A Study on the Interaction of Eu2+(aq) with Pyridinecarboxylic Acids

Europous ion forms with isonicotinic, N-methylisonicotinic, nicotinic, and picolinic acids one to one complexes having several features, which are rather unusual for a lanthanide ion. They are formed in strongly acidic aqueous solutions and have absorption maxima around 420 nm. The formation constants are 0.15 1. mol-1 for nicotinic acid, 0.2 1. mol-1 for picolinic acid, 1.9 1. mol-1 for isonicotinic acid, and 0.4 1. mol-1 for N-methylisonicotinic acid, respectively. Evidence is presented that the complexes involve charge transfer from the metal ion to the ligand. The complexes of nicotinic and picolinic acids are stable toward further redox reaction. The complexes of isonicotinic acid and its N-methyl derivative, however, undergo further reduction leading in the first case to isonicotinaldehyde and in the second very likely to the dihydro derivative. In the presence of Eu3+(aq) the kinetics of the redox reaction of isonicotinic acid and its N-methyl derivative are second order in europous ion, first order in the organic acid, first order in hydrogen ion, and inverse first order in Eu3+(aq). A unified mechanism is proposed to explain the results for both of these acids, which is also consistent with the results obtained on complex formation and with the postulate of a charge transfer from europous ion to the ligand. © 1975, American Chemical Society. All rights reserved

Reduction of pyruvate by titanium(III)

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. © 1984

A new series of organochromium complexes formed in aqueous solutions

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 schemeA figure is presented. © 1980

Reduction of pyruvate by titanium(III)

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. © 1984

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

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 107M-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 k2 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. © 1985

Kinetics and Mechanisms of Aquation of Some π-Bonded Organochromium Complexes

The study of the aquation of the organochromium complexes formed during reactions of chromous ion with 3- and 4-pyridineacrylic acids on one hand and maleic and fumaric acids on the other showed essential kinetic and mechanistic differences between the two groups. In the first group the rate of aquation has no acid-independent term, is accelerated by oxygen, but is unaffected by excess chromous ion. In the second group the rate of aquation has both an acid-dependent and an acid-independent term, it is not affected by oxygen, but it is considerably enhanced by chromous and vanadous ions. The aquation of the second group is also accelerated by carboxylic acids. Activation parameters of all the catalytic and noncatalytic paths studied are reported. For the organometallic complexes obtained from pyridineacrylic acids it is postulated that the first step toward aquation is an intramolecular transfer of the Cr(OH2)5 moiety from carbon to the carboxylic oxygen. For the organometallic complexes obtained from maleic and fumaric acids it is postulated that, in the chelate ring containing both a Cr-C and a Cr-O bond, the weaker link is the latter. An interpretation is also given for the observation that Cr2+ does not catalyze the aquation of all organochromium complexes, as it seems to do for the ordinary ones. © 1981, American Chemical Society. All rights reserved