P. Martin-Zarza

Equilibria of chromate(VI) species in acid medium and ab initio studies of these species

Abstract The study of the H+ −CrO42− system in 3.0 M KCl as ionic medium at 25°C by means of emf (glass electrode) and direct calorimetric measurements, in the total CrVI concentration (B), average number of H+ bounds per central group CrO42− (Z), and pH ranges: 25 ⩽ B ⩽ 100 mM, 0 ⩽ Z ⩽ 1.16 and 1 ⩽ pH ⩽ 8, respectively, indicates the formation of the following complexes Hp(CrO4)q(p−2q), stability constants (logβpq(± 3σ)) and partial molar enthalpies (ΔHpq(±3 σ) kcal.mol−1): HCrO4−, 5.888(4), −0.6(1); Cr2O72−, 13.900(3), −5.7(1); H2CrO4, 7.004(7), 1.8(2) and HCr2O7−, 15.007(5), −5.0(1), respectively, according to the general reaction: pH+ + qCrO42− ⇄ Hp(CrO4)q(p−2q). The results previously obtained by Raman spectroscopy for this system are better adjusted when the HCrO4− species is included. The energies and optimized structures from ab initio calculations for the CrO42−, HCrO4−, Cr2O72−, H2CrO4 and HCr2O7− species have also been obtained. For the HCrO4− species the values of bond lengths and angles theoretically calculated are in good agreement with the experimental data of the anion in the crystal structure of the compound (PPh4)[CrVIO3(OH)]. Finally, correlations between thermodynamic experimental and structural theoretical parameters are discussed.

Preparation and characterization of the dichromates of the organic bases: 2,4′-bipyridine, 2,2′-dipyridylamine and 4,4′-bipyridine

Abstract Dichromates of 2,4′-bipyridine (two forms), [(H2-2,4′-bipy)(Cr2O7)] (1) and [(H-2,4′-bipy)2(Cr2O7)] (2), 2,2′-dipyridylamine, [(H-dpam)2(Cr2O7)] (3), and 4,4′-bipyridine (two forms), [(H2-4,4′-bipy)(Cr2O7)] (4) and [(H-4,4′-bipy)2(H2-4,4′-bipy)(Cr2O7)2] (5), were prepared by reaction of chromium(VI) oxide with the free organic bases in the appropriate stoichiometric ratio in water at 25°C. The structures of compounds 2, 3 and 4 were determined by X-ray crystallography and revealed the existence of discrete dichromate anions formed by two tetrahedral CrO4 groups joined through shared O atoms, and the respective protonated organic bases hydrogen bonded to respective dinegative anions. A short CH…O interaction [2.996(4) A] between the organic cation and the dichromate anion was observed in compound 4. All organic dichromates were also characterized by IR, UV-vis 1H NMR measurements. For compounds 2, 3 and 4, a comparison was made and the crystallographic data of a series of mono- and polychromates and showed a correlation of the terminal and bridging CrO atomic distances with an increasing degree of polymerization. A graph theory cluster expansion was employed for these correlation studies.

Molybdates of aromatic heterocyclic compounds. Synthesis and structure of an octamolybdate containing coordinately bound pyrazole: [(C3H4N2)2(Mo8O26)]4−

Abstract Molybdenum trioxide reacts with pyrazole to give the octamolybdate: (C3H4N2H)4[(C3H4N2)2(Mo8O26)]. The anion [(C3H4N2)2(Mo8O26)]4−, is built of two C3H4N2MoO5 and six MoO6 edge-sharing octahedra. The MoNNC3H4 bond length is 2.243(4) A, giving a bond strength of 0.35.

