Modification of 4-(2-pyridylazo)-resorcinol postcolumn reagent selectivity through competitive equilibria with chelating ligands

Aminopolycar☐ylate ligands were added to the 4-(2-pyridylazo)-resorcinol (PAR) postcolumn reagent to alter the reagent selectivity towards transition metals. Addition of ethylenediaminetetraacetic acid (EDTA) completely suppressed the reaction between PAR and the metal ions. Addition of 0.1 mM nitrilotriacetic acid (NTA) to 1 mM PAR lowered the response to specific transition metal ions, but completely suppressed the PAR response to the lanthanides. Increasing the NTA concentration to 8 mM resulted in complete suppression of the PAR response to all metal ions except Cu²⁺ and Co²⁺ for which the detection limits were 3 and 1 ng, respectively. The observed selectivity results from the slow rate of conversion of metal ions from the M(NTA)₂⁴⁻ form to M(PAR)₂.     

Precipitation-dissolution system for silver preconcentration and determination by flow injection flame atomic absorption spectrometry

In this paper, flame atomic absorption spectrometry was used for the determination of silver in various materials. The proposed preconcentration method is based on the continuous precipitation of silver as p-dimethylaminobenzilidene-rhodanine (PDBR) complex and dissolution of the precipitate with potassium cyanide. EDTA was added to the sample solution to mask large concentrations of Fe(III), Ni(II), Cu(II), Zn(II), Pb(II), Co(II) and Al(III). An enhancement factor of 20 was obtained for a preconcentration time of 3 min, resulting in a sampling frequency of 16 h(-1). The detection limit (3sigma) in the sample solution was 5 ngml(-1). The relative standard deviation at 30 ngml(-1) level was 4.7%. Analytical results obtained for alloy, biological reference material and ore samples analyzed were in good agreement with the certified values and comparable to those obtained with other techniques.     

Photodissociation of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)L and Fe(CO)(4)L: a theoretical study

The photochemistry of the phosphine-substituted transition metal carbonyl complexes Cr(CO)(5)PH(3) and ax-Fe(CO)(4)PH(3) is studied with time-dependent DFT theory to explore the propensity of the excited molecules to expel their ligands. The influence of the PH(3) ligand on the properties of these complexes is compared with the photodissociation behavior of the binary carbonyl complexes Cr(CO)(6) and Fe(CO)(5). The lowest excited states of Cr(CO)(5)PH(3) are metal-to-ligand charge transfer (MLCT) states, of which the first three are repulsive for PH(3) but modestly bonding for the axial and equatorial CO ligands. The repulsive nature is due to mixing of the initial MLCT state with a ligand field (LF) state. A barrier is encountered along the dissociation coordinate if the avoided crossing between these states occurs beyond the equilibrium distance. This is the case for expulsion of CO but not for the PH(3) group as the avoided state crossing occurs within the equilibrium Cr-P distance. The lowest excited state of ax-Fe(CO)(4)PH(3) is a LF state that is repulsive for both PH(3) and the axial CO. Excited-state quantum dynamics calculations for this state show a branching ratio of 99 to 1 for expulsion of the axial phosphine ligand over an axial CO ligand. The nature of the phosphorus ligand in these Cr and Fe complexes is only of modest importance. Complexes containing the three-membered phosphirane or unsaturated phosphirene rings have dissociation curves for their lowest excited states that are similar to those having a PH(3) ligand. Analysis of their ground-state Cr-P bond properties in conjunction with frontier orbital arguments indicate these small heterocyclic groups to differ from the PH(3) group mainly by their enhanced sigma-donating ability. All calculations indicate that the excited Cr(CO)(5)L and Fe(CO)(4)L molecules (L = PH(3), PC(2)H(5), and PC(2)H(3)) prefer dissociation of their phosphorus substituent over that of an CO ligand. This suggests that the photochemical approach may be a viable complement to the ligand exchange and redox methods that are currently employed to demetalate transition metal complexed organophosphorus compounds.     

