Dinuclear Manganese Research Articles

A dinuclear manganese(II) complex {[Na2(H2O)4Mn2(μ-pmtz)4(NCS)2]·xH2O}n with 5-(pyrimidyl)tetrazolato bridges involved in 1D ladder-like chains: Synthesis, X-ray structure, magnetic properties and DFT calculations

A dinuclear manganese(II) unit of the composition {[Na2(H2O)4Mn2(μ-pmtz)4(NCS)2]·xH2O}n (1·xH2O) (x=3 or 0.5), where pmtz=5-(pyrimidyl)tetrazolate(1−), involved in 1D ladder-like chains mediated through pmtz bridges, was prepared and characterized by elemental analysis, FT-IR spectroscopy, simultaneous thermogravimetric (TG) and differential thermal (DTA) analyses, single crystal X-ray analysis and magnetic measurements, accompanied by DFT calculations. The magnetic data, treated with the spin Hamiltonian formalism, revealed a weak antiferromagnetic exchange mediated via the κ2N1,N7:κN2 chelating/bridging mode of pmtz. This finding was also supported by the DFT calculations performed on the experimentally determined geometry of the dinuclear manganese(II) unit at the B3LYP/TZVP and B2PLYP/TZVP levels of theory.

Synthesis of nanocrystalline zinc manganese oxide by thermal decomposition of new dinuclear manganese(III) precursors

Nanocrystalline manganese-doped zinc oxide was synthesized by thermal decomposition of a zinc oxide sol with two new dinuclear manganese(III) complexes as precursor. Thermal analysis results indicated that the decomposition of manganese precursors occurred at 269 and 314 °C. X-ray structural analysis shows the presence of dimanganese core in the complexes and the binding of the ligands to the manganese(III) is through N2O2. The manganese-doped zinc oxide composite was characterized by means of X-ray diffraction, scanning electron microscopy, and UV–Vis spectroscopy. Structural properties of the composites elucidated that the manganese ions have substituted the zinc ions without changing the wurtzite structure of zinc oxide.

Hydroxamic Acids as Potent Inhibitors of FeII and MnIIE. coli Methionine Aminopeptidase: Biological Activities and X‐ray Structures of Oxazole Hydroxamate–EcMetAP‐Mn Complexes

New series of acids and hydroxamic acids linked to five-membered heterocycles including furan, oxazole, 1,2,4- or 1,3,4-oxadiazole, and imidazole were synthesized and tested as inhibitors against the Fe(II) , Co(II) , and Mn(II) forms of E. coli methionine aminopeptidase (MetAP) and as antibacterial agents against wild-type and acrAB E. coli strains. 2-Aryloxazol-4-ylcarboxylic acids appeared as potent and selective inhibitors of the Co(II) MetAP form, with IC(50) values in the micromolar range, whereas 5-aryloxazol-2-ylcarboxylic acid regioisomers and 5-aryl-1,2,4-oxadiazol-3-ylcarboxylic acids were shown to be inefficient against all forms of EcMetAP. Regardless of the heterocycle, all the hydroxamic acids are highly potent inhibitors and are selective for the Mn(II) and Fe(II) forms, with IC(50) values between 1 and 2 μM. One indole hydroxamic acid that we previously reported as a potent inhibitor of E. coli peptide deformylase also demonstrated efficiency against EcMetAP. To gain insight into the positioning of the oxazole heterocycle with reversed substitutions at positions 2 and 5, X-ray crystal structures of EcMetAP-Mn complexed with two such oxazole hydroxamic acids were solved. Irrespective of the [metal]/[apo-MetAP] ratio, the active site consistently contains a dinuclear manganese center, with the hydroxamate as bridging ligand. Asp 97, which adopts a bidentate binding mode to the Mn2 site in the holoenzyme, is twisted in both structures toward the hydroxamate bridging ligand to favor the formation of a strong hydrogen bond. Most of the compounds show weak antibacterial activity against a wild-type E. coli strain. However, increased antibacterial activity was observed mainly for compounds with a 2-substituted phenyl group in the presence of the nonapeptide polymyxin B and phenylalanine-arginine-β-naphthylamide as permeabilizer and efflux pump blocker, respectively, which boost the intracellular uptake of the inhibitors.

