Aurelio Beltrán-Porter

Generalised syntheses of ordered mesoporous oxides: the atrane route

Abstract A new simple and versatile technique to obtain mesoporous oxides is presented. While implying surfactant-assisted formation of mesostructured intermediates, the original chemical contribution of this approach lies in the use of atrane complexes as precursors. Without prejudice to their inherent unstability in aqueous solution, the atranes show a marked inertness towards hydrolysis. Bringing kinetic factors into play, it becomes possible to control the processes involved in the formation of the surfactant–inorganic phase composite micelles, which constitute the elemental building blocks of the mesostructures. Independent of the starting compositional complexity, both the mesostructured intermediates and the final mesoporous materials are chemically homogeneous. The final ordered mesoporous materials are thermally stable and show unimodal porosity, as well as homogeneous microstructure and texture. Examples of materials synthesised on account of the versatility of this new method, including siliceous, non siliceous and mixed oxides, are presented and discussed.

Enhanced surface area in thermally stable pure mesoporous TiO2

Abstract We describe here for the first time the surfactant-assisted synthesis of thermally stable mesoporous pure TiO 2 having a high surface area. Our synthetic approach to the chemistry of this system is based on the equilibrium between the hydrolysis and condensation reactions of the inorganic species and the organic–inorganic self-assembling processes. The use of titanatrane complexes helps to retard the hydrolysis and condensation reactions, thus allowing us to overcome the difficulties in preparing titanium dioxide mesoporous materials starting from highly reactive Ti-alkoxides. The mesoporous material has been characterized by TEM, XRD and N 2 adsorption–desorption isotherms and displays a typical wormhole-like pore structure.

Crystal structure and spectroscopic studies of bis(N-2-pyridinylcarbonyl-2-pyridinecarboximidato)copper(II) monohydrate. Local bonding effects

Abstract The crystal and molecular structure of bis(N-2- pyridinylcarbonyl-2-pyridinecarboximidato)copper(II) monohydrate, Cu(BPCA)2·H2O, has been determined from single crystal X-ray data. It crystallizes in the monoclinic space group P21/c with four formula units in a cell of dimensions: a = 8.917(1), b = 8.932(1), c = 28.794(17) A, β= 95.49(2)°.Least- squares refinement of 2754 reflections with I > 2.5σ(I) and 379 parameters gave a final R = 0.037 (Rw = 0.036). The structure consists of discrete neutral Cu(BPCA)2 entities linked two by two through water molecules hydrogen bonded to ligand carbonyl groups. The coordination geometry around copper ions can approximately be described as orthorhombically distorted octahedral. EPR and ligand field spectroscopic results have been analyzed in terms of a static Jahn-Teller distortion, and bonding parameters have been derived from EPR hyperfine and superhyperfine structures in the spectra of copper(II)-doped Zn(BPCA)2·H2O powder samples and frozen solution.

New trends in V–P–O solids

On the basis of the industrial interest of the oxovanadium phosphate catalysts, current research effort in this field is focused mainly on the development of synthetic strategies directed towards obtaining open-framework materials. There is a growing body of work describing preparations using hydrothermal procedures under a diversity of conditions. A great number of new solids, whose nets range from lamellar arrays to micro- and mesostructured organizations, has been prepared in last years. In this context, the applicability of concepts and procedures from the zeolites chemistry to systems involving transition elements is critically analyzed.

Crystal structure and spectroscopic study of [Cu(BPCA)(OH2)(O2CCH3)·H2O complex; BPC = N-2-pyridinylcarbonyl-2-pyridinecarboximidate anion

Abstract The crystal and molecular structure of the [Cu(BPCA)(OH2)(O 2CCH3]·H2O complex (BPCA = N-2-pyridinylcarbonyl-2-pyridinecarboximidate anion) has been determined by X-ray diffraction methods. It crystallizes in the triclinic space group P l with two formula units in a cell of dimensions: a = 7.416(2), b = 8.632(4), c = 13.034(3) A, α = 74.55(3), β = 84.84(3), γ = 81.04(5)°. Least-squares refinement of 2534 reflections with I > 2.5σ(I) and 218 parameters gave a final R = 0.047 and Rw = 0.049. The structure consists of discrete [Cu(BPCA)(OH2)(O2CCH3)] neutral units linked through hydrogen bonding. The coordination geometry around copper ions can be described as tetragonal square pyramidal, the oxygen atom from the coordinated water molecule occupying the axial position. The compound loses both coordinated and lattice water upon heating, and quickly rehydrates to the initial form when it cools down to room temperature. The powder X-band ESR spectra of both the hydrated and the anhydrous forms exhibit the ΔMs = 2 absorption, indicating magnetic exchange between two copper(II) ions. The nature of this coupling is discussed on the basis of the structural features.

