Dithallium(III)-Containing 30-Tungsto-4-phosphate, [Tl2Na2(H2O)2(P2W15O56)2]16-: Synthesis, Structural Characterization, and Biological Studies.

Here we report on the synthesis and structural characterization of the dithallium(III)-containing 30-tungsto -4-phosphate [Tl2Na2(H2O)2{P2W15O56}2]16- (1) by a multitude of solid-state and solution techniques. Polyanion 1 comprises two octahedrally coordinated Tl3+ ions sandwiched between two trilacunary {P2W15} Wells-Dawson fragments and represents only the second structurally characterized, discrete thallium-containing polyoxometalate to date. The two outer positions of the central rhombus are occupied by sodium ions. The title polyanion is solution-stable as shown by 31P and 203/205Tl NMR. This was also supported by Tl NMR spectra simulations including several spin systems of isotopologues with half-spin nuclei (203Tl, 205Tl, 31P, 183W). 23Na NMR showed a time-averaged signal of the Na+ counter cations and the structurally bonded Na+ ions. 203/205Tl NMR spectra also showed a minor signal tentatively attributed to the trithallium-containing derivative [Tl3Na(H2O)2(P2W15O56)2]14-, which could also be identified in the solid state by single-crystal X-ray diffraction. The bioactivity of polyanion 1 was also tested against bacteria and Leishmania.

Synthesis, Structure, and Antibacterial Activity of a Thallium(III)-Containing Polyoxometalate, [Tl2{B-ß-SiW8O30(OH)}2]12.

We have synthesized and structurally characterized the first discrete thallium-containing polyoxometalate, [Tl2{B-ß-SiW8O30(OH)}2]12- (1). Polyanion 1 was characterized in the solid-state and shown to be solution-stable by 203/205Tl NMR, electrospray ionization mass spectrometry, and electrochemical studies. The antibacterial activity of 1 was also investigated.

[Tl(III)(dota)](-): An Extraordinarily Robust Macrocyclic Complex.

The X-ray structure of {C(NH2)3}[Tl(dota)]·H2O shows that the Tl(3+) ion is deeply buried in the macrocyclic cavity of the dota(4-) ligand (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate) with average Tl-N and Tl-O distances of 2.464 and 2.365 Å, respectively. The metal ion is directly coordinated to the eight donor atoms of the ligand, which results in a twisted square antiprismatic (TSAP') coordination around Tl(3+). A multinuclear (1)H, (13)C, and (205)Tl NMR study combined with DFT calculations confirmed the TSAP' structure of the complex in aqueous solution, which exists as the Λ(λλλλ)/Δ(δδδδ) enantiomeric pair. (205)Tl NMR spectroscopy allowed the protonation constant associated with the protonation of the complex according to [Tl(dota)](-) + H(+) ⇆ [Tl(Hdota)] to be determined, which turned out to be pK(H)Tl(dota) = 1.4 ± 0.1. [Tl(dota)](-) does not react with Br(-), even when using an excess of the anion, but it forms a weak mixed complex with cyanide, [Tl(dota)](-) + CN(-) ⇆ [Tl(dota)(CN)](2-), with an equilibrium constant of Kmix = 6.0 ± 0.8. The dissociation of the [Tl(dota)](-) complex was determined by UV-vis spectrophotometry under acidic conditions using a large excess of Br(-), and it was found to follow proton-assisted kinetics and to take place very slowly (∼10 days), even in 1 M HClO4, with the estimated half-life of the process being in the 10(9) h range at neutral pH. The solution dynamics of [Tl(dota)](-) were investigated using (13)C NMR spectroscopy and DFT calculations. The (13)C NMR spectra recorded at low temperature (272 K) point to C4 symmetry of the complex in solution, which averages to C4v as the temperature increases. This dynamic behavior was attributed to the Λ(λλλλ) ↔ Δ(δδδδ) enantiomerization process, which involves both the inversion of the macrocyclic unit and the rotation of the pendant arms. According to our calculations, the arm-rotation process limits the Λ(λλλλ) ↔ Δ(δδδδ) interconversion.

The role of equilibrium and kinetic properties in the dissociation of Gd[DTPA-bis(methylamide)] (Omniscan) at near to physiological conditions.