A dinuclear manganese(III) and a rare pentanuclear manganese(III) compound with a phenol–pyrazole ligand

Reacting Mn(ClO4)2·6H2O and H2phpzEt (H2phpzEt=5(3)-(2-hydroxyphenyl)-3(5)-ethylpyrazole) in methanol in the presence of sodium hydroxide yields the compound [Mn2(HphpzEt)4(H2O)2](ClO4)2 (1). The pentanuclear [Mn5(μ3-O)2(HphpzEt)3(phpzEt)3(OCH3)(HOCH3)2(O2CPh)] (2) is obtained when performing a similar reaction with Mn(O2CPh)2·2H2O. Compound 1 consists of a symmetric dinuclear manganese(III) cationic unit, [Mn2(HphpzEt)4(H2O)2]2+ with two non-coordinated perchlorate anions. Compound 2 is a rare pentanuclear complex containing all the manganese ions in the oxidation state III and it also has μ3-oxido ligands. Temperature- and field-dependent magnetization studies indicate dominant antiferromagnetic interactions between the manganese(III) ions in both compounds.

ChemInform Abstract: Manganese‐Catalyzed Cleavage of a Carbon—Carbon Single Bond Between Carbonyl Carbon and α‐Carbon Atoms of Ketones.

AbstractThe treatment of aromatic or aliphatic ketones with carbodiimides in the presence of a tri‐ or dinuclear manganese catalyst affords amides in good to excellent yields and quinoline derivatives as side products.

Synthesis, temperature-controlled structures and polyphenol oxidase activities of dinuclear manganese(II) complexes with N,N,N′,N′-tetrakis(2′-benzimidazolylmethyl)-1,4-diethylene amino glycol ether

Three dinuclear Mn(II) complexes, [Mn2(EGTB)(NO3)2(H2O)4](NO3)2 (1), [Mn2(EGTB)(NO3)4] (2) and [Mn2(EGTB)(NO3)(H2O)](NO3)3 (3) (EGTB=N,N,N′,N′-tetrakis(2′-benzimidazolylmethyl)-1,4-diethylene amino glycol ether) were prepared and characterized. Their crystal structures revealed a relationship between synthesis temperature and structure. With increasing the reaction temperature from 60 to 120°C or 160°C, the Mn–Mn distances are shortened from 14.019Å in 1, 7.662Å in 2 to 5.662Å in 3. Additionally, polyphenol oxidase (PPO) activity of those three complexes was studied by measuring the rate of increase in absorbance at 320nm of pyrogallol substrate. At pH 7.8 and 30°C, the turnover numbers are 5.58h−1 for 1, 11.38h−1 for 2 and 13.97h−1 for 3, respectively. Furthermore, in pH 7.8–8.2, their catalytic activity increased with rising pH.

Manganese complexes with planar or tridimensional acyclic or cyclic Schiff base ligands

The coordination properties toward manganese(II) salts of a variety of [1+2], [2+1], [2+2] and [3+1] acyclic and cyclic ligands with different shapes and coordinating moieties have been studied. These ligands, obtained by condensation of suitable formyl- and primary amine precursors, give rise to mono- or dinuclear manganese(II) or manganese(III) complexes, characterized by elemental analyses, IR spectroscopy, ESI-MS spectrometry together with X-ray structural determinations. In particular, in the structure of [Mn(H3–LC2)(H2O)2](Cl)(H2O), where the planar, potentially dinucleating ligand H5–LC2 forms by the [2+1] condensation of pyridoxal hydrochloride and 1,3-diamino-2-propanol, the central octahedral manganese(III) ion resides into the N2O2 Schiff base moiety of [H3–LC2]2−, which does not behave as a compartmental system as the central alcoholic group does not take part in the coordination. In [Mn(H3–LA5)]·0.25H2O·CH3OH, where the potentially dinucleating ligand H6–LA5 forms by the [3+1] condensation of 3-formylsalicylic acid and tris(aminoethyl)amine, the central octahedral manganese(III) ion resides into the external O3O3 site while the inner N4O3 site does not participate in the coordination because of the protonation of the imine groups.Although the formulation and properties in the solid state of the prepared manganese complexes can be recognized, also with the aid of the related zinc(II) analogs, by usual physico-chemical measurements, their solubility and stability in organic solvents or in water is not predictable in advance, because of the occurrence of demetalation–metalation reactions, mainly detected by ESI-MS experiments. These metal encapsulation–release processes, however, represent a necessary prerequisite for further investigations aimed at the use of these complexes in medicine, based on metal ion release in consequence of specific external stimuli.

