Mononuclear Manganese Complexes Research Articles

Oxidation of iodide with a mononuclear manganese(IV) complex ion: Mechanistic investigation of autocatalytic behaviour

One example of rare water soluble Mn(IV) species is the chelate complex formed between Mn(IV) and biguanide, the tris(biguanide)manganese(IV) complex ion, [Mn(LH2)3]4+ (1), LH2 = biguanide (C2N5H7). When this Mn(IV) complex is reacted with iodide ion in mildly acidic solution, reaction profiles of an autocatalytic nature are observed. These profiles show an initial slow reduction followed by a faster and nearly linear progress, and then become curved again near the end of the reaction. The observed kinetic behaviour is explained by a mechanism involving the reduction of Mn(IV) to Mn(II) via Mn(III) and a comproportionation between Mn(IV) and Mn(II) to regenerate Mn(III). The Mn(III) species is believed to be a steady state intermediate for the most part of the reaction. This sequence of reaction explains the observed profiles pretty well and the second order rate constants for successive electron transfers were estimated using KINSIM simulations. The effect of variation of pH and added biguanide on the reaction rates are small and accounted for logically, assuming the involvement of a ligand dissociation equilibrium for the Mn(III) complex.

Synthesis, crystal structure, and supramolecular interactions in a bis(tetrachlorocatecholate) chelated manganese(III) complex

A mononuclear manganese(III) complex [DABH2][Mn2(Cl4Cat)2(H2O)2]·4DMF (1) (DAB = 1,4-diaminobutane, Cl4CatH2 = tetrachlorocatechol) is synthesized by a one-pot reaction involving manganese(II) chloride tetrahydrate and tetrachlorocatechol in a 1:2 molar ratio in the presence of 1,4-diaminobutane in a DMF-water mixture under aerobic conditions, and is structurally characterized. X-ray crystallography reveals that the complex anion is constructed with two tetrachlorocatecholate ligands coordinated to the manganese(III) center in equatorial positions, and two axial positions are occupied by two water molecules. The crystal packing of 1 is stabilized by complex networks of hydrogen bonding interactions in which oxygen atoms of coordinated tetrachlorocatecholate ligands and lattice DMF molecules serve as hydrogen bond acceptors, while the axially coordinated water molecules together with doubly protonated 1,4-diaminobutane act as hydrogen bond donors. The solid state packing of 1 is further stabilized by Cl⋯Cl halogen bonding interactions within tetracholorocatecholate units and the C—H⋯π and π⋯π interactions involving DMF solvates and aromatic rings of the tetracholorocatecholate ligands. The complex is further characterized by IR spectroscopy and cyclic voltammetry, and the results are analyzed.

Formation, stability and catalase-like activity of mononuclear manganese(ii) and oxomanganese(iv) complexes in protic and aprotic solvents

Catalytic and stoichiometric H2O2oxidation by [MnII(N4Py*)]2+and [MnIV(N4Py*)(O)]2+complexes as catalase mimics have been carried out.

Synthesis, structure and catalytic activity of manganese(ii) complexes derived from bis(imidazole)methane-based ligands.

New mononuclear manganese(ii) complexes [Mn(κ2-L1)(OAc)2] ([Mn]-1), [Mn(κ2-L2)(OAc)2] ([Mn]-2) and [Mn(κ2-L3)(OAc)2] ([Mn]-3) with imidazole based ligands {4,4'-(phenylmethylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L1), {(4,4'-((2-methoxy phenyl)methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L2) and {4,4'-((2-chlorophenyl) methylene)bis(2-ethyl-5-methyl-1H-imidazole)} (L3) are synthesized and fully characterized by a variety of techniques. Furthermore, the molecular structures of complexes [Mn]-1 and [Mn]-2 are established by single crystal X-ray structure analysis. The synthesized manganese(ii) complexes exhibited efficient catalytic oxidative coupling of primary amines in air under solvent-free conditions to the corresponding imines in moderate to good yields.

Stepwise spin-state switching in a manganese(III) complex.

A mononuclear manganese(iii) complex containing a flexible hexadentate chelating ligand has been prepared and characterized by performing, at various temperatures, single-crystal X-ray diffraction analyses and magnetic, spectroscopic, and electrochemical studies. The complex was shown to consist of an MnN4O2 octahedral coordination environment, and to exhibit reversible two-step thermally induced spin-state switching, a gradual one at 168 K and an abrupt one at 103 K. Structural analyses revealed the existence of three spin-states, namely high-spin, low-spin, and intermediate states, during the spin-state switching process. Electrochemistry studies showed the quasi-reversible reduction and oxidation of the manganese(iii) center with a comparatively easily accessible reduced state.