[Gd(DTPA-BMA)] is the principal constituent of Omniscan, a magnetic resonance imaging (MRI) contrast agent. In body fluids, endogenous ions (Zn(2+), Cu(2+), and Ca(2+)) may displace the Gd(3+). To assess the extent of displacement at equilibrium, the stability constants of DTPA-BMA(3-) complexes of Gd(3+), Ca(2+), Zn(2+), and Cu(2+) have been determined at 37 °C in 0.15 M NaCl. The order of these stability constants is as follows: GdL≈CuL>ZnL≫CaL. Applying a simplified blood plasma model, the extent of dissociation of Omniscan (0.35 mM [Gd(DTPA-BMA)]) was found to be 17% by the formation of Gd(PO4), [Zn(DTPA-BMA)](-) (2.4%), [Cu(DTPA-BMA)](-) (0.2%), and [Ca(DTPA-BMA)](-) (17.7%). By capillary electrophoresis, the formation of [Ca(DTPA-BMA)](-) has been detected in human serum spiked with [Gd(DTPA-BMA)] (2.0 mM) at pH 7.4. Transmetallation reactions between [Gd(DTPA-BMA)] and Cu(2+) at 37 °C in the presence of citrate, phosphate, and bicarbonate ions occur by dissociation of the complex assisted by the endogenous ligands. At physiological concentrations of citrate, phosphate, and bicarbonate ions, the half-life of dissociation of [Gd(DTPA-BMA)] was calculated to be 9.3 h at pH 7.4. Considering the rates of distribution and dissociation of [Gd(DTPA-BMA)] in the extracellular space of the body, an open two-compartment model has been developed, which allows prediction of the extent of dissociation of the Gd(III) complex in body fluids depending on the rate of elimination of the contrast agent.

Solution structures, stabilities, kinetics, and dynamics of DO3A and DO3A-sulphonamide complexes.

The Gd(3+)-DO3A-arylsulphonamide (DO3A-SA) complex is a promising pH-sensitive MRI agent. The stability constants of the DO3A-SA and DO3A complexes formed with Mg(2+), Ca(2+), Mn(2+), Zn(2+), and Cu(2+) ions are similar, whereas the logKLnL values of Ln(DO3A-SA) complexes are 2 orders of magnitude higher than those of the Ln(DO3A) complexes. The protonation constant (log KMHL) of the sulphonamide nitrogen in the Mg(2+), Ca(2+), Mn(2+), Zn(2+), and Cu(2+) complexes is very similar to that of the free ligand, whereas the logKLnHL values of the Ln(DO3A-SA) complexes are lower by about 4 logK units, indicating a strong interaction between the Ln(3+) ions and the sulphonamide N atom. The Ln(HDO3A-SA) complexes are formed via triprotonated *Ln(H3DO3A-SA) intermediates which rearrange to the final complex in an OH(-)-assisted deprotonation process. The transmetalation reaction of Gd(HDO3A-SA) with Cu(2+) is very slow (t1/2 = 5.6 × 10(3) h at pH = 7.4), and it mainly occurs through proton-assisted dissociation of the complex. The (1)H and (13)C NMR spectra of the La-, Eu-, Y-, and Lu(DO3A-SA) complexes have been assigned using 2D correlation spectroscopy (COSY, EXSY, HSQC). Two sets of signals are observed for Eu-, Y-, and Lu(DO3A-SA), showing two coordination isomers in solution, that is, square antiprismatic (SAP) and twisted square antiprismatic (TSAP) geometries with ratios of 86-14, 93-7, and 94-6%, respectively. Line shape analysis of the (13)C NMR spectra of La-, Y- , and Lu(DO3A-SA) gives higher rates and lower activation entropy values compared to Ln(DOTA) for the arm rotation, which indicates that the Ln(DO3A-SA) complexes are less rigid due to the larger flexibility of the ethylene group in the sulphonamide pendant arm. The fast isomerization and the lower activation parameters of Ln(DO3A-SA) have been confirmed by theoretical calculations in vacuo and by using the polarizable continuum model. The solid state X-ray structure of Cu(H2DO3A-SA) shows distorted octahedral coordination. The coordination sites of Cu(2+) are occupied by two ring N- and two carboxylate O-atoms in equatorial position. The other two ring N-atoms complete the coordination sphere in axial positions. The solid state structure also indicates that a carboxylate O atom and the sulphonamide nitrogen are protonated and noncoordinated.