Mononuclear and dinuclear manganese compounds stabilized by supramolecular interactions

New manganese compounds [Mn(HphpzMe)(2)(H(2)phpzMe)(HCO(2))] (1), [Mn(2)(phpzMe)(2)(HphpzMe)(2)(OCH(3))]·2CH(3)OH (2), Na{[Mn(HphpzPh)(phpzPh)(MeOH)(2)](2)}(HCO(2)) (3), [Mn(HphpzPh)(2)(EtOH)(2)]ClO(4)·2EtOH (4) and [Mn(HphpzPh)(2)N(3)] (5) were synthesized and characterized with various techniques. 1, 4 and 5 are mononuclear manganese(III) compounds, 2 is a mixed-valence dinuclear manganese(III/IV) compound, and 3 is a trinuclear compound containing two manganese(III) ions and a sodium(I) ion. A remarkable feature is the spontaneous formation of the formate ion as a result of the methanol or methoxide oxidation in compounds 1 and 3. Using ethanol precludes the formation of the formate and compound 4 is obtained. The molecular structure of all compounds is stabilized by supramolecular interactions, including strong hydrogen bonding and π-π interactions.

Wateroxidation catalysed by manganese compounds: from complexes to ‘biomimetic rocks’

One of the most fundamental processes of the natural photosynthetic reaction sequence is the light-driven oxidation of water to molecular oxygen. In vivo, this reaction takes place in the large protein ensemble Photosystem II, where a μ-oxido-Mn(4)Ca- cluster, the oxygen-evolving-complex (OEC), has been identified as the catalytic site for the four-electron/four-proton redox reaction of water oxidation. This Perspective presents recent progress for three strategies which have been followed to prepare functional synthetic analogues of the OEC: (1) the synthesis of dinuclear manganese complexes designed to act as water-oxidation catalysts in homogeneous solution, (2) heterogeneous catalysts in the form of clay hybrids of such Mn(2)-complexes and (3) the preparation of manganese oxide particles of different compositions and morphologies. We discuss the key observations from the studies of such synthetic manganese systems in order to shed light upon the catalytic mechanism of natural water oxidation. Additionally, it is shown how research in this field has recently been motivated more and more by the prospect of finding efficient, robust and affordable catalysts for light-driven water oxidation, a key reaction of artificial photosynthesis. As manganese is an abundant and non-toxic element, manganese compounds are very promising candidates for the extraction of reduction equivalents from water. These electrons could consecutively be fed into the synthesis of "solar fuels" such as hydrogen or methanol.

Theory‐Guided Experiments on the Mechanistic Elucidation of the Reduction of Dinuclear Zinc, Manganese, and Cadmium Complexes

