Lanthanide(III) nitrobenzenesulfonates and p-toluenesulfonate complexes of lanthanide(III), iron(III), and copper(II) as novel catalysts for the formation of calix[4]resorcinarene

Lanthanide(III) salts of p-toluenesulfonic acid [lanthanide(III) tosylates, Ln(TOS)₃] and nitrobenzenesulfonic acid [Ln(NBSA)₃], and p-toluenesulfonate complexes of iron(III) and copper(II) were prepared, characterized, and examined as catalysts for the synthesis of resorcinol-derived calix[4]resorcinarenes. The reaction of resorcinol with benzaldehyde yields two isomers, the all-cis isomer (rccc) and the cis-trans-trans isomer (rctt) with the relative isomer ratios depending on the reaction conditions. However, in the reaction of resorcinol with octanal only one isomer, the all-cis isomer, is formed in high yields with less than 0.1 mol % of Yb(TOS)₃. Examination of lanthanide(III) tosylates and lanthanide(III) nitrobenzenesulfonates revealed that ytterbium(III) 4-nitrobenzenesulfonate [ytterbium(III) nosylate, Yb(4-NBSA)₃] and ytterbium(III) 2,4-dinitrobenzenesulfonate [Yb(2,4-NBSA)₃] are the most active catalysts. The catalysts could be easily recovered and reused several times for resorcinarene formation without loss of efficiency. Surprisingly good results were also obtained with iron(III) and copper(II) p-toluenesulfonates. Besides optimizing the reaction conditions, new insights into the reaction mechanism were also obtained.     

Cobalt(III) corrole-tethered semiconducting graphdiyne film for efficient electrocatalysis of oxygen reduction reaction

Inspired from nature, transition metal porphyrins and corroles have been designed and synthesized for electrocatalytic oxygen reduction reaction (ORR). However, the efficiency is limited by their low conductivity and thus carbonization is usually required. Herein, we report a new strategy by covalently linking cobalt(III) corrole and cobalt(II) porphyrin onto a semiconducting fluoro-graphdiyne (F-GDY) film through nucleophilic substitution reaction. The crystalline F-GDY film was prepared by Glaser-Hay coupling at the water/dichloromethane interface, followed by ultrasonic-assisted exfoliation in liquid. The Co(III) corrole-tethered F-GDY material exhibited excellent four-electron ORR activity, with a half-wave potential of 0.875 V (vs reversible hydrogen electrode). It also displayed high discharge performance and capacity in a zinc-air battery device, superior to the commercial Pt/C. Our study provides a pyrolysis-free approach toward biomimetic catalysts for efficient small molecule activation.     

Ru(III)-EDTA catalyzed oxidation of ascorbic acid by hydrogen peroxide in aqueous solution

The kinetics of oxidation of ascorbic acid to dehydroascorbic acid by hydrogen peroxide catalyzed by ethylenediaminetetraacetatoruthenate(III) has been studied over the pH range 1.50 – 2.50, at 30°C and μ = 0.1 M KNO₃. The reaction has a first-order dependence on ascorbic acid and Ru(III)-EDTA concentrations, an inverse first-order dependence on hydrogen ion concentration, and is independent of hydrogen peroxide concentration in the pH range studied. A mechanism has been proposed in which ascorbate anion forms a kinetic intermediate with the catalyst in a pre-equilibrium step. Ruthenium(III) is reduced to ruthenium(II) in a rate-determining step and is reoxidized with hydrogen peroxide back to the Ru(III) complex in a fast step.     

Electrochemical reduction of the lanthanide ions: Part I. First reduction step of ytterbium(III) in acidic 1 M NaClO4 solution

The reduction of Yb(III) to Yb(II) in 1 M NaClO₄ in the pH range 1.9–6.6 was studied by d c. polarography, cyclic voltammetry and electrode impedance measurements as a function of frequency and electrode potential. It results that the d.c. reversible reduction is followed by a homogeneous chemical reaction and is accompanied, by an irreversible process which is attributed to a lowering of the overpotential of the reduction of the hydrogen ions. Values of the rate constant and transfer coefficient pertaining to the charge transfer step were deter mined.     

