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.

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.

High-temperature experimental and theoretical study of magnetic interactions in diamond and pseudo-diamond frameworks built up from hexanuclear tantalum clusters.

Magnetic interactions in solid-state tantalum cluster compounds have been evidenced by using magnetic susceptibility measurements and corroborated by broken-symmetry DFT calculations. The three selected compounds are based on [Ta(6)X(12)(H(2)O)(6)](3+) (X=Cl or/and Br) units with edge-bridged Ta(6) octahedral clusters. Although two of them crystallise in the tetragonal space group I4(1)/a, all compounds exhibit a similar arrangement of paramagnetic clusters related to the diamond structural framework (Fd ̅3m space group). Magnetic parameters were fitted by using the [5,4] Padé approximant of high-temperature series expansion of susceptibility for the Heisenberg model (S=1/2) in the diamond framework, assuming only nearest-neighbouring interactions. Such a model appropriately describes magnetic-susceptibility measurements at temperatures T>0.7|J|/k. The magnetic interaction parameter J between two [Ta(6)Cl(12)(H(2)O)(6)](3+) clusters is estimated to be -64.28(7) cm(-1) ; it has been enhanced by replacing several chlorine inner ligands with bromine ones (J=-123(3) cm(-1) for two [Ta(6)Br(7.7(1))Cl(4.3(1))(H(2)O)(6)](3+) clusters) and is strongest between two bromine [Ta(6)Br(12)(H(2)O)(6)](3+) clusters with a value of -155(1) cm(-1) . Broken-symmetry DFT calculations within spin-dimer analysis confirmed this trend. Those interactions can be explained on the basis of the overlap between singly occupied a(2u) orbitals localised on neighbouring clusters.

Single crystals of DPPH grown from diethyl ether and carbon disulfide solutions - crystal structures, IR, EPR and magnetization studies.

Single crystals of the free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH) obtained from diethyl ether (ether) and carbon disulfide (CS2) were characterized by the X-ray diffraction, IR, EPR and SQUID magnetization techniques. The X-ray structural analysis and IR spectra showed that the DPPH form crystallized from ether (DPPH1) is solvent free, whereas that one obtained from CS2 (DPPH2) is a solvate of the composition 4DPPH·CS2. Principal values of the g-tensor were estimated by the X-band EPR spectroscopy at room and low (10 K) temperatures. Magnetization studies revealed the presence of antiferromagnetically coupled dimers in both types of crystals. However, the way of dimerization as well as the strength of exchange couplings are different in the two DPPH samples, which is in accord with their crystal structures. The obtained results improved parameters accuracy and enabled better understanding of properties of DPPH as a standard sample in the EPR spectrometry.

Supramolecular motifs and solvatomorphism within the compounds [M(bpy)3]2[NbO(C2O4)(3)]Cl.nH2O (M = Fe2+, Co2+, Ni2+, Cu2+ and Zn2+; n = 11, 12). Syntheses, structures and magnetic properties.

Solvatomorphism has been found between two series of complexes of the composition [M(bpy)3]2[NbO(C2O4)3]Cl.nH2O [M = Fe2+ (1, 2), Co2+ (3, 4), Ni2+ (5, 6), Cu2+ (7) and Zn2+ (8, 9); bpy = 2,2'-bipyridine)], crystallizing in the monoclinic space group P2 1/c [3, 5, 8 (n = 11)] or in the orthorhombic space group P21 21 21 [2, 4, 6, 7 (n = 12)]. All the structures contain two symmetry independent [M(bpy)3]2+ cations, one [NbO(C2O4)3]3- anion, one Cl(-) anion, and crystal water molecules. The cations possess a trigonally distorted octahedral geometry, with an additional tetragonal distortion in 7. Analysis of crystal packing reveals a specific type of supramolecular contact comprising four bipyridine ligands from two neighbouring [M(bpy)3]2+ cations--quadruple aryl embrace (QAE) contact. The contact is realized by the alignment of two molecular two-fold rotation axes, preserving the parallel orientation of the molecular three-fold rotation axes. The resulting two-dimensional honeycomb lattices of [M(bpy)3]2+ cations are placed between the hydrogen bonding layers made of [NbO(C2O4)3]3- and Cl(-) anions and the majority of the crystal water molecules. The temperature-dependent magnetic susceptibility measurements (1.8-300 K) show a significant orbital angular momentum contribution for 3 and 4 (high-spin Co2+), the influence of zero-field splitting for 5 and 6(Ni2+) and a substantially paramagnetic Curie behaviour for the Cu2+ compound (7).

Tetrakis(tetramethylammonium) dodeca-mu-chloro-hexachloro-octahedro-hexatantalate chloride.

The title compound, (C(4)H(12)N)(4)[Ta(6)Cl(18)]Cl, crystallizes in the cubic space group Fm-3m. The crystal structure contains two different types of coordination polyhedra, i.e. four tetrahedral [(CH(3))(4)N](+) cations and one octahedral [(Ta(6)Cl(12))Cl(6)](3-) cluster anion, and one Cl(-) ion. The presence of three different kinds of Cl atoms [bridging (mu(2)), terminal and counter-anion] in one molecule makes this substance unique in the chemistry of hexanuclear halide clusters of niobium and tantalum. The Ta(6) octahedron has an ideal O(h) symmetry, with a Ta-Ta interatomic distance of 2.9215 (7) A.

Hexaaquadodeca-mu-bromo-octahedro-hexatantalum bromide chloride octahydrate.

The title compound, [Ta(6)Br(12)(H(2)O)(6)](Br(0.4)Cl(1.6)) x 8H(2)O, crystallizes in space group P 1 macro. The structure contains two crystallographically independent [Ta(6)Br(12)(H(2)O)(6)](2+) cluster cations forming distinct layers parallel to the ab plane. The compound is isoconfigurational with the double salts [Ta(6)Br(12)(H(2)O)(6)]X(2) x trans-[Ta(6)Br(12)(OH)(4)(H(2)O)(2)] x 18H(2)O (X = Cl, Br).

Bis(tetramethylammonium) hexaaquadodeca-mu-bromo-octahedro-hexatantalum tetrabromide dihydrate.

The novel title compound, [(CH(3))(4)N](2)[Ta(6)Br(12)(H(2)O)(6)]Br(4) x 2H(2)O, with a [Ta(6)Br(12)]2+ cluster unit, has been prepared and structurally characterized. The compound crystallizes in space group C2/c, with a twofold axis passing through the cluster and the centre of symmetry located between the clusters. The nearest neighbouring cluster units are aligned along the crystallographic c axis, forming a one-dimensional chain pattern.