Since the recognition of the first Zn Zn bond in the dinuclear sandwich decamethyldizincocene [(h-C5Me5)Zn Zn(h-C5Me5)], [1] the chemistry of Zn Zn-bonded species has grown so rapidly that many complexes of the type LZn ZnL have been characterized and studied. Regardless of the denticity of the supporting ligands L, they all coordinate to Zn in a terminally chelating mode. However, formation of these dinuclear compounds has not been mechanistically examined. We recently described the characterization of dinuclear Zn Zn-bonded species [{k-Me2Si(NDipp)2}Zn Zn{k-Me2Si(NDipp)2}] 2 (2) (Dipp= 2,6-iPr2C6H3) from KC8 reduction of dinuclear zinc complex [Zn2(m-k -Me2Si(NDipp)2)2] (1), whereby the coordination mode of the diamido ligands dramatically changes from bridging to chelating. We thus became interested in the structural preference and the formation mechanism of Zn Zn-bonded complexes. Elaborate calculations were performed to understand the reduction of 1, and a plausible mechanism was then proposed (Scheme 1). On two-electron reduction of 1, two intermediates, Ia and Ib, are generated, and the energy difference between them is only 0.3 kcalmol . The Zn Zn-bonded mixed-valent intermediate Ia is produced by one-electron reduction of 1, and subsequently undergoes a dramatic structural rearrangement to give Ib, in which one threecoordinate and one one-coordinate Zn atoms are proposed. The exact valence of the Zn atoms in Ib is still not clear. Although the application of quantum chemical methods (ab initio molecular orbital and density functional theory) to elucidate reaction mechanisms has been very successful, most of the time it is difficult to prove the theoretically developed reaction mechanisms by experiments. This is indeed the case for the transformation from 1 to 2. Attempts to probe both intermediates Ia and Ib failed. To this end, we turned our attention from zinc to manganese and cadmium, because they not only show structural similarity in the reported M M-bonded dinuclear complexes [(k-Nacnac)M M(k-Nacnac)] (M=Zn, Mn; Nacnac= HC[C(Me)NDipp]2) and [Ar’M MAr’] (M=Zn, Cd; Ar’= 2,6-(2,6-iPr2C6H3)2C6H3), but also feature an identical M M s-bonding scheme. Herein we report structural transformations on reduction of dinuclear manganese and cadmium complexes [Mn2{k -Me2Si(NDipp)2}2] (3) and [Cd2{mk-Me2Si(NDipp)2}2] (4). Characterization of the products supports the computed mechanism shown in Scheme 1. As shown in Scheme 2, reactions of the dilithiated diamido ligand and 1 equiv of anhydrous MnCl2 and CdCl2 in diethyl ether and THF, respectively, yielded the corresponding dimeric compounds 3 and 4 in good yields. The dinuclear nature of 3 and 4 was deciphered by single-crystal X-ray crystallography, and their molecular structures are provided in Figures S1 and S2 of the Supporting Information. Complex 3 is essentially composed of two MnN2Si fourmembered rings, which are brought together by two Mn N bonds, and consequently exhibit a boat conformation with two manganese atoms at the stern and two Si atoms at the bow. Each Mn atom is embraced by three nitrogen atoms and adopts a distorted T-shaped geometry. The central Mn2N2 Scheme 1. Calculated mechanism of transformation of 1 into 2.

Binding mode prediction and inhibitor design of anti-influenza virus diketo acids targeting metalloenzyme RNA polymerase by molecular docking.

Influenza is a yearly seasonal threat and major cause of mortality, particularly in children and the elderly. Although neuraminidase inhibitors and M2 protein blockers are used for medication, drug resistance has gradually emerged. Thus, the development of effective anti-influenza drugs targeting different constituent proteins of the virus is urgently desired. In this light, we carried out molecular docking to predict the binding modes of anti-influenza diketo acid inhibitors in the active site of the PAN subunit of the metalloenzyme RNA polymerase of influenza virus. The calculations suggested that the dianionic forms of the diketo acids should chelate the dinuclear manganese center as dinucleating ligands and sequester it. They also indicated that the diketo acid derivatives with larger hydrophobic substituents should block a hydrophobic cavity in the active site more tightly. These assumptions could adequately explain the enzyme inhibition by these compounds. Furthermore, we designed potential inhibitors by lead optimization of a diketo acid inhibitor from the thermodynamic points of view. Molecular docking results showed that the newly designed diketo acid derivatives might inhibit the metalloenzyme RNA polymerase more strongly than the lead inhibitor.

Dinuclear manganese centers in the manganese–lead–tellurate glasses

FTIR, UV–VIS and EPR spectra of manganese doped lead–tellurate glasses with composition xMnO·(100 − x)[4TeO 2·PbO 2] where x = 0, 1, 5, 10, 20, 30, 40 mol% have been studied. The FTIR spectra show the formation of the Mn–O–Pb and Mn–O–Te bridging bonds by increasing of MnO concentration. The UV–VIS spectra show the Mn +3 species exhibit pronounced absorption, which masks the Mn +2 spin-forbidden absorption bands when Mn +2 ions are in high concentrations in these glasses. The EPR spectra exhibit resonance signals characteristic of Mn +2 ions. The resonance signal located at g ≈ 2 is due to Mn +2 ions in an environment close to octahedral symmetry, whereas the resonance at g ≈ 4.3 and 3.3 are attributed to the rhombic surroundings of the Mn +2 ions. The increase in the MnO content gives rise to absorption at g ≈ 2.4 and the paramagnetic ions are involved in dinuclear manganese centers.

