Search

A one-dimensional (1D) ladder-like coordination polymer {NH4[{Cu(bpy)}2(C2O4)Fe(C2O4)3]·H2O}n(1; bpy = 2,2′-bipyridine) containing [Cu(bpy)(μ-C2O4)Cu(bpy)]2+cationic units linked by oxalate groups of [Fe(C2O4)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 [CuII(bpy)(μ-C2O4)CuII(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 CO2.

The heterodimetallic [CuFe] compounds [CuII4(terpy)4Cl5][FeIII(C2O4)3]·10H2O (1;terpy = 2,2′:6′,2′′-terpyridine), [CuII2(H2O)2(terpy)2(C2O4)][CuIIFeIII(CH3OH)(terpy)(C2O4)3]2(2), and {[Cu2IIFeIII(H2O)(terpy)2(C2O4)7/2]·6H2O}n(3) were obtained using building block approach, from reaction of aqueous solution of [Fe(C2O4)3]3–and a methanol solution containing Cu2+ions and terpy by the layering technique. Interestingly, by changing only the anion of the starting salt of copper(II), Cu(NO3)2·3H2O instead of CuCl2·2H2O, an unexpected change in the type of bridge, oxalate (2and 3) versus chloride (1), was achieved, thus affecting the overall structural architecture. Two polymorphs of 3D coordination polymer [CuIIFeII2(H2O)(terpy)(C2O4)3]n(4), crystallizing in the triclinic (a) and monoclinic (b) space groups, were formed hydrothermally, depending on whether CuCl2·2H2O or Cu(NO3)2·3H2O was added to the water, besides K3[Fe(C2O4)3]·3H2O and terpy, respectively. Under hydrothermal conditions iron(III) from initial building block is reduced to the divalent state, creating 2D honeycomb [FeII2(C2O4)3]n2n–layers, which are bridged by [Cu(H2O)(terpy)]2+cations. Compounds were investigated by single-crystal X-ray diffraction, IR, and impedance spectroscopies, magnetization measurements, and density functional theory (DFT) calculations. In compounds 1and 2, 0D magnetism is observed, with 1having a ground-state spin of 1 due to different interactions through chloride bridges of Cu2+ions in tetramer [CuII4(terpy)4Cl5]3+and 2showing strong antiferromagnetic coupling of Cu2+ions mediated by oxalate ligand in [CuII2(H2O)2(terpy)2(C2O4)]2+and weak ones between Cu2+and Fe3+ions through oxalate bridge in [CuIIFeIII(CH3OH)(terpy)(C2O4)3]−. Polymer 4exhibits antiferromagnetic phase transition at 25 K: The [FeII2(C2O4)3]n2n–layers are antiferromagnetically ordered, and a small amount of interlayer interaction is transferred through [Cu(H2O)(terpy)]2+cations via Oox–Cu–Ooxbridges. Additionally, compounds 1and 2are electrical insulators, while 4aand 4bshow proton conductivity.

Although γ-Ba4Nb2O9was considered to be metastable at room temperature (RT) and, therefore, prepared only by quenching from high temperatures, a new procedure for preparation of pure γ-Ba4Nb2O9from a single-molecular precursor, the oxalate-based complex {Ba2(H2O)5[NbO(C2O4)3]HC2O4}·H2O, is described. This study focuses on controlling the phase composition and the crystallite domain lengths by altering preparation conditions, namely, (i) the time for which samples were held at the given temperature and (ii) the cooling rate. The high-temperature γ-Ba4Nb2O9polymorph has been successfully retained and stabilized at RT; the desired crystallite size in the nanoscale regime, ranging from ∼5 to 20 nm, can easily be tuned. The crystallite domain length and lattice strain were calculated from X-ray diffraction line-broadening analysis performed during Rietveld structure refinement.

Herein, we present a detailed X-ray charge density study of the electron delocalization in five species of chloranilic acid: neutral molecule, mono- and dianion, and two chelating modes: bidendate and (bis)bidentate. The experiments provide the electron density at the bond critical points, which yields an accurate measure of bond order (and therefore electron delocalization), and complement previous literature and our data on bond lengths extracted from crystal structures and infrared spectra. Mapping of the electrostatic potential indicates electron-rich and electron-poor areas in the molecule, corresponding to single (electron-poor), double, and delocalized bonds (electron-rich) that can explain stacking interactions of quinoid rings in crystal packing.

