reaction of [Mn(X-salpn)]0/+ with KO2 demonstrates that in diluted solutions these complexes behave as catalysts supporting several additions of excess O2 •-, but at high complex concentrations (≥0.75 mM) catalyst self-inhibition occurs by the formation of a catalytically inactive dimer. The correlation of spectroscopic, electrochemical, and kinetics data suggest that second-sphere effects control the oxidation states of Mn involved in the O2 •- dismutation cycle catalyzed by complexes 1-6 and modulate the strength of the Mn-substrate adduct for electron-transfer through an inner-sphere mechanism.

Functional modeling of the MnCAT active site with a dimanganese(III) complex of an unsymmetrical polydentate N3O3 ligand.

A new diMnIII complex, [Mn2L(OAc)2(H2O)](BPh4)·3H2O (1), obtained with the unsymmetrical N3O3-ligand H3L = 1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(benzyl)amino]propan-2-ol, has been prepared and characterized. The unsymmetrical hexadentate ligand L3- leads to coordination dissymmetry (dissimilar donor atoms) around each Mn ion (N2O4 and NO4(solvent), respectively) leaving one labile site on one of the two Mn ions that facilitates interaction of the metal center with H2O2, as in Mn catalase. 1 is able to catalyze H2O2 disproportionation in acetonitrile, with second-order rate constant kcat = 23.9(2) M-1 s-1. The accessibility of the MnII2 state and the closeness of the two one-electron reduction processes suggest 1 employs MnIII2/MnII2 oxidation states for catalysis.

Tuning the MnII2/MnIII2 redox cycle of a phenoxo-bridged diMn catalase mimic with terminal carboxylate donors.

A new phenoxo-bridged diMnIII complex, Na[Mn2L(OH)2(H2O)2]·5H2O (1), obtained with the ligand L5- = 5­methyl­2­hydroxo­1,3­xylene­α,α­diamine­N,N,N',N'­tetraacetato, has been prepared and characterized. Mass spectrometry, conductivity, UV-visible, EPR and 1H NMR spectroscopic studies showed that the complex exists in solution as a monoanionic diMnIII complex. Complex 1 catalyzes H2O2 disproportionation with second-order rate constant kcat = 305(9) M-1 min-1 and without a time-lag phase. Based on spectroscopic results, the catalase activity of complex 1 in methanol involves a MnIII2/MnII2 redox cycle, which distinguishes this catalyst from other phenoxo-bridged diMn complexes that cycle between MnIIMnIII/MnIIIMnIV species. Addition of base stabilizes the catalyst, restrains demetallation during catalysis and causes moderate enhancement of catalase activity. The terminal carboxylate donors of 1 not only contribute as internal bases to assist deprotonation of H2O2 but also favor the formation of active homovalent diMn species, just as observed for the enzyme.

Crystal structure of cis-1-phenyl-8-(pyridin-2-ylmeth-yl)dibenzo[1,2-c:2,1-h]-2,14-dioxa-8-aza-1-borabi-cyclo-[4.4.0]deca-3,8-diene.

The title compound, C26H23BN2O2, was obtained as by product during synthetic attempts of a complexation reaction between the tripodal ligand H2L [N,N-bis-(2-hy-droxy-benz-yl)(pyridin-2-yl)methyl-amine] and manganese(III) acetate in the presence of NaBPh4. The isolated B-phenyl dioxaza-borocine contains an N→B dative bond with a cis conformation. In the crystal, C-H⋯O hydrogen bonds define chains parallel to the b-axis direction. A comparative analysis with other structurally related derivatives is also included, together with a rationalization of the unexpected production of this zwitterionic heterocycle.

Dimerization, redox properties and antioxidant activity of two manganese(III) complexes of difluoro- and dichloro-substituted Schiff-base ligands.

