Light-Induced Intramolecular Electron Transfer in a 1D [CuFe] Coordination Polymer Containing the [Fe(C2O4)3]3- Core.

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.

High Proton Conductivity of Magnetically Ordered 2D Oxalate-Bridged [MnIICrIII] Coordination Polymers with Irregular Topology.

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 fes topology 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)-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 (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.

Oxalate-based [CuIICrIII] coordination compounds affected by the tridentate ligand, simple anion, and reactant ratio: structural and magnetic features.

Seven novel oxalate-based [CuIICrIII] compounds: [Cu4(terpy)4Cl5][Cr(C2O4)3]·9H2O (1; terpy = 2,2':6',2''-terpyridine), {[Cr2Cu4(H2O)2(terpy)4(C2O4)7]·10H2O}n (2), [Cr2Cu4(H2O)2(terpy)4(C2O4)7]·12H2O (3), [Cu(H2O)3(terpy)][CrCu(H2O)(terpy)(C2O4)3]2·9H2O (4), [Cu(H2O)(terpy)(NO3)][CrCu(H2O)(terpy)(C2O4)3]·6H2O (5), [CrCu2(terpy)2(C2O4)3(NO3)]·1.5H2O·CH3OH (6) and [Cr2Cu4(H2O)4(terpy)4(C2O4)6][Cr2Cu2(terpy)2(C2O4)6]·9H2O·CH3OH (7) were obtained from the reaction of an aqueous solution of the building block [Cr(C2O4)3]3- and a methanol solution containing Cu2+ ions and terpyridine ligand by the layering technique. Interestingly, changing only the anion of the starting salt of copper(II), NO3- instead of Cl-, resulted in an unexpected modification in the bridge type, namely oxalate (compounds 2-7) versus chloride (compound 1). During the crystallization process in the test tube, the partial decomposition of the tris(oxalato)chromate(III) anion leads to the release of the oxalate ligand from the coordination sphere of chromium(III). Consequently, this oxalate ligand is coordinated to copper(II) ions in the reaction mixture, resulting in the oxalate-bridged cationic moieties of copper(II) ions [(terpy)Cu(µ-C2O4)Cu(terpy)]2+ of 2 and 3. Compounds 4-7 were formed in the same test tube using identical components as for 2 and 3, but in a different ratio; during preparation, the starting material did not decompose and retained its original role as a building block. The compounds were studied by single-crystal X-ray diffraction, IR spectroscopy, magnetization measurements and density functional theory (DFT) calculations. Compound 1 exhibits a ground-state spin of 1 due to antiferromagnetic and ferromagnetic interactions of Cu2+ ions across the chloride bridges in the tetramer; ferromagnetic coupling transferred through the oxalate bridge was found between Cu2+ ions in compound 2 and between Cu2+ and Cr3+ in compounds 4 and 5. Since compound 3 is considered to be a very similar fragment of compound 2, a ferromagnetic interaction between two Cu2+ ions bridged by a bis(bidentate) oxalate group is also expected. The performed calculations for compound 7 indicate that the main interaction is ferromagnetic.

Humidity-Sensing Properties of an 1D Antiferromagnetic Oxalate-Bridged Coordination Polymer of Iron(III) and Its Temperature-Induced Structural Flexibility.

A novel one-dimensional (1D) oxalate-bridged coordination polymer of iron(III), {[NH(CH3)(C2H5)2][FeCl2(C2O4)]}n (1), exhibits remarkable humidity-sensing properties and very high proton conductivity at room temperature (2.70 × 10-4 (Ω·cm)-1 at 298 K under 93% relative humidity), in addition to the independent antiferromagnetic spin chains of iron(III) ions bridged by oxalate groups (J = -7.58(9) cm-1). Moreover, the time-dependent measurements show that 1 could maintain a stable proton conductivity for at least 12 h. Charge transport and magnetic properties were investigated by impedance spectroscopy and magnetization measurements, respectively. Compound 1 consists of infinite anionic zig-zag chains [FeCl2(C2O4)]nn- and interposed diethylmethylammonium cations (C2H5)2(CH3)NH+, which act as hydrogen bond donors toward carbonyl oxygen atoms. Extraordinarily, the studied coordination polymer exhibits two reversible phase transitions: from the high-temperature phase HT to the mid-temperature phase MT at T ~213 K and from the mid-temperature phase MT to the low-temperature phase LT at T ~120 K, as revealed by in situ powder and single-crystal X-ray diffraction. All three polymorphs show large linear thermal expansion coefficients.