Anderson-Evans type molybdotellurates with diazolium cations: preparation, structure and properties of (H2imz)6 [TeMo6O24]·4H2O and (H2pyz)6 [TeMo6O24]·Te(OH)6

Abstract Two new Anderson-Evans molybdotellurates with imidazolium and pyrazolium cations have been prepared. X-ray crystallographic studies have been made from crystals of both compounds. The imidazolium salt (C3H5N2)6 [TeMo6O24]·4H2O (1) crystallizes in the monoclinic space group P2 1 /n with a = 14.372(3), b = 11.275(2), c = 13.019(3) A , β = 91.17(3)°, V = 2109.2(2) A 3 at 20°C and Z = 2; structural analysis is based on 6172 independent reflections (l ≥ 2σ(l)) with least-squares refinement of 297 parameters converged to R = 0.049. The crystal structure consists of a [TeMo6O24]6− with the TeO6 octahedron at the centre surrounded by six MoO6 sharing edges. The protonated organic bases bond with the anion in the crystal by hydrogen bonds. The pyrazolium salt (C3H5N2)6 [TeMo6O24]·Te(OH)6 (2) crystallizes in the triclinic space group P-1 with a = 10.920(2), b = 11.518(2), c = 10.056(2) A , α = 93.92(3), β = 103.97(3), γ = 109.80(3)°, V = 1139.1(4) A 3 at 20° C and Z = 2 . Structure analysis with least-squares refinement of 311 parameters for 6599 independent reflections (l ≥ 2σ(l)) converged to R = 0.0526. The crystal structure of this compound consists in discrete [TeMo6O24]6− anions and Te(OH)6 units bonded to pyrazolium cations by hydrogen bonds. The IR spectra of both compounds are discussed on the D3d symmetry of the [TeMo6O24]6 anion. The compounds have been studied by 1H, 95Mo and 125Te NMR spectroscopy, indicating in DMF the presence of octahedral species for both compounds, and a dissimilar behavior in H2O solution. While compound 1 undergoes a clear hydrolitic process forming tetrahedral MoO42− species, compound 2 is more stable in aqueous media. Thermogravimetry and differential scanning calorimetry of 1 show that this compound loses the four water molecules through an endothermic process ΔH = 137 kJ mol−1. The residual products were characterized by chemical analysis, IR and XPS spectroscopy. The best kinetic parameter values, corresponding to a solid state process involving the loss of the water molecules, were found on the basis of dynamic and isothermal methods. The best representation of a reaction mechanism is the random nucleation-one nucleus on each particle: −ln(1 − α) = kt. The values of activation energy (Ea = 93.3 kJmol−1), entropy of activation (ΔS≠ = −42.2 J K−1 mol−1), enthalpy of activation (ΔH≠ = 76.7 kJ mol−1) and free energy of activation (ΔG340.15 K≠ = 91.1 kJ mol−1) have been determined.

Synthesis and characterization of the new peroxo complex aquaoxodiperoxoimidazolemolybdenum(VI)

Abstract A new peroxo complex of Mo(VI), MoO(O 2 ) 2 (Him)(H 2 O) (Him=imidazole), has been prepared from octamolybdate of imidazole, [H 2 im] 4 [Mo 8 O 26 (Him) 2 ], and H 2 O 2 . The crystal and molecular structure was determined by X-ray crystallography and showed that the molybdenum atom is seven-coordinated with the nitrogen atom of the imidazole and the four oxygen atoms of the two peroxo groups in the equatorial plane while the oxygen atom and one water molecule occupy the apical positions of a pentagonal bipyramid.

Dimer species in aqueous solutions of m–phenylenediamine-N,N,N′,N′-tetra-acetic acid (m-H4pdta) with copper(II) and of pyridine-2,6-diamine-N,N,N′,N′-tetra-acetic acid (2,6-H4pydta) with nickel(II). X-Ray crystal structures of Na4[Cu2(m-pdta)2]·18H2O, Na4[Co2(m-pdta)2]·10H2O, and Na4[Ni2(2,6-pydta)2]·8H2O