A comparative study of capillary zone electrophoresis and pH-potentiometry for determination of dissociation constants.

Acidity constants of six cephalosporin antibiotics, cefalexin, cefaclor, cefadroxil, cefotaxim, cefoperazon and cefoxitin are determined using capillary zone electrophoresis (CZE) and pH-potentiometric titrations. Since CZE is a separation method, it is not necessary for the samples to be of high purity and known concentration because only mobilities are measured. The effect on determination of dissociation constants of different matrices (serum, 0.9% NaCl, fermentation matrix) was examined. The advantages of CZE can be utilized in those fields where potentiometry has limitations (sample quantity, solubility, purity, simultaneous determinations), although pK(a) values that are close to each other can be determined by potentiometry with more accuracy.

Equilibrium and structure of the Al(III)-ethylenediamine-N,N'-bis(3-hydroxy-2-propionate) (EDBHP) complex. A multi-method study by potentiometry, NMR, ESI MS and X-ray diffraction.

The equilibrium and structure of the complex formed by Al(III) and ethylenediamine-N,N'-bis(3-hydroxy-2-propionate) (EDBHP2-) have been studied using pH-potentiometry, 1H and 27Al NMR, ESI MS and single crystal X-ray diffraction methods. The EDBHP ligand is a strong Al-binder in aqueous solution for pH between 4 and 8 and for c(Al) = c(EDBHP)> or = 0.1 mmol dm(-3). The dominating complex identified by ESI MS and potentiometry is a neutral dimer, Al2L2(OH)2, with logbeta(22-2) = 14.16 +/- 0.03. In the solid Al2(EDBHP)2(OH)2.2H2O the Al(III) ions are connected through a double hydroxo bridge. Both four-dentate organic ligands are coordinated terminally through two carboxylate groups and two N-donors forming three five-membered chelate rings. The hydroxyl groups of the ligand EDBHP remain protonated and are not coordinated to the aluminium ions. The structure and composition of the dimer are very likely the same in solution and the solid state.

Studies on the kinetic stabilities of the Gd(3+) complexes formed with the N-mono(methylamide), N'-mono(methylamide) and N,N"-bis(methylamide) derivatives of diethylenetriamine-N,N,N',N",N"-pentaacetic acid.

High kinetic stability is an important requirement for the Gd(3+) complexes used as contrast enhancement agents in magnetic resonance imaging. The kinetic stabilities of the Gd(3+) complexes formed with DTPA-N-mono(methylamide) (L(3)), DTPA-N'-mono(methylamide) (L(2)) and DTPA-bis(methylamide) (L(1)) are characterized by the rates of the exchange reactions with Eu(3+) and the endogenous Cu(2+) and Zn(2+). The exchange reactions occur via the proton-assisted dissociation of the complexes and direct attack of the exchanging metal ions on the complex. On the basis of the line-shape analysis of the 1H NMR spectra of the LaL(2), obtained in the pH range 2.5-3.5, we assume that for the proton-assisted dissociation of the complexes the formation of an intermediate containing a free iminodiacetate group must be followed with the rupture of the metal-central nitrogen bond. At about pH > or = 5, the reactions between GdL(2) or GdL(3) and Cu(2+) or Zn(2+) proceed predominantly by direct reaction of the reactants, through the formation of dinuclear intermediates. The contribution of the proton-assisted dissociation is highly important for GdL(1), but its reaction with Zn(2+) is significantly slower than the reactions of GdL(2) and GdL(3). The overall rates of dissociation of GdL(1), GdL(2), GdL(3) and Gd(DTPA)(2-) through H(+) (pH 7.4), Cu(2+) (1 x 10(-6) M) and Zn(2+) (1 x 10(-5) M)-assisted reactions are surprisingly very similar. Replacement of one or two carboxylates with amide groups results in significantly decreased stability constants, but has practically no effect on the kinetic stability of the Gd(3+) complexes, indicating the lower reactivity of the amide groups with Cu(2+) and Zn(2+).