Structural, Electrical, and Magnetic Versatility of the Oxalate-Based [CuFe] Compounds Containing 2,2':6',2″-Terpyridine: Anion-Directed Synthesis.

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 (2 and 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 1 and 2, 0D magnetism is observed, with 1 having a ground-state spin of 1 due to different interactions through chloride bridges of Cu2+ ions in tetramer [CuII4(terpy)4Cl5]3+ and 2 showing 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 4 exhibits 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-Oox bridges. Additionally, compounds 1 and 2 are electrical insulators, while 4a and 4b show proton conductivity.

Magnetic and Electrical Behaviors of the Homo- and Heterometallic 1D and 3D Coordination Polymers Based on the Partial Decomposition of the [Cr(C2O4)3]3- Building Block.

One-dimensional (1D) oxalate-bridged homometallic {[Mn(bpy)(C2O4)]·1.5H2O}n (1) (bpy = 2,2'-bipyridine) and heterodimetallic {[CrCu3(bpy)3(CH3OH)(H2O)(C2O4)4][Cu(bpy)Cr(C2O4)3]·CH2Cl2·CH3OH·H2O}n (2) coordination polymers, as well as the three-dimensional (3D) heterotrimetallic {[CaCr2Cu2(phen)4(C2O4)6]·4CH3CN·2H2O}n (3) (1,10-phenanthroline) network, have been synthesized by a building block approach using a layering technique, and characterized by single-crystal X-ray diffraction, infrared (IR) and impedance spectroscopies and magnetization measurements. During the crystallization process partial decomposition of the tris(oxalate)chromate(III) happened and 1D polymers 1 and 2 were formed. The antiferromagnetic interactions between the manganese(II) ions was mediated by oxalate ligands in the chain [Mn(bpy)(C2O4)]n of 1, with intra-chain super-exchange interaction ? = (-3.134 ± 0.004) K; magnetic interaction between neighbouring chains is negligible making this system closer than other known Mn-chains to the ideal 1D Heisenberg antiferromagnet. Compound 2 comprises a 1D coordination anion [Cu(bpy)Cr(C2O4)3]nn- (Cr2-Cu4) with alternating [Cr(C2O4)3]3- and [Cu(bpy)]2+ units mutually bridged through the oxalate group. Another chain (Cr1-Cu3) is similar, but involves a homodinuclear unit [Cu(bpy)(H2O)(µ-C2O4)Cu(bpy)(CH3OH)]2+ (Cu1-Cu2) coordinated as a pendant group to a terminal oxalate oxygen. Magnetic measurements showed that the Cu1-Cu2 cationic unit is a strongly coupled antiferromagnetic dimer, independent from the other magnetic ions within ferromagnetic chains Cr1-Cu3 and Cr2-Cu4. A 3D polymer {[CaCr2Cu2(phen)4(C2O4)6]·4CH3CN·2H2O}n (3) comprising three different metal centers (Ca2+, Cr3+ and Cu2+) oxalate-bridged, contains Ca2+ atoms as nodes connected with four Cr3+ atoms through oxalate ligands. The network thus formed can be reduced to an underlying graph of diamondoid (dia) or (66) topology. Magnetization of 3 shows the ferromagnetic oxalate-bridged dimers [CuIICrIII], whose mutual interaction could possibly originate through the spin polarization of Ca2+ orbitals. Compounds 1 and 3 exhibit lower electrical conductivity at room temperature (RT) in comparison to compound 2.

Magnetic oxygen stored in quasi-1D form within BaAl2O4 lattice.