Two mononuclear MnIII complexes [Mn(3,5-F2salpn)(H2O)2][B(C6H5)4]·2H2O (1·2H2O) and [Mn(3,5-Cl2salpn)(OAc)(H2O)]·H2O (2·H2O), where H2salpn=1,3-bis(salicylidenamino)propane, have been prepared and characterized. The crystal structure of 1·H2O shows that this complex forms µ-aqua dimers with a short Mn⋯Mn distance of 4.93Å. Under anaerobic conditions, the two complexes are stable in solution and possess trans-diaxial symmetry with the tetradentate Schiff base ligand symmetrically arranged in the equatorial plane. When left in air, these complexes slowly dimerize to yield high-valent [MnIV2(3,5-X2-salpn)2(µ-O)2] in which each X2-salpn ligand wraps the two Mn ions. This process is favored in basic medium where the deprotonation of the bound water molecule is concomitant with air oxidation. The two complexes catalyze the dismutation of superoxide (superoxide dismutase (SOD) activity) and peroxide (catalase (CAT) activity) in basic medium. The phenyl-ring substituents play an important role on the CAT reaction but have little effect on SOD activity. Kinetics and spectroscopic results indicate that 1 and 2 catalyze H2O2 disproportionation through a cycle involving MnIII2 and MnIV2 dimers, unlike related complexes with a more rigid and smaller chelate ring, which employ MnIII/MnVO monomers.

Synthesis, characterization and activity of imidazolate-bridged and Schiff-base dinuclear complexes as models of Cu,Zn-SOD. A comparative study.

Two imidazolate-bridged diCuII and CuIIZnII complexes, [CuZn(dien)2(µ-Im)](ClO4)3·MeOH (1) and [Cu2(dien)2(µ-Im)](ClO4)3 (2) (Im = imidazole, dien=diethylenetriamine), and two complexes formed with Schiff base ligands, [CuZn(salpn)Cl2] (3) and [Cu2(salbutO)ClO4] (4) (H2salpn=1,3-bis(salicylidenamino)propane, H3salbutO=1,4-bis(salicylidenamino)butan-2-ol) have been prepared and characterized. The reaction of [Cu(dien)(ImH)](ClO4)2 with [Zn(dien)(H2O)](ClO4)2 at pH≥11 yields complex 1; at lower pH, the Cu3Zn tetranuclear complex [{(dien)Cu(µ-Im)}3Zn(OH2)(ClO4)2](ClO4)3 (1a) forms as the main reaction product. X-ray diffraction of 1a reveals that the complex contains a metal centered windmill-shaped cation having three blades with a central Zn ion and three peripheral capping Cu(dien) moieties bound to the central Zn ion through three imidazolate bridges. The four complexes are able to disproportionate O2- in aqueous medium at pH7.8, with relative rates 4>1>2≫3. [Cu2(salbutO)]+ (4) is the most easily reducible of the four complexes and exhibits the highest activity among the SOD models reported so far; a fact related to the ligand flexibility to accommodate the copper ion in both CuI and CuII oxidation states and the lability of the fourth coordination position of copper facilitating stereochemical rearrangements.

A new mononuclear manganese(III) complex of an unsymmetrical hexadentate N3O3 ligand exhibiting superoxide dismutase and catalase-like activity: synthesis, characterization, properties and kinetics studies.

A mononuclear Mn(III) complex MnL·4H2O (H3L=1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(4-methylbenzyl)amino]propan-2-ol) has been prepared and characterized. This complex catalyzes the dismutation of superoxide efficiently, with catalytic rate constant kcat=1.7×10(6)M(-1)s(-1) and IC50 1.26µM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH7.8. MnL is also able to disproportionate more than 300 equivalents of H2O2 in CH3CN, with initial rate of H2O2 decomposition given by ri=kcat [MnL](2) [H2O2] and kcat=1.32(2)mM(-2)min(-1). The accessibility of the Mn(IV) state (E(p)=0.53V vs. saturated calomel electrode), suggests MnL employs a high-valent catalytic cycle to decompose O2(-) and H2O2.

Synthesis, characterization, and reactivity studies of a water-soluble bis(alkoxo)(carboxylato)-bridged diMn(III) complex modeling the active site in catalase.

