Reaction of chromium(III) with 3,4-Dihydroxybenzoic acid: Kinetics and mechanism in weak acidic aqueous solutions

The interactions between chromium(III) and 3,4-dihydroxybenzoic acid (3,4-DHBA) were studied resulting in the formation of oxygen-bonded complexes upon substitution of water molecules in the chromium(III) coordination sphere. The experimental results show that the reaction takes place in at least three stages, involving various intermediates. The first stage was found to be linearly dependent on ligand concentration k 1 (obs) - = k 0 + k 1 (obs) [ 3, 4 -DHBA ], and the corresponding activation parameters were calculated as follows: Δ H 1 (obs) = 51.2 ± 11.5 kJ mol - 1, Δ S 1 (obs) = - 97.3 ± 28.9 J mol - 1 K - 1 (composite activation parameters). The second and third stages, which are kinetically indistinguishable, do not depend on the concentrations of ligand and chromium(III), accounting for isomerization and chelation processes, respectively. The corresponding activation parameters are Δ H 2 (obs) = 44.5 ± 5.0 kJ mol - 1, Δ S 2 (obs) = - 175.8 ± 70.3 J mol - 1 K - 1. The observed stages are proposed to proceed via interchange dissociative (I d, first stage) and associative (second and third stages) mechanisms. The reactions are accompanied by proton release, as is shown by the pH decrease

Kinetics and mechanism of the reaction between chromium(III) and 2,3-dihydroxybenzoic acid in weak acidic aqueous solutions

The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1:1. The first stage is suggested to be the reaction of [Cr(H 2O)5(OH)]2+ with the ligand in weak acidic aqueous solutions that follows an Id mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are ΔH2(obs)≠ = 61.2±3.1kJ mol-1, ΔS2(obs)≠ = -91.1±11.0 JK-1 mol-1, ΔH 3(obs)≠ = 124.5±8.7 kJ mol-1, and ΔS3(obs)≠ = 95.1±29.0 JK-1 mol-1. These two stages are proposed to proceed via associative mechanisms. The positive value of ΔS3(obs)≠ can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease. Copyright © 2010 Athinoula L. Petrou et al

Kinetics and mechanism of the reaction between chromium(III) and 3,4-dihydroxy-phenyl-propenoic acid (caffeic acid) in weak acidic aqueous solutions

Our study of the complexation of 3,4-dihydroxy-phenyl-propenoic acid by chromium(III) could give information on the way that this metal ion is available to plants. The reaction between chromium(III) and 3,4-dihydroxy-phenyl- propenoic acid in weak acidic aqueous solutions has been shown to take place by at least three stages. The first stage corresponds to substitution (I d mechanism) of water molecule from the Cr (H 2 O) 5 OH 2+ coordination sphere by a ligand molecule. A very rapid protonation equilibrium, which follows, favors the aqua species. The second and the third stages are chromium(III) and ligand concentration independent and are attributed to isomerisation and chelation processes. The corresponding activation parameters are Δ H 2(obs) = 28.6 ± 2.9kJ mol - 1, Δ S 2(obs) = -220± 10J K - 1 mol - 1, Δ H 3(obs) = 62.9 ± 6.7kJ mol - 1 and Δ S 3(obs) = -121± 22J K - 1 mol - 1. The kinetic results suggest associative mechanisms for the two steps. The associatively activated substitution processes are accompanied by proton release causing pH decrease

Palladium(II) complexes with S-benzyl dithiocarbazate and S-benzyl-N-isopropylidenedithiocarbazate: Synthesis, spectroscopic properties and X-ray crystal structures

Two new palladium(II) bis(NS) chelates, bis(S-benzyl dithiocarbazato)palladium(II) (1) and bis(S-benzyl-N-isopropylidenedithiocarbazato)palladium(II) (2), have been prepared and characterized using single-crystal X-ray diffraction and spectroscopic (electronic, IR and NMR) techniques. Complex 1 has a perfectly square planar trans configuration (point group Ci), while complex 2 has a distorted square planar cis configuration. © 2007 Elsevier Ltd. All rights reserved

Development of an electrochemical biosensor for the rapid detection of naphthalene acetic acid in fruits by using air stable lipid films with incorporated auxin-binding protein 1 receptor

This work describes the investigations of electrochemical interactions of naphthalene acetic acid (NAA) with stabilized lipid films supported on a methacrylate polymer on a glass fiber filter with incorporated auxin-binding protein 1 receptor for the development of a biosensor for the rapid detection of this compound in fruits. NAA was injected into the flowing streams of a carrier electrolyte solution, the flow of the electrolyte solution stops and an ion current transient was obtained; the magnitude of the signal was correlated to NAA concentration, which could be determined at the micromolar level. NAA preconcentrates at the lipid membrane surface which causes dynamic alterations of the electrostatic fields and phase structure of membranes. The response times were ca. 5 min and naphthalene acetic acid was determined at concentration levels of μM. The effect of potent interferences included a wide range of compounds. The results showed no interferences from these compounds in concentration levels usually found in real samples. The method was applied for the determination of NAA in fruits and the reproducibility of the method was checked in about 100 samples. A quantitative method for the detection of NAA in fruits that can be complimentary to HPLC methods is provided in the present paper. These lipid films can be used as portable sensors for the rapid detection of NAA in fruits by non-skilled personnel. © 2008 Bentham Science Publishers Ltd