Mixed-valence Manganese Research Articles

Oligonuclear homo- and mixed-valence manganese complexes based on thiophene- or aryl-carboxylate ligation: Synthesis, characterization and magnetic studies

Mixed-valence trinuclear manganese compounds [Mn3O(O2CTh)6(L)x(H2O)y]·n(solvent) (Th=thiophene, 1·CH3CN: L=pyridine (py), x=3, y=0, n=1, solvent=CH3CN; 1·∼H2O: L=py, x=3, y=0, n=∼1, solvent=H2O; 2: L=py, x=2, y=1, n=0.25, solvent=CH3CN; 3: L=3-Mepy, x=2, y=1, n=0) containing a [MnIIMn2III(μ3-O)]6+ core have been prepared. Homo-valence tetranuclear manganese complexes (NBun4)[Mn4O2(O2CAr)9(L)] (4·ThCO2H·CH2Cl2: Ar=–Th, L=EtOH; 6: Ar=–Ph, L=H2O; 7: Ar=–Ph-p-Me, L=H2O; 8·CH2Cl2: –Ph-3,5-Me2, no L) with a [MnIII4(μ3-O)2]8+ core and [MnIIMnIII3O2(O2CTh)7(bpy)2] (5) were synthesized, structurally and magnetically characterized. Compounds 1–3 were obtained by comproportionation of MnII(O2CMe)2·4H2O with (NBun4)MnVIIO4 in aprotic pyridine (1) or aprotic/protic py/EtOH (2) or Mepy/EtOH (3) solvent mixtures. Clusters 4–8 were synthesized by comproportionation of MnII(O2CAr)2·xH2O/ArCOOH with (NBun4)MnVIIO4 in EtOH/CH3CN. X-ray structural characterization of the 2-thiophenecarboxylate (ThCO2−) containing compounds 1·CH3CN, 1·∼2H2O and 4·ThCO2H·CH2Cl2 revealed a thiophene ring disorder about the (O2)C–C(Th) bond so that the S atom and opposite (5–)CH group are distributed over two ring positions each. The MnII atom in the approximately isosceles triangular [MnIIMn2III(μ3-O)]6+ cores in 1 could be clearly assigned from bond valence sum calculations and bond distances. The [Mn4(μ3-O)2]8+ core in the anionic complexes in the structures of 4·ThCO2H·CH2Cl2 and 8·CH2Cl2 has an all MnIII oxidation level and “butterfly-like” arrangement. The magnetic properties of 4–8 were investigated by variable temperature magnetic susceptibility and magnetization measurements. Similar magnetic behavior was observed for the [MnIII4(μ3-O)2]8+ butterfly-core compounds 4, 6–8, with antiferromagnetic interactions between pairs of manganese ions with amplitude of ∼14.5cm−1 and ∼4.6cm−1 for the Mnb⋯Mnb and Mnb⋯Mnw coupling, respectively (b=body, w=wing). Changing the Ar-ligand size in [Mn4O2(O2CAr)9(L)]− does not have a significant effect on the magnetic properties of these butterfly clusters.

The effect of MnO2 loading on the glycerol electrooxidation activity of Au/MnO2/C catalysts

Manganese oxides have shown promise as co-catalysts for platinum- and palladium-based alcohol electrooxidation catalysts. In this investigation, the effect of adding MnO2 to a gold-based catalyst system was explored for the glycerol electrooxidation reaction. It was found that modification of the carbon support with MnO2 had the effect of stabilising gold nanoparticles, giving rise to smaller gold particles with a more uniform particle size distribution than on the carbon alone. All gold-based catalysts gave very high currents for glycerol oxidation (1–5Amg−1cat) and catalysts with MnO2 contents of 5 and 9wt% yielded the highest mass activities (35–37Amg−1Au). This is believed to be partly due to MnO2 preventing gold agglomeration, creating a larger gold surface area for reaction. In addition, it is believed that MnO2 may act as an active co-catalyst, especially at higher potentials, by means of oxygen spillover onto gold – leading to higher activities and lower peak potentials. MnO2 was also found to activate the glycerol oxidation reaction earlier, giving lower onset potentials. Stability and impedance studies showed that MnO2-modified catalysts were less stable at a low potential (0V) than Au/C. This could be due to blocking of mixed-valence manganese oxide sites or poisoning of the smaller gold particles on these catalysts. However, at a higher potential of 0.2V, Au/5% MnO2/C demonstrated the best activity and stability overall. All our catalysts are more active than the palladium- and platinum-based catalysts reported in literature, with our best-performing catalyst on a mass activity basis (Au/9% MnO2/C) being at least 12 times more active. This effectively demonstrates the advantage of using a gold-based catalyst for glycerol oxidation, especially when supported by MnO2 which aids in lowering the overpotential for glycerol oxidation over gold.

