Kinetic analysis of the complexation of aqueous lanthanide(III) ions by arsenazo III †

Abstract

The complexation behavior of Ln 3ϩ (aq) (Ln = Eu, Gd or Tb) ions by arsenazo III [3,-4,5-dihydroxynaphthalene-2,7-disulfonic acid] has been investigated by stopped-flow kinetic techniques (ambient and high-pressure). The reaction takes place in three stages. The first kinetic stage differs in pressure dependence for the different metal ions, probably related to a co-ordination number changeover. The second and third kinetic stages were also studied under a variety of conditions. A step by step ring-closure mechanism is suggested to account for the three kinetic stages. The results are discussed with reference to related literature data.

Kinetics and mechanisms of the complexation of aqueous lanthanide ions by 4-(2-pyridylazo)resorcinol †

“…The k 1 values shown in Fig. 6, and the other rate constants of aqueous Ln 3ϩ complexations by H 2 O, , 21,22 acetate, 23 picolinic acid, 24 methyl red, 26 malonate, 27 arsenazo III, 13 and acyclic aminopolycarboxylates, such as EDTA and DTPA, 18,29 vary by several orders of magnitude for the same metal ion. The ligands do not have the same steric properties and electronic charge and will have very different outer-sphere parameters.…”

“…1,2 A changeover in the co-ordination number of the lanthanide complexes from nine to eight near the middle of the series 3-6 gives rise to kinetics for the complexation or solvent exchange of lanthanide ions in solution that has been studied extensively using high-pressure NMR relaxation techniques. [7][8][9][10][11][12][13] Interest in these kinetic phenomena has increased with the development of some lanthanide complexes as contrast agents in magnetic imaging (MRI). [14][15][16] Another significant feature of lanthanide element behavior in aqueous solution is the very high stability of the trivalent state although cerium() and, in strongly reducing solutions, divalent samarium, europium and ytterbium can be formed.…”

“…murexide (E-jump), 25 methyl red (pulse-radiolytic pH-jump), 26 anthranilate (temperature-jump), 26 malonate (ultrasonic relaxation), 27 oxalate (pressure-jump), 28 arsenazo III [3,6-bis-(o-arsonophenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid] (stopped-flow) 13 and acyclic aminopolycarboxylates, such as EDTA (ethylenedinitrilotetraacetate) and DTPA [carboxymethyliminobis(ethylenenitrilo)tetraacetate]. 18, 29 The rate of complexation is affected either by the size of the central ion or by the nature of the ligand.…”

“…Some kinetic studies[71][72][73] indicate that the catalytic effect results from a trans labilization effect by co-ordinated acetate and involves the attack by a solvent water molecule on the metal-carbon bond for the heterolysis reaction of (α-hydroxyalkyl)chromium(). The co-ordinated acetate accelerates dissociation of water molecules from the inner coordination sphere of the metal ions, resulting in kinetic differences with reactions in unbuffered or other buffered solutions 13,74. No protonated pyridine nitrogen exists in the acetate buffered pH range (3.61-5.40) since the pK a1 is as low as 2.35, 44 thus [PAR] tot = [PAR], and k 1 is independent of pH in acetate buffer.Pressure dependence.…”

Ferrate(VI) oxidation of aniline

“…Associative mechanisms characteristically exhibit negative ∆V ‡ values and dissociative mechanisms positive ∆V ‡ values for reactions involving exchange of neutral ligands. 22,23 For redox reactions, intermolecular electron transfer is usually accompanied by a significantly negative activation volume. 24-26 Formation of highly charged complex species and bond-making also cause negative volumes of activation.…”