Three new dinuclear manganese(II) complexes with bis(μ-phosphinato)-bridges

A new series of dinuclear phosphinato-bridged manganese(II) complexes [Mn(μ-bmp)(bpy)(NO 3)] 2 ( 1), [Mn(μ-bmp)(phen)(NO 3)] 2·4CH 2Cl 2 ( 2) and [Mn 2(μ-bmp) 2(5-dmbpy) 2(NO 3)] 2 ( 3) where Hbmp is bis(4-methoxyphenyl)phosphinic acid, bpy = 2,2′-bipyridyl, phen = 1,10-phenanthroline and 5-dmbpy = 5,5′-dimethyl-2,2′-dipyridyl, have been synthesized and structurally characterized by X-ray crystallography. In this series, the structures consist in bis(4-methoxyphenyl)phosphinato anions (bmp) bridging the two Mn(II) centers in a syn– syn coordination mode. The coordination geometry around the Mn(II) ions in 1– 3 is six-coordinate with distorted octahedral environment. The magnetic behavior of these complexes is reported. The complexes show weak antiferromagnetic coupling with | J| in the range 0.1–0.6 cm −1. The magnetic properties are discussed in relation to the structural data.

Crystal Structure and Magnetic Properties of the Dinuclear MnII Compound [Mn2(bpy)4(2-ClC6H4COO)2](ClO4)2·2EtOH

The synthesis and crystal structure of the dinuclear manganese (II) compound [Mn2(bpy)4(2-ClC6H4COO)2](ClO4)2·2EtOH is described. The complex crystallizes in the monoclinic system, space group P21/n with a = 12.2067(18), b = 17.335(3), c = 13.706(3) A and β = 92.606(8)°. In this structure, two manganese ions are bridged by two 2-chlorobenzoate ligands in a syn–anti mode. The hexa-coordination of each manganese is completed by two 2,2′-bipyridine ligands. Two perchlorate anions and two molecules of ethanol complete the packing. In order to check the magnetic properties previously reported from a powder sample, new magnetic studies have been carried out from a crystal sample, obtaining J = −1.79 cm−1 and g = 2.00 (H = −JS 1 ·S 2 ). The X-ray crystal structure of the dinuclear MnII compound [Mn2(bpy)4(2-ClC6H4COO)2](ClO4)2·2EtOH has been determined and its magnetic properties studied.

K10 montmorillonite supported manganese catalysts for the oxidation of water to dioxygen

Two dimanganese complexes and two manganese salts were adsorbed on K10 montmorillonite clay to prepare water oxidation catalysts inspired by the manganese-containing active site of Photosystem II, the enzyme where water oxidation occurs in vivo. The montmorillonite hybrids of the dinuclear manganese(III,III) complex [Mn 2 III,III(µ-O)(tpdm) 2(µ-OAc) 2] 2+ (2@Mt, tpdm = tris(2-pyridy1)methane), manganese(II) sulphate (Mn 2+@Mt) and manganese(III) acetate (Mn 3+@Mt) are presented as an addition to the well studied clay hybrid system of [Mn 2 III,IV(µ-O) 2(terpy) 2(H 2O) 2] 3+ (1@Mt, terpy = 2,2′:6′,2″-terpyridine). As indicated by UVVis and EPR spectroscopy, the immobilization of the manganese compounds on the clay surfaces was associated with changes of their structure and their electronic properties (with the exception of Mn 2+@Mt). No remarkable changes in clay interlayer distance could be observed suggesting that the manganese compounds do not intercalate between the montmorillonite layers but adsorb only on the surface of the clay mineral. Three of the four heterogeneous systems were found to be able to catalyse water oxidation with the single-electron oxidation agent Ce IV. The manganese(III) acetate clay hybrid Mn 3+@Mt showed the highest activity, nearly three times higher than the best catalytic system of this class found so far, the montmorillonite hybrid 1@Mt. Because there are only very few examples of functional, manganese-based catalysts for water oxidation, these new materials represent interesting additions to the original hybrid systems. The spectroscopic results additionally indicate that the formation of manganese oligomers on the clay surface seems to be the most likely explanation for the fact that catalytically active materials are generated.

