ML Species Research Articles

Speciation Study of L-ascorbic Acid and its Chelated Cu(II) & Ni(II) Complexes: an Experimental and Theoretical Model of Complex Formation

Different species of L-ascorbic acid and their corresponding complex formation ability with Cu(II) and Ni(II) metal ions in aqueous medium has been studied in the pH range from 2.0-12.5. The stability constants of different complexes of Cu(II) and Ni(II) with the bidentate ligand, L-ascorbic acid were determined theoretically using MINIQUARD software. Speciation of ligand and complex of Cu(II)/Ni(II) ascorbate were experimentally investigated by the titration method in solution within this pH range. Different Cu (II) and Ni(II)-L-ascorbic acid species percentages with variation of pH were calculated within the studied pH range with the help of another computer programs SIM and SPECIES. Different species distribution diagrams and the equilibria for the formation of the species were also investigated and at higher pH, ML2 species was found to be the major species in the case of both the metal complexes. All the theoretical possible structures of Cu(II) and Ni(II) complexes with L-ascorbic acid were optimized and square pyramidal and square planer geometry have been evaluated for Cu(II) and Ni(II) respectively by Gaussian09 software. Their corresponding HOMO-LUMO energy and reactivity parameters such as chemical hardness (ç), ionization potential (I), electron affinity (A), electro negativity (χ), chemical potential («), electrophilicity index (ω) have been calculated in order to provide a better understanding of the electronic structure of complexes with the experimental results. Keywords: Cu(II)and Ni(II) complexes, ascorbic acid, molecular speciation, DFT, reactivity parameters.

EPR Evidence for the Origin of Nonlinear Effects in an Enantioselective Cu(II)-Catalyzed Spiroannulation

Herein, we describe an enantioselective Cu(II)-catalyzed spiroannulation of N-Boc-iminooxindoles with allylsilanes where a significant positive nonlinear effect (NLE) is observed. EPR spectroscopic studies of the copper(II) species present under synthetically relevant conditions reveal explicit spectroscopic evidence based on analysis of the metal center for the species responsible for the positive NLE in a metal-catalyzed system. EPR spectroscopy indicates that formation of a heterochiral ML2 species under scalemic conditions enriches the effective enantiopurity of the catalytically active species, leading to the asymmetric amplification observed in the spiroannulation. Mathematical analysis of the positive NLE reveals a high thermodynamic preference toward formation of the heterochiral ML2, which has a low relative reactivity when compared to the homochiral ML2.

Study of Al3+ interaction with AMP, ADP and ATP in aqueous solution

The interaction of Al3+ and nucleotide ligands, namely adenosine-5′-monophosphate, (AMP), adenosine-5′-diphosphate, (ADP), adenosine-5′-triphosphate, (ATP), has been studied in aqueous solution at T = 298.15 K and I = 0.15 mol L−1 in NaCl (only for Al3+-ATP system at I = 0.1 mol L−1). Formation constants and speciation models for the species formed are discussed on the basis of potentiometric results. The speciation models found for the three systems include ML and ML2 species in all the cases, and for Al3+-ADP and ATP systems, MLH, MLOH and ML2OH species as well. The formation constant value for ML species shows the trend, AMP < ADP < ATP. 1H NMR spectroscopy was also employed for the study of Al3+-ATP system. The 1H NMR results are in agreement with the speciation model obtained from analysis of potentiometric titration data, confirming the stabilities of the main species. Enthalpy change values were obtained by titration calorimetry; for the main Al3+-ATP species (at T = 298.15 K and I = 0.1 mol L−1 in NaCl), they resulted always higher than zero, as typical for hard-hard interactions. The dependence of formation constants on ionic strength over the range I = 0.1 to 1 mol L−1 in NaCl is also reported for Al3+-ATP system. The sequestering ability of the nucleotides under study towards Al3+ was also evaluated by the empirical parameter pL0.5.

Complexing Properties of Pyridine-4-methylene Derivatives: Diethyl(pyridine-4-ylmethyl)phosphate, 4-Pyridylmethylphosphonic Acid and 4-Hydroxymethylpyridine with Cu(II) in Aqueous Solution

