Binuclear Manganese Research Articles

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

Abstract The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N , N ′-bis(2-hydroxybenzyl)- N , N ′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H 3 bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.

A novel (μ-OAc) 2 bridged unsymmetric coordinated binuclear Mn(II) macrocyclic complex with ligating pendant-arm

A novel binuclear manganese Mn(II) macrocyclic complex with two pyridylmethyl pendant arms, [Mn 2 II(H 2L)(μ-OAc) 2](ClO 4) 2 · H 2O, has been synthesized and characterized crystallographically and magnetically. The crystal structure of the complex shows that two manganese ions locate in the same head of the macrocycle, leaving an uncoordinating cavity to catch protons through oxide of phenolate and the nearby imine groups in another head. The electrochemical study demonstrates that the complex gives two couples of redox peaks with E 1/2 of 0.3775 V and 0.8409 V, respectively. The variable temperature magnetic susceptibility measurement on the sample displays weak antiferromagnetic interaction between two manganese (II) with the J = −3.733(7) cm −1. This complex exhibits a moderate activity for catalyzing disproportionation of H 2O 2 to O 2.

Synthesis, Spectroscopic and Thermal Investigations of O‐Phd and P‐Phd Bridged Binuclear Manganese (III) Schiff Base Complexes Derived from Pyrazolone Base Ligands

Acyl pyrazolone and their derivatives are an important class of chelating compounds called heterocyclic β‐diketones. β‐diketones have played, and continue to play, a key role in coordination compounds that have found wide application in several fields, from new materials to catalysts, as precursors for CVD in the microelectronic industry and as potential antitumourals. Due to variety of application of this class of ligand, there are studies on metal complexes of many researchers, including ours. We herein report the structural variability of pyrazolone moiety and its chelating nature with manganese (III) ions, in mononuclear and binuclear complexes. The mononuclear complex [Mn(PMFP)(bipy)(OAc)]ClO4 and [Mn(PMFP‐Me) (bipy)(OAc)]ClO4 was prepped by reaction of Mn(OAc)3 · 2H2O with ligand PMFP or PMFP‐Me (where PMFP=5‐Hydroxy‐3‐methyl‐1‐phenyl‐1H‐pyrazole‐4‐carbaldehyde; PMFP‐Me=5‐hydroxy‐3‐methyl‐1‐p‐tolyl‐1H‐pyrazole‐4‐carbaldehyde; bipy=2,2′bipyridyl). The corresponding binuclear complexes were prepared by bridging molecule, o‐phD or p‐phD (o‐phD= where o‐phenelene diamine or p‐phD=p‐phenelen diamine). Both the ligand PMFP and PMFP‐Me were characterized by elemental analysis and 1H‐NMR, GC‐MS, FT‐IR spectroscopic study. The resulting complexes have been characterized by elemental analysis, magnetic susceptibility and conductometry measurements and spectroscopic methods FT‐IR, electronic spectra and mass spectrometry. All the complexes were electrolytic in nature and consist of 1∶2 electrolytes in solution. Electronic spectral and magnetic susceptibility measurements indicate square pyramidal geometry around manganese (III) ion. TGA and DSC determined thermal stability, heat capacity and activation energy of thermal degradation for these complexes.

One-dimensional co-ordination polymer of aqua-bridged binuclear manganese(II) carboxylate

A simple route for a novel sulphur containing one-dimensional aqua-bridged carboxylate co-ordination polymer of manganese(II) that have structural resemblance to biological aqua-bridged manganese systems is described. The manganese(II) co-ordination polymer is derived from dicarboxylate of (4-carboxymethoxy-2,3-bis-4-methylphenylsulfanyl-naphthalene-1-yloxy)-acetic acid and complex has pyridine as ancillary ligand.

Binuclear manganese and rhenium carbonyls M2(CO)n (n = 10, 9, 8, 7): comparison of first row and third row transition metal carbonyl structures

The equilibrium geometries, thermochemistry, and vibrational frequencies of the homoleptic binuclear rhenium carbonyls Re2(CO)n (n = 10, 9, 8, 7) were determined using the MPW1PW91 and BP86 methods from density functional theory (DFT) with the effective core potential basis sets LANL2DZ and SDD. In all cases triplet structures for Re2(CO)n were found to be unfavorable energetically relative to singlet structures, in contrast to corresponding Mn2(CO)n derivatives, apparently owing to the larger ligand field splitting of rhenium. For M2(CO)10 (M = Mn, Re) the unbridged structures (OC)5M-M(CO)5 are preferred energetically over structures with bridging CO groups. For M2(CO)9 (M = Mn, Re) the two low energy structures are (OC)4M(micro-CO)M(CO)4 with an M-M single bond and a four-electron donor bridging CO group and (OC)4M[double bond, length as m-dash]M(CO)5 with no bridging CO groups and an M[double bond, length as m-dash]M distance suggesting a double bond. The lowest energy structures for Re2(CO)8 have Re[triple bond, length as m-dash]Re distances in the range 2.6-2.7 A suggesting the triple bonds required to give the Re atoms the favored 18-electron configuration. Low energy structures for Re2(CO)7 are either of the type (OC)(4)M[triple bond, length as m-dash]M(CO)3 with short metal-metal distances suggesting triple bonds or have a single four-electron donor bridging CO group and longer M-M distances consistent with single or double bonds. The 18-electron rule thus appears to be violated in these highly unsaturated Re2(CO)7 structures.