Inorganic materials that enable a link between the storage and release of molecular oxygen offer a fertile ground in continuous quest for the applications that can potentially reduce energy consumption and thus minimize adverse effects on the environment. Herein, we address reversible intake/release of an oxygen within the BaAl2O4 material as evidenced by unexpected magnetic ordering. Magnetic measurements unveil that an oxygen is stored in the form of condensed matter, creating a kind of low dimensional, chain-like assembly within the tunnels of BaAl2O4 structure. We demonstrate that oxygen is adsorbed simply by staying in air, at ambient conditions, and released relatively quickly by staying in the He or other gas atmosphere of several millibars pressure even at 300 K.

Ladder-like [CrCu] coordination polymers containing unique bridging modes of [Cr(C2O4)3]3- and Cr2O72.

Three heterometallic one-dimensional (1D) coordination polymers {A[CrCu2(bpy)2(C2O4)4]·H2O}n [A = K+ (1) and NH4+ (2); bpy = 2,2'-bipyridine] and [(Cr2O7)Cu2(C2O4)(phen)2]n (3; phen = 1,10-phenanthroline) with uncommon topology have been synthesized using a building block approach and characterized by single-crystal X-ray diffraction, IR and impedance spectroscopies, magnetization measurements, and DFT calculations. Due to the partial decomposition of the building block [Cr(C2O4)3]3-, all three compounds contain oxalate-bridged [Cu2(L)2(µ-C2O4)]2+ units [L = bpy (1 and 2) and phen (3)]. In compounds 1 and 2 these cations are mutually connected through oxalate groups from [Cr(C2O4)3]3-, thus forming ladder-like topologies. Unusually, three different bridging modes of the oxalate ligand are observed in these chains. In compound 3 copper(ii) ions from cationic units are bridged through the oxygen atoms of Cr2O72- anions in a novel ladder-like mode. Very strong antiferromagnetic coupling observed in all three compounds, determined from the magnetization measurements and confirmed by DFT calculations (J = -343, -371 and -340 cm-1 for 1, 2 and 3, respectively), appears between two copper(ii) ions interacting through the oxalate bridge.

From a square core to square opening: structural diversity and magnetic properties of the oxo-bridged [CrIIINbV] complexes.

Three heterometallic oxo-bridged compounds, [Cr2(phen)4(µ-O)4Nb2(C2O4)4]·2H2O (1; phen = 1,10-phenanthroline), [Cr2(terpy)2(H2O)2(µ-O)4Nb2(C2O4)4]·4H2O (2; terpy = 2,2';6',2''-terpyridine) and [Cr(terpy)(C2O4)(H2O)][Cr2(terpy)2(C2O4)2(µ-O)2Nb(C2O4)2]·3H2O (3), have been synthesized using a building block approach and characterized by IR spectroscopy, single-crystal and powder X-ray diffraction, magnetization measurements, and DFT calculations. The molecular structures of 1 and 2, crystallizing in P42212 and P21/n space groups, respectively, contain a square-shaped {Cr(µ-O)4Nb} unit, while that of complex salt 3 (P1[combining macron] space group) consists of a mononuclear cation containing CrIII and trinuclear anions in which two CrIII ions are bridged by a -O-NbV-O- fragment. Besides hydrogen-bonding patterns resulting in a 1D- or 3D-supramolecular arrangement in 1-3, an unusual intermolecular contact has been noticed between parallel oxalate moieties occurring due to the electrostatic attraction of electron-rich carbonyl oxygen and severely electron-depleted carbon atoms in the crystal packing of 2. The antiferromagnetic coupling observed in all three compounds, determined from magnetization measurements (J = -13.51(2), -8.41(1) and -7.44(4) cm-1 for 1, 2 and 3, respectively) and confirmed by DFT calculations, originates from two CrIII ions with spin 3/2 interacting through diamagnetic -O-NbV-O- bridge(s).

The influence of metal centres on the exchange interaction in heterometallic complexes with oxalate-bridged cations.

