Search

A novel heterometallic oxalate-based compound of the formula {[Co(bpy)3][Mn2(C2O4)3]·H2O}n (1; bpy = 2,2'-bipyridine) was synthesized and characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction (XRD), and magnetization measurement. The molecular structure of 1 is made of a three-dimensional (3D) anionic network, [Mn2(C2O4)3]n2n-, and tris-chelated cations [Co(bpy)3]2+ occupying the vacancies of the framework. Splitting between the zero-field-cooled (ZFC) and field-cooled (FC) branches of susceptibility below the small peak at 13 K indicates magnetic ordering. Compound 1 was used as a single-source precursor for the formation of the mixed-metal oxide CoMn2O4. This conversion via thermal decomposition was explored by thermal analysis (TGA and DTA), IR spectroscopy, powder XRD, and magnetic susceptibility measurement. From refined structural parameters, it could be seen that the spinel obtained by the thermal treatment of 1 at 800 °C is characterized by the inversion parameter ε = 21%, and therefore the structural formula at room temperature can be written as tet[Co0.79Mn0.21]oct[Co0.105Mn0.895]2O4. The temperature dependence of magnetization for CoMn2O4 points to at least three magnetic phases: the ferrimagnetic state is observed below 83 K, and up to 180 K blocking of the magnetic moments of nanocrystallites of 31 nm appears, transforming to paramagnetic-like behavior above 180 K. Microstructural characterization of the CoMn2O4 sample was carried out by means of XRD line-broadening analysis. [ABSTRACT FROM AUTHOR]

A novel heterometallic oxalate-based compound, {Ba2(H2O)5[TaO(C2O4)3]HC2O4}·H2O (1), was obtained by using an (oxalato)tantalate(V) aqueous solution as a source of the complex anion and characterized by X-ray single-crystal diffraction, IR spectroscopy, and thermal analysis. Compound 1 is a three-dimensional (3D) coordination polymer with the Ta atom connected to eight neighboring Ba atoms through the oxalate ligands and the oxo oxygen group. Thermal treatment of 1 up to 1200 °C leads to molecular precursor-to-material conversion that yields the mixed-metal γ-Ba4Ta2O9 phase. This high-temperature γ-Ba4Ta2O9 polymorph has the 6H-perovskite structure (space group P63/m), in which the Ta2O9 face-sharing octahedral dimers are interconnected via corners to the regular BaO6 octahedra. To date, γ-Ba4Ta2O9 has never been obtained at room temperature, because of the reduction of symmetry (P63/m → P21/c) that usually occurs during the cooling. Spectroscopic, optical, photocatalytic, and electrical properties of the obtained γ-Ba4Ta2O9 phase were investigated. In addition to the experimental data, an absorption spectrum and band structure of the γ-Ba4Ta2O9 polymorph were calculated using density functional theory. [ABSTRACT FROM AUTHOR]

A novel oxalate-based complex of the formula {Ba2(H2O)5[NbO(C2O4)HC2O4}·H2O (1) was prepared from an aqueous solution containing the [NbO(C2O4)3]3- and Ba2+ entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, LR spectroscopy, and thermal analysis. The crystal packing of 1 reveals a complex three-dimensional (3D) network: the Nb polyhedron is connected to eight neighboring Ba polyhedra through the oxalate ligands and the oxo-oxygen group, whereas the Ba polyhedra share edges and vertices. The ability of compound 1 to act as a single-source precursor for the formation of bimetallic oxides was investigated by the thermal analysis (TGA and DSC) and X-ray powder diffraction. Thermal processing of 1 resulted in the formation of mixed-metal oxide phases, Ba4Nb2O9 and Ba5Nb4O15. Three stable polymorphs of Ba4+Nb2O9 were isolated: the known, hexagonal α- and orthorhombic β-Ba4Nb2O9, and another one, not previously reported, hexagonal δ-Ba4Nb2O9 polymorph. The new, δ-Ba4Nb2O9 polymorph has the 6H-perovskite structure (space group P63/m), in which the Nb2O98- face-sharing octahedral dimers are interconnected via corners to the regular BaO610- octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba5Nb4O15 oxide occurred at ~700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba4Nb2O9 form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, β-Ba4Nb2O9 and δ-Ba4Nb2O9. Special focus was set on the electrical properties of the prepared mixed BaII--NbV oxides obtained by this molecular pathway in a single-step preparation. [ABSTRACT FROM AUTHOR]