Mechanisms of sensitization of lanthanide(III)-based luminescence in transition metal/lanthanide and anthracene/lanthanide dyads

Four sets of dyads are discussed, in all of which near-infrared emitting lanthanide(III) ions such as Nd(III), Er(III) or Yb(III) are energy-acceptors which provide sensitized luminescence following energy-transfer from an antenna group. In three sets of dyads the antenna (energy-donor) group is a luminescent transition metal fragment; in the fourth the antenna is an anthracene group. A combination of photophysical studies and calculations has been used to understand the mechanisms by which energy-transfer to the lanthanide(III) ion occurs. Although definitive answers are not possible in every case due to the presence of several possible energy-transfer pathways, the relative contributions of Förster-type, Dexter-type and redox-mediated energy-transfer pathways have been analysed. Interesting results include (i) the demonstration of pure Dexter energy-transfer over 20 Å in a Ru(II)/Nd(III) dyad, and (ii) the demonstration of a redox-based mechanism for energy-transfer in anthracene/Ln(III) dyads in which the first step is photoinduced electron-transfer from the excited anthracene chromophore to a diimine ligand on the lanthanide(III) to generate a charge-separated state.     

Oxidation of manganese(II) by ozone and reduction of manganese(III) by hydrogen peroxide in acidic solution

Manganese(II) is oxidized by ozone in acid solution, k=(1.5±0.2)×10³ M⁻¹ s⁻¹ in HClO₄ and k=(1.8±0.2)×10³M⁻¹ s⁻¹ in H₂SO₄. The plausible mechanism is an oxygen atom transfer from O₃ to Mn²⁺ producing the manganyl ion MnO²⁺, which subsequently reacts rapidly with Mn²⁺ to form Mn(III). No free OH radicals are involved in the mechanism. The spectrum of Mn(III) was obtained in the wave length range 200–310 nm. The activation energy for the initial reaction is 39.5 kJ/mol. Manganese(III) is reduced by hydrogen peroxide to Mn(II) with k(Mn(III)+H₂O₂)=2.8×10³M⁻¹ s⁻¹ at pH 0–2. The mechanism of the reaction involving formation of the manganese(II)-superoxide complex and reaction of H₂O₂ with Mn(IV) species formed due to reversible disproportionation of Mn(III), is suggested. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 207–214, 1998.     

Chelate des nitrosylruthenium(III)-ions mit nitroso-naphtholen

A spectrophotometric study of the reactions of nitrosylruthenium(III)-ion with nitrosonaphthols in dilute nitric and hydrochloric acid media is described. The complexes formed contain Ru : R ratios of 1:1 to 1:3, depending on the hydrogen ion concentration and on the molar ratios of the complex-forming species. Mixtures are usually obtained.     

Electrochemistry of the bis(1,4,7-triazacyclodecane) cobalt(III) complex and its role in the catalytic reduction of hydrogen

The electrochemistry of the bis(1,4,7-triazacyclodecane) cobalt(III) complex at a mercury electrode, HMDE, in aqueous Britton–Robinson buffer solutions was investigated using cyclic voltammetry, double-potential-step chronoamperometry and chronocoulometry. The cyclic voltammetric data were analyzed by digital simulation to confirm and to measure the heterogeneous and homogeneous parameters for the suggested electrode mechanism. Generally, the complex is electrochemically reduced giving rise to two cyclic voltammetric waves. The first wave is a diffusion-controlled reversible wave. It is assigned to the stable Co(III)/Co(II) redox couple. The second one is found to be irreversible and corresponding to a reduction of Co(II) to Co(I) species. The monovalent cobalt, highly unstable, is rapidly protonated, and then forms cobalt hydride. The hydride decomposes to hydrogen molecules and regenerates Co(II) species following a disproportionation pathway. The overall reduction mechanism is concluded to be an EECC kinetics.     

Spectroscopic Study of Cm(III) Sorption onto gamma-Alumina

The surface sorption of Cm(III) onto aqueous suspensions of alumina is investigated by time-resolved laser fluorescence spectroscopy (TRLFS). The experiment is performed under an Ar atmosphere at an ionic strength of 0.1 M NaClO(4). The pH is varied between 2 and 10 and the metal ion concentration between 2.7x10(-8) and 4.5x10(-5) mol/L. With increasing pH, two Cm(III)-alumina surface species are identified which are attributed to identical withAl-O-Cm(2+)(H(2)O)(5) and identical withAl-O-Cm(+)(OH)(H(2)O)(4). The two curium-alumina surface complexes are characterized by their emission spectra (peak maxima at 601.2 nm and 603.3 nm, respectively) and fluorescence emission lifetime (both 110&mgr;s). In the concentration range investigated, the surface complex formation is not dependent on the metal ion concentration but only on the pH. Additionally, the concentration ratio of the two surface species is found to be independent of the metal ion concentration. No spectroscopic evidence for the presence of "strong" and "weak" sites can be found at different surface coverages. Copyright 2001 Academic Press.     