A Binuclear Manganese Cluster That Catalyzes Radical‐Mediated N‐Oxygenation

Site-directed mutagenesis was used to explore the ligand sphere and geometry of a novel binuclear manganese biocatalyst (see picture; Mn green, C gray, O red, N blue) that oxidizes amino to nitro groups with high specificity. The involvement of Mn in catalysis and radical formation was unequivocally proven by ESR spectroscopy.

A Binuclear Manganese Cluster That Catalyzes Radical‐Mediated N‐Oxygenation

Ortsgerichtete Mutagenese wurde zur Untersuchung der Ligandensphäre und Geometrie eines neuartigen zweikernigen Mangan-Biokatalysators verwendet (siehe Bild; Mn grün, C grau, O rot, N blau), der auf hoch spezifische Weise Amino- zu Nitrogruppen oxidiert. Die Beteiligung von Mn am Katalysevorgang und an der Radikalbildung wurde durch EPR-Spektroskopie zweifelsfrei belegt. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2007/z703089_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Binuclear Schiff Base Complex of Manganese(III) as a Neutral Carrier for a Highly Selective Iodide Electrode

A new highly selective iodide electrode incorporating a binuclear manganese(III) complex, bis(salicylaldehyde-aminopropanol)dichloroaceticdimanganese(III) [Mn(III)(2)-BSAPDCA], as a neutral carrier is described. The electrode displays an anti-Hofmeister selectivity sequence: iodide >> perchlorate > salicylate > thiocyanate > nitrate > bromide > nitrite > chloride > sulfate. The excellent selectivity for iodide is related to a direct interaction between the central Mn(III) atom and iodide and a steric effect associated with the structure of the carrier, which is supported by UV spectroscopy and AC impedance techniques. The electrode exhibits a near-Nernstian potentiometric linear response range to iodide from 1.0 x 10(-1) to 2.0 x 10(-5) mol/L with a detection limit of 8.0 x 10(-6) mol/L and a slope of -60.3 mV/decade in pH 3.0 of phosphate buffer solutions at 20 degrees C. From a comparison of the potentiometric response characteristics between a binuclear manganese(III) complex, Mn(III)(2)-BSAPDCA, and a mononuclear manganese(III) complex, Mn(III)-BSAPB, an enhanced response towards iodide from a binuclear metallic complex-based electrode was observed. The electrode, based on binuclear manganese(III) complex, was successfully applied to the determination of inorganic total iodine in iodized table salt with satisfactory results.

Binuclear manganese(II) complexes in biological systems

The paper reports calculations on binuclear manganese(II) clusters related to those found in biological systems. B3LYP/6-311g(d,p) density functional theory (DFT) calculations are carried out specifically on a set of clusters that derive from the X-ray crystal structure of DNA polymerase β [Batra, V. K. et al., Structure 14, 757–766 (2006)]. Issues investigated are: (a) the relative energy of the lowest and highest spin states (antiferromagnetic (low spin/broken symmetry = LS/BS) or ferromagnetic); (b) the Mn(II)–Mn(II) distance; (c) the effect of water versus CH3OH (a sugar analogue) as a direct Mn ligand; and (d) the effect of the kind of oxygen bridge (mono-oxygen, single-oxygen from a carboxylate or phosphate, or a two-oxygen bridge from a carboxylate), on the geometry and energy. It was found generally that the antiferromagnetic state is lower in energy, that the Mn(II)–Mn(II) distance (which varies from 2.91 to 3.57 Å in the series studied) is longer for ferromagnetic states than for the corresponding low spin states, that the interchange of CH3OH and water does not affect the results, and that a single (carboxylate) oxygen as bridge leads to a lower energy than a corresponding two (carboxylate) oxygen bridge, as well as to a more compact structure.