Structure and magnetism of a mixed-valence octanuclear manganese(ii/iii) cluster derived from carbamoylcyanonitrosomethanide (ccnm)

A mixed-valence octanuclear manganese complex of formula [Mn(II)(4)Mn(III)(4)O(4)(ccnm)(12)]·4MeCN·7H(2)O (1·4MeCN·7H(2)O) (ccnm = carbamoylcyanonitrosomethanide) has been synthesized and characterized by single crystal X-ray diffraction, IR spectroscopy, elemental analysis and magnetic measurements. The metal core of 1 adopts a tetra-capped (with Mn(II) ions) distorted [Mn(III)(4)O(4)](4+) cubane topology, with a three-J magnetic analysis indicating that the cluster displays dominant antiferromagnetic interactions. Three equally possible fits were obtained, all leading to a spin ground state of S = 0 with S = 1, 2 and 3 close in energy. Comparisons of the magnetic analysis are made with that published for a related Mn(8) cluster and, more generally, with other Mn(III)Mn(III) and Mn(III)Mn(II) fragments in clusters of various types.

Mn4 Single-Molecule-Magnet-Based Polymers of a One-Dimensional Helical Chain and a Three-Dimensional Network: Syntheses, Crystal Structures, and Magnetic Properties

Two Mn(4) single-molecule-magnet (SMM)-based coordination polymers, {[Mn(4)O(salox)(3)(N(3))(3)(DMF)(2)(H(2)O)(dpp)]·0.5MeOH}(n) (1·0.5MeOH; H(2)salox = salicylaldoxime; dpp = 1,3-di-4-pyridylpropane; DMF = N,N-dimethylformamide) and {[Mn(4)O(Me-salox)(3)(N(3))(3)(dpp)(1.5)]·1.5Et(2)O}(n) (2·1.5Et(2)O; Me-H(2)salox = hydroxyphenylethanone oxime), are self-assembled from Mn(ClO(4))(2)·6H(2)O/H(2)salox and Mn(ClO(4))(2)·6H(2)O/Me-H(2)salox systems with dpp and NaN(3) in DMF/MeOH, respectively. Both compounds comprise a mixed-valence tetranuclear manganese core, [Mn(II)Mn(III)(3)O](9+), which serves as a building unit for subsequent assembly via oximate and azido ligands. The flexible dpp ligand links with a Mn(4) unit, leading to the formation of a one-dimensional helical structure in 1·0.5MeOH and a three-dimensional pcu network in 2·1.5Et(2)O. The magnetic data analysis shows that antiferromagnetic interactions within the Mn(4) units resulted in S = (3)/(2) and (7)/(2) ground states for 1·0.5MeOH and 2·1.5Et(2)O, respectively. Both compounds show SMM behavior, as evidenced by frequency-dependent out-of-phase signals in alternating-current magnetic susceptibility and magnetic hysteresis loop studies with an energy barrier of U(eff) = 37 K for 2·1.5Et(2)O.

Effect of processing kinetics on the structure of ferromagnetic-ferroelectric-ferromagnetic interfaces

Trilayer heterostructures consisting of a ferroelectric bismuth ferrite (BFO) film sandwiched between ferromagnetic lanthanum strontium manganese oxide (LSMO) films were fabricated using pulsed laser deposition. Both BFO thicknesses (20 nm, 5 nm) and cooling rates were varied to investigate the role of processing parameters on the chemistry of the interfaces. The interfaces were investigated using a dedicated aberration corrected scanning transmission electron microscope (STEM) operated at 100 kV via STEM-high angle annular dark field (STEM-HAADF) and STEM-electron energy loss spectroscopy (STEM-EELS) modes. Combined analysis through STEM-HAADF and STEM-EELS revealed the formation of lattice distortion in certain regions of the BFO layer for the ∼5 nm film. Piezoresponse force microscopy (PFM) studies of the ∼5 nm BFO sample revealed weak ferroelectric domain switching. Stacking fault defects with mixed valence manganese (Mn-B site cation) were formed in the top LSMO layer when the heterostructure was cooled at a slower rate irrespective of BFO thickness, thereby demonstrating the effect of processing kinetics on the physical integrity of the heterostructure.