A new diMn(III) complex, Na[Mn2(5-SO3-salpentO)(µ-OAc)(µ-OMe)(H2O)]·4H2O, where 5-SO3-salpentOH = 1,5-bis(5-sulphonatosalicylidenamino)pentan-3-ol, has been prepared and characterized. ESI-mass spectrometry, paramagnetic (1)H NMR, EPR and UV-visible spectroscopic studies on freshly prepared solutions of the complex in methanol and 9 : 1 methanol-water mixtures showed that the compound retains the triply bridged bis(µ-alkoxo)(µ-acetato)Mn2(3+) core in solution. In the 9 : 1 methanol-water mixture, slow substitution of acetate by water molecules took place, and after one month, the doubly bridged diMn(III) complex, [Mn2(5-SO3-salpentO)(µ-OMe)(H2O)3]·5H2O, formed and could be characterized by X-ray diffraction analysis. In methanolic or aqueous basic media, acetate shifts from a bridging to a terminal coordination mode, affording the highly stable [Mn2(5-SO3-salpentO)(µ-OMe)(OAc)](-) anion. The efficiency of the complex in disproportionating H2O2 depends on the solvent and correlates with the stability of the complex (towards metal dissociation) in each medium: basic buffer > aqueous base > water. The buffer preserves the integrity of the catalyst and the rate of O2 evolution remains essentially constant after successive additions of excess of H2O2. Turnovers as high as 3000 mol H2O2 per mol of catalyst, without significant decomposition and with an efficiency of k(cat)/K(M) = 1028 M(-1) s(-1), were measured for the complex in aqueous buffers of pH 11. Kinetic and spectroscopic results suggest a catalytic cycle that runs between Mn(III)2 and Mn(IV)2 oxidation states, which is consistent with the low redox potential observed for the Mn(III)2/Mn(III)Mn(IV) couple of the catalyst in basic medium.

Trinuclear manganese complexes of unsymmetrical polypodal diamino N3O3 ligands with an unusual [Mn3(µ-OR)4]5+ triangular core: synthesis, characterization, and catalase activity.

Two new tri-Mn(III) complexes of general formula [Mn3L2(µ-OH)(OAc)]ClO4 (H3L = 1-[N-(2-pyridylmethyl),N-(2-hydroxybenzyl)amino]-3-[N'-(2-hydroxybenzyl),N'-(4-X-benzyl)amino]propan-2-ol; 1ClO4, X = Me; 2ClO4, X = H) have been prepared and characterized. X-ray diffraction analysis of 1ClO4 reveals that the complex cation possesses a Mn3(µ-alkoxo)2(µ-hydroxo)(µ-phenoxo)(4+) core, with the three Mn atoms bound to two fully deprotonated N3O3 chelating L(3-), one exogenous acetato ligand, and one hydroxo bridge, the structure of which is retained upon dissolution in acetonitrile or methanol. The three Mn atoms occupy the vertices of a nearly isosceles triangle (Mn1···Mn3 = 3.6374(12) Å, Mn2···Mn3 3.5583(13) Å, and Mn1···Mn2 3.2400(12) Å), with one substitution-labile site on the apical Mn ion occupied by terminally bound monodentate acetate. Temperature-dependent magnetic susceptibility studies indicate the presence of predominant antiferromagnetic intramolecular interactions between Mn(III) ions in 1ClO4. Complexes 1ClO4 and 2ClO4 decompose H2O2 at comparable rates upon initial binding of peroxide through acetate substitution, with retention of core structure during catalysis. Kinetic and spectroscopic studies suggest that these complexes employ the [Mn-(µ-oxo/aquo)-Mn](4+) moiety to activate peroxide, with the additional (µ-alkoxo)(µ-phenoxo)Mn(µ-alkoxo) metallobridge carrying out a structural function.

Synthesis, characterization, and catalase activity of a water-soluble diMn(III) complex of a sulphonato-substituted Schiff base ligand: an efficient catalyst for H2O2 disproportionation.

A new diMn(III) complex, Na[Mn(2)(3-Me-5-SO(3)-salpentO)(µ-MeO)(µ-AcO)(H(2)O)]·4H(2)O (1), where salpentOH = 1,5-bis(salicylidenamino) pentan-3-ol, was synthesized and structurally characterized. The complex possesses a bis(µ-alkoxo)(µ-acetato) triply bridged diMn(III) core, the structure of which is retained upon dissolution. Complex 1 is highly efficient to disproportionate H(2)O(2) in an aqueous solution of pH ≥ 8.5 or in DMF, with only a slight decrease of activity. Electrospray ionization mass spectrometry, EPR, and UV-vis spectroscopy used to monitor the H(2)O(2) disproportionation in buffered basic medium, suggest that the major active form of the catalyst during cycling occurs in the Mn(III)(2) oxidation state and that the starting complex retains the dinuclearity and composition during catalysis, with the acetate that moves from bridging to terminal ligand. UV-vis and Raman spectroscopy of H(2)O(2) + 1 + Bu(4)NOH mixtures in DMF suggest that the catalytic cycle involves Mn(III)(2)/Mn(IV)(2) oxidation levels. At pH 10.6 in an Et(3)N/Et(3)NH(+) buffer, complex 1 catalyzes dismutation of H(2)O(2) with saturation kinetics on the substrate, first order dependence on the catalyst, and k(cat)/K(M) = 16(1) × 10(2) s(-1) M(-1). During catalysis, the exogenous base contributes to retain the integrity of the bis(µ-alkoxo) doubly bridged diMn core and favors the formation of the catalyst-peroxide adduct (low value of K(M)), rendering 1 a highly efficient catalyst for H(2)O(2) disproportionation.

