Alfredo Mederos

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.

Ferromagnetic coupling in the malonato-bridged copper(II) chains [Cu(Im)2(mal)]n and [Cu(2-MeIm)2(mal)]n(H2mal = malonic acid, Im = imidazole and 2-MeIm = 2-methylimidazole)

Two new malonato-bridged copper(II) complexes of formula [Cu(Im)2(mal)]n (1) and [Cu(2-MeIm)2(mal)]n (2) (Im=imidazole, 2-MeIm=2-methylimidazole and mal=malonate dianion) have been prepared and their structures solved by X-ray diffraction methods. The [Cu(Im)2(mal)] and [Cu(2-MeIm)2(mal)] neutral entities act as monodentate ligands towards the adjacent copper(II) units through one of the two carboxylate groups, the OCO bridge exhibiting an anti-anti conformation. The environment of each copper atom in 1 and 2 is distorted square pyramidal: two carboxylate oxygen atoms from a bidentate malonate and two nitrogen atoms from two imidazole (1) or 2-methylimidazole (2) ligands form the equatorial plane whereas the apical position is occupied by a carboxylate oxygen from the malonate group of the neighbouring complex unit. The intrachain copper-copper separation is 6.036(2) (1) and 6.099(2) A (2). The magnetic properties of 1 and 2 have been investigated in the temperature range 1.9–290 K. Overall, ferromagnetic behaviour is observed in both cases and the intrachain magnetic coupling (J) between the copper(II) ions through the carboxylato group is found to be 1.64 (1) and 0.39 cm−1 (2) (the Hamiltonian being H=−JΣiSi·Si+1).

Cucurbituril as a New Macrocyclic Ligand for Complexation of Lanthanide Cations in Aqueous Solutions

(Aqua)lanthanide complexes with cucurbituril {[Gd(NO3)(H2O)4](C36H36N24O12)}(NO3)2·7H2O (1), {[Gd(NO3)(C2H5OH)(H2O)3](C36H36N24O12)}(NO3)2·5.5H2O (2), {[Ho(NO3)(H2O)4](C36H36N24O12)}(NO3)2·7H2O (3), {[Yb(NO3)(H2O)4](C36H36N24O12)}(NO3)2·6H2O (4), {[La(H2O)6(SO4)](C36H36N24O12)}(NO3)·12H2O (5), {[Gd(H2O)4]2(C36H36N24O12)3}Br6·45H2O (6), and {[Ce(H2O)5]2(C36H36N24O12)2}Br6·26H2O (7) were obtained in high yield by reaction of cucurbituril with aqueous solutions of lanthanide(III) species. The crystal structures of the compounds show a packing of 1:1, 2:2, and 2:3 in the (cucurbituril)lanthanide complexes in which cucurbituril plays a bidentate ligand role, and water molecules of the (aqua)lanthanide complexes form hydrogen bonds with carbonyl groups of the cucurbituril molecule. The guest water molecule is situated in the cucurbituril molecule cavity of 2 and 5. The crystal structure of 6 is a packing of three-deck sandwiches, built from alternating cucurbituril molecules and Gd(H2O)43+ ions. The largest distance between outermost oxygen atoms in the sandwiches is 30.04 A. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Coordinating ability of phenylenediamines

Abstract The coordinating ability of aromatic diamines is conditioned both by the low basicity of the nitrogen atoms and their situation in ortho, meta or para positions on the aromatic ring. Important aspects of the aromatic diamines taking into account their possible applications are: processes based on electron transfer forming Wurster type radical colored cations; trace analysis of metal ions and other species based on catalysis of the oxidative coloration system; the development of biosensors coated with poly(o-phenylenediamine) and their mutagenic and toxic character. o-Phenylenediamine ligands coordinate in a monodentate, chelating bidentate and bridging bidentate fashion in dinuclear complexes; p-phenylenediamine is mainly a bridging bidentate ligand as expected.

Emf(H) data analysis of weak metallic complexes using reduced formation functions

A general method for the study of weak metal complexes by emf(H) measurements has been developed using reduced formation functions instead of classical formation functions. This approach consists of subtracting the contribution of the products of the hydrolysis (protolysis) of the metallic cation (anion), as well as the possible protonated species of the ligand from the total number of associated H+, and from the total concentrations of metal or ligand, observing only the contribution of the reaction of interest. This was carried out using the FONDO version of the generalized least-squares computer program LETAGROP, written to analyze these reduced formation functions. The aim of this communication was to show in greater detail than in previous publications the data analysis of reactions in solution using these reduced formation functions. The method is illustrated using emf(H) data for the three-component systems H+–Be(II)–serine, H+–Mo(VI)–NTA and H+–V(IV)–V(V) investigated recently.

Coordinating ability of ligands derived from phenylenediamines

Abstract The preparation of coordinating agents derived from aromatic diamines is of special interest since the use of nitrogen atoms for coordination to a single cation is directly related to their situation in ortho, meta or para positions. In the case of diaminetetramethylenecarboxylic acids or Schiff bases derived from o-phenylenediamines, the proximity of the nitrogen atoms permits their simultaneous coordination to the same metal cation, leading mainly to monomer species. Combined with the lesser basicity of the nitrogen atoms of the aromatic diamines, the o-phenylenediaminetetramethylene-carboxylic acids are good sequestering agents for some metal ions. Complexes derived from o-phenylenediamine Schiff bases provide new and interesting environment for M–C funcionalities. Iron(III) complexes with o-phenylenediamine Schiff bases may present spin-crossover. On the other hand, the ligands derived from m- or p-phenylenediamines can only coordinate one nitrogen atom to any one metal cation. The formation of species with excess of ligand (2:1 ligand:metal ratio), with excess of metal (1:2 ratio), and monomers (1:1 ratio) is now possible. Moreover, the special conformation of the ligands with nitrogen atoms in meta or para positions on the aromatic ring facilitates the formation of dimer complexes since the ligands act as a bridge. The dinuclear complexes of those ligands with paramagnetic ions may present magnetic coupling. With the nitrogen atoms in para position, polymer complexes are also possible since the ligand acts as a bridge.