The synthesis and properties of a novel hetero-tetranuclear compound [Cr2(bpy)4(μ-O)4Nb2(C2O4)4]·3H2O (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 Pcabspace group) consists of a square-shaped macrocyclic {Cr2(μ-O)4Nb2} core in which CrIIIand NbVions are alternately bridged by oxo ions and three uncoordinated water molecules. The intramolecular CrIII···CrIIIdistances through the −O–NbV–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 CrIIIions of spin 3/2 through two triatomic −O–NbV–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 gCr= 1.992(3), J= −12.77(5) cm–1, and |D| = 0.17(4) cm–1. The CrIII···CrIIIdimer 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–1and E= 0.024 cm–1.

The sizes of CoMn2O4nanoparticles can easily be tuned, from 40 to 8 nm, depending on the temperature of decomposition of the single-source molecular precursor {[Co(bpy)3][Mn2(C2O4)3]·H2O}n. The structural features of the CoMn2O4spinel 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 CoMn2O4was 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.

Two heterometallic coordination polymers {[NH(CH3)2(C2H5)]8[Mn4Cl4Cr4(C2O4)12]}n(1) and {[NH(CH3)-(C2H5)2]8[Mn4Cl4Cr4(C2O4)12]}n(2) were obtained by slow evaporation of an aqueous solution containing the building block [A]3[Cr(C2O4)3] [A = (CH3)2(C2H5)NH+or (CH3)(C2H5)2NH+] and MnCl2·2H2O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers [Mn4Cl4Cr4(C2O4)12]n8n–with a Shubnikov plane net festopology designated as (4·82), interleaved by the hydrogen-bonded templating cations (CH3)2(C2H5)NH+(1) or (CH3)(C2H5)2NH+(2). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature [1.60 × 10–3(Ω·cm)−1at 90% relative humidity (RH) of 1and 9.6 × 10–4(Ω·cm)−1at 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 1compared to that in 2can be tentatively attributed to the higher hydrophilicity of the cations (CH3)2(C2H5)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 Mn2+and Cr3+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.

Two new heterotemetallic complexes [{Cu(μ‐C2O4)(bpy)2}2Cr(C2O4)]NO3·H2O (1) and [Cu(bpy)3]2[Cr(C2O4)3]NO3·9H2O (2) (bpy = 2,2′‐bipyridine) have been synthesized. The crystal structures were determined and magnetic and spectroscopic (IR, UV/Vis) characterization accomplished. In 1, the central [Cr(C2O4)3]3–unit acts as a bridge between two copper(II) ions, each of them having a distorted octahedral geometry. The apical positions are occupied by one oxygen atom belonging to the bridging oxalate anion [Cu–O2 = 2.6439(18) Å] and one nitrogen atom from one of the bpy ligands [Cu–N1 = 2.1079(18) Å]. The N3O equatorial environment consists of three bpy nitrogen atoms [Cu–N = 1.995 Å (mean value)] and the second oxygen atom from the bridging oxalate anion [Cu–O1 = 2.1168(15) Å]. The chromium(III) atom is in a slightly distorted octahedral environment of four oxygen atoms from two bridging oxalate groups [Cr–O = 1.988 Å (mean value)] and two oxygen atoms from the terminally bonded bidentate oxalate group [Cr–O = 1.9454(16) Å]. Complex 2consists of two mononuclear [Cu(bpy)3]2+cations, one mononuclear [Cr(C2O4)3]3–anion, one NO3–anion and nine molecules of water of crystallisation. The copper(II) ions in two crystallographically independent [Cu(bpy)3]2+cations are in a tetragonally distorted octahedral coordination environment of six N atoms from the bpy ligands. The chromium(III) ion in [Cr(C2O4)3]3–has a slightly distorted octahedral environment of oxygen atoms from three oxalate groups [with the Cr–O bond lengths ranging from 1.964(3) to 1.982(3) Å]. Analysis of the magnetic properties (1.8–290 K) of 1showed a ferromagnetic exchange interactionbetween CuIIand CrIIIwith J= +1.20 cm–1[using HINT=–J(SCu1SCr+ SCu2SCr)].(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)