Weak interactions modulating the dimensionality in supramolecular architectures in three new nickel(II)-hydrazone complexes, magnetostructural correlation, and catalytic potential for epoxidation of alkenes under phase transfer conditions.

Three different ONO donor acetyl hydrazone Schiff bases have been synthesized from the condensation of acetic hydrazide with three different carbonyl compounds: salicylaldehyde (HL(1)), 2-hydroxyacetophenone (HL(2)), and 2, 3-dihydroxybenzaldehyde (HL(3)). These tridentate ligands are reacted with Ni(OOCCF(3))(2)·xH(2)O to yield three new Ni(II) complexes having distorted octahedral geometry at each Ni center: [Ni(L(1))(OOCCF(3))(CH(3)OH)](2) (1), [Ni(L(2))(OOCCF(3))(H(2)O)](2) (2), and [Ni(L(3))(L(3)H)](OOCCF(3))(H(2)O)(1.65)(CH(3)OH)(0.35) (3). The ligands and the complexes have been characterized by elemental analysis and IR and UV-vis spectroscopy, and the structures of the complexes have been established by single crystal X-ray diffraction (XRD) study. 1 and 2 are centrosymmetric dinuclear complexes and are structural isomers whereas 3 is a bis chelated cationic monomer coordinated by one neutral and one monoanionic ligand. O-H···O hydrogen bonds in 3 lead to the formation of a dimer. Slight steric and electronic modifications in the ligand backbone provoke differences in the supramolecular architectures of the complexes, leading to a variety of one, two, and three-dimensional hydrogen bonded networks in complexes 1-3 respectively. Variable temperature magnetic susceptibility measurements reveal that moderate antiferromagnetic interactions operate between phenoxo bridged Ni(II) dimers in 1 and 2 whereas very weak antiferromagnetic exchange occurs through hydrogen bonding and π-π stacking interactions in 3. All complexes are proved to be efficient catalysts for the epoxidation of alkenes by NaOCl under phase transfer condition. The efficiency of alkene epoxidation is dramatically enhanced by lowering the pH, and the reactions are supposed to involve high valent Ni(III)-OCl or Ni(III)-O· intermediates. 3 is the best epoxidation catalyst among the three complexes with 99% conversion and very high turnover number (TON, 396).

Synthesis, characterization and antioxidant activity of water soluble MnIII complexes of sulphonato-substituted Schiff base ligands.

Two new Mn(III) complexes Na[Mn(5-SO(3)-salpnOH)(H(2)O)]5H(2)O (1) and Na[Mn(5-SO(3)-salpn)(MeOH)]4H(2)O (2) (5-SO(3)-salpnOH=1,3-bis(5-sulphonatosalicylidenamino)propan-2-ol, 5-SO(3)-salpn=1,3-bis(5-sulphonatosalicylidenamino)propane) have been prepared and characterized. Electrospray ionization-mass spectrometry, UV-visible and (1)H NMR spectroscopic studies showed that the two complexes exist in solution as monoanions [Mn(5-SO(3)-salpn(OH))(solvent)(2)](-), with the ligand bound to Mn(III) through the two phenolato-O and two imino-N atoms located in the equatorial plane. The E(1/2) of the Mn(III)/Mn(II) couple (-47.11 (1) and -77.80mV (2) vs. Ag/AgCl) allows these complexes to efficiently catalyze the dismutation of O(2)(-), with catalytic rate constants 2.4x10(6) (1) and 3.6x10(6) (2) M(-1)s(-1), and IC(50) values of 1.14 (1) and 0.77 (2) muM, obtained through the nitro blue tetrazolium photoreduction inhibition superoxide dismutase assay, in aqueous solution of pH 7.8. The two complexes are also able to disproportionate up to 250 equivalents of H(2)O(2) in aqueous solution of pH 8.0, with initial turnover rates of 178 (1) and 25.2 (2) mM H(2)O(2) min(-1)mM(-1)catalyst(-1). Their dual superoxide dismutase/catalase activity renders these compounds particularly attractive as catalytic antioxidants.