Preparative, potentiometric and NMR studies of the interaction of beryllium(II) with oxalate and malonate. X-ray structure of K3[Be3(OH)3(O2C–CH2–CO2)3]·6H2O

Abstract The complexes formed by beryllium(II) with the bidentate ligands oxalate, L=(O2C–CO2)2−, and malonate, L=(O2C–CH2–CO2)2−, have been investigated in aqueous solution using both potentiometric and 9Be NMR measurements. The species [BeL(H2O)2], [BeL2]2−, [Be3(OH)3L3]3− and [Be3(OH)3(H2O)3L]+ have been identified and their formation constants have been determined at 25°C in 0.5 mol dm−3 NaClO4. The malonate complexes are much more stable than the oxalate ones. New crystalline salts of formula K3[Be3(OH)3L3]·nH2O have been isolated using conditions established with the aid of speciation calculations. The structure of K3[Be3(OH)3(malonate)3]·6H2O has been determined by an X-ray structure analysis: orthorhombic, space group Pc21n, a=9.011(3), b=14.041(4), c=18.761(9) A, Z=4. Each beryllium atom is tetrahedrally coordinated by two hydroxo groups and two oxygen atoms from the chelating malonate. The (Be(OH))3 core is a puckered six-membered ring with each hydroxo group bridging two beryllium centres.

Crystal structures of two new heptamolybdates and of a pyrazole incorporating a gamma-octamolybdate anion

Two new heptamolybdates, one containing calcium and imidazole and the other with urea and ammonium cations, have been prepared. X-ray crystallographic studies of single crystals of both compounds have been carried out. In the mixture of calcium and imidazole [(Himi)(4)][Ca(H2O)(6)(mu-O)(2)][Mo7O24]. 2(imi). 3H(2)O (1), the calcium(II) cation is surrounded by six water molecules and two oxygen atoms from the polyanion, and bridges in trans position two [Mo7O24](6-) polyanions forming a linear composite with alternating Mo7O24 and Ca(H2O)(6) units. The urea compound [CO(NH2)H](3)(NH4)(9)[Mo7O24](2). 5[CO(NH2)(2)]. 4H(2)O (2) presents [Mo7O24](6-) heptamolybdate anions bound by hydrogen bonds from NH4+, [CO(NH2)H] cations and H2O molecules. A new crystalline phase of a gamma-octamolybdate containing pyrazole coordinatively bound to molybdenum has also been prepared [(Hpyr)(4)][(pyr)(2)(Mo8O26)]CH3COCH3. 2H(2)O (3). The structure has been solved by X-ray diffraction from a single crystal. The Mo-N bond length is 2.262(3) Angstrom. All compounds have been characterised by IR,H-1 NMR and thermal analysis

Speciation study of the anti-inflammatory drug tenoxicam (Htenox) with Cu(II): X-ray crystal structure of [Cu(tenox)(2)(py)(2)].EtOH.

A speciation study was carried out in aqueous solution of the anti-inflammatory drug tenoxicam (Htenox), under quasi-physiological conditions (temperature of 37 degrees C and ionic strength 0.15 M NaCl) in order to determine the acidity constants from spectrophotometric studies, the pK(a) values found being pK(1)=1.143+/-0.008 and pK(2)=4.970+/-0.004. Subsequently, the spectrophotometrical speciation of the different complexes of Cu(II) with the drug was performed under the same conditions of temperature and ionic strength, observing the formation of Cu(Htenox)(2)(2+) with log beta(212)=20.05+/-0.01, Cu(tenox)(2) with log beta(012)=13.6+/-0.1, Cu(Htenox)(2+) with log beta(111)=10.52+/-0.08, as well as Cu(tenox)(+) with log beta(011)=7.0+/-0.2, all of them in solution, and solid species Cu(tenox)(2)(s) with an estimated value of log beta(012)(s) approximately 18.7. The crystalline structure of the complex [Cu(tenox)(2)(py)(2)]. EtOH, was also determined, and it was observed that tenoxicam employs the oxygen of the amide group and the pyridyl nitrogen to bond to the cation.

Preparation and characterization of triangular clusters [M3Q4(acac)3(py)3]+ (M=Mo, W; Q=S, Se)

Abstract Reactions of [M3Q4(H2O)9]4+ with acetylacetone (Hacac) and pyridine (py) in weakly alkaline solutions (pH 8–9) give mixed-ligand complexes [M3Q4(acac)3(py)3]+ (M=Mo, W; Q=S, Se), isolated as hexafluorophosphate salts. For M=Mo, Q=S and for M=W, Q=both S and Se, X-ray structural analysis has been carried out. The three compounds are isostructural and crystallize as [M3Q4(acac)3(py)3]PF6·CHCl3. Each M is attached to one capping (μ3) and two bridging (μ2) chalcogen atoms, one pyridine ligand and one chelating acetylacetonate ligand. The latter are coordinated symmetrically in positions trans to the μ2-Q bridges to give cluster cations with crystallographic threefold rotation symmetry (point group 3m or C3v). The structure is preserved in solution. CV experiments show two almost reversible waves due to consecutive one electron reductions with the ease of reduction decreasing in the order, Mo3S4>Mo3Se4>W3Se4>W3S4. FAB MS data are also discussed.