Syntheses, molecular structures, and spectroscopic properties of manganese(II)/(III) complexes with tetraphenylimidodiphosphinato and bi-pyridine or salicylaldehyde ligands

Treatment of [Mn(CH3COO)2·4H2O] with two equivalents of K[N(Ph2PO)2] in the presence of one equivalent of 2,2′-bipyridine (bpy) or 5,5′-dimethyl-2,2′-bipyridine(dmbpy) in ethanol resulted in the formation of the mono-nuclear manganese(II) complexes [Mn{η1ON(Ph2PO)2}{N(Ph2PO)2}(EtOH)(bpy)] (1) and [Mn{N(Ph2PO)2}2(dmbpy)] (2), respectively. Interaction of [Mn(CH3COO)2·4H2O], K[N(Ph2PO)2] and salicylaldehyde or 5-chlorosalicylaldehyde or 3,5-dibromosalicylaldehyde in the presence of triethylamine in methanol gave the bi-nuclear manganese(II) complexes [Mn2{N(Ph2PO)2}2(μ,η2-O,O′-Sal)2(MeOH)2] (3) and [Mn2{N(Ph2PO)2}2(μ,η2-O,O′-5-Cl-Sal)2(MeOH)2] (4), and a tetra-nuclear manganese(II)/(III) complex [Mn4{N(Ph2PO)2}2(μ,η2-O,O′-3,5-Br2-Sal′)2(MeOH)4(μ-OMe)2(μ3-OMe)2] (5), respectively. All complexes were characterized by infrared and ultraviolet spectroscopy, their molecular structures were unambiguously established by single crystal X-ray diffraction. The electrochemical properties of complexes 1–5 were also investigated in the paper.

Synthesis and characterization of a manganese(III) schiff base complex and exploration of Br···Br interaction in the solid state structure of the complex

A mononuclear manganese(III) complex, [MnL(CH3OH)(H2O)]ClO4, has been synthesized and structurally characterized {H2L = N,N'-bis(5-bromo-3-methoxysalicylidene)2,2-dimethyl-1,3-propanediamine}. The energetic features of significant supramolecular interactions present in the complex, i.e. Br···Br, hydrogen-bonding and π···π stacking interactions, have been calculated using DFT calculations and further corroborated with NCI plot index computational tool. Catalase mimicking activity (catalytic decomposition of hydrogen peroxide into oxygen and water) of the complex has been investigated. The complex catalyzes the decomposition of hydrogen peroxide effectively in solution. The efficiency of the complex toward catalytic decomposition of hydrogen peroxide is related to its structure.

Syntheses, structures and catalytic properties of new mononuclear terpyridine-manganese(II) complexes with tetraphenylimido-diphosphinate and diphenylphosphinate ligands

Treatment of manganese(II) acetate tetrahydrate [Mn(CH3COO)2·4H2O] with one equivalent of 2,2′:6′,2′′-terpyridine (terpy) and two equivalents of potassium tetraphenylimido-diphosphinate K[N(Ph2PO)2] in methanol afforded a mononuclear manganese(II) complex, [(terpy)Mn{η1-O-N(Ph2PO)2}2(H2O)] (1), with two terminal [N(Ph2PO)2]– ligands. Interaction of [Mn(CH3COO)2·4H2O] with one equivalent of terpy in the presence of both K[N(Ph2PO)2] and Ph2PO2K in methanol gave a mononuclear manganese(II) complex [(terpy)Mn(η1-O-O2PPh2){N(Ph2PO)2}] (2) with a chelated [N(Ph2PO)2]– ligand. Treatment of manganese(II) dichloride tetrahydrate [MnCl2·4H2O] with three equivalents of K[N(Ph2PO)2] in methanol resulted in isolation of a mononuclear manganese(III) complex [Mn{η1-O-N(Ph2PO)2}-{N(Ph2PO)2}2] (3) with one terminal and two chelated [N(Ph2PO)2]– ligands. Reaction of [Mn(CH3COO)2·4H2O] with one equivalent of 4′-phenyl-[2,2′:6′,2′′]-terpyridine (4-Ph-terpy) and two equivalents of Ph2PO2K in methanol gave [(4-Ph-terpy)Mn(η1-O-O2PPh2)2(H2O)] (4) with a labile water molecule. Complexes 1–4 have been spectroscopically characterized and their structures have been established by single-crystal X-ray diffraction. Catalytic behavior of 1 and 4 for sulfide oxidation was also investigated.

Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(II) Complex.

A mononuclear manganese(II) complex, [Mn(4-bzpy)4Cl2] (4-bzpy = 4-benzylpyridine), exhibits a slow magnetic relaxation with three relaxation modes that is supported by the external magnetic field. The low-frequency mode shows an exceptionally slow relaxation time: τ = 798 ms at T = 1.9 K and BDC = 0.35 T. The high-frequency domain exhibits unusual acceleration of the relaxation time upon cooling below 3.5 K.

Mononuclear manganese(iii) complexes with reduced imino nitroxide radicals by single-electron transfer and intermolecular hydrogen bonds as an intramolecular structural driving force.

Manganese(iii) complexes were synthesized by one-electron transfer from a Mn(ii) ion to the imino nitroxide radical 2-(2-imidazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (IMImH) in methanol. After the manganese ions attained the +III oxidation state, the imino nitroxide radicals were found to be irreversibly reduced in the complexes. Depending on the synthesis conditions, two complexes differing by their counter-anions were isolated as single crystals. These are [Mn(IMHIm)2(MeOH)2]ClO4·H2O (1) and [Mn(IMHIm)2(MeOH)2]PF6 (2), which crystallize in the monoclinic P21/n and triclinic P1[combining macron] space groups, respectively. The two complexes show Jahn-Teller distortions typical of Mn(iii) centres and only reduced radicals are coordinated, as indicated by the N-O bond lengths and electroneutrality. In addition, the crystal structure analyses reveal two intermolecular hydrogen bonding networks. One involves counter-anions, water molecules and reduced radicals, and the other involves coordinated methanol molecules and imidazole moieties. These intermolecular interactions are driving forces that stabilize the two complexes. They also suggest that the tautomer is in the amino imine-oxide form after reduction of the radical and reveal the deprotonation of the imidazole ring, which is required for electroneutrality. This assessment is supported by single-crystal X-ray diffraction, EPR and Raman spectroscopy as well as magnetic and electrochemical studies.

Mn2+-COMPLEXES OF N,O-DIHYDRAZONE: STRUCTURAL STUDIES, INDIRECT BAND GAP ENERGY AND CORROSION INHIBITION ON ALUMINUM IN ACIDIC MEDIUM

Mononuclear manganese complexes of dihydrazone derived from the condensation of oxaloyldihydrazide with 2-hydroxynaphthaldehyde and 2-methoxybenzaldehyde have been synthesized. The hydrazone Schiff base ligands and their chelates were characterized on the basis of their elemental analyses, spectral (UV-Vis., IR, mass, 1H/13C NMR), magnetism, ESR and thermal (TGA) measurements. The dihydrazone has been suggested to coordinate to the metal center in bi-dentate manner forming 1:1[M:L] complex. The complexes are suggested to have tetrahedral/octahedral stereochemistry. Optical transmission spectra were recorded in the range 190–2100 nm and optical band gap energy was determined. The band gap energy (Eg) for all separated compounds lies in the range of semiconductors. On the other hand, the inhibition and adsorptive properties of the ligands for the corrosion of aluminum in 1 M HCl solutions were studied using traditional weight loss measurements. The results revealed that bis(2-methoxy-benzaldehyde)oxaloyldihydrazone has a greater inhibition than bis(2-hydroxy-1- naphthaldehyde)oxaloyldihydrazone. The adsorption of the inhibitors on metal surface was found to be spontaneous first order reaction and consistent well with the mechanism of physical adsorption. The adsorption data fitted well to Freundlich, Langmuir and Frumkin adsorption isotherms.

Using theoretical calculations to predict the redox potential of mononuclear manganese complexes

Prediction of redox potential allows chemists to rationally design metal complexes with a desired redox activity.

Oxidation of Organic Functionalities by PhI(OAc)2 Catalysed by Magnetically Separable Fe3O4@dopa‐Supported Mn(III) Complexes: Combined Experimental and Theoretical Approach

