Showing papers by "Wolfgang Linert published in 2013"

Mutual Lewis Acid–Base Interactions of Cations and Anions in Ionic Liquids

Abstract: Solute properties are known to be strongly influenced by solvent molecules due to solvation. This is due to mutual interaction as both the properties of the solute and of the solvent strongly depend on each other. The present paper is based on the idea that ionic liquids are cations solvated by anions and anions solvated by cations. To show this (in this system strongly pronounced) interaction the long time established donor–acceptor concept for solvents and ions in solution by Viktor Gutmann is extended to ionic liquids. A number of solvent parameters, such as the Kamlet–Abboud–Taft and the Dimroth–Reichardt ET scale for ionic liquids neglect this mutual influence, which, however, seems to be in fact necessary to get a proper description of ionic liquid properties. It is shown how strong such parameters vary when the influence of the counter ion is taken into account. Furthermore, acceptor and donor numbers for ionic liquids are presented.

Thermal and Photoinduced Spin Crossover in a Mononuclear Iron(II) Complex with a Bis(pyrazolyl)pyridine Type of Ligand

Abstract: A mononuclear iron(II) compound 1 of the general formula [Fe(L)2](ClO4)2 {L = 4-[2,6-bis(pyrazol-1-yl)pyridin-4-yl]benzaldehyde} was prepared and structurally characterised. Single-crystal X-ray structure analysis revealed the presence of a complex dication [Fe(L)2]2+ and two ClO4– counteranions within the unit cell. The bond lengths and angles within the coordination polyhedron FeN6 indicate the low-spin state of the central iron(II) metal ion at T = 180 K. Magnetic investigations elucidate spin crossover with T1/2 = 285 K. The experimental magnetic susceptibility data could be satisfactorily fitted with the Curie law in combination with the Ising-like model. The room-temperature character of the spin crossover was further studied by variable-temperature far-IR, Vis and Mossbauer spectroscopy. Laser irradiation of 1 carried out at 10 K gives rise to a complete low-spin to high-spin photoconversion. A subsequent temperature-dependent investigation revealed the existence of a photoexcited metastable HS state up to TLIESST = 70 K (LIESST = light-induced excited-spin-state trapping) as well as the presence of a light-induced thermal hysteresis loop with a width of 10 K.

Isokinetic relationships: preface

Abstract: Almost 90 years have passed since Constable reported a relationship between the activation energies and activation entropies measured in his investigation of the dehydrogenation of ethanol, using a series of different catalysts. Decreasing activation energies were found to be linearly related to increasing activation entropies and vice versa. Thus, the increase of reaction rate with a ‘‘better’’ catalyst, exhibiting lower activation energy, was not as large as anticipated, since it was partly compensated by the ‘‘disadvantageous’’ decrease in entropy changes. This led to the term ‘‘compensation effect’’. This effect, and similar ones relating to kinetics in other fields of science, were (partly independently) observed over subsequent decades and given various names such as h-rule, Meyer–Neldel rule and Zawadzki–Bretszanjder rule. They are frequently called ‘‘isokinetic relationships’’ (IKR), since the linear relation implies that the rates of all of Constable’s catalytic reactions, for example, will be the same at an isokinetic (or Meyer–Neldel) temperature. A major obstacle to the acceptance of the reality and the understanding of such effects has been that incorrect statistical analysis can easily produce effects that look similar, often with the isokinetic temperature within the experimental range, but that are simply artefacts due to experimental errors; For example, it is a mistake to perform a linear regression analysis on temperature-dependent kinetic or equilibrium data for a given series of reactions, in order to obtain enthalpies and entropies, and then use these in a further linear regression analysis to prove the existence of an IKR or isoequilibrium relationship (IER). For a long time, another obstacle to the understanding of these effects was the failure to realize that they are observed whenever the activation energy in a system or in related systems is large compared with thermal and excitation energies. In contrast, the effect is not observed when the activation energies are small. In such cases, the entropy may be independent of activation energy or it may increase or decrease slightly. To avoid confusion with statistical artefacts, and to present correctly proven real effects, the term IKR (including IER, if thermodynamic data rather than kinetic results are investigated) has been used in the literature. We have used this term as the description of the present issue. Based on experimental and theoretical results, we strongly believe, not only that there can be little doubt that these effects are real, but also that they can be advantageously used for understanding, predicting and analysing the temperature dependence of series of similar reactions. Constable may have been frustrated in his efforts to increase the rates of catalytic reactions, but it is reasonable to suppose that the existence of catalysis depends upon compensation. The investigation of compensation is almost surely more prominent in chemistry than in any other field, and a large fraction of the papers in this issue are devoted to chemical subjects. However, as pointed out above, it is a general W. Linert (&) Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria e-mail: wlinert@mail.zserv.tuwien.ac.at; wolfgang.linert@tuwien.ac.at

