E. Vrachnou-Astra

Pyrazole and imidazole complexes of vanadium(II) chloride

Complexes of vanadium(II) chloride with pyrazole and imidazole of the type VCl2(pyrazole)4 and VCl2 (imidazole)6 were prepared. The complexes are unstable in air and substitution labile in solution. The electronic spectra of their methanolic solutions suggest octahedral symmetry. The participation of methanol in the first coordination sphere appears to be a function of the concentration of the ligand in solution. The values of Dq and B are calculated. The far infrared spectrum of the pyrazole complex in the crystalline state indicates a trans configuration. Magnetic measurements give values for the magnetic moments lower than the spin-only value.

Homogeneous catalytic action of a nickel dithiolene complex, leading to dihydrogen formation from N,N'-dimethyl-4,4'-dipyridinium radical ion solutions

Abstract Bis(2-chlorodithiobenzyl)nickel(II) reacts with the methylviologen free-radical ion and is reduced to its mono-anion. The second-order rate constant for this reaction was found to be greater than 10 7 M −1 s −1 . The mono-anion of the complex acts as a homogeneous catalyst for the electron transfer reaction between the remaining excess of the methylviologen free-radical ion and water, leading to dihydrogen production. The kinetics and mechanism of the catalytic reaction were studied in acetone-water solutions. The reaction is second order with respect to the catalyst and depends on the water content of the solutions. The value of k 2 is 8.2 ± 0.3 M −2 s −1 at 21 °C and at a 30:70 water: acetone ratio. The Arrhenius activation energy is 77 ± 3 kJ mol −1 . A mechanism is proposed for the reaction.

A new series of organochromium complexes formed in aqueous solutions

Abstract Chromous ion reacts with 3-pyridineacrylic acid, 4-pyridineacrylic acid, maleic acid and fumaric acid to give remarkably inert organochromium(III) species. These species were isolated by ion exchange from acidic aqueous media and were characterized by a variety of techniques. An organochromium(III) compound was even isolated from the Cr(III)-3-pyridineacrylic acid aqueous mixture in solid form. In all the organometallic compounds reported in this paper chromium(III) is believed to be σ-bonded to carbonyl by the general scheme .

The mechanism of hydrogenation by aqueous chromium(II) ion of the carbon–carbon double bond of olefinic compounds with polar substituents

The kinetics of the reaction of Cr2+ with maleic acid, fumaric acid, methylmaleic acid, chloromaleic acid, dichloromaleic acid, and methylfumaric acid have been investigated over a wide range of chloromaleic > maleic ≃ methylmaleic > dichloromaleic > fumaric and maleic > dichloromaleic ≃ ligand is in excess. In excess of Cr2+ the rate law is as shown below and k3 follows the trend: Rate =k3[Cr2+]2[L] chloromaleic > maleic > dichloromaleic > methylmaleic > methylfumaric. With excess ligand, L, the rate law has two terms (below) and the two rate constants, k′3 and k′2 follow the order: Rate =k′2[Cr2+][L]+k′3[Cr2+][L]2 chloromaleic > maleic ≃ methylmaleic > dichloromaleic > fumaric and maleic > dichloromaleic ≃ methylmaleic > chloromaleic respectively. The kinetic data are supplemented by stoicheiometric data, by determinations of product distribution, and by spectroscopic data, and they are discussed in terms of a model involving at least partial attack by Cr2+ directly on the CC double bonds.

Reduction of pyruvate by titanium(III)

Abstract Titanium(III) in aqueous solutions reacts with carbonate-like pyruvic acid and/or pyruvate to give a product of reductive coupling. The reaction was investigated kinetically over a range of hydrogen ion concentrations from 0.007 M to 2.5 M and over a wide range of concentrations of the other reactants. Under all conditions only one path was identified, corresponding to a second order rate law in {TiIII}, first order in {Pyr}, and inverse second order in {H+}. The data are interpreted by postulating the formation of an η2 precursor complex between TiIII and the carbonyl group.

A kinetic study of the complexation of cysteine and related compounds with aqueous vanadium(II) and vanadium(III) at approximately neutral pH; the mediating role of sulphur compounds in electron transfer

Vanadium(II) and vanadium(III) form with cysteine (cys) and other sulphhydryl compounds intensely yellow complexes soluble in neutral and weakly alkaline solutions. Some of the vanadium(II) complexes are powerful reductants. Thus, VII–cys reduces water to dihydrogen under mild conditions. The formation of the reducing species, which is [VII(cysOS)3]4–[cysOS = cysteinate(2–)] proceeds in two stages, i.e. a stage corresponding to a jump in the absorbance at zero time and a second stage, which was followed kinetically in a stopped-flow instrument. The complexation of cysteine with vanadium(III) also proceeds in two stages, but leads to the formation of a species containing two cysteines instead of three (at pH values around neutral). The observed activation energy (Ea) and pre-exponential factor (A) for the stages that were followed kinetically are as follows: Ea= 41 ± 4 kJ mol–1, A= 1.4 × 107(pH 8.2) for VII and Ea= 41 ± 4 kJ mol–1, A= 1.4 × 1010(pH 8.8)for VIII. The reduction of water to dihydrogen by VII–cys proceeds with a rate first order in [VII]total and in the pH range 7.5–8.5 it is independent of hydrogen-ion concentration. The activation parameters are: Ea= 54 ± 2 kJ mol–1, A= 5 × 106. Dihydrogen is also obtained with VII–cys (cysa = cysteamine) and VII–cysme (cysme = cysteine methyl ester). The corresponding reaction of VII–ser (ser = serine) is ca. one thousand times slower compared to the reaction of VII–cys, even though the polarographic half-wave potentials have comparable values.