Hydrolysis of thorium(iv) at variable temperatures.

Hydrolysis of Th(iv) was studied in tetraethylammonium perchlorate (0.10 mol kg(-1)) at variable temperatures (283-358 K) by potentiometry and microcalorimetry. Three hydrolysis reactions, mTh(4+) + nH2O = Thm(OH)n((4m-n)+) + nH(+), in which (n,m) = (2,2), (8,4), and (15,6), were invoked to describe the potentiometric and calorimetric data for solutions with the [hydroxide]/[Th(iv)] ratio ≤ 2. At higher ratios, the formation of (16,5) cannot be excluded. The hydrolysis constants, *ß2,2, *ß8,4, and *ß15,6, increased by 3, 7, and 11 orders of magnitude, respectively, as the temperature was increased from 283 to 358 K. The enhancement is mainly due to the significant increase of the degree of ionization of water as the temperature rises. All three hydrolysis reactions are endothermic at 298 K, with enthalpies of (118 ± 4) kJ mol(-1), (236 ± 7) kJ mol(-1), and (554 ± 4) kJ mol(-1) for ΔH2,2, ΔH8,4, and ΔH15,6 respectively. The hydrolysis constants at infinite dilution have been obtained with the specific ion interaction approach. The applicability of three approaches for estimating the equilibrium constants at different temperatures, including the constant enthalpy approach, the constant heat capacity approach and the DQUANT equation was evaluated with the data from this work.

Protonation and lanthanide(III) complexation equilibria of a new tripodal polyaza-polycatechol-amine.

The sulfonated tripodal polyaza-polycatechol-amine ligand tris(2,3-hydroxy-5-sulfobenzylamine) ethyl)amine (STRENCAT) was synthesized and its protonation constants determined by potentiometry at 25 degrees C and in 0.1 mol dm(-3) NaClO(4). In the adopted experimental conditions (2.5 < p[H(+)] < 11.5), six of the virtual thirteen protonation constants of STRENCAT were determined. The deprotonation sequence of STRENCAT, identified by UV-vis and NMR studies, is characterized by the initial loss of three catechol protons, between p[H(+)] 5 and 8, followed by the loss of three secondary ammonium protons at higher p[H(+)] (9 < p[H(+)] < 11.5). No deprotonation of the three, more basic, catechol sites of STRENCAT was observed even at the highest p[H(+)] studied. Complexation measurements run on lanthanide(iii)-STRENCAT systems (Ln = La, Gd and Yb), in the same ionic medium and temperature, show that the ligand (partly protonated or completely deprotonated) is able to form soluble 1 : 1 and 1 : 2 metal-ligand complexes over the whole p[H(+)] range investigated. Due to the high affinity of lanthanide(iii) for catecholate units, even at p[H(+)] near 10 and before ammonium deprotonation, STRENCAT forms tri-chelate complexes, in which the metal ions are hosted in the cage formed by the six catechol oxygen atoms, completely deprotonated. Deprotonation of secondary ammonium groups, although not directly involved in complexation, increases the stability of the 1 : 1 complexes. They prevail in solution at p[H(+)] > 10, even in excess of ligand, in all the metal-ligand systems.

Thermodynamic and spectroscopic studies of lanthanides(III) complexation with polyamines in dimethyl sulfoxide.

The thermodynamic parameters of complexation of Ln(III) cations with tris(2-aminoethyl)amine (tren) and tetraethylenepentamine (tetren) were determined in dimethyl sulfoxide (DMSO) by potentiometry and calorimetry. The excitation and emission spectra and luminescence decay constants of Eu3+ and Tb3+ complexed by tren and tetren, as well as those of the same lanthanides(III) complexed with diethylenetriamine (dien) and triethylenetetramine (trien), were also obtained in the same solvent. The combination of thermodynamic and spectroscopic data showed that, in the 1:1 complexes, all nitrogens of the ligands are bound to the lanthanides except in the case of tren, in which the pendant N is bound. For the larger ligands (trien, tren, tetren) in the higher complexes (ML2), there was less complete binding by available donors, presumably due to steric crowding. FT-IR studies were carried out in an acetonitrile/DMSO mixture, suitably chosen to follow the changes in the primary solvation sphere of lanthanide(III) due to complexation of amine groups. Results show that the mean number of molecules of DMSO removed from the inner coordination sphere of lanthanides(III) is lower than ligand denticity and that the coordination number of the metal ions increases with amine complexation from approximately 8 to approximately 10. Independently of the number and structure of the amines, linear trends, similar for all lanthanides, were obtained by plotting the values of DeltaGj degrees, DeltaHj degrees, and TDeltaSj degrees for the complexation of ethylenediamine (en), dien, trien, tren, and tetren as a function of the number of amine metal-coordinated nitrogen atoms. The main factors on which the thermodynamic functions of lanthanide(III) complexation reactions in DMSO depend are discussed.

Complexation behaviour and stability of Schiff bases in aqueous solution. The case of an acyclic diimino(amino) diphenol and its reduced triamine derivative.

The copper(II), nickel(II), and zinc(II) complexes of the acyclic Schiff base H(2)L(A), obtained by [1 + 2] condensation of 1,2-ethanediamine,N-(2-aminoethyl)-N-methyl with 3-ethoxy-2-hydroxybenzaldehyde, and of H(2)L(B), the reduced derivative of H(2)L(A), were prepared and their properties studied by IR, NMR and SEM-EDS. In these complexes, the metal ion is always located in the coordination chamber of the ligand delimited by two phenol oxygens and nitrogen atoms (either aminic or iminic). The coordination behaviour of H(2)L(A) and H(2)L(B) towards H(+), Cu(2+), Ni(2+) and Zn(2+) in aqueous solution at 298 K and mu = 0.1 mol dm(-3) (Na)ClO(4) was also studied by potentiometric, NMR and UV-VIS measurements. In particular, potentiometric equilibrium studies indicate that H(2)L(A) is not stable enough to have a pH range in which it is the sole species in aqueous solution. In such a solution, the Schiff base forms over a limited pH range, between 6 and 10, with a maximum formation percentage at pH approximately 9. In addition, the involvement of imine nitrogens in the complexes markedly stabilises the azomethylene linkage, so that the metal complexes of H(2)L(A), particularly those of copper(II), are the species largely prevailing in solutions with pH >3.5. The stability constants of the complexes formed by metal ions with H(2)L(A) and H(2)L(B) follow the order Cu(2+) >> Ni(2+) > Zn(2+); distribution plots show that copper(II) gives complexes more stable with H(2)L(A), whereas Ni(2+) and Zn(2+) prefer the reduced ligand, H(2)L(B).