Ln(III) complexes of a bis(5-(pyridine-2-yl)-1,2,4-triazol-3-yl)methane ligand: synthesis, structure and fluorescent properties

Abstract: The new tetradentate ligand H2L bis(5-(pyridine-2-yl)-1,2,4-triazol-3-yl)methane ligand H2L has been used to prepare corresponding Ln(III) complexes [Ln(L)(HL)H2O]·6H2O where Ln(III) is Tb(III), Sm(III), Eu(III), Gd(III), Nd(III). The resulting complexes were investigated by various means including elemental- and thermogravimetric analysis and IR-, UV-, VIS spectroscopy. The complexes form an isostructural series in the solid state according to X-ray diffraction. The optimal value of triple state energy of the ligand sensitizes both fluorescent visible and near-IR emissions of the lanthanides. However, the overall emission is moderate as a result of the quenching action of coordinated water molecules. The displacement of water molecules from the inner coordination sphere both in solid samples and the solutions results in a significant increase of the efficiency of the luminescence. The best value of the quantum yield was obtained for the Tb(III) complex with 41%. Luminescence titrations demonstrate that [Tb(L)(HL)H2O] is a pH-sensitive emitter which is able to detect pH values in both near neutral pH and acidic pH ranges.

Effect of the counter-anion on the spin-transition properties of a family of Fe(II) tetrazole complexes, [Fe(i4tz)6]X2 (X = ClO4−, PF6−, SbF6−, BF4−)

Abstract: The monodentate ligand 1-isobutyl-1H-tetrazole (i4tz) was used to synthesize a series of iron(II) spin-crossover (SCO) complexes with the general formula [Fe(i4tz)6]X2, where X = ClO4− (1), PF6− (2), SbF6− (3), or BF4− (4). Upon cooling, magnetic susceptibility measurements of compounds 1, 2 and 3 show a reversible one-step spin crossover (SCO) behaviour between HS (S = 2) and LS (S = 0) states without hysteresis. The ClO4− compound shows a gradual spin transition at T1/2 = 208 K at a considerably higher temperature than the other compounds. The larger anions PF6− (T1/2 = 114 K) and SbF6− (T1/2 = 116 K) show more gradual transitions, and a “frozen-in effect” was observed after fast cooling. The crystal structure of [Fe(i4tz)6](SbF6)2 (3) was determined at 296, 200 and 100 K, revealing a reversible first-order phase transition between 200 and 100 K, accompanied by a triplication of the unit cell volume at 100 K. Temperature-dependent vibration and electronic spectroscopic measurements of compound 1 were compared with the magnetic susceptibility measurements.

Study of total antioxidant activity of human serum blood in the pathology of alcoholism.

Abstract: The total antioxidant activity (TAA) of human serum blood of patients suffering from alcoholism was tested by cathode voltammetry with a model process of oxygen electroreduction. A known spectrophotometrical method was used for comparison. As results the total antioxidant activity of serum blood of patients with alcoholism was estimated by voltammetry during therapy in hospital. It was shown the TAA of serum blood of patients in pathology before and after treatment is lower than that one of healthy people. However, during the process of 10 days of alcoholism treatment the TAA coefficient increases. The relationship between the coefficient of total antioxidant activity of human serum blood and the stage of treatment was detected.

Redox activity of some non-innocent amino acids

Abstract: Cyclic voltammetry has been applied in order to scan the redox characteristics of selected amino acids, namely tyrosine, tryptophan, glycine, and cysteine. These species in aqueous solution (NaCl supporting electrolyte) show anodic oxidation peaks E pa ~ 1.0 V vs. Ag/AgCl reference electrode. The calculated reduction potential of these species shows an interference with the iron salts and iron hydroxides/oxides, including ferritin. Several types of quantum-chemical methods were applied to study the ionization of the amino acid as well as zwitterion forms of glycine: semiempirical AM1, density functional B3LYP/UHF, 6-31G**, and ab initio-Hartree-Fock/UHF, 6-31G**.

Spacer-armed copper(II) complexes with benzenecarboxylic acids and trifluoroacetylacetone aroylhydrazones.

Abstract: New bi- and trinuclear copper(II) complexes of aroylhydrazones of TFA and benzenecarboxylic acids were synthesized and characterized. Despite long range intramolecular Cu⋯Cu distances of about 10 A, exchange couplings between paramagnetic centres are observed with J values of 0.33 and 0.37 cm−1 in binuclear complexes, and −0.33 cm−1 in trinuclear complexes. Seven hyperfine peaks with a value of aCu = 37.0 × 10−4 cm−1 are observed in EPR spectra of binuclear complexes. The possible mechanisms responsible for the observed weak long range coupling via aromatic spacers are discussed.

