Sixto Domínguez

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.

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.

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.

Solution studies of complexes of iron(III) with iminodiacetic, alkyl-substituted iminodiacetic and nitrilotriacetic acids by potentiometry and cyclic voltammetry

Abstract Potentiometric studies have been performed for the iminodiacetic (IDA), methyliminodiacetic (MIDA), ethyliminodiacetic (EIDA), propyliminodiacetic (PIDA), H2L, and nitrilotriacetic (NTA, H3L)–iron(III) systems at 25°C and I=0.5 mol dm−3 in KNO3. The model and the stability constants of the complexes formed have been determined. Deprotonation and dimerisation constants have been calculated and the species distribution diagrams are explained, together with the cyclic voltammetric studies performed for IDA– and NTA–iron(III) systems.

Chelatable iron pool: inositol 1,2,3-trisphosphate fulfils the conditions required to be a safe cellular iron ligand.

Mammalian cells contain a pool of iron that is not strongly bound to proteins, which can be detected with fluorescent chelating probes. The cellular ligands of this biologically important “chelatable”, “labile” or “transit” iron are not known. Proposed ligands are problematic, because they are saturated by magnesium under cellular conditions and/or because they are not “safe”, i.e. they allow iron to catalyse hydroxyl radical formation. Among small cellular molecules, certain inositol phosphates (InsPs) excel at complexing Fe3+ in such a “safe” manner in vitro. However, we previously calculated that the most abundant InsP, inositol hexakisphosphate, cannot interact with Fe3+ in the presence of cellular concentrations of Mg2+. In this work, we study the metal complexation behaviour of inositol 1,2,3-trisphosphate [Ins(1,2,3)P3], a cellular constituent of unknown function and the simplest InsP to display high-affinity, “safe”, iron complexation. We report thermodynamic constants for the interaction of Ins(1,2,3)P3 with Na+, K+, Mg2+, Ca2+, Cu2+, Fe2+ and Fe3+. Our calculations indicate that Ins(1,2,3)P3 can be expected to complex all available Fe3+ in a quantitative, 1:1 reaction, both in cytosol/nucleus and in acidic compartments, in which an important labile iron subpool is thought to exist. In addition, we calculate that the fluorescent iron probe calcein would strip Fe3+ from Ins(1,2,3)P3 under cellular conditions, and hence labile iron detected using this probe may include iron bound to Ins(1,2,3)P3. Therefore Ins(1,2,3)P3 is the first viable proposal for a transit iron ligand.

Dimer complexes of 2,4-toluenediamine-N,N,N′,N′-tetraacetic acid (2,4-TDTA) with copper(II), nickel(II), cobalt(II), zinc(II) and manganese(II). Studies in aqueous solution and solid state. X-ray crystal structures of Na4[Ni2(2,4-TDTA)2]·15H2O and Na4[Cu2(2,4-TDTA)2]·20H2O

Abstract Potentiometric investigations in aqueous solution at 25°C and ionic strength 0.1 mol dm−3 KCl show that 2 2,4-toluenediamine-N,N,N′N′-tetraacetic acid (2,4-TDTA) forms with Cu(II), Ni(II), Co(II), Zn(II) and Mn(II) the following complexes (ligand, H4L): monomers [MH2L], [MHL]− and [ML]2−; dimers [M2HL2]3− and [M2L2]4−; with excess of ligand [MH2L2]4−, [MHL2]5− and [ML2]6− (Cu(II), Ni(II) and Zn(II)) and with excess of metal [M2HL]+ (Ni(II), Co(II)and Mn(II) and [M2L] (Cu(II)). The formation constants of the complexes have been determined. The species distribution diagrams indicate that a concentrated solution with a ligand:metal ratio 1:1 at pH 6 is suitable for the synthesis of crystalline salts of the dimetric complex species of Cu(II), Ni(II), Co(II), Zn(II) and Mn(II). X-ray diffraction structural analysis of the complexes Na4[Ni2(2,4-TDTA)2] · 15H2O and Na4[Cu2(2,4-TDTA)2]· 20H2O revealed that the dimer complex of Ni(II) is C1 and the dimer complex of Cu(II) is centrosymmetrical (C1). In both complexes each metal atom is surrounded by four carboxylic oxygens and two amine nitrogens in a distorted octahedron, for the Ni(II) complex, and an elongated bipyramid for the Cu(II) complex. X-ray diffraction analysis in powder was carried out for the complex Na4[Co2(2,4-TDTA)2] · 12H2O. The electronic properties have been characterised by spectroscopic techniques and variable-temperature magnetic susceptibility measurements. An orbital interpretation of the pathway of exchange interaction in this series of dimeric complexes is presented. The complexes were also characterised by mass and IR spectra and thermogravimetric analysis.

Synthesis, crystal structure, thermal and magnetic properties of [Mn(H2O)6][Cu(pdta)]·2H2O (pdta=1,3-propylenediamine-N,N,N′N′-tetraacetate)

Abstract On mixing concentrated aqueous solutions of pdta, Cu(II) and Mn(II) in the ratio 1:1:1 at pH 4.5, single crystals of the complex [Mn(H2O)6][Cu(pdta)]·2H2O (1) (pdta=1,3-propylenediamine-N,N,N′N′-tetraacetate) were obtained. X-ray structural analysis revealed that in the anion [Cu(pdta)]2− the coordination polyhedron around the Cu(II) ion can be described as a tetragonaly distorted CuN2O4 octahedron, whereas the cation [Mn(H2O)6]2+ can be described as a very regular MnO6 octahedron. Thermogravimetric analysis shows that at 110°C compound 1 loses its eight water molecules yielding the anhydrous compound [MnCu(pdta)] (2). Variable-temperature magnetic susceptibility measurements indicate that no magnetic interactions are present in 1 while weak antiferromagnetic interactions occur in 2.

Synthesis, characterization and potentiometric studies of trans-dioxorhenium(V) complexes. X-ray crystal structure of [ReO2(tn)2]I·H2O

Abstract [ReO2(PPh3)2I] has been used as starting material to prepare [RevO2(tn)2]I (tn = trimethylenediamine) in high yield. The complex was characterized by elemental analysis and spectroscopic methods. The complex [ReO2(en)2]I (en = 1,2-diaminoethane) was prepared analogously for comparison. The crystal structure of [ReO2(tn)2]I·H2O was determined from X-ray data. It consists of a six-coordinate rhenium atom bonded to two trans-oxo groups. The amines lie in the equatorial plane, almost perpendicular to the ReO2 core. The average value for the ReO bond is 1.76 A, similar to that in other Re(V) dioxo complexes. The successive protonations of the oxo groups were determined for both complexes.