The coordinating properties in aqueous solution of two novel pyridine-4-methylene derivatives, diethyl(pyridine-4-ylmethyl)phosphate (4-PO(Oet)2CH2-py = 4-pmOpe) and 4-pyridylmethylphos phonic acid (4-PO(OH)2CH2-py = 4-pmpa), towards Cu(II) are compared with those of 4-hydroxymethyl-pyridine (4-OHCH2-py = 4-pmOH). The two new derivatives were chosen due to their expected interesting biological activity that is observed for related solid complexes of several metals with various pyridine-methylene derivatives. The studies were carried out using pH potentiometry, UV–Vis spectrophotometry and EPR spectroscopy. The results show that 4-pmOpe and 4-pmpa, similarly to 4-OHCH2-py, form mononuclear L:M = 1:1 and 2:1 complexes with Cu(II). However, in the case of 4-pmpa, complexation occurs both via oxygen atoms of the R- $$ {\text{PO}}_{3}^{2 - } $$ group (ML and ML2 species) and the N(1) nitrogen (protonated MLH and ML2H2 complexes). The possible attendance of the third ligand in the coordination sphere was indicated for 4-OHCH2-py by EPR spectroscopy.

Complexation and DFT studies of lanthanide ions by (2-pyridylmethoxy)homooxacalixarene derivatives

The binding of lanthanide cations by 2-pyridylmethoxy derivatives of p-tert-butyldihomooxacalix[4]arene (1b), in the cone conformation, and p-tert-butylhexahomotrioxacalix[3]arene (2b), in both cone and partial cone conformations, was studied. These properties were assessed by extraction studies of the metal picrates from water into dichloromethane and stability constant measurements in methanol and acetonitrile, using spectrophotometric and microcalorimetric techniques. Proton NMR titrations with La3+ and Yb3+ cations were done in order to get information on the binding sites. Computational methods (density functional theory (DFT) calculations) were also used to complement the NMR data. The p-tert-butylcalix[4]arene analogue (3b) was also studied, and the results of the four ligands were compared. Partial cone-2b is the best extractant for lanthanide ions, showing some preference for the heavy lanthanides. In complexation, all four ligands show the same trend and a high selectivity for Yb3+ (ML, log β ≥ 7). Besides the formation of ML complexes, ML2 species were also obtained. In most cases, these species were corroborated by the proton NMR studies. For partial cone-2b with Pr3+ the complexation process is enthalpically driven, whereas for 3b the formation of the ML2 species with this cation is due to a favourable entropy term. DFT studies indicate that ligand 3b forms the most stable complex with La3+, followed by partial cone-2b.

Interaction of niacin with copper ions

The interaction of copper(II) ions with nicotinic acid and nicotinamide in aqueous solutions was examined. Potentiometric and spectroscopic methods indicate formation of the MHL, ML and ML2 species in Cu(II)–nicotinic acid system whereas in Cu(II)–nicotinamide system the ML and ML(OH) species have been identified. The polarographic measurements have shown the stepwise reduction of Cu(II) complexes on the mercury electrode through an intermediate state of Cu(I). The values of stability constants for Cu(I) and Cu(II)–nicotinate and nicotinamide complexes were determined. The coordination mode of nicotinic acid and nicotinamide to copper ions is discussed.

Binding Ability of Sodium Catechol Disulfonate (Tiron) toward Hg2+, CH3Hg+, (CH3)3Sn+, and (CH3)2Sn2+ Cations

A potentiometric study on the interactions between catechol disulfonate (disodium 4,5-dihydroxybenzene-1,3-disulfonate, tiron) and Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ cations is reported in aqueous NaCl solution with ionic strength in the range (0 ≤ I ≤ 1) mol·L–1 and at a temperature of 298.15 K. In the presence of tiron, CH3Hg+ and (CH3)3Sn+ form only the two species ML and MLH; Hg2+ forms ML, MLH, and ML(OH) species; and (CH3)2Sn2+ forms ML, MLH, and ML2 species. The interactions of tiron with Hg2+ and (CH3)2Sn2+ are stronger than those with CH3Hg+ and (CH3)3Sn+. As an example, at the ionic strength I = 0.1 mol·L–1, equilibrium constants for the formation of ML species are log β = 18.87, 17.076, 10.39, and 7.55, for M = Hg2+, (CH3)2Sn2+, CH3Hg+, and (CH3)3Sn+, respectively. On the basis of the speciation models proposed, the sequestering ability of tiron toward Hg2+, CH3Hg+, (CH3)2Sn2+, and (CH3)3Sn+ was quantitatively evaluated by determining the concentration of the ligand able to complex half of the metal ion fraction. In the range 5 ≤ pH ≤ 8, the sequestering ability is high toward Hg2+ and (CH3)2Sn2+ (at the (10–11 to 10–6) mol·L–1 level), fairly low toward CH3Hg+ (at the 10–3 mol·L–1 level), and very low toward (CH3)3Sn+ (at the 10–1 mol·L–1 level).