Compleximetric titration of chromium (III) with catalytic end-point detection

Chromium (III) (2–20 mg) is determined by reaction with excess of EDTA and backtitration with a standard copper(II) solution to a catalytic end-point. The indicator reaction is the copper (II) -catalyzed autodecomposition of hydrogen peroxide, which is observed biamperometrically. For 10 mg of chromium (III) , the relative standard deviation was < 0.5 % (n=10).     

Sparkle model for the calculation of lanthanide complexes: AM1 parameters for Eu(III), Gd(III), and Tb(III)

Our previously defined Sparkle model (Inorg. Chem. 2004, 43, 2346) has been reparameterized for Eu(III) as well as newly parameterized for Gd(III) and Tb(III). The parameterizations have been carried out in a much more extensive manner, aimed at producing a new, more accurate model called Sparkle/AM1, mainly for the vast majority of all Eu(III), Gd(III), and Tb(III) complexes, which possess oxygen or nitrogen as coordinating atoms. All such complexes, which comprise 80% of all geometries present in the Cambridge Structural Database for each of the three ions, were classified into seven groups. These were regarded as a "basis" of chemical ambiance around a lanthanide, which could span the various types of ligand environments the lanthanide ion could be subjected to in any arbitrary complex where the lanthanide ion is coordinated to nitrogen or oxygen atoms. From these seven groups, 15 complexes were selected, which were defined as the parameterization set and then were used with a numerical multidimensional nonlinear optimization to find the best parameter set for reproducing chemical properties. The new parameterizations yielded an unsigned mean error for all interatomic distances between the Eu(III) ion and the ligand atoms of the first sphere of coordination (for the 96 complexes considered in the present paper) of 0.09 A, an improvement over the value of 0.28 A for the previous model and the value of 0.68 A for the first model (Chem. Phys. Lett. 1994, 227, 349). Similar accuracies have been achieved for Gd(III) (0.07 A, 70 complexes) and Tb(III) (0.07 A, 42 complexes). Qualitative improvements have been obtained as well; nitrates now coordinate correctly as bidentate ligands. The results, therefore, indicate that Eu(III), Gd(III), and Tb(III) Sparkle/AM1 calculations possess geometry prediction accuracies for lanthanide complexes with oxygen or nitrogen atoms in the coordination polyhedron that are competitive with present day ab initio/effective core potential calculations, while being hundreds of times faster.     

AM1 Sparkle modeling of Er(III) and Ce(III) coordination compounds

The recently defined Sparkle/AM1 model is now extended to Er(III) and Ce(III), using the same parameterization scheme. Thus, a set of fifteen complexes for each lanthanide ion, with various representative ligands of high crystallographic quality (R factor < 0.05 Å), and which possess oxygen and/or nitrogen as coordinating atoms, was used as the training set. In the validation procedure we used a set of twenty-two more complex structures for the Ce(III) ion and twenty-four more for the Er(III) ion, all of high crystallographic quality. For the thirty-seven cerium(III) complexes and thirty-nine erbium(III) complexes considered, the Sparkle/AM1 unsigned mean error, for all interatomic distances between the Ln(III) ion and the ligand atoms of the first sphere of coordination, is 0.08 and 0.06 Å, a level of accuracy comparable to present day ab initio/ECP geometries, while being hundreds of times faster. The Sparkle/AM1 model may thus prove useful for luminescent complex design.     

Probing the binding of Tb(III) and Eu(III) to the hammerhead ribozyme using luminescence spectroscopy