A new terephthalato-bridged binuclear manganese(II) complex [Mn2(dmptacn)2( p -bdc)](ClO4)2: synthesis, structure and properties

A binuclear manganese complex, [Mn2(dmptacn)2(p-bdc)] (ClO4)2 [dmptacn = 1,4-bis(2-pyridylmethyl)-1,4,7-triazacyclononane, p-bdc = terephthalate] has been synthesized and characterized. The complex crystallizes in triclinic space group , with a = 9.0325(7), b = 11.2851(8), c = 11.9907(9) Å; α = 90.7900(10), β = 101.3700(10), γ = 103.5040(10)°. IR, UV--Vis, element analysis, interaction with DNA and magnetic property of the complex has been studied. The magnetic susceptibility measured over the range 2–300 K showsthat there is a very weak antiferromagnetic interaction with J = −0.035 cm−1, g = 1.98.

Structure and Action of the N-oxygenase AurF from Streptomyces thioluteus

Nitro groups are found in a number of bioactive compounds. Most of them arise by a stepwise mono-oxygenation of amino groups. One of the involved enzymes is AurF participating in the biosynthesis of aureothin. Its structure was established at 2.1 Å resolution showing a homodimer with a binuclear manganese cluster. The enzyme preparation, which yielded the analyzed crystals, showed activity using in vitro and in vivo assays. Chain fold and cluster are homologous with ribonucleotide reductase subunit R2 and related enzymes. The two manganese ions and an iron content of about 15% were established by anomalous X-ray diffraction. A comparison of the cluster with more common di-iron clusters suggested an additional histidine in the coordination sphere to cause the preference for manganese over iron. There is no oxo-bridge. The substrate p-amino-benzoate was modeled into the active center. The model is supported by mutant activity measurements. It shows the geometry of the reaction and explains the established substrate spectrum.

Syntheses, crystal structures and magnetic study of two binuclear manganese(II) complexes with aromatic N-oxide as bridging ligand

Two new binuclear complexes, [Mn2(μ-dmpo)2(SCN)4(H2O)2] (1) (where dmpo = 3,5-dimethylpyridine N-oxide), [Mn2(μ-po)2(H2O)6I2]I2 (2) (where po = pyridine N-oxide), have been synthesized and their crystal structures determined by X-ray crystallography. Complexes 1 and 2 crystallize in monoclinic, space group P21/c, with unit cell dimensions a = 8.8836(18) Å, b = 15.450(3) Å, c = 15.484(3) Å, β = 91.020(3)° for 1, and a = 8.8352(13) Å, b = 17.927(3) Å, c = 8.3338(12) Å, β = 103.765(2)° for 2. In each binuclear complex two Mn(II) were bridged by two 3,5-dimethylpyridine N-oxides or by two pyridine N-oxides and the distances between the bridged Mn(II) ions are 3.599 Å for 1 and 3.552 Å for 2. Variable temperature (4–300 K) magnetic measurements were performed for 1 and the susceptibility data were fitted by using a binuclear Mn(II) magnetic coupling formula producing the 2J = −2.17 cm−1.

Crystal Structure of Human Arginase I Complexed with Thiosemicarbazide Reveals an Unusual Thiocarbonyl μ-Sulfide Ligand in the Binuclear Manganese Cluster

The crystal structure of the human arginase I−thiosemicarbazide complex reveals an unusual thiocarbonyl μ-sulfide ligand in the binuclear manganese cluster. The CS moiety of thiosemicarbazide bridges Mn2+A and Mn2+B with coordination distances of 2.6 and 2.4 Å, respectively. Otherwise, the binding of thiosemicarbazide to human arginase I does not cause any significant structural changes in the active site. The crystal structure of the unliganded enzyme reveals a hydrogen-bonded water molecule that could support proton transfer between a μ-water molecule and H141 to regenerate the nucleophilic μ-hydroxide ion in the final step of catalysis.

Magnetic transitions in manganese benzoquinone coordination polymers

Coordination polymers exhibiting magnetic properties can be considered as promising materials for magnetic applications. We obtained such materials by means pf mechanochemical incorporation of binuclear metal complexes Mn–O–Mn into polystyrene. Incorporation of binuclear manganese complexes was achieved by using substituted benzoquinones (organic acceptors) as intermediate ligands that react with manganese acetylacetonate and provide new coordination and stabilization of manganese in different valence states. Magnetic transitions in this system are due to reversible disproportionation process Mn3+–O–Mn3+⇔Mn2+–O–Mn4+ in the binuclear complex. The structure of the binuclear complex was obtained using ab initio calculations.

Magnetic Investigation of an Unusual Dissymmetric Binuclear Manganese Carboxylate Complex

The magnetic susceptibility (χT) of an unusual dissymmetric binuclear manganese corboxylate complex has been measured from 2 to 300K. The magnetic data which have been fitted with help of the Heisenberg Dirac Van Vleck HDVV spin-exchange Hamiltonian H = − J S 1 S 2 , indicate that an antiferromagnetic interaction equal to J = -0.90(1) cm-1 is present. A correlation between J values and Mn-H2O-Mn angles has been tempted.