Redox and complexation chemistry of the Cr(VI)/Cr(V)/Cr(IV)-D-glucuronic acid system.

When excess uronic acid over Cr(VI) is used, the oxidation of D-glucuronic acid (Glucur) by Cr(VI) yields D-glucaric acid (Glucar) and Cr(III) as final products. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species, with Cr(VI) and Cr(V) reacting with Glucur at comparable rates. The rate of disappearance of Cr(VI), and Cr(V) increases with [H(+)] and [substrate]. The experimental results indicated that Cr(IV) is a very reactive intermediate since its disappearance rate is much faster than Cr(VI)/Cr(V) and decreases when [H(+)] rises. Even at high [H(+)] Cr(IV) intermediate was involved in fast steps and does not accumulate in the reaction. Kinetic studies show that the redox reaction between Glucur and Cr(VI) proceeds through a mechanism combining one- and two-electron pathways for the reduction of intermediate Cr(IV) by the organic substrate: Cr(VI) --> Cr(IV) --> Cr(II) and Cr(VI) --> Cr(IV) --> Cr(III). The mechanism is supported by the observation of free radicals, CrO(2)(2+) (superoxoCr(III) ion) and Cr(V) as reaction intermediates. The EPR spectra show that five-co-ordinate oxo-Cr(V) bischelates are formed at pH < or = 4 with the uronic acid bound to Cr(V) through the carboxylate and the alpha-OH group of the furanose form. Five-co-ordinated oxo-Cr(V) monochelates are observed as minor species in addition to the major five-co-ordinated oxo-Cr(V) bischelates. At pH 7.5 the EPR spectra show the formation of a Cr(V) complex where the cis-diol groups of Glucur participate in the bonding to Cr(V). In vitro, our studies on the chemistry of Cr(V) complexes can provide information on the nature of the species that are likely to be stabilized in vivo. In particular, the EPR pattern of Glucur-Cr(V) species can be used as a finger print to identify Cr(V) complexes formed in biological systems.

Synthesis, structure, and catalase-like activity of dimanganese(III) complexes of 1,5-bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]pentan-3-ol (X = H, Br, OCH3).

New diMn(III) complexes of general formula [Mn(2)L(mu-OR)(mu-OAc)]BPh(4) (H(3)L = 1,5-bis[(2-hydroxy-5-X-benzyl)(2-pyridylmethyl)amino]pentan-3-ol, 1: X = H, R = Me, 2: X = OMe, R = Me, 3: X = Br, R = Me, 4: X = Br, R = Et) have been prepared and structurally characterized. The synthesized complexes possess a triply bridged (mu-alkoxo)(2)(mu-acetato)Mn(2)(3+) core, a short intermetallic distance of 2.95/6 A modulated by the aliphatic spacers between the central alcoholato and N-amino donor sites, and the remaining coordination sites of the two Mn(III) centers occupied by the six donor atoms of the polydentate ligand. In dimethylformamide, complexes 1-3 are able to disproportionate more than 1500 equiv of H(2)O(2) without significant decomposition, with first-order dependence on catalyst and saturation kinetic on [H(2)O(2)]. Spectroscopic monitoring of the reaction mixtures revealed that the catalyst converts into [Mn(2)(III)(mu-O)(mu-OAc)L], which is the major active form during cycling. Overall, kinetics and spectroscopic studies of H(2)O(2) dismutation by these complexes converge at a catalytic cycle between Mn(III)(2) and Mn(II)(2) oxidation levels. Comparison to other alkoxo-bridged complexes suggests that the binding mode of peroxide to the metal center of the Mn(III)(2) form of the catalyst is a key factor for tuning the Mn oxidation states involved in the H(2)O(2) dismutation mechanism.