The reaction of bis(phenanthroline)metal(ii) cations (M = Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) with bis(oxalato)chromium(iii) anions in a water/ethanol solution gives rise to a series of compounds with oxalate-bridged cations, [{M(phen)2}2(µ-C2O4)][Cr(phen)(C2O4)2]2·4H2O [Mn2Cr2 (1), Co2Cr2 (2), Ni2Cr2 (3), Cu2Cr2 (4) and Zn2Cr2 (5)]. Their structural analysis reveals that all the prepared compounds crystallize in the triclinic system, space group P1[combining macron], having similar unit cell parameters, molecular structures and crystal packing features. All metal centres in 1-5 are octahedrally coordinated: M2+ in homodinuclear cations are coordinated with two phen molecules and one bridging oxalate ligand; Cr3+ in anions is coordinated with one phen ligand and two bidentate oxalate groups. The copper atom in Cu2Cr2 (4) exhibits a Jahn-Teller-distorted octahedral coordination. Owing to the considerable number of pyridyl groups present in 1-5 (from phen ligands) the crystal packing of cations and anions is driven by stacking interactions appearing in offset-face-to-face (OFF) and edge-to-face (EF) orientations. The hydrogen bonds between the anions and water molecules of crystallization form 1D ladder-like motifs. In addition to the single crystal X-ray diffraction studies, the characterization of the new complexes was accomplished by means of IR and UV/Vis spectroscopy and magnetization measurements on a SQUID magnetometer. The temperature dependence of magnetic susceptibility reveals different magnetic super exchange interactions taking place in homodinuclear oxalate-bridged cations depending on the transition metal centre (Mn2+, Co2+, Ni2+ and Cu2+). Oxalate ligands mediate the ferromagnetic coupling of Cu2+ metal cations in Cu2Cr2 (4), whereas in Mn2Cr2 (1), Co2Cr2 (2) and Ni2Cr2 (3), antiferromagnetic interactions are observed between Mn2+, Co2+ and Ni2+ cations, respectively. Also, relatively large zero-field splitting parameters for Cr3+ ions (from mononuclear anions), D ≈ 1 cm-1, were observed.

Experimental and Theoretical Investigation of the Anti-Ferromagnetic Coupling of CrIII Ions through Diamagnetic -O-NbV-O- Bridges.

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 Pcab space group) consists of a square-shaped macrocyclic {Cr2(µ-O)4Nb2} core in which CrIII and NbV ions are alternately bridged by oxo ions and three uncoordinated water molecules. The intramolecular CrIII···CrIII distances 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 CrIII ions 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···CrIII 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.

Effect of the Cation Distribution and Microstructure on the Magnetic Behavior of the CoMn2O4 Oxide.

The sizes of CoMn2O4 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][Mn2(C2O4)3]·H2O}n. The structural features of the CoMn2O4 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 CoMn2O4 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.

Magnetic order in a novel 3D oxalate-based coordination polymer {[Cu(bpy)3][Mn2(C2O4)3]·H2O}n.

A heterometallic coordination polymer {[Cu(bpy)3][Mn2(C2O4)3]·H2O}n (1; bpy = 2,2'-bipyridine) was synthesized using a building-block approach and characterized by IR spectroscopy, single-crystal X-ray diffraction, magnetization measurement, and X-band ESR spectroscopy both on a single crystal and a polycrystalline sample. The molecular structure of 1 is made of a three-dimensional (3D) anionic network [Mn2(C2O4)3]n(2n-) and tris-chelated cations [Cu(bpy)3](2+) occupying the vacancies of the framework. In compound 1 magnetic order is confirmed below 12.8 K - magnetization measurements reveal an antiferromagnetic-like network of canted Mn(2+) spins with incorporated paramagnetic Cu(2+) centres. The ESR spectroscopy distinctly shows the phase transition; above T≈ 13 K, single isotropic Lorentzian lines of Mn(2+) ions in the high spin state S = 5/2 were observed, while below this temperature, only characteristic Cu(2+) signals from cations were detected. Thermal decomposition residues of 1 at different temperatures (800-1000 °C) were analyzed by powder X-ray diffraction; by heating the sample up to 1000 °C the spinel oxide CuMn2O4 [94.1(2) wt%] was formed. From the refined structural parameters, it could be seen that the obtained spinel is characterized by the inversion parameter δ∼ 0.8, and therefore the structural formula at room temperature can be written as (tet)[Cu0.17Mn0.83](oct)[Mn1.17Cu0.83]O4.