Complexation of the N,N′,O-donor ligand N-trans-(2′-hydroxycyclohexyl)-2-aminomethylpyridine

The potentially tridentate N,N′,O-donor N-trans-(2′-hydroxycyclohexyl)-2-aminomethylpyridine (1) forms ML2 complexes with M(II) = Cu, Ni, and Zn. X-ray crystal structures of the isostructural Ni(II) and Zn(II) complexes confirm bis-tridentate coordination in significantly distorted octahedral geometries as the all-cis facial isomer. Structural comparisons with the previously reported all-trans facial Cu(II) and cis,cis,trans(Npy) facial Co(III) complexes are presented. Protonation constants for 1 and stability constants with Cu(II), Ni(II), and Zn(II) are reported, with both ML and ML2 species defined. The trend for ML (log K 1 values for Cu, Ni, and Zn of 8.3, 6.9, and 5.3, respectively) is conventional. Protonation and stability constants with Cu(II) for N,N-bis(2-pyridylmethyl)amine (2) were also defined. The log K 1 value measured for 2 of 7.4 is very similar to that found for 1 of 8.3, despite the marked difference in the third donor group; it appears that the third donor of the tridentate ligand generally binds only poorly to Jahn–Teller elongated Cu(II) in solution.

Potentiometric, Calorimetric, and1H NMR Investigation on Hg2+-Mercaptocarboxylate Interaction in Aqueous Solution

The interaction between Hg2+ and three different mercaptocarboxylic acids (thiolactic, 3-mercaptopropanoic, and thiomalic acids) was studied in aqueous solution by potentiometry, calorimetry, and for thiolactic acid 1H NMR spectroscopy. Potentiometric measurements were performed in NaNO3 in the presence of a small amount of iodide (NaI) as a competitive ligand. For all systems, the formation of the MLH, ML, and ML2 species was found, with very high values of formation constants (for the ML species, log β ranges from 32.10 to 35.10 at I = 0.1 mol·L−1). For the system containing thiolactic acid, the 1H NMR spectroscopy fully supported the chemical model proposed, providing fairly similar formation constant values to those obtained by potentiometry. In confirmation of the very high stabilities of Hg2+-mercaptocarboxylate species, the speciation diagrams show that the metal fractions of the complex species are very high over a wide pH range, suppressing almost completely the hydrolysis of the cation. All enth...

Potentiometric and spectrophotometric studies of MnII and NiII cimetidine complexes

Cimetidine is an important hydrogen histamine receptor which has the ability to chelate metal ions in blood plasma and in different tissues. This study aimed to determine the stability constants for the cimetidine ligand with MnII and NiII metallic ions, synthesizing complexes and characterizing them by infrared spectroscopy, IR, and hydrogen nuclear magnetic resonance, ¹H NMR. Cimetidine protonation constant regarding to the imidazole group was logK 7.05 and the stability constants for MnII and NiII complexes, ML2 species were logK 3.75 and 2.97, respectively, in 0.100 mol L-1 KCI. The interpretation of IR and H¹ NMR spectra for complexes MnII-cim2 and NiII-cim2 indicated that their formation occurs through the sulfur atoms in the thiol group, nitrogen atoms of imidazole ring, and nitrogen atoms of secondary amine. The nitrile group seems to be involved in the complexation of the NiII-cim2 complex.

Complex Formation of Acetic Acid with Ca(II) and Mg(II) Under Physiological Conditions

Potentiometric titrations of aqueous acetic acid alone and in the presence of Ca(II) or Mg(II) ions have been carried out under physiological conditions at the temperature 37 °C and ionic strength 0.15 mol⋅dm−3 (NaCl) at different ligand-to-metal ratios. Changes in pH were monitored with a glass electrode calibrated daily in terms of the hydrogen ion concentrations. Titration data within the pH range 2.5 to 6.6 were analyzed to determine stability constants using the SUPERQUAD program. Different combinations of complexes were considered during the calculation procedure for both systems, but evidence was found only for mononuclear ML and ML2 species. Speciation calculations based on the corresponding constants were then used to simulate the species’ distributions.

Spectroscopic and thermodynamic studies of complexation of some divalent metal ions with isatin-β-thiosemicarbazone

AbstractMetal complexes of some divalent metal ions (Co, Ni, Cu, Zn, Hg, and Pd) with isatin-β-thiosemicarbazone (ITS) as the Schiff base have been investigated potentiometrically and spectrophotometrically. The dissociation constants of the ligand and formation constants of the metal complexes, as well as the corresponding thermodynamic functions (ΔG, ΔH and ΔS) have been determined at different temperatures in ethanol—water solution. The full stability constants were also evaluated spectrophotometrically by the Job method. The experimental results indicate that Cu(II), Zn(II), Pd(II), and Hg(II) form one-to-one molecular complexes (ML) with the studied ligand, whereas Co(II) and Ni(II) form both ML and ML2 species.