AbstractThree newly designed ligands N, N′‐bis (3, 5‐diX)‐2, 2‐dimethylpropane‐1, 3 diamine, where X=Cl/Br/I (H2L1‐H2L3) having N2O2binding sites, have been chosen to synthesize mononuclear manganese(III) complexes 1–3 with an aim to study their catalytic activity towards oxidation of various substrates using PhI(OAc)2as a terminal oxidant. All these complexes have been characterized by routine physico‐chemical techniques. Complexes 1 (MnL1Cl ) and 2 (MnL2Cl ) have further been structurally characterized by X‐ray single crystal structure analysis. Detailed analysis of ESI‐MS, UV‐Vis and cyclic voltammetry studies along with theoretical calculations helped us to put forward probable mechanistic pathway for these efficient catalytic reactions. Complex 1exhibited the highest catalytic efficiency in every reaction and therefore it was selected for building a magnetically separable catalyst by attaching it to surface modified magnetic nanoparticles. The new Fe3O4@dopa@MnL1Cl(dopa=dopamine hydrochloride) [FDM‐1] catalyst was characterized by solid state UV‐VIS, FT‐IR, SEM, TEM, TGA, PXRD, EDX and SQUID analyses. The catalytic activity towards oxidation of various substrates by FDM‐1 was studied using PhI(OAc)2 and results indicated excellent efficiency along with reusability.

Techniques in the synthesis of mononuclear manganese complexes: a review

AbstractThis article describes an overview of the synthetic techniques and protocols for the preparation of new ligands and respective manganese (Mn) complexes to be tested for biomedical applications. Mn is an essential and biocompatible element, the complexes of which have diverse medicinal applications. The most significant use of Mn complexes is their application against reactive oxygen species in biological systems, and due to this, three Mn-incorporated complexes (AEOL-10150, EUK-134, and M40403) are already under clinical trials. Hence, the interest in synthesizing biologically active Mn complexes is rapidly increasing. Mn complexes are commonly synthesized using either water or ethanol as a reaction medium for their possible usage in biological systems. Using common Mn salts along with suitable organic ligand works well in the presence of little heat to obtain Mn complexes of interest.

Synthesis, crystal structure, and magnetic characterization of two manganese Schiff-base-containing complexes

The reactions of [MnIII(3-MeOSalen)(H2O)2]+ (Salen = N,N-ethylenebis(salicylideneaminato) dianion) with (Et4N)4[M(CN)8] (M = Mo, W) have been investigated and one mononuclear manganese(II) complex [MnII(Salen)(H2O)] (I) and one bimetallic ion-pair complex [MnIII(3-MeO-Salen)(H2O)2]4[W(CN)8] · DMSO · 4H2O (II) were obtained unexpectedly and characterized by element and single crystal structure analysis. Single crystal X-ray diffraction (CIF files CCDC nos. 1456365 (I) and 1456366 (II)) showed that the Mn2+ ion in complex I is five-coordinated involving in a distorted square pyramid. Furthermore, with the help of the intermolecular hydrogen bond interactions, this complex can be constructed into interesting one-dimensional zig-zag chain structure. For complex II, the coordination sphere of Mn3+ ion is an elongated octahedron. Additional, the four mononuclear manganese(III) units are self-complementary through the coordinated aqua ligand from one molecule and the free O(4) compartment from the neighboring molecule, giving supramolecular dimmers structure. Investigation of the magnetic susceptibility of the two complexes reveals the overall weak antiferromagnetic interactions between the adjacent manganese centers caused by H-bond interactions.

Mechanism of Water Oxidation Catalyzed by a Mononuclear Manganese Complex.

The design and synthesis of biomimetic Mn complexes to catalyze oxygen evolution is a very appealing goal because water oxidation in nature employs a Mn complex. Recently, the mononuclear Mn complex [LMnII (H2 O)2 ]2+ [1, L=Py2 N(tBu)2 , Py=pyridyl] was reported to catalyze water oxidation electrochemically at an applied potential of 1.23 V at pH 12.2 in aqueous solution. Density functional calculations were performed to elucidate the mechanism of water oxidation promoted by this catalyst. The calculations showed that 1 can lose two protons and one electron readily to produce [LMnIII (OH)2 ]+ (2), which then undergoes two sequential proton-coupled electron-transfer processes to afford [LMnV OO]+ (4). The O-O bond formation can occur through direct coupling of the two oxido ligands or through nucleophilic attack of water. These two mechanisms have similar barriers of approximately 17 kcal mol-1 . The further oxidation of 4 to generate [LMnVI OO]2+ (5), which enables O-O bond formation, has a much higher barrier. In addition, ligand degradation by C-H activation has a similar barrier to that for the O-O bond formation, and this explains the relatively low turnover number of this catalyst.