Selective interaction of lower rim calix[4]arene derivatives and bivalent cations in solution. Crystallographic evidence of the versatile behavior of acetonitrile in lead(II) and cadmium(II) complexes

The interaction of lower rim calix(4)arene derivatives containing ester (1) and ketone (2) functional groups and bivalent (alkaline-earth, transition- and heavy-metal) cations has been investigated in various solvents (methanol, N,N-dimethylformamide, acetonitrile, and benzonitrile). Thus, 1H NMR studies in CD3OD, C3D7NO, and CD3CN show that the interaction of these ligands with bivalent cations (Mg2+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+, Cd2+) is only observed in CD3CN. These findings are corroborated by conductance measurements in these solvents including benzonitrile, where changes upon the addition of the appropriate ligand (1 or 2) to the metal-ion salt only occur in acetonitrile. Thus, in this solvent, plots of molar conductance against the ligand/metal cation ratio reveal the formation of 1:1 complexes between these ligands and bivalent cations. Four metal-ion complex salts resulting from the interaction of 1 and 2 with cadmium and lead, respectively, were isolated and characterized by X-ray crystallography. All four structures show an acetonitrile molecule sitting in the hydrophobic cavity of the ligand. The mode of interaction of the neutral guest in the cadmium(II) complexes differs from each other and from that found in the lead(II) complexes and provides evidence of the versatile behavior of acetonitrile in binding processes involving calix(4)arene derivatives. The thermodynamics of complexation of these ligands and bivalent cations in acetonitrile is reported. Thus, the selective behavior of 1 and 2 for bivalent cations is for the first time demonstrated. The role of acetonitrile in the complexation process in solution is discussed on the basis of 1H NMR and X-ray crystallographic studies. It is suggested that the complexation of 1 and 2 with bivalent cations is likely to involve the ligand-solvent adducts rather than the free ligand. Plots of complexation Gibbs energies against the corresponding data for cation hydration show a selectivity peak which is explained in terms of the predominant role played by cation desolvation and ligand binding energy in complex formation involving metal cations and macrocycles in solution. A similar peak is found in terms of enthalpy suggesting that for most cations (except Mg2+) the selectivity is enthalpically controlled. The ligand effect on the complexation process is quantitatively assessed. Final conclusions are given highlighting the role of the solvent in complexation processes involving calix(4)arene derivatives and metal cations.     

Cyclodextrin derivatives in GC separation of racemic mixtures of volatiles – part XII: Thick-film wide-bore columns for enantiomer GC preparation

Preliminary results on thick-film wide-bore (0.53mm i.d.) columns for GC preparation of pure enantiomers are described. In particular the loading capacity for several racemates of a 3μm, 30% 2,6-di-O-methyl-3-O-Pentyl-β-CD/OV1701 column and of a 2 μm, 30% 2,3-di-O-acetyl-6-O-dimethyl-t-butylyl-β-CD/PS-086 collumn were determined. Consideration is also given to the relation-ship between resolution values of two enantiomers on anayltical columns and their loading capacity and scaling-up to the corresponding micropreparative columns.     

Beyond a T-shape

Varying the steric bulk of either the phosphine or the halide in Au(PR3)2X complexes allows intuitive tuning of the phosphorescence energy to multiple visible colors, including the coveted blue for LED applications. The excited-state structure involves distortion of the trigonal coordination sphere beyond a T-shape. The [Au(TPA)2]Cl complex exhibits orange phosphorescence due to exciplex formation with the counterion to form the same type of excited state, representing the first example of a luminescent two-coordinate Au(I) complex in absence of both Au...Au interactions and aromatic moieties.     

Solvation effect of guest, supramolecular host, and host-guest compounds on the thermodynamic selectivity of calix(4)arene derivatives and soft metal cations

This paper reports thermodynamic data for the transfer of calixarene derivatives and their metal-ion complexes in dipolar aprotic solvents. These data are used to assess the effect of solvation of these compounds on the selective complexation shown by these macrocycles for soft metal cations in different media. Thus, solubilities and derived Gibbs energies of solution of 5,11,17,23-tetra-tert-butyl[25,27-bis(hydroxyl)-26,28-bis(ethylthioethoxy)]calix(4)arene, 1, and 5,11,17,23-tetra-tert-butyl-[25,27-bis(ethylenethanoate)-26,28-bis(ethylthioethoxy)]-calix(4)arene, 2, in various solvents at 298.15 K are reported. Solvation of these ligands in one medium relative to another is analyzed from their standard transfer Gibbs energies using acetonitrile as the reference solvent. These data are combined with transfer enthalpies (derived from standard solution enthalpies obtained calorimetrically) to calculate the corresponding entropies of transfer of these calix(4)arene derivatives from acetonitrile to methanol and N,N-dimethylformamide. As far as the metal-ion salts (silver and mercury) in their free and complex forms are concerned, standard solution enthalpies were determined in acetonitrile, methanol, and N,N-dimethylformamide. These data are used to derive their transfer enthalpies from one medium to another. It is concluded that the extent of complexation of these macrocycles with soft metal cations is controlled by not only the solvation changes that the free cation undergoes in moving from one medium to another but also those for the ligand and its complex cation in these solvents.     