Simultaneous determination of stoichiometry, degree of condensation and stability constant: A generalization of the molar-ratio method

A generalization of the molar-ratio method is proposed, which allows study of relatively weak complexes. The method is based on treatment of the data from a molar-ratio saturation curve. From the mathematical expression derived, the stoichiometry, degree of condensation and stability constant are easily evaluated. Graphical representations of the results can be used advantageously.

Crystal structure, spectroscopic and magnetic properties of the complex [Cu(paphy)(NCS)(SCN)](paphy = pyridine-2-carbaldehyde 2′-pyridylhydrazone). An unusual di-µ-thiocyanato-N bridged copper(II) dimer

The crystal and molecular structure of [Cu(paphy)(NCS)(SCN)](paphy = pyridine-2-carbaldehyde 2′-pyridylhydrazone) has been determined from three-dimensional X-ray data. It crystallizes in the monoclinic space group P21/a with four formula units in a cell of dimensions a= 15.778(3), b= 9.937(2), c= 9.795(3)A, and β= 91.31(2)°. Least-squares refinement of 1 163 reflections with I 2.5σ(I) gave a final R= 0.073 (R′= 0.073). The structure consists of isolated centrosymmetric [Cu(paphy)(NCS)(SCN)]2 dimeric units where the two copper(II) ions are linked in an unusual way through the N end of two N-thiocyanate bridging groups. The resulting co-ordination geometry around each copper(II) ion is elongated tetragonal octahedral. The N bridging atoms occupy simultaneously an equatorial position in the co-ordination sphere of one of the copper atoms and an axial one in the other. The remaining axial positions are occupied by sulphur atoms of the S-bonded thiocyanate groups. Magnetic susceptibility measurements down to 2.8 K and e.s.r. spectra provide evidence of the existence of very weak exchange coupling. These results are discussed on the basis of the structural features. A comparison is made with previously published data on related copper(II) thiocyanate complexes.

SYNTHETIC PATHWAYS TO VANADYL PHOSPHATES

Abstract A general scheme concerning the synthesis and reactivity of oxovanadium phosphates is presented. In particular, the isolation of new materials of composition NH 4 (VO 2 )(HPO 4 ), [M(en) 2 ] x VOPO 4 ·(2−2 x )H 2 O (M=Cu or Ni; 0.03⩽ x ⩽0.20; en=ethylendiamine) and Na x VOPO 4 is described. The structures of α-NH 4 (VO 2 ) (HPO 4 ), β-A(VO 2 ) (HPO 4 ) (A=NH + 4 , k + , Rb + , Cs + ) and VOHPO 4 ·2H 2 O are reported. New spectroscopic (IR, EPR, NMR) results are furnished. The magnetic properties of vanadium (IV)-containing derivatives are discussed on the basis of the structural results.

Compounds of WVI with 1(+)-sorbitol: Study of formation and interconversion equilibria

SummaryThe polarimetric study of the WVI-sorbitol system shows the formation of three stable complexes. The species stable at higher pH is monomeric ([WO(OH)(C6H12O6)2]−), while the other two are dinuclear complexes of different stoichiometries ([W2O3(OH)4(C6H10O6)]−2 and [W3O3(OH)(C6H11O6)2]−). The interconversion equilibria have been established and the formation constants determined. The behaviour of sorbitol is like that displayed by other polyhydroxylic ligands having one or two carboxylic groups. The substitution of a carboxylic group for an alcoholic one seems only to result in lower stability constants for the complexes formed.

Synthesis and spectral studies ofN-2-pyridinylcarbonyl-2-pyridinecarxoximidate copper(II) complexes

SummaryDimeric and polymeric copper(II) complexes containing BPCA (N-2-pyridinylcarbonyl-2-pyridinecarboximidate), having general formulae Cu(BPCA)X·nH2O (X=Cl, Br, NCS, NCO, N3, or CN) and Cu2(BPCA)2-X·nH2O [X=oxalate anion (OX), chloranilate anion (CA) or the dianion of 2,5-dihydroxy-1,4-benzoquinone (DHBQ)] have been synthesized by the copper(II)-assisted hydrolysis of 2, 4, 6-tris(2-pyridyl)-1, 3, 5-triazine. Spectroscopic results indicate five-coordinate, approximately square-pyramidal, geometry around the copper(II) ion. Half-field absorption in the ΔMs=±2 region of the X-band e.p.r. powder spectra has been observed for the dimeric species.