Structural, spectral, electrochemical and DFT studies of two mononuclear manganese(II) and zinc(II) complexes

Two mononuclear M(II) complexes, [M(L2)2]·CH3OH (M=Mn(1) and Zn(2), HL2=1-(2-{[(E)-3,5-dibromo-2-hydroxybenzylidene]amino}phenyl)ethanone oxime), were synthesized via complexation of corresponding M(II) acetate with HL1 (2-(3,5-dibromo-2-hydroxyphenyl)-4-methyl-1,2-dihydroquinazoline-3-oxide, H is the deprotonatable hydrogen) originally. During the reaction, the C–N bond in the ligand HL1 is converted into the CNOH group in the HL2. The spectral data of both complexes were compared with the ligand HL1. Both complexes were determined by single crystal X-ray diffraction and display similar coordination geometry and have a 2:1 ligand-to-metal ratio. In the crystal structure, complexes 1 and 2 form an infinite 1-D chain and 2 into 3-D supramolecular frameworks. The electrochemical property of complex 1 was investigated by cyclic voltammetry. The electronic transitions and spectral features of HL1 and both complexes were discussed by DFT and TD-DFT calculations. Time dependent DFT calculations have been carried out on the optimised geometry to further understand the electronic transitions in the UV–Vis spectra of the compounds. In addition, the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and HOMO–LUMO gap were also calculated.

Two new manganese(III) complexes with salicylaldimine Schiff bases: Synthesis, structure, self-assembly and phenoxazinone synthase mimicking activity

Two new mononuclear manganese(III) complexes, [Mn(L1)2]ClO4 (1) and [Mn(L2)(NCS)(H2O)]·DMSO (2), where HL1=3-(N,N-dimethylamino)propyliminomethyl-6-ethoxyphenol, H2L2=N,N′-bis(3-ethoxysalicylidene)ethane-1,2-diamine have been prepared and characterized by elemental analysis, IR, UV–Vis spectroscopy and single crystal X-ray diffraction studies. Manganese(III) in each complex assumes distorted octahedral geometry. Supramolecular interactions in both complexes were explored. Both complexes show phenoxazinone synthase mimicking activity but presence of a solvent molecule in the coordination site of complex 2 makes it more efficient catalyst compound to complex 1.

Tris{2-[(5-fluorosalicylidene)amino]ethyl}amine and its manganese(III) complex: Synthesis, characterization, crystal structures, and catalytic property

ABSTRACT A tris-Schiff base tris{2-[(5-fluorosalicylidene)amino]ethyl}amine (H3L) was prepared by the reaction of 1:3 molar ratio of tris(2-aminoethy1)amine with 5-fluorosalicylaldehyde in methanol. The Schiff base reacted with manganese perchlorate to give a mononuclear manganese(III) complex, [MnL]. The Schiff base and the complex were characterized by elemental analysis, IR spectra, and single-crystal X-ray determination. The Schiff base crystallized in triclinic space group P , with unit cell dimensions a = 10.191(2) Å, b = 10.778(2) Å, c = 13.353(2) Å, α = 79.727(2)°, β = 81.986(2)°, γ = 64.717(2)°, V = 1301.6(3) Å3, Z = 2, R 1 = 0.0522, wR 2 = 0.1155. The complex crystallized in cubic space group Ia-3, with unit cell dimensions a = b = c = 21.6266(5) Å, V = 10115.0(4) Å3, Z = 16, R 1 = 0.0779, wR 2 = 0.2442. The Mn atom in the complex is in octahedral coordination. The complex has good catalytic capability in the oxidation of selected olefins to the corresponding epoxides.

Tetrabromocatecholato Mn(III) complexes of bis(phenol) diamine ligands as models for enzyme–substrate adducts of catechol dioxygenases

Two mononuclear manganese(III) complexes of bis(phenol) diamine ligands (H2LNEX) and 2,3,4,5-tetrabromocatechol (TBC), MnLNEX(TBC), were synthesized as models for enzyme–catechol adducts of the intradiol Fe-dependent catechol dioxygenases. X-ray analysis of the complexes has revealed that the manganese center in the model compounds has a distorted octahedral coordination sphere and is surrounded by the LNEX ligand and two oxygen atoms of TBC. The phenolate moieties of MnLNEX(TBC) were electrochemically oxidized to phenoxyl radicals. Consistent with the electrochemical results, quantum chemical calculations showed that the HOMO and LUMO levels on both complexes are on the phenolate moieties. The paramagnetic properties of the manganese(III) center of the complexes have been investigated by magnetic susceptibility measurements. H2LNEX/MnCl2 showed quite good enzyme-mimicking reactivity. It utilized molecular oxygen in carrying out the cleavage of di-tert-butyl catechol at room temperature. To the best of our knowledge this is the second report where a manganese complex can perform oxygenase and not oxidase activity.