A 3D oxalate-based network as a precursor for the CoMn2O4 spinel: synthesis and structural and magnetic studies.

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]n(2n-), 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 δ = 21%, and therefore the structural formula at room temperature can be written as (tet)[Co(0.79)Mn(0.21)](oct)[Co(0.105)Mn(0.895)]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.

A novel type of coordination mode of chloranilic acid leading to the formation of polymeric coordination ribbon in the series of mixed-ligand copper(II) complexes with 1,10-phenanthroline.

A series of four novel mixed-ligand complexes of copper(II) with 3,6-dichloro-2,6-dihydroxy-1,4-benzoquinone (chloranilic acid) and 1,10-phenanthroline was prepared and characterised by X-ray structure analysis and IR spectroscopy. Three complexes exhibit square-pyramidal coordination, whereas one exhibits octahedral coordination. The ligand 1,10-phenanthroline acts in a bidentate chelating mode with N,N-metal binding. The chloranilate dianion coordinates to the Cu(II) atom in a terminal bidentate ortho-quinone-like mode, forming a mononuclear complex species. However, in one structure a novel type of coordination mode of chloranilate is observed. In addition to the bidentate mode, a monodentate bridging mode through a carboxy oxygen of a symmetry-related dianion leads to the formation of polymeric coordination ribbon. The crystal packing of penta-coordinated species, in addition to hydrogen bonding, involves less common stacking interactions of chelate rings with the π-systems of the ligands.

Ba4Ta2O9 oxide prepared from an oxalate-based molecular precursor-characterization and properties.

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 P6(3)/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 (P6(3)/m → P2(1)/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.

Stacking of metal chelating rings with π-systems in mononuclear complexes of copper(II) with 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone (chloranilic acid) and 2,2'-bipyridine ligands.

A series of four novel mononuclear complexes of copper(II) with chloranilic acid and 2,2'-bipyridine were prepared and structurally characterised by X-ray structure analysis and IR spectroscopy. The complexes exhibit square-planar, square-pyramidal and octahedral coordination. The chloranilate dianion coordinates the Cu(II) atom in a terminal bidentate o-quinone-like mode forming a mononuclear complex species. The crystal packing of the aqua complex and the complexes with crystal water molecules are defined by hydrogen bonds. However, significant contributions are from interactions involving five-membered chelate rings with both types of ligands and intermolecular π-systems (bipyridine and chloranilate rings). The crystal packing of the complex with square planar Cu(II) coordination is dominated by interactions of the chelate rings with π-delocalised bonds and intermolecular π-systems. The occurrence of dimorphism could be related to the different types of these interactions in the stacks. The crystal packing of the octahedral complexes of the mono- and dihydrate solvates reveal 3D-networks with pores occupied by non-coordinated bipyridine molecules.

Single-step preparation of the mixed Ba(II)-Nb(V) oxides from a heteropolynuclear oxalate complex.

