Singlet ground state in compounds with [MnIII4O2]8+ core due to broken degeneration

Two new tetranuclear compounds with a formula [MnIII4(μ-O)2(μ-4-RC6H4COO)7−m(L)2m(phen)2](ClO4)1+m, where R = MeO or tBu and m = 0 or 1, were synthesised and studied structurally and magnetically. The core of these compounds comprises a central Mn2O2 rhombus to which two terminal ions are attached - one to each oxo bridge. There are two types of bridges that alternately bind the central and terminal ions, those having a triple (μ-O)(μ-RCOO)2 or a double (μ-O)(μ-RCOO) bridge. The fit of the magnetic data of analogous compounds has so far been performed considering two different magnetic interactions, that between central ions (J1) and those between terminal and central ions (Jct), leading to ground states with ST = 2 or 3, or to five energetically degenerate ground states with ST = 0-4, depending on the J1/Jct ratio. In contrast, the compounds presented herein show an isolated ST = 0 ground state, and it was necessary to distinguish the two types of magnetic interactions between central and terminal ions (J2 and J3) to achieve a good fit of the experimental data. The differentiation of these interactions causes a spin state redistribution: the degeneration of ST = 0-4 breaks and the states with ST ≠ 0 become unstable as J2 and J3 become more different. Nevertheless, the assignment of these states to a particular spin configuration was unachievable because the composition of these states changes upon decreasing the J3/J2 ratio. The importance of considering the relative orientation of Jahn-Teller axes is also highlighted

New insights into the comprehension of the magnetic properties of dinuclear Mn(III) compounds with general formula [{MnL(NN)}2(μ-O)(μ-n-RC6H4COO)2]X2

Five new dinuclear Mn(III) compounds with benzoato derivative bridges [{Mn(bpy)L}2(μ-O)(μ-n-RC6H4COO)2]X2 (n-R = 3-MeO, 4-MeO and 4-tBu, X = NO3− and ClO4−) were synthesised and characterised. According to X-ray diffraction, the X anions tend to be coordinated to the Mn ions and may occupy the place of the monodentate ligand L. Two structural isomers that only differ in one of their monodentate ligands have been obtained with the 3-MeOC6H4COO− bridges. For all compounds, the Mn(III) ions display elongated octahedra with a pronounced rhombic distortion. To quantify these distortions separately, the elongation and rhombicity parameters Δ and ρ have been defined. The magnetic study shows a good relationship between the distortion of the coordination polyhedra and the zero field splitting parameters (DMn and EMn). From the magnetic data of a powder sample, it is possible to determine the sign and magnitude of DMn for ferromagnetic systems or weak antiferromagnetic systems with DMn < 0. For this kind of dinuclear compound, the R group at the meta position, the rhombic distortion of the octahedra, and large torsion angles between the Jahn-Teller axes lead to ferromagnetic interactions

Determination of ZFS parameters from the EPR spectra of mono-, di- and trinuclear MnII complexes: impact of magnetic coupling

A family of new MnII compounds, consisting of seven dinuclear, three mononuclear, and four trinuclear ones, were synthesised using benzoic acid derivatives n-RC6H4COOH, where n-R = 2-MeO, 3-MeO, 4-MeO, or 4-tBu, and 2,2′-bipyridine (bpy) or 1,10-phenantroline (phen) as blocking ligands. The crystal structures of nine of these compounds and the magnetic studies of all of them are reported here. Each type of compound was formed depending on the presence or absence of ClO4− ions, the solvent used, and/or the presence of a small amount of water in the reaction medium. The use of the tert-buthylbenzoate ligand gave unexpected results, very likely due to the steric hindrance caused by the voluminous tBu groups. The EPR spectra of each type of compound give some peculiar features that allow its identification. Attempts to fit these spectra have been made in order to determine the ZFS parameters, D and E, of the MnII ion (for mononuclear and dinuclear systems) or of the ground state (for trinuclear systems). For trinuclear systems, the single-ion ZFS parameters estimated from those of the ground state provided a good simulation of the EPR spectra of these compounds. The EPR signals observed in each case have been rationalised according to the energy level distribution and the plausible population in the excited states. In some particular situations, the sign of DMn could be determined from the fit of the EPR spectra of the antiferromagnetic dinuclear compounds, the source of the difference between the spectra lying in the second excited state

Biomimetic Mn-catalases based on dimeric manganese complexes in mesoporous silica for potential antioxidant agent

Two new structural and functional models of the Mn-catalase with formula [{MnIII(bpy)(H2O)}(μ-2-MeOC6H4CO2)2(μ-O){MnIII(bpy)(X)}]X, where X = NO3 (1) and ClO4 (2) and bpy = 2,2′-bipyridine, were synthesized and characterized by X-ray diffraction. In both cases, a water molecule and an X ion occupy the monodentate positions. The magnetic properties of these compounds reveal a weak antiferromagnetic behavior (2J = −2.2 cm 1 for 1 and −0.7 cm 1 for 2, using the spin Hamiltonian H = −2J S1·S2) and negative zero-field splitting parameter DMn (−4.6 cm 1 and −3.0 cm 1 for 1 and 2, respectively). This fact, together with the nearly orthogonal orientation of the Jahn Teller axes of the MnIII ions explain the unusual shape of χMT versus T plot at low temperature. Compound 1 presents a better catalase activity than 2 in CH3CN H2O media, probably due to a beneficial interaction of the NO3− ion with the Mn complex in solution. These compounds were successfully inserted inside two-dimensional hexagonal mesoporous silica (MCM-41 type) leading to the same hybrid material ([Mn2O]@SiO2), without the X group. The manganese complex occupies approximately half of the available pore volume, keeping the silica's hexagonal array intact. Magnetic measurements of [Mn2O]@SiO2 suggest that most of the dinuclear unit is preserved, as a non-negligible interaction between Mn ions is still observed. The X-ray photoelectron spectroscopy analysis of the Mn 3s peak confirms that Mn remains as MnIII inside the silica. The catalase activity study of material [Mn2O]@SiO2 reveals that the complex is more active inside the porous silica, probably due to the surface silanolate groups of the pore wall. Moreover, the new material shows catalase activity in water media, while the coordination compounds are not active