Potentiometric investigations in aqueous solution at 25 °C and ionic strength 0.1 mol dm–3 KCl have shown that the dimer complex species [Cu2L2]4–(ligand H4L, m-phenylenediamine-N,N,N,′N′-tetra-acetic acid, m-H4pdta) and [Ni2L2]4–(ligand H4L, pyridine-2,6-diamine-N,N,N′,N′-tetra-acetic acid, 2,6-H4pydta) are present in significant amounts at CM > 2 × 10–3 mol dm–3. The formation constants of the monomer and dimer have been determined. The formation of the dimer [M2L2]4– from the monomer [ML]2– or [M(HL)]– is affected by the Jahn–Teller effect (CuII–m-H4pdta) or by the effect of the withdrawal of electronic density by the pyridine nitrogen of the ring (NiII–2,6-H4pydta system). From a concentrated solution with a ligand: metal ratio of 1 : 1 at pH 6, single crystals of the complexes Na4[Cu2(m-pdta)2]·18H2O, Na4[Co2(m-pdta)2]·10H2O, and Na4[Ni2(2,6-pydta)2]·8H2O, respectively, were obtained. X-Ray diffraction structural analysis revealed that the dimer molecules are centrosymmetrical (Ci), with each metal atom surrounded by four carboxylic oxygens and two amine nitrogens in a distorted octahedron, the copper(II) complex being the most distorted (Jahn–Teller effect). In the nickel(II) complex the pyridine nitrogen is not bonded. Each sodium is co-ordinated to water molecules and carboxyl groups, being six-co-ordinated in the copper(II) complex and five-co-ordinated in the complexes of COII and NiII. The complexes were also characterized by i.r., electronic, and mass spectra, magnetic measurements, and thermogravimetric analysis.

Structure and properties of Cu(II)(o-hydroxypropiophenone-salicylhydrazide)(2,2′-bipyridyl) [Cu(OHP-SHZ)(bipy)] and Cu(II)(o-hydroxypropiophenone-salicylhydrazide)(2,2′-bipyridylamine) monohydrate [Cu(OHP-SHZ)(bipyam)]H2O

Abstract The preparation of stable Lewis base adducts of CuL (H2L=Schiff base derived from the condensation of o-hydroxypropiophenone (OHP) and salicylhydrazide (SHZ)) is described. The N-donors bipy and bipyam ligands (bipy=2,2′-bipyridyl; bipyam=2,2′-bipyridylamine) give the complexes [Cu(OHP-SHZ)(bipy)] (I) and [Cu(OHP-SHZ)(bipyam)]H2O (II) which have been characterized by physical and spectroscopic techniques. Crystals of I are orthorhombic, space group Pcab, with a=10.221(3), b=12.643(3), c=35.448(3) A, V=4581(2) A3 and Z=8. The structure of complex I is shown to comprise discrete monomeric [Cu(OHP-SHZ)(bipy)] molecules in which the Cu(II) atom environment exhibits a highly distorted five-coordinated geometry. Crystals of II are monoclinic, space group C2/c, with a=24.804(3), b=13.231(3), c=14.739(3) A, β=99.57(2)°, V=4770(2) A3 and Z=8. The structure of this complex consists of discrete entities associated into H-bonded binuclear units via the lattice water molecules. The coordination sphere around the copper center is nearly square pyramidal. The transition from a six-membered chelated ring to a five-membered one results in a less distorted coordination polyhedron around the Cu(II) ion. In both complexes the OHP-SHZ ligand occupies three of the coordination sites through the same (ONO) donor atoms while the remaining positions are occupied by the pyridine N atoms. EPR spectra provide evidence of the existence of very weak magnetic exchange coupling in the complex [Cu(OHP-SHZ)(bipyam)]H2O (II). Thermogravimetric and differential scanning calorimetric studies of complex I show that this complex loses the bipy molecule through an endothermic process with ΔH=81.75 kJ mol−1. The best kinetic parameter values corresponding to the process involving the loss of the bipy molecule were found on the basis of the agreement between non-isothermal and isothermal methods and are given by the Abou-Shaaban–Simonelli equation in isothermal conditions. The better representation of the reaction mechanism is the phase-boundary, spherical symmetry reaction.

Preparation and characterization of pyridinium-n-carboxylate trioxochromate (VI) (n=3, 4) and pyridinium-4-carboxylic pyridine-4 carboxylate trioxochromate (VI) hemihydrate