Ferromagnetic cluster spin waves in molecular disks studied by inelastic neutron scattering

Structurally, the two mixed-valence manganese disks Mn${}_{7}$-11 and Mn${}_{7}$-16 differ only in the peripheral ligand but, as a result of a subtle interplay of intramolecular exchange interactions, differ strongly in their magnetic properties, e.g., Mn${}_{7}$-11 possesses a ground-state spin of $S=11$ and Mn${}_{7}$-16 of $S=16$. The exchange interactions in the disks were studied by inelastic neutron scattering. The analysis of the $Q$ dependence of the observed magnetic transition intensities reveals that ferromagnetic cluster spin-wave excitations were observed. In this framework, it was possible to successfully model the experimental data and provide a physical understanding of the magnetism in the two disks.

Correction to Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storage

Correction to Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storage

Anomalous Pseudocapacitive Behavior of a Nanostructured, Mixed-Valent Manganese Oxide Film for Electrical Energy Storage

While pseudocapacitors represent a promising option for electrical energy storage, the performance of the existing ones must be dramatically enhanced to meet today's ever-increasing demands for many emerging applications. Here we report a nanostructured, mixed-valent manganese oxide film that exhibits anomalously high specific capacitance (∼2530 F/g of manganese oxide, measured at 0.61 A/g in a two-electrode configuration with loading of active materials ∼0.16 mg/cm(2)) while maintaining excellent power density and cycling life. The dramatic performance enhancement is attributed to its unique mixed-valence state with porous nanoarchitecture, which may facilitate rapid mass transport and enhance surface double-layer capacitance, while promoting facile redox reactions associated with charge storage by both Mn and O sites, as suggested by in situ X-ray absorption spectroscopy (XAS) and density functional theory calculations. The new charge storage mechanisms (in addition to redox reactions of cations) may offer critical insights to rational design of a new-generation energy storage devices.

A New Partially Deprotonated Mixed-Valence Manganese(II,III) Hydroxide–Arsenate with Electronic Conductivity: Magnetic Properties of High- and Room-Temperature Sarkinite

A new three-dimensional hydroxide-arsenate compound called compound 2 has been synthesized by heating (in air) of the sarkinite phase, Mn(2)(OH)AsO(4) (compound 1), with temperature and time control. The crystal structure of this high-temperature compound has been solved by Patterson-function direct methods. A relevant feature of this new material is that it is actually the first member of the adamite-type family with mixed-valence manganese(II,III) and electronic conductivity. Crystal data: a = 6.7367(5) Å, b = 7.5220(6) Å, c = 9.8117(6) Å, α = 92.410(4)°, β = 109.840(4)°, γ = 115.946(4)°, P1̅. The unit cell content derived from Rietveld refinement is Mn(8)(O(4)H(x))(AsO(4))(4). Its framework, projected along [111], is characterized by rings of eight Mn atoms with the OH(-)/O(2-) inside the rings. These rings form an almost perfect hexagonal arrangement with the AsO(4) groups placed in between. Bond-valence analysis indicates both partial deprotonation (x ≅ 3) and the presence of Mn in two different oxidation states (II and III), which is consistent with the electronic conductivity above 300 °C from electrochemical measurements. The electron paramagnetic resonance spectra of compound 1 and of its high-temperature form compound 2 show the presence of antiferromagnetic interactions with stronger magnetic coupling for the high-temperature phase. Magnetization measurements of room-temperature compound 1 show a complex magnetic behavior, with a three-dimensional antiferromagnetic ordering and magnetic anomalies at low temperatures, whereas for compound 2, an ordered state is not reached. Magnetostructural correlations indicate that superexchange interactions via oxygen are present in both compounds. The values of the magnetic exchange pathways [Mn-O-Mn] are characteristic of antiferromagnetic couplings. Notwithstanding, the existence of competition between different magnetic interactions through superexchange pathways can cause the complex magnetic behavior of compound 1. The loss of three-dimensional magnetic ordering by heating of compound 1 could well be based on the presence of Mn(3+) ions (d(4)) in compound 2.

Mixed-valence manganese cluster containing a sandwich-type polyoxometalate

Reaction of Na12[α-P2W15O56] · 18H2O with Mn(CH3COO)2 · 4H2O in the presence of K2S2O8 in acidic aqueous solution leads to isolation of a {} cluster-containing sandwich-type compound: αββα-K3Na11{(H2O)2(P2W15O56)2} · 40H2O (1). Single-crystal X-ray diffraction analysis revealed that 1 shows the well-known Weakley-type sandwich-type structure consisting of two trivacant [α-P2W15O56]12− Dawson moieties linked by a central symmetric rhomb-like mixed-valence manganese cluster via W–O–Mn and P–O–Mn connections. Mixed-valence Mn-cluster-substituted sandwich-type complexes based on trivacant Dawson units are rarely observed in polyoxometalate chemistry. Further, the sandwich-type polyoxoanions were connected by [K3Na2] clusters into a 44-layer. Magnetic study indicates that antiferromagnetic interactions exist in this compound.