Protocrystallinity of Monodispersed Ultra-Small Templated Mesoporous Silica Nanoparticles

Monodisperse and semi-faceted ultra-small templated mesoporous silica nanoparticles (US-MSNs) of 20–25 nm were synthesized using short-time hydrolysis of tetraethoxysilane (TEOS) at room temperature, followed by a dilution for nucleation quenching. According to dynamic light scattering (DLS), a two-step pH adjustment was necessary for growth termination and colloidal stabilization. The pore size was controlled by cetyltrimethylammonium bromide (CTAB), and a tiny amount of neutral surfactant F127 was added to minimize the coalescence between US-MSNs and to favor the transition towards internal ordering. Flocculation eventually occurred, allowing us to harvest a powder by centrifugation (~60% silica yield after one month). Scanning transmission electron microscopy (STEM) and 3D high-resolution transmission electron microscopy (3D HR-TEM) images revealed that the US-MSNs are partially ordered. The 2D FT transform images provide evidence for the coexistence of four-, five-, and sixfold patterns characterizing an “on-the-edge” crystallization step between amorphous raspberry and hexagonal pore array morphologies, typical of a protocrystalline state. Calcination preserved this state and yielded a powder characterized by packing, developing a hierarchical porosity centered at 3.9 ± 0.2 (internal pores) and 68 ± 7 nm (packing voids) of high potential for support for separation and catalysis

Selective C–C Bond Cleavage in Diols and Lignin Models: High-Throughput Screening of Metal Oxide-Anchored Vanadium in Mesoporous Silica

International audienceThe development of selective and robust heterogeneous oxidation catalysts is an enabling technology for conversion of biomass-derived platform chemicals. Vanadium active sites were incorporated into the structure of mesoporous silica via an ultra-fast, one-pot synthesis method based on microwave-assisted heating. In addition, Al/Ti/Zr/Ce anchoring ions were introduced in order to minimize vanadium leaching and better control its dispersion. The supported V-(Al/Ti/Zr/Ce)-MCM-41 composite materials were assessed as catalysts for aerobic C–C bond cleavage of simple models for lignin (1,2-diphenyl-2-methoxyethanol) and sugar-derived polyalcohols (meso-hydrobenzoin). The TiIV ions proved to be the best anchors to prevent V leaching, while AlIII and ZrIV ions were the best to improve selective conversion of the substrates. The active sites in these catalysts are shown to be 2D VOx layers stabilized on the anchors. In a screen of twelve solvents, weakly polar solvents like toluene were found to be most suitable for this reaction, as was environmentally friendly ethyl acetate. The above properties, together with the high selectivity for C–C bond cleavage, advocate for a heterogeneous catalytic pathway, intrinsically different from that reported previously for molecular oxovanadium (V) catalysts

Incorporation of methylene blue into mesoporous silica nanoparticles for singlet oxygen generation

Different strategies to incorporate methylene blue (MB) into silica nanoparticles are described, leading to either its monomeric or dimeric form. The supported photosensitizer displays increased ability to generate 1 O 2 with regard to MB in solution

New insights into the comprehension of the magnetic properties of dinuclear Mn(III) compounds with general formula [{MnL(NN)}2(μ-O)(μ-n-RC6H4COO)2]X2

Five new dinuclear Mn(III) compounds with benzoato derivative bridges [{Mn(bpy)L}2(μ-O)(μ-n-RC6H4COO)2]X2 (n-R = 3-MeO, 4-MeO and 4-tBu, X = NO3− and ClO4−) were synthesised and characterised. According to X-ray diffraction, the X anions tend to be coordinated to the Mn ions and may occupy the place of the monodentate ligand L. Two structural isomers that only differ in one of their monodentate ligands have been obtained with the 3-MeOC6H4COO− bridges. For all compounds, the Mn(III) ions display elongated octahedra with a pronounced rhombic distortion. To quantify these distortions separately, the elongation and rhombicity parameters Δ and ρ have been defined. The magnetic study shows a good relationship between the distortion of the coordination polyhedra and the zero field splitting parameters (DMn and EMn). From the magnetic data of a powder sample, it is possible to determine the sign and magnitude of DMn for ferromagnetic systems or weak antiferromagnetic systems with DMn < 0. For this kind of dinuclear compound, the R group at the meta position, the rhombic distortion of the octahedra, and large torsion angles between the Jahn-Teller axes lead to ferromagnetic interactions

Reactivity of 9-anthracenecarboxylate in the presence of Mn(II) and Mn(III) ions: Biomimetic aerobic oxidative decarboxylation catalysed by a manganese(III) 2,2′-bipyridine complex

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