Synthesis, characterization, antioxidant, cytotoxic, and DNA cleavage studies of ruthenium(III) complexes containing N-substituted thiosemicarbazone/semicarbazone

Abstract: The synthesis, characterization, and biological studies of ruthenium(III) complexes containing ONS/ONO tridentate ligands are reported. Reactions of 1,2-naphthoquinone N-substituted thiosemicarbazone/semicarbazone (HL n ; n = 1–4) with [RuCl3(EPh3)3] (E = P or As) gave [RuCl2(EPh3)(L n )], which were characterized by elemental analysis, FT-IR, UV–Vis, magnetic susceptibility measurement, ESR spectra, ESI–MS, electrochemical studies, and powder X-ray diffraction pattern. The free radical scavenging ability, assessed by antioxidant assays involving DPPH and hydroxyl radicals, showed that the complexes possess excellent radical scavenging properties. The in vitro cytotoxicity of the complexes against MCF-7 and A431 cell lines was assayed which showed that the ruthenium(III) complexes exhibited effective cytotoxic activity against selected cancer cells. Furthermore, the complexes were found to promote the cleavage of plasmid pBR322 DNA.

Interaction of Glutathione With Cis-(2-Aminomethylpyridine)diaqua platinum(II) Perchlorate in Aqueous Medium: Their Kinetics and Mechanism

Abstract: The kinetic interaction of glutathione with cis-[Pt(pic)(OH2)2] (ClO4)2 (pic = 2-Aminomethylpyridine) has been studied spectrophotometrically as a function of cis-[Pt(pic)(OH2)2]2+, glutathione concentration, and temperature at pH 4.0, where the complex exists predominantly as the diaqua species and glutathione as a zwitter ion. The substitution reaction shows two distinct consecutive steps: the first is the ligand assisted anation and the second is the chelation step. The association equilibrium constant (KE ) for the outer sphere complex formation has been evaluated with rate constants k1 (≈10−3) for the first step. The activation parameters for both the steps were evaluated using Eyring equation. The low ΔH1 ≠ = (52.37 ± 2.10 kJmol−1) and large negative value of ΔS1 ≠ = (−112.35 ± 2.98 JK−1mol−1) as well as ΔH2 ≠ = (37.29 ± 1.84 kJmol−1) and ΔS2 ≠ = (−130.12 ± 3.16 JK−1mol−1) indicate an associative mode of activation for both the aqua ligand substitution processes.

Kinetics and Mechanism of the Interaction of Di-μ-hydroxobis(1,10-phenanthroline)dipalladium(II) Perchlorate with Thioglycolic Acid and Glutathione in Aqueous Solution

Abstract: The kinetics of interaction between di-μ-hydroxobis(1,10-phenanthroline)dipalladium(II) perchlorate and thioglycolic acid and with glutathione has been studied spectrophotometrically in aqueous medium as a function of the complex concentration as well as the ligand concentrations, pH, and temperature at constant ionic strength. The observed pseudo-first-order rate constants k obs (s−1) obeyed the equation k obs = k 1[Nu] (Nu = nucleophile). At pH = 6.5, the interaction with thioglycolic acid shows two distinct consecutive steps and both steps are dependent on the concentration of thioglycolic acid. The rate constants for the process are: k 1 ≈ 10−5 s−1 and k 2 ≈ 10−3 dm3 · mol−1 · s−1. The association equilibrium constant (K E) for the outer sphere complex formation has been evaluated together with the rate constants for the two subsequent steps. The other bio-active ligand, glutathione, showed a single step reaction depending on [ligand] with a second-order anation rate constant: the 102 (k 2) values are (61.72, 79.20, 109.24 and 154.33) dm3 · mol−1 · s−1 at 20, 25, 30 and 35 °C, respectively. On the basis of the kinetic observations and evaluated activation parameters, plausible associative mechanisms are proposed for both interaction processes.

Isokinetic and isoequilibrium relationships in the gaseous phase

Abstract: Known cases of isoequilibrium and isokinetic relationships (IER, IKR) in gas phase reactions were analysed by employing classical collision theory which allows the order of magnitude of pre-exponential factors to be predicted. The vibrational relaxation must be taken into account. In the majority of cases IER and IKR are caused by the fundamental correlations between such parameters as the reduced mass of colliding particles, vibrational frequency, and bond strength.