The anion-directed self-assembly of manganese complexes derived from 2-ethoxy-6-[(2-phenylaminoethylimino)methyl]phenol

The mixed valence manganese(II/IV) complex, [MnIIL2(MeOH)2]·[MnIVL2(OAc)2]·2(MeOH) (1), and the chloride-bridged 1D polymeric manganese(III) complex, [MnIIIL2(μ-Cl)]n (2), where L is the deprotonated form of 2-ethoxy-6-[(2-phenylaminoethylimino)methyl]phenol (HL), have been prepared and structurally characterized by single-crystal X-ray diffraction analysis and IR spectra. The Mn atoms in both complexes are octahedrally coordinated. The self-assembly of the complex structures is apparently directed by the anions of the manganese salts.

Unusual oxidation state distributions observed for two mixed-valence heptanuclear manganese disc-like clusters

The synthesis, structures and magnetic properties of two new mixed-valence heptanuclear manganese clusters are described. Both complexes utilize triethanolamine (teaH(3)) as a bridging ligand, displaying near planar, disc-like metal topologies and are of formulae [Mn(II)(4)Mn(IV)(3)(tea)(teaH(2))(3)(peolH)(4)](BF(4))(2)·solv (1) and [Mn(II)(4)Mn(III)(3)F(3)(tea)(teaH)(teaH(2))(2)(piv)(4)(Hpiv)(chp)(3)]·0.5MeCN (2). Compound 1 is a rare mixed-valence compound containing Mn(II) and Mn(IV) ions only and is the first example of a heptanuclear disc with a {Mn(II)(4)Mn(IV)(3)} oxidation state distribution. Compound 2 is a {Mn(II)(4)Mn(III)(3)} complex and displays a unique arrangement of oxidation states within the disc, when compared to other known {Mn(II)(4)Mn(III)(3)} examples. Variable temperature DC and AC magnetic susceptibility studies were carried out for 1 and 2 in the 2-300 K temperature range. Compound 1 displayed an increase in the χ(M)T susceptibility values as the temperature is decreased indicating dominant ferromagnetic interactions are present within the cluster. Fits of the χ(M)T vs. T data reveals an S = 23/2 ground state, with several close lying excited states within 1 cm(-1). Compound 2 displays an overall decrease in the χ(M)T value as the temperature is decreased down to 2 K indicating dominant antiferromagnetic interactions present with a probable S = 4 ground state as determined from the DC and AC susceptibility data.

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.

Characterization of the Fe-Doped Mixed-Valent Tunnel Structure Manganese Oxide KOMS-2

A sol–gel-assisted combustion method was used to prepare Fe-doped manganese oxide octahedral molecular sieve (Fe-KOMS-2) materials with the cryptomelane structure. Characterization of the nanopowder samples over a wide range of Fe-doping levels (0 ≤ Fe/Mn ≤ 1/2) was carried out using a variety of experimental techniques. For each sample, Cu Kα XRD and ICP-AES were used to index the cryptomelane structure and determine the elemental composition, respectively. A combination of SEM and TEM images revealed that the morphology changes from nanoneedle to nanorod after Fe doping. Furthermore, TGA scans indicated that the thermal stability is also enhanced with the doping. Anomalous XRD demonstrated that the Fe ions replace the Mn ions in the cryptomelane structure, particularly in the (211) planes, and results in a lattice expansion along the c axis, parallel to the tunnels. Reasonable fits to EXAFS data were obtained using a model based on the cryptomelane structure. Mössbauer spectra for selected Fe-KOMS-2 samples indicated that the Fe is present as Fe3+ in an octahedral environment similar to Mn in the MnO6 building blocks of KOMS-2. Magnetization measurements detected a small amount of γ-Fe2O3 second phase (e.g., 0.6 wt % for the Fe/Mn = 1/10 sample), the vast majority of the Fe being in the structure as Fe3+ in the high-spin state.