Reevaluation of the kinetics of polynuclear mimics for manganese catalases.

Considerable effort has been expended in order to understand the mechanism of manganese catalases and to develop functional mimics for these enzymes. For many years, the most efficient reactivity mimic was [MnIVsalpn(mu-O)]2 [H2salpn = 1,3-bis(salicylideneiminato)propane], a compound that cycles between the MnIV2 and MnIII2 oxidation levels instead of the MnII2 and MnIII2 oxidation states used by the enzyme, with kcat = 250 s(-1) and kcat/KM = 1000 M(-1) s(-1). Recently, a truly exceptional high value of kcat was reported for the complex [Mn(bpia)(mu-OAc)]22+ [bpia = bis(picolyl)(N-methylimidazol-2-yl)amine]. On the basis of a calculated kcat value of 1100 s(-1) and an efficiency kcat/KM of 34 000 M(-1) s(-1), this complex has been suggested to represent a significant breakthrough in catalytic efficiencies of manganese catalase mimics. However, a plot of ri/[cat]T vs [H2O2]0, where the saturation value approaches 1.5 s(-1), is inconsistent with the 1100 s(-1) value tabulated for kcat. Similar discrepancies are observed for two other families of manganese complexes containing either a Mn2(mu-OPh)22+ core and different substituted tripodal ligands or complexes of methyl and ethyl salicylimidate, with an Mn2(mu-OPh)24+ core. Reevaluation of the kinetic parameters for these three systems reveals that the originally reported values were overestimated by a factor of approximately 1000 for both kcat and kcat/KM. We discuss the origin of the discrepancy between the previously published kinetic parameters and the newly derived values. Furthermore, we provide a short analysis of the existing manganese catalase mimics in an effort to provide sound directions for future investigations in this field.

Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity.

The diMn(III) complexes [Mn2(5-Me-salpentO)(mu-MeO)(mu-AcO)(H2O)Br] (1) and [Mn2(3-Me-salpentO)(mu-MeO)(mu-AcO)(MeOH)2]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(micro-alkoxo)(micro-acetato) triply-bridged diMn(III) core with an Mn...Mn separation of 2.93-2.94 A, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H2O2, with first-order dependence on the catalyst, and saturation kinetics on [H2O2], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H2O2 without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn(II) species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn2(III) oxidation state during cycling. The correlation between log(k(cat)) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H2O2 dismutation by these complexes converge at a catalytic cycle that involves the Mn2(III) and Mn2(IV) oxidation states.

New dimanganese(III) complexes of pentadentate (N2O3) Schiff base ligands with the [Mn2(mu-OAc)(mu-OR)2]3+ core: synthesis, characterization and mechanistic studies of H2O2 disproportionation.

Two new diMn(III) complexes [Mn(2)(III)L(1)(mu-AcO)(mu-MeO)(methanol)(2)]Br (1) and [Mn(2)(III)L(2)(mu-AcO)(mu-MeO)(methanol)(ClO(4))] (2) (L(1)H(3)=1,5-bis(2-hydroxybenzophenylideneamino)pentan-3-ol; L(2)H(3)=1,5-bis(2-hydroxynaphtylideneamino)pentan-3-ol) were synthesized and structurally characterized. Structural studies evidence that these complexes have a bis(mu-alkoxo)(mu-carboxylato) triply bridged diMn(III) core in the solid state and in solution, with two substitution-labile sites--one on each Mn ion--in cis-position. The two complexes show catalytic activity toward disproportionation of H(2)O(2), with saturation kinetics on [H(2)O(2)], in methanol and dimethyl formamide at 25 degrees C. Spectroscopic monitoring of the H(2)O(2) disproportionation reaction suggests that (i) complexes 1 and 2 dismutate H(2)O(2) by a mechanism involving redox cycling between Mn(2)(III) and Mn(2)(IV), (ii) the complexes retain the dinuclearity during catalysis, (iii) the active form of the catalyst contains bound acetate, and (iv) protons favors the formation of inactive Mn(II) species. Comparison to other dimanganese complexes of the same family shows that the rate of catalase reaction is not critically dependent on the redox potential of the catalyst, that substitution of phenolate by naphtolate in the Schiff base ligand favors formation of the catalyst-substrate adduct, and that, in the non-protic solvent, the bulkier substituent at the imine proton position hampers the binding to the substrate.