BACKGROUND: Divalent metal ions serve as structural as well as catalytic cofactors in the hammerhead ribozyme reaction. The natural cofactor in these reactions is Mg(II), but its spectroscopic silence makes it difficult to study. We previously showed that a single Tb(III) ion inhibits the hammerhead ribozyme by site-specific competition for a Mg(II) ion and therefore can be used as a spectroscopic probe for the Mg(II) it replaces. RESULTS: Lanthanide luminescence spectroscopy was used to study the coordination environment around Tb(III) and Eu(III) ions bound to the structurally well-characterized site on the hammerhead ribozyme. Sensitized emission and direct excitation experiments show that a single lanthanide ion binds to the ribozyme under these conditions and that three waters of hydration are displaced from the Tb(III) upon binding the RNA. Furthermore, we show that these techniques allow the comparison of binding affinities for a series of ions to this site. The binding affinities for ions at the G5 site correlates linearly with the function Z(2)/r of the aqua ion (where Z is the charge and r is the radius of the ion). CONCLUSIONS: This study compares the crystallographic nature of the G5 metal-binding site with solution measurements and gives a clearer picture of the coordination environment of this ion. These results provide one of the best characterized metal-binding sites from a ribozyme, so we use this information to compare the RNA site with that of typical metalloproteins.     

Spectrophotometric determination of the stability constant of the Eu(III)-murexide complex

The values of the stability constants of the Ca(II) and lanthanide(III) complexes of murexide reported in the literature were determined without proper correction for binding of buffer ions to the metal ion. The constants are best determined without a buffer present. Accurate values of conditional stability constants for the Eu(III)—murexide complex (relative standard deviation better than 3%), of the differential molar absorptivity of the Eu(III)—murexide complex with respect to murexide at 480 nm (relative standard deviation better than 0.5%) and of the molar absorptivity of murexide at 520 and at 506 nm (precision better than 0.4%) at pH 5.0 and 6.5 at 15, 25 and 35° are reported. The accuracy and precision of the concentration of metal ion solution determined by using these conditional stability constants are discussed.     

Experimental evidence of consecutive electron transfers for the Sb(III)−Sb(Hg) couple in 4 M H(Cl,ClO4) at low chloride concentrations

A dc polarographic and cyclic voltammetric study has been made of the reduction of Sb(III) ions from 0.01 M HCl+3.99 M HClO₄ and 0.001 M HCl+3.999 M HClO₄ supporting electrolytes in which a quasi-reversible, respectively irreversible behaviour is observed. It is shown that the Sb(III) reduction can be explained on the assumption of a reaction mechanism that consists of three successive one-electron transfers. Along the reduction wave the Sb(III)→Sb(II) and Sb(II)→Sb(I) step are rate determining, respectively at more negative and more positive potentials. Kinetic parameters were determined and the rate constants are shown to increase with chloride ion concentration.     

Coordination ability of trans-cyclohexane-1,2-diamine-N,N,N',N'-tetrakis(methylenephosphonic acid) towards lanthanide(III) ions

The proton and metal complex equilibria of trans-cyclohexane-1,2-diamine-N,N,N',N'-tetrakis(methylenephosphonic acid) (CDTP) with lanthanide(iii) ions, where Ln(III) = La(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Ho(III) and Lu(III) were studied. The stoichiometry, protonation and complex formation constants were determined by potentiometric titration at 25.0 degrees C and ionic strength of 0.1 mol dm(-3) (KCl). All metal ions form several species: [LnH4L]-, [LnH3L](2-), [LnH2L](3-), [LnHL](4-), [LnL](5-), [LnH(-1)L](6-) and [LnH(-2)L](7-) in the pH range between 2 and 11. The stability constants log beta(LnL) were found to be between 14.7 and 16.7. The studied complexes were also characterized by spectroscopic methods (31P NMR, UV-Vis absorption and emission spectroscopy). These studies allowed to reveal a distinct structural change of the Ln(III)-CDTP complex which occurs between protonated and hydroxy species in solutions at pH around 7.5. The major change is caused by the involvement of both nitrogen donors in the metal ion coordination occurring in ML species. The data obtained from UV-Vis spectroscopy allowed to draw conclusions about complex symmetry and to estimate a number of coordinated water molecules. The hydration number or more precisely the number of two OH oscillators was found to be approximately one in all species formed over the pH range between 5 and 10. The structure of the major hydroxy complex was supported by X-ray crystallographic data. The crystal structures of the Eu(III) and Tb(III) complexes clearly show that the CDTP ligand is coordinated to the Ln(III) ion by two nitrogen and four oxygen atoms in such a way that only one oxygen atom from each phosphonic group is placed in the lanthanide inner sphere. The monomeric complex anion is connected to a symmetry related ion through short hydrogen bonds formed by two hydroxy ions and one water molecule. In this way the two neighbouring anions form a quasi-dimer in which one of the Ln(III) ion is seven-coordinate (two N atoms, four O atoms and one hydroxy ion) and the other is eight-coordinate (two N atoms, four O atoms, one hydroxy ion and one water molecule).     