Abstract Three chromium (VI) compounds have been synthesized at room temperature by reaction of CrO3 with pyridinium carboxylic acids in water. An X-ray diffraction study of pyridinium-3-carboxylate trioxochromate, [3-HNicCrO3] (1), indicates crystallization in the triclinic system, space group P l (No. 2) with Z = 2 and lattice parameters a = 7.548(3) A , b = 6.593(3) A , c = 7.820(3) α = 90.01(2)°, β = 91.65(2)°, γ = 89.91(2)° and V = 389.0(3) A 3 at 25(2)°C. The chromium atom presents a distorted tetrahedral coordination, with the three CrO (terminal oxygen) bond lengths ranging from 1.588(6) A to 1.603(6) A, and the fourth CrO bond being 1.881(4) A long. The symmetry is C81 all atoms lying in the same plane with the exception of two terminal oxygens. 3-HNicCrO3 molecules interact by hydrogen bonds. This experimental work has been complemented with a theoretical study to discuss geometries, relative energies for the two possible isomers of compound 1, IR and UV-visible spectroscopic data, and electronic analyses. Powder data for pyridinium-4-carboxylic pyridine-4- carboxylate trioxochromate hemihydrate, [4-HNic][4-NicCrO3] (2), and pyridinium-4-carboxylate trioxochromate, [4-HNicCrO3] (3), are also reported.

Complexes of copper(II) and nickel(II) with the new ligand N-[2-(3-ethyl-indole]pyridoxaldimine. X-ray crystal structure of bis{N-[2-(3-ethyl-indole)]pyridoxaldiminate}nickel(II)

Abstract The monoanionic bidentate ligand N-[2-(3-ethyl-indole)]pyridoxaldimine prepared by 1:1 Schiff-base condensation of 3-(2-amino-ethyl)indole (tryptamine, TPA) with [3-hydroxy-5-(hydroxy-methyl)-2-methyl-4-pyridinecarboxaldehyde] (pyridoxal, pyrdox) and three different copper complexes are reported. The nickel(II) compound with the same ligand is also studied. All complexes were characterized by IR, UVvis and 1H NMR spectra, magnetic measurements and thermoanalytical data. A single-crystal of the nickel complex was subjected to X-ray diffraction analysis. The coordination of the nickel(II) complex is trans planar (NiN2O2) with NiO and NiN bond lengths of 1.841(l) and 1.926(2) A, respectively. The spectroscopic data for the copper(II) complexes suggest the same chromophore group (CuN2O2) with a trans configuration in the solid phase for all. Finally, the dehydration reaction Cu(pyrdoxTPA)2·yH2O(s) → Cu(pyrdoxTPA)2(s) +yH2O(g) for the two copper complexes is studied.

Molybdotellurates containing the 1-methyl-, 2-methyl- and 4-methyl imidazolium cations

Four molybdotellurates containing the 1-methylimidazolium, 2-methylimidazolium and 4-methylimidazolium cations have been synthesized and their structures: [2-H2-methyl-imz]6[TeMo6O24 ]·2H2O (2), [2-H2-methyl-imz]6[TeMo6O24 ]·2(2-H-methyl-imz)·2H2O (3) and [4-H2-methyl-imz]6[TeMo6O24 ]·Te(OH)6 (4) determined by X-ray diffraction methods. The protonated organic bases are bonded to the anion in the crystal by hydrogen bonds, except for (4) where the crystal structure consists of discrete [TeMo6O24]6− anions and Te(OH)6 units, both bonded to 4-methylimidazolium cations by hydrogen bonds. The hydrogen bonds were studied as a function of the unit charge of the oxygen atoms of the [TeMo6O24]6− anion. Distortions of the central octahedron of polyanions of formula [XMo6O24]n− (X=AlIII, MoVI, TeVI and IVII), and polyanions of formula [H6YMo6O24]n−, (Y=CoII, CuII, ZnII, CrIII, RhIII and PtIV) are discussed. 95Mo n.m.r spectroscopy of compounds [1-H2-methyl-imz]6[Te-Mo6O24]·Te(OH)6 (1), (2) and (4) indicates the existence of an octahedral oxygen atom arrangement around the molybdenum and a pH variation experiment, carried out with compound (1), confirmed the existence of hydrolytic processes of the compounds in aqueous solution. 125Te n.m.r. studies permitted identification of the Te atom in the [TeMo6O24]6− kernel in all compounds; the presence of two different Te(OH)6 moieties in compounds (1) and (4) was also detected. The similarity between the spectra of both compounds could indicate that (1) has the same structural arrangement as (4). Finally, the thermal behaviour and the thermal stabilities of the complexes as a function of the organic cation were studied.