Synthesis and Crystal Structure of [Mn2(H2sal)2(Hsal)2(H2O)4]. First Example of the Reductive Synthesis of a Binuclear Manganese(I) Salicylate Complex

The synthesis and characterization of the first unique binuclear manganese(I) salicylate complex is described. The structure of the complex is unequivocally established with the help of an X-ray crystallographic study. The structure consists of two [MnO6] units, containing octahedral MnI ions, linked together by a bridging salicylato (Hsal−) ligand and each MnI is chelated with a (H2sal) ligand. The complex possesses a metal oxidation state of +1 and is a rare example of a noncarbonyl or cyanide complex of a binuclear MnI species. The effective magnetic moment per binuclear molecule (µeff. at 27 °C is 8.07 µ b , that also describes the manganese (+1) oxidation state.

Synthesis, characterization and crystal structure of manganese (IV) complex derived from salicylic acid

The binuclear manganese (IV) [Mn 2(Hsal) 4(OH) 4] (H 2sal = salicylic acid) complex has been obtained from a complex reaction mixture in methanol consisting of Mn(II)(OAc) 2 · 4H 2O, GS ( a reagent obtained by refluxing glycine and salicylaldehyde in 1:1 molar ratio in methanol), monosodium salicylate and pyridine. The compound contains a distorted octahedral MnO 6 coordination unit of potential importance to high oxidation state manganese bimolecules.

Magnetism and electronic structure of binuclear manganese complexes in ortho-quinone ligand and polymer environments

By means of mechanochemical synthesis we obtained a coordination polymer containing binuclear manganese complexes with organic catechol∕ortho-quinone ligands. The reaction system contained polystyrene, manganese acetylacetonate, and donor-acceptor mixture catechol∕ortho-quinone. Electron paramagnetic resonance (EPR) and infrared spectroscopic data show that the binuclear manganese complex is covalently linked to the chain of polystyrene. Theoretical EPR spectrum analysis revealed a triplet state of the complex with two nonequivalent manganese ions coupled to each other by spin exchange. The type of magnetic coupling between manganese ions in this system is similar to the one in manganese based inorganic magnetic materials.

Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes—implications for photosynthetic water-oxidation

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem II (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes ( 1 and 2), which differ in the set of first sphere ligands to Mn (N 3O 3 in 1, N 2O 4 in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: Mn II - ( μ -OR ) ( μ -OCO ) 2 - Mn II ⇔ Mn II - ( μ -OR ) ( μ -OCO ) 2 - Mn III ⇒ Mn III - ( μ -OR ) ( μ -OCO ) ( μ -O ) - Mn III . In 2: Mn II - ( μ -OR ) ( μ -OCO ) 2 - Mn III ⇔ Mn III - ( μ -OR ) ( μ -OCO ) 2 - Mn III ⇒ Mn III - ( μ -OR ) ( μ -OCO ) ( μ -O ) - Mn IV . In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5–1 V. The second one-electron oxidation likely is coupled to μ-oxo-bridge (or μ-OH) formation which seems to counteract a further potential increase. In both complexes, μ-O(H) bridge formation is associated with a redox transition proceeding at ∼1 V, but the μ-O(H) bridge is observed at the Mn 2 III , III level in 1 and at the Mn III,IV level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential.

Binuclear manganese(III) complexes with carboxylate bridges: Synthesis, structure, electrochemistry and magnetic properties

A series of μ-oxo, μ-carboxylato binuclear manganese(III) complexes derived from bipy, phen and different carboxylate ligands have been prepared and characterized by IR and UV–vis spectroscopies, cyclic voltammetry, magnetic susceptibility and the X-ray crystallography. The electronic spectra of all the complexes show very similar features consisting of two absorptions in the visible region, one high intensity LMCT band and two intra-ligand transitions in the UV region. The cyclic voltammograms of the complexes exhibit four steps in most cases, each consisting of a one-electron transfer reaction. These probably correspond to Mn 2 III ↔ Mn IV Mn III ↔ Mn 2 IV oxidation and Mn 2 III ↔ Mn III Mn II ↔ Mn 2 II reduction processes. The structure of complex 2 has been determined by the X-ray crystallography and consists of a binuclear [Mn 2(bipy) 2(μ- O)(μ-OOCR) 2(NO 3)(H 2O)] + cation (R = CH 2Cl) with a ClO 4 - anion. Low temperature magnetic susceptibility measurements show that there is a weak ferromagnetic interaction in 1 (R = Ph) ( J = +0.43 cm −1) and a weak antiferromagnetic interaction between the metal centers in 2 (R = CH 2Cl) ( J = −0.15 cm −1).