Mixed-valent tetranuclear manganese complexes with an {MnII3MnIV} core

Abstract Two new examples of mixed-valent phenyl 2-pyridyl ketoxime complexes with an {Mn II 3 Mn IV -μ 4 -O} core are presented: cationic {Mn II 3 Mn IV O(OH)[Ph(py)CNO] 3 [Ph(py)CNOH](ClC 6 H 4 COO) 3 } + ( 1 ) and neutral {Mn II 3 Mn IV O[Ph(py)CNO] 4 (IC 6 H 4 COO) 4 } ( 2 ), representing two different structural types (types I and II, respectively). Also preliminary data are mentioned for two further complexes with (2-pyrimidylthio)acetate ( 3 ) or 2-(methylthio)nicotinate ( 4 ) ligands. 1 and 2 differ in magnetic properties, 1 representing the first {Mn II 3 Mn IV -μ 4 -O}-core cationic complex with dominating antiferromagnetic interactions.

A novel tetranuclear MnII2MnIII2 cluster [Mn4(hmp)6(N3)4]: Synthesis, crystal structure and magnetic properties

A novel tetranuclear mixed-valence manganese(II/III) cluster [Mn4(hmp)6(N3)4] (1) where hmp is the anion of 2-hydroxymethylpyridine, was obtained by the reaction of manganese perchlorate with hmpH and NaN3 in acetonitrile. Complex 1 was characterized by chemical analysis, IR spectroscopy, single crystal X-ray diffraction and variable-temperature magnetic susceptibility studies. Complex 1 crystallizes in triclinic system with space group Pī consisting of mixed-valence Mn ions whose coordination geometry can be described as distorted octahedron. The magnetic properties were studied in the temperature range of 1.9–300 K and the results show that mixed-valence Mn ions are ferromagnetically coupled in the cluster leading to an S = 9 ground state.

Single Crystalline La0.7Sr0.3MnO3Molecular Sieve Nanowires with High Temperature Ferromagnetism

Porous mixed-valent manganese oxides are a group of multifunctional materials that can be used as molecular sieves, catalysts, battery materials, and gas sensors. However, material properties and thus activity can vary significantly with different synthesis methods or process conditions, such as temperature and time. Here, we report on a new synthesis route for MnO(2) and LaSr-doped molecular sieve single crystalline nanowires based on a solution chemistry methodology combined with the use of nanoporous polymer templates supported on top of single crystalline substrates. Because of the confined nucleation in high aspect ratio nanopores and of the high temperatures attained, new structures with novel physical properties have been produced. During the calcination process, the nucleation and crystallization of ε-MnO(2) nanoparticles with a new hexagonal structure is promoted. These nanoparticles generated up to 30 μm long and flexible hexagonal nanowires at mild growth temperatures (T(g) = 700 °C) as a consequence of the large crystallographic anisotropy of ε-MnO(2). The nanocrystallites of MnO(2) formed at low temperatures serve as seeds for the growth of La(0.7)Sr(0.3)MnO(3) nanowires at growth temperatures above 800 °C, through the diffusion of La and Sr into the empty 1D-channels of ε-MnO(2). Our particular growth method has allowed the synthesis of single crystalline molecular sieve (LaSr-2 × 4) monoclinic nanowires with composition La(0.7)Sr(0.3)MnO(3) and with ordered arrangement of La(3+) and Sr(2+) cations inside the 1D-channels. These nanowires exhibit ferromagnetic ordering with strongly enhanced Curie temperature (T(c) > 500 K) that probably results from the new crystallographic order and from the mixed valence of manganese.

Mixed-valence manganese calcium oxides as efficient catalysts for water oxidation

Incorporation of calcium to mixed-valence manganese oxides improved the water oxidation activity of these manganese oxides.

A mixed valence manganese triangle in a trigonal lattice: structure and magnetism

The synthesis, structural and magnetic characterisation of trinuclear manganese cluster, [Mn(3)O(O(2)C-anth)(6)(HOCH(3))(3)] 1 (where O(2)C-anth = 9-anthracenecarboxylate), with crystallographic three-fold (C(3)) symmetry, are described. The cluster was prepared by a carboxylate exchange reaction between HO(2)C-anth and [Mn(12)O(12)(O(2)CMe)(16)(H(2)O)(4)] with concomitant fragmentation of the dodecanuclear Mn core of the starting material to form a trinuclear Mn(3)(μ(3)-O) cluster capped by six carboxylate ligands. Bond valence sum calculations and SQUID magnetometric measurements establish the oxidation states of the metal ions as Mn(II)·2 Mn(III) which are antiferromagnetically coupled.

New high-nuclearity manganese clusters containing mixed chelating ligands: Syntheses, crystal structures and magnetochemical characterization

Two polynuclear mixed-valence manganese clusters, [Mn(13)] and [Mn(16)], containing mixed chelating ligands were synthesized and structurally characterized. The alternating current (AC) susceptibilities of both complexes reveal nonzero frequency-dependent out-of-phase (χ(M)'') signals.