ChemInform Abstract: The Dinuclear Manganese Complex Mn2O(OAc)2(TPTN) as a Catalyst for Epoxidations with Hydrogen Peroxide.

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A dinuclear manganese(II) complex with 2,6-pyridinedicarboxylate: Preparation, crystal structure and oxygen evolution activity in the presence of oxone

Abstract A dinuclear complex of Mn(II) with 2,6-pyridinedicarboxylate, [Mn2(dipic)2(H2O)6].2H2dipic, where dipic = 2,6-pyridinedicarboxylate, has been synthesized and characterized by elemental analysis, IR, EPR, electronic absorption spectroscopy, electrochemistry and single-crystal X-ray diffraction. The complex acts as an oxygen evolving complex with oxone (2KHSO5.KHSO4 .K2SO4) as primary oxidant in aqueous solution with a turnover number of ∼ 5 (mol of O2/mol of the complex).

Catalase-like catalytic reaction of the dinuclear manganese–salen complex

Catalase-like activity of a dinuclear manganese-salen (Mn–salen) complex, [Mn(salen)(H2O)]2(ClO4)2 (salen = N,N ′-bis(salicylidene)-1,2-diaminoethane), was investigated. The dinuclear Mn–salen complex exhibits higher catalase-like activity than that of the mononuclear Mn–salen compound, and its activity can be enhanced by an external base. Different reaction intermediates in the presence and absence of an external base were observed, and the catalytically active species was dimeric as evidenced by UV-Vis spectroscopic studies and mass spectrometry data.

Diversity of coordination modes in the polymers based on 3,3′,4,4′-biphenylcarboxylate ligand

Four new compounds [Ni 2(4,4′-bpy)(3,4-bptc)(H 2O) 4] n ( 1), [Ni(4,4′-bpy)(3,4-H 2bptc)(H 2O) 3] n ( 2), [Mn 2(2,2′-bpy) 4(3,4-H 2bptc) 2] ( 3) and {[Mn(1,10-phen) 2(3,4-H 2bptc)]·4H 2O} n ( 4) (3,4-H 4bptc=3,3′,4,4′-biphenyltetracarboxylic acid, 4,4′-bpy=4,4′-bipyridine, 2,2′-bpy=2,2′-bipyridine, 1, 10-phen=1, 10-phenanthroline), have been prepared and structurally characterized. In all compounds, the derivative ligands of 3,4-H 4bptc (3,4-bptc 4− and 3,4-H 2bptc 2−) exhibit different coordination modes and lead to the formation of various architectures. Compounds 1 and 2 display the three-dimensional (3D) framework: 1 shows a 3,4-connected topological network with (8 3)(8 5·10) topology symbol based on the coordination bonds while in 2, the hydrogen-bonding interactions are observed to connect the 1D linear chain generating a final 3D framework. 3 exhibits the 2D layer constructed from the hydrogen-bonding interactions between the dinuclear manganese units. Complex 4 shows the double layers motif through connecting the 1D zigzag chains with hydrogen-bonded rings. The thermal stability of 1– 4 and magnetic property of 1 were also reported.

Hydrothermal Syntheses and Crystal Structures of Two New Three‐dimensional Coordination Polymers Based on 4‐Pyrid‐3‐ylbenzoic Acid

AbstractTwo new coordination polymers [Co2(pbc)4(H2O)]n (1) and [Mn(pbc)2] (2) (Hpbc = 4‐pyrid‐3‐ylbenzoic acid) were obtained by hydrothermal reaction and characterized by elemental analysis, IR spectroscopy, and single‐crystal X‐ray diffraction. Compound 1 features a 3D network with a four‐connected 66 net constructed from secondary building units of dinuclear cobalt. Compound 2 exhibits a six‐connected 412·63 topology based on dinuclear manganese.