A one-dimensional (1D) ladder-like coordination polymer {NH 4 [{Cu(bpy)} 2 (C 2 O 4 )Fe(C 2 O 4 ) 3 ]·H 2 O} n ( 1 ; bpy = 2,2'-bipyridine) containing [Cu(bpy)(μ-C 2 O 4 )Cu(bpy)] 2+ cationic units linked by oxalate groups of [Fe(C 2 O 4 ) 3 ] 3- building blocks was investigated as a new type of photoactive solid-state system. It exhibits a photocoloration effect when exposed to direct sunlight or UV/vis irradiation. The photochromic properties and mechanism were studied by powder and single-crystal X-ray diffraction, UV/vis diffuse reflectance, IR and electron paramagnetic resonance spectroscopy, magnetization and impedance measurements, and density functional theory calculations. The process of photochromism involves simultaneous intramolecular electron transfers from the oxalate ligand to Fe(III) and to [Cu II (bpy)(μ-C 2 O 4 )Cu II (bpy)] 2+ , leading to the reduction of the metal centers to the electronic states Fe(II) and Cu(I), accompanied by the release of gaseous CO 2 .

Two heterometallic coordination polymers {[NH(CH 3 ) 2 (C 2 H 5 )] 8 [Mn 4 Cl 4 Cr 4 (C 2 O 4 ) 12 ]} n ( 1 ) and {[NH(CH 3 )-(C 2 H 5 ) 2 ] 8 [Mn 4 Cl 4 Cr 4 (C 2 O 4 ) 12 ]} n ( 2 ) were obtained by slow evaporation of an aqueous solution containing the building block [A] 3 [Cr(C 2 O 4 ) 3 ] [A = (CH 3 ) 2 (C 2 H 5 )NH + or (CH 3 )(C 2 H 5 ) 2 NH + ] and MnCl 2 ·2H 2 O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn 4 Cl 4 Cr 4 (C 2 O 4 ) 12 ] n 8n - with a Shubnikov plane net fes topology designated as (4·8 2 ), interleaved by the hydrogen-bonded templating cations (CH 3 ) 2 (C 2 H 5 )NH + ( 1 ) or (CH 3 )(C 2 H 5 ) 2 NH + ( 2 ). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10 -3 (Ω·cm) -1 at 90% relative humidity (RH) of 1 and 9.6 × 10 -4 (Ω·cm) -1 at 94% RH of 2 ]. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH 3 ) 2 (C 2 H 5 )NH + , which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn 2+ and Cr 3+ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.

The synthesis and properties of a novel hetero-tetranuclear compound [Cr 2 (bpy) 4 (μ-O) 4 Nb 2 (C 2 O 4 ) 4 ]·3H 2 O (1; bpy = 2,2'-bipyridine), investigated by single-crystal X-ray diffraction, magnetization measurements, IR, UV/visible spectroscopy, electron paramagnetic resonance (EPR; X- and Q-bands and high-field), and density functional theory (DFT) calculations, are reported. Crystal structure of 1 (orthorhombic Pcab space group) consists of a square-shaped macrocyclic {Cr 2 (μ-O) 4 Nb 2 } core in which Cr III and Nb V ions are alternately bridged by oxo ions and three uncoordinated water molecules. The intramolecular Cr III ···Cr III distances through the -O-Nb V -O- bridges are 7.410(2) and 7.419(2) Å, while diagonal separation is 5.406(2) Å. The temperature dependence of magnetization M(T) evidences an anti-ferromagnetic ground state, which originates from a magnetic interaction between two Cr III ions of spin 3/2 through two triatomic -O-Nb V -O- diamagnetic bridges. A spin Hamiltonian appropriate for polynuclear isolated magnetic units was used. The best-fitting curve for this model is obtained with the parameters g Cr = 1.992(3), J = -12.77(5) cm -1 , and |D| = 0.17(4) cm -1 . The Cr III ···Cr III dimer model is confirmed by EPR spectra, which exhibit a pronounced change of their shape around the temperature corresponding to the intradimer coupling J. The EPR spectra simulations and DFT calculations reveal the presence of a single-ion anisotropy that is close to being uniaxial, D = -0.31 cm -1 and E = 0.024 cm -1 .

The sizes of CoMn 2 O 4 nanoparticles can easily be tuned, from 40 to 8 nm, depending on the temperature of decomposition of the single-source molecular precursor {[Co(bpy) 3 ][Mn 2 (C 2 O 4 ) 3 ]·H 2 O} n . The structural features of the CoMn 2 O 4 spinel are also affected by the heat treatment temperature, showing a pronounced expansion of unit cell parameters as a consequence of thermally induced cation redistribution between tetrahedral and octahedral sites. Moreover, the magnetic behavior of CoMn 2 O 4 was successfully tailored as well; depending on the heat treatment, it is possible to switch between the superparamagnetic and ferrimagnetic ordering and to tailor the magnetic transition temperatures, i.e., the boundaries between the hard and soft magnetic behavior.