A novel oxalate-based complex of the formula {Ba(2)(H(2)O)(5)[NbO(C(2)O(4))(3)]HC(2)O(4)}·H(2)O (1) was prepared from an aqueous solution containing the [NbO(C(2)O(4))(3)](3-) and Ba(2+) entities in the molar ratio 1:2, and characterized by X-ray single-crystal diffraction, IR 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, Ba(4)Nb(2)O(9) and Ba(5)Nb(4)O(15). Three stable polymorphs of Ba(4)Nb(2)O(9) were isolated: the known, hexagonal α- and orthorhombic γ-Ba(4)Nb(2)O(9), and another one, not previously reported, hexagonal δ-Ba(4)Nb(2)O(9) polymorph. The new, δ-Ba(4)Nb(2)O(9) polymorph has the 6H-perovskite structure (space group P6(3)/m), in which the Nb(2)O(9)(8-) face-sharing octahedral dimers are interconnected via corners to the regular BaO(6)(10-) octahedra. Formation of the mixed-metal oxides takes place at different temperatures: the Ba(5)Nb(4)O(15) oxide occurred at ∼700 °C, as the major crystalline oxide phase; by heating the sample up to 1135 °C, the α-Ba(4)Nb(2)O(9) form was obtained, whereas the heating at 1175 °C caused the crystallization of two polymorphs, γ-Ba(4)Nb(2)O(9) and δ-Ba(4)Nb(2)O(9). Special focus was set on the electrical properties of the prepared mixed Ba(II)-Nb(V) oxides obtained by this molecular pathway in a single-step preparation.

Supramolecular architectures of novel chromium(III) oxalate complexes: steric effects of the ligand size and building-blocks approach.

Five new oxalate complexes of chromium(III), [Hphen][Cr(phen)(C(2)O(4))(2)]·2H(2)O (1), [Cr(phen)(2)(C(2)O(4))][Cr(phen)(C(2)O(4))(2)]·3H(2)O (2), [Cr(phen)(2)(C(2)O(4))]NO(3)·H(2)C(2)O(4)·H(2)O (3), [Cr(bpy)(2)(C(2)O(4))][Cr(bpy)(C(2)O(4))(2)]·3H(2)O (4) and [Cr(bpy)(2)(C(2)O(4))]NO(3)·1/2H(2)C(2)O(4)·4H(2)O (5) (phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine), were prepared by using an (oxalato)tantalate(V) solution as a source of oxalate ligands. The compounds contain either the discrete mononuclear [Cr(L)(2)(C(2)O(4))](+) cation [L = phen (3); L = bpy (5)] or the discrete mononuclear [Cr(L)(C(2)O(4))(2)](-) anion [L = phen (1)], or both types of mononuclear ions [L = phen (2); L = bpy (4)]. The crystal structures are dominated by the hydrogen-bonding and π···π-stacking interactions that give rise to the overall two- (compounds 1, 2, 4, 5) or three-dimensional (compound 3) architectures. Compounds 2 and 4 represent a borderline case between isostructurality and non-isostructurality; they exhibit an analogous packing of the cation and the anion units, but the crystallization water molecules occupy different positions - due to a difference in size between the phen and bpy ligands. The influence of steric factors is evident also in the case of 3 and 5, which, despite very similar chemical formulae, exert a completely different packing of the constituents. By the self-assembling of 1 and 4, used as building blocks in the reaction with calcium(II) cations, the heterobimetallic polymeric compounds {[CaCr(2)(phen)(2)(C(2)O(4))(4)]·5H(2)O}(n) (6) and {[CaCr(2)(bpy)(2)(C(2)O(4))(4)]·0.83H(2)O}(n) (7) were obtained. The crystal structure of 7 is reported: the [Cr(bpy)(C(2)O(4))(2)](-) unit, through the two oxalate groups, acts as a chelating ligand towards Ca cations, resulting in heterometallic one-dimensional double zigzag chains, formed of diamond-shaped units. The characterization of the compounds obtained was accomplished by the spectroscopy and thermal analysis methods.

The supramolecular architecture in 4,4'-bipyridinium bis(hydrogen oxalate).

The asymmetric unit of the title compound, C(10)H(10)N(2)(2+)·2C(2)HO(4)(-), consists of one half of a 4,4'-bipyridinium cation, which has inversion symmetry, and a hydrogen oxalate anion, in which an intramolecular hydrogen bond exists. The cations and anions are connected by O-H···O, N-H···O and C-H···O hydrogen bonds, forming a two-dimensional network, whereas π-π stacking interactions involving the 4,4'-bipyridinium cations lead to the formation of a three-dimensional supramolecular structure. An unusual deca-atomic ring is formed between two hydrogen oxalate anions, which are linked side-to-side via O-H···O hydrogen-bonding interactions.