Extraction of Sm(III), Gd(III) and Ho(III) ions with picrolonic acid in methylisobutylketone

Summary Picrolonic acid (HPA) in methylisobutylketone (MIBK) (0.01 mol^. dm^-3) has been used for the extraction of lanthanide(III) ions such as Sm(III), Gd(III) and Ho(III) (Me) (~3^. 10^-6mol^. dm^-3) from pH 1-2 buffer solutions of 0.1 mol^. dm^-3(H⁺, Cl^-) ionic strength and quantitative extraction (>95%) was found at pH 2. Through slope analysis the composition of the organometallic adduct responsible for the extraction came out to be M(PA)₃. The conditional equilibrium constant values, log K_ex, were deduced to be 2.60±0.01, 2.09±0.01 and 1.44±0.03 for these lanthanide(III) ions, respectively. The metals in concentration up to ~2.5^. 10^-4mol^. dm^-3can be quantitatively extracted by the proposed system. Among the various anions, fluoride, oxalate and cyanide ions (~3.0^. 10^-4mol^. dm^-3) and, among the cations, Zn(II) Cu(II), Co(II) and Fe(III) reduced the lanthanide extraction. The extraction of various other metal ions at the optimized conditions of Me extraction for this series of lanthanide ions was also studied and high separation factors (10²-10³) were obtained showing the good selectivity of this extraction system.     

Ruthenium(III)-catalyzed oxidation of thallium(i) by 12-tungstocobaltate(III) in hydrochloric acid medium

The reaction between thallium(I) and [Co^IIIW₁₂O₄₀]^5- in the presence of ruthenium(III) as catalyst proceeds viainitial outer-sphere oxidation of the catalyst to ruthenium(VI). The ruthenium(IV) thus generated will oxidize thallium(I) to an unstable thallium(II) which by reacting with oxidant gives the final product, thallium(III). The formation of ruthenium(II) by direct two-electron reduction of the catalyst by thallium(I) is thermodynamically less favorable. The reaction rate is unaffected by the [ H⁺ ], whereas it is catalyzed by chloride ion . The formation of reactive chlorocomplex,TlCl, in a prior equilibrium is the reason for the chloride ion catalysis. Increasing the relative permittivity of the medium increases the rate of the reaction, which is attributed to the formation of an outer-sphere complex between the catalyst and oxidant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Preparation, Characterization and Properties of Two New Lanthanide(III)-5-(2-pyridyl)tetrazolate Complexes

Two new mononuclear lanthanide(III) complexes Ln(pytz)3(H2O)3·(H2O)3.5[Ln=Tb(1); Eu(2); Hpytz= 5-(2-pyridyl)tetrazole] were synthesized by reacting Hpytz with the corresponding lanthanide(III) ions and characterized. The single crystal X-ray diffraction analysis reveals that complexes 1 and 2 are isostructural and the lanthanide(III) ions in both complexes 1 and 2 are nine-coordinated, with three oxygen atoms of three coordination water molecules and six nitrogen atoms of three pytz ligands, forming a monocapped square antiprism. Extensive hydrogen bonds exist, resulting in a three-dimensional supramolecular network structure by hydrogen-bonds in both complexes 1 and 2, respectively. Complex 1 exhibits typical green fluorescence of Tb(III) ion and complex 2 red fluorescence of Eu(III) ion, in solid state at room temperature.     

Kinetics of Fe(II) reduction of cis‐halogeno(dodecylamine) bis(ethylenediamine)‐ cobalt(III) complex ion in aqueous solutions

Kinetics of reduction of the surfactant complex ions, cis‐chloro/bromo (dodecylamine)bis(ethylenediamine)cobalt(III) by iron(II) in aqueous solution was studied at 303, 308, and 313 K by spectrophotometry method under pseudo‐first‐order conditions, using an excess of the reductant. The second‐order rate constant remains constant below critical micelle concentration (cmc), but increases with cobalt(III) concentration above cmc, and the presence of aggregation of the complex itself alters the reaction rate. The rate of reaction was not affected by the added [H⁺]. Variation of ionic strength (μ) influences the reaction rate. Activation and thermodynamic parameters have been computed. It is suggested that the reaction of Fe²⁺ (aq) with cobalt(III) complex proceeds by the inner‐sphere mechanism. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 38: 98–105, 2006