Synthesis and binding properties of calix[4]arenes with [2 + 2'] mixed ligating functional groups.

A series of mixed [2 + 2'] p-tert-butylcalix[4]arene have been synthesised by selective 1,3-dialkylation of phenolic groups using various alkylating agents such as benzyl bromide, methyl iodide, ethyl bromoacetate, and 2-methoxyethyl tosylate. The extraction and complexation properties of the synthesized calixarenes towards alkali and alkaline earth metal cations have been investigated in acetonitrile by means of UV spectrophotometry and 1H NMR spectroscopy. The results show the formation of ML and/or ML2 species depending on the ligand and the cation. The enthalpies and entropies of complexation of alkali metal cations by a tetraglycol, diglycol-dibenzyl and diglycol-diester derivatives have been obtained from calorimetric measurements. The results revealed that the formation of ML species is controlled by enthalpy while the formation of ML2 from ML is entropy driven.

A Potential Model for Metal-Peptide Interactions

In order to investigate hitherto undescribed dioxygen interactions in the cobalt (II)-DGEN system (DGEN: N, N-diglycylethylenediamine, l, 8-diamino-3, 6-diaza-2, 7-octanedione) all the equilibria possible under an oxygen-free atmosphere had to be reexamined. Two new equilibria have been confirmed by potentiometric and spectrophotometric measurements: M + L + H ⇌ MLH and M i+ 2L ⇌ ML2. Unlike Co (II)-dipeptide systems, metal-promoted deprotonation of the amide group has not been observed. The ML2 species may act as an “active” complex, able to take up dioxygen in a partially reversible way (reversibility up to 50%). The reaction mechanism is similar to that known for diamines. The structure of the bridging unit in the oxygenated μ-peroxo-μ-hydroxodicobalt (III) complexes has been confirmed by volumetric and spectroscopic experiments.

Chelating equilibria of 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol and 4-(2-pyridylazo)resorcinol with La(III) ions

2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol (BrPADAP) with lanthanum(III) ions in 50% (v/v) aqueous ethanol or 30-50% (v/v) aqueous dimethylformamide (DMF) forms unprotonated chelates ML and ML2 characterized by a double absorption band with maxima at 550 and 570 nm. The ML2 species is rather unstable, hydrolyzing readily to the M(OH)L chelate, which exhibits an absorption maximum at 570 nm. The molar absorptivities of the ML, ML2, and M(OH)L species, lying in the regions of 5.3-6.5, 7.0-7.2, and 6.5-7.2 m2 mmol-1, respectively, depend on the kind and fraction of the organic component in the solvent. The conditional equilibrium constants (-log βpqr = 3.3-4.0, 12.2-12.5, and 7.8-8.2, respectively) decrease with increasing fraction of the organic solvent; in systems with high proportions of ethanol orDMF, the formation of higher species is greatly suppressed. 4-(2-Pyridylazo)resorcinol (PAR) with La(III) in acid aqueous solutions forms the MLH and ML species with absorption maxima at 490 and 506 nm, respectively ( ε = 1.2 and 2.5 m2 mmol-1, respectively) and with conditional equilibrium constants –log βpqr = -3.4 and 3.5, respectively. In alkaline solutions with excess PAR, the ML2 species with the absorption maximum at 509 mn (ε = 4.6 m2 mmol-1) and conditional equilibrium constant –log βpqr = 8.1 is formed. This chelate hydrolyses readily forming the M(OH)L species and other hydrolysis products. The ML2 species of the two reagents, forming in ammoniacal buffer at pH 9.0 – 9.5 (PAR) and 8.2 – 8.4 (BrPADAP), can be used for a sensitive (ε = 3.8 and 6.1 m2 mmol-1, respectively) post-column derivatization of lanthanoids after their separation by IEC or HPLC or for their FIA determination.

Lanthanide-daunomycin interactions. Predominance of ML2 at low metal to ligand ratios

Spectroscopic investigations have been undertaken on the effects of lanthanide ions on the spectrum of the anticancer agent, daunomycin. The visible spectral perturbations due to the metal ions resulted in hypochromic effects in the 480 nm region and in shifts towards longer wavelength. The different metal ions investigated did not give any significant difference in the, spectral character. Data analysis according to the methods of molar ratio, continuous variation and graphical approaches based on simplex matrix theory indicated the presence of predominantly a ML2 species at metal to ligand ratios, less than one. Data also suggested the presence of ML and ML3 as well.