Reactivity,Syntheses, and Crystal Structures of Na5[MO2][X] with M = Co+, Ni+, Cu+; X = CO32—, SO42—, SO32—, S2—, and Na25[CuO2]5[SO4]4[S]

Na₅[CuO₂][CO₃], Na₅[CuO₂][SO₃], Na₅[CuO₂][S], and Na₅[CuO₂][SO₄] were obtained as single crystals and powders from reactions of Na₂O, Cu₂O, and Na₂X with X = CO₃^2—, SO₃^2—, S^2—, and SO₄^2—, respectively. A redox reaction between CdO and Co metal occurs in the presence of Na₂O and Na₂X, yielding Na₅[CoO₂][X] with X = CO₃^2— and S^2—. From a mixture of Na₂SO₄, CdO and Na₂O in Ni‐containers we observed the formation of Na₅[NiO₂][S] single crystals. Single crystals of Na₂₅[CuO₂]₅[SO₄]₄[S] can be grown by annealing Na₅[CuO₂][SO₃] at 600 °C, leading to the decomposition of SO₃^2—, yielding SO₄^2— and S^2— at 550 °C. The structures have been determined from single crystal data and powder data. All structures contain the isolated complex [MO₂]^3— in a dumb‐bell like arrangement. The main feature of these compounds is that the anions SO₄^2—, SO₃^2—, CO₃^2— and S^2— are not connected to the transition metal. The formation of Na₅[CuO₂][X] (X = S^2—, SO₄^2—, SO₃^2—, CO₃^2—) has been studied by thermal analysis and in situ X‐ray diffraction techniques. Infrared spectra confirm the presence of SO₄^2—, SO₃^2—, and CO₃^2—, respectively, in the structures.     

Synthesis and structural characterization of unprecedented bis-asymmetric heteroscorpionate U(III) complexes: [U(kappa3-H2B(pztBu,Me)(pzMe,tBu))2I] and [U(kappa3-H2B(pztBu,Me)(pzMe2))2I

[UI(3)(THF)(4)] reacts at room temperature with 2 equiv of KBp(tBu,Me), in toluene, yielding [U(kappa(3)-H(mu-H)B(pz(tBu,Me))(pz(Me,tBu)))(2)I] (1). This unprecedented complex, stabilized by two asymmetric heteroscorpionate ligands, is formed due to an isomerization process promoted in situ by the metal center. To find a general method for preparing this type of compound, we synthesized the novel asymmetric K[H(2)B(pz(tBu,Me))(pz(Me2))], and by a straightforward salt metathesis with [UI(3)(THF)(4)] the novel bis-asymmetric complex [U(kappa(3)-H(mu-H)B(pz(tBu,Me))(pz(Me2)))(2)I] (2) was isolated and characterized in the solid state and in solution. As indicated by X-ray crystallographic analysis, the U(III) in 1 and 2 is seven-coordinated by two tridentate asymmetric dihydrobis(pyrazoly)borates and by an iodide. In both cases, the coordination geometry around the metal is very distorted, the pentagonal bipyramid being the one which better describes the arrangement of the atoms around the U(III). An approximate C(2) axis can be defined in the solid state, and is maintained in solution as indicated by the (1)H NMR spectrum of 1 and 2. In the course of attempting to crystallize some of the compounds, monocrystals of the dimer [U(kappa(3)-Bp(tBu,Me))(Hpz(tBu,Me))I(mu-I)](2) (3) were isolated. In this compound each U(III) atom is seven-coordinated by one kappa(3)-Bp(tBu,Me), by one terminal and by two bridging iodide ligands, and by a monodentate Hpz(tBu,Me), exhibiting a distorted 4:3 tetragonal base-trigonal geometry.