Preparation and properties of mono, homo- and heterobinuclear complexes with a new heptadentate Schiff base ligand

A new heptadentate Schiff base, containing an inner N₃O₂ and an outer O₂O₂ site, has been obtained by the reaction of 3-formylsalicylic acid and diethylenetriamine. By reaction of this ligand with copper(II), nickel(II) or uranyl(VI) salts, mononuclear and dinuclear complexes have been synthesized. The mononuclear complexes can act as ligands towards a second metal ion giving rise to homodinuclear or heterodinuclear complexes. The enlargement of the inner coordination chamber allows the synthesis of dinuclear uranyl(VI) species, impossible to obtain with the inner N₂O₂ site of the ligands previously employed. The equatorial pentacoordination of the UO₂²⁺ group in the outer O₂O₂ chamber is reached with the coordination of a solvent molecule to the central metal ion. The electrochemical behaviour of some complexes prepared is also reported.     

Complexation thermodynamics and the formation of the binary and the ternary complexes of tetravalent plutonium with carboxylate and aminocarboxylate ligands in aqueous solution of high ionic strength

The stability constants of the binary and the ternary complexes of Pu⁴⁺ have been measured for certain carboxylate and aminocarboxylate ligands in aqueous solution of I = 5.0 M (1 M perchloric acid + 4 M ionic strength perchlorate media) and temperatures of 0-45 °C by the solvent extraction technique. The stability constants of the binary and the ternary complexes increased with increased temperature. The complexation enthalpy and entropy of the binary Pu-Ox and the ternary Pu-EDTA-Ox and DGA complexes indicated the stability of these complexes is due to the highly favorable entropy contribution while complexation enthalpies either oppose complexation or are weakly favorable.     

Cd(II) and Cu(II) complexes of polydentate Schiff base ligands: synthesis, characterization, properties and biological activity

We report the synthesis of the Schiff base ligands, 4-[(4-bromo-phenylimino)-methyl]-benzene-1,2,3-triol (A₁), 4-[(3,5-di-tert-butyl-4-hydroxy-phenylimino)-methyl]-benzene-1,2,3-triol (A₂), 3-(p-tolylimino-methyl)-benzene-1,2-diol (A₃), 3-[(4-bromo-phenylimino)-methyl]-benzene-1,2-diol (A₄), and 4-[(3,5-di-tert-butyl-4-hydroxy-phenylimino)-methyl]-benzene-1,3-diol (A₅), and their Cd(II) and Cu(II) metal complexes, stability constants and potentiometric studies. The structure of the ligands and their complexes was investigated using elemental analysis, FT-IR, UV-Vis, ¹H and ¹³C NMR, mass spectra, magnetic susceptibility and conductance measurements. In the complexes, all the ligands behave as bidentate ligands, the oxygen in the ortho position and azomethine nitrogen atoms of the ligands coordinate to the metal ions. The keto-enol tautomeric forms of the Schiff base ligands A₁-A₅ have been investigated in polar and non-polar organic solvents. Antimicrobial activity of the ligands and metal complexes were tested using the disc diffusion method and the strains Bacillus megaterium and Candida tropicalis.Protonation constants of the triol and diol Schiff bases and stability constants of their Cu²⁺ and Cd²⁺ complexes were determined by potentiometric titration method in 50% DMSO-water media at 25.00 ± 0.02 °C under nitrogen atmosphere and ionic strength of 0.1 M sodium perchlorate. It has been observed that all the Schiff base ligands titrated here have two protonation constants. The variation of protonation constant of these compounds was interpreted on the basis of structural effects associated with the substituents. The divalent metal ions of Cu²⁺ and Cd²⁺ form stable 1:2 complexes with Schiff bases.The Schiff base complexes of cadmium inhibit the intense chemiluminescence reaction in dimethylsulfoxide (DMSO) solution between luminol and dioxygen in the presence of a strong base. This effect is significantly correlated with the stability constants K_CdL of the complexes and the protonation constants K_OH of the ligands; it also has a nonsignificant association with antibacterial activity.     

Copper(II) complexes of a hydroxamic acid from maize

The thermodynamics of formation for DIMBOA-Cu(II) complexes (where DIMBOA = 2,4-dihydroxy-7-methoxy-2H-1,4-benzoxazin-3-4H-one, a hydroxamic acid from maize) has been investigated in aqueous solutions by a potentiometric method. DIMBOA forms 1:1 and 1:2 chelates with Cu(II) at ionic strength 0.05 M (NaCl04). The stability constants measured were about 10⁵ and 10⁴ for the 1:1 and 1:2 complexes respectively, determined at 10, 20 and 30°. The contribution of ΔH and ΔS to the stability of complexes is examined and the pK values are compared with other ligands found in maize. Although DIMBOA has similar or higher constants to form copper complexes than other plant ligands, its possible role as a transport agent in maize remains to be established.     

Metal complexes of N-hydroxy-imino-di-α-propionic acid and related ligands

N-hydroxy-imino-di-α-propionic acid, the ligand present in the natural oxovanadium(IV) complex ‘amavadin’ which occurs in the toadstool Amanita muscaria, has been synthesised, as well as two related ligands—N-hydroxy-iminodiacetic acid and imino-di-α-propionic acid—useful for comparison purposes. The formation of complexes of these ligands with VO²⁺, Ni²⁺ has been studied and their stability constants have been determined.The two N-hydroxy-substituted ligands, of low basicity, form ML₂ complexes with VO²⁺, unlike the more basic derivatives of iminodiacetic acid. Since substitution of ligands bonded to the apical site trans to the oxo ligand is very fast and the formation of ML₂ complexes of VO²⁺ exposes that apical site to the reaction media, this may be the reason why oxovanadium(IV) and the unusual derivative of iminodiacetic acid present in ‘amavadin’ were selected for the biological role that this complex plays in the toadstool.     

Uranyl nitrate complexes of some Schiff bases of 4-aminoantipyrine and certain carbonyl compounds

A series of nine complexes of uranyl nitrate with some Schiff bases derived from 4-aminoantipyrine and certain carbonyl compounds, such as benzaldehyde, 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, 4- methylbenzaldehyde, 4-N,N-dimethylaminobenzaldehyde, 2-hydroxybenzaldehyde, 2-hydroxy-1-naphthaldehyde, acetylacetone and benzoylacetone have been synthesized. These complexes have been characterized by elemental analysis, molecular weight determination and IR spectral, conductance and magnetic studies. From these studies they can be formulated as [UO₂L₂(NO₃)₂], in which the first five ligands (in the order given above) and nitrate ions are coordinated bidentately, while the last four ligands (which have either a phenolic hydroxyl group or a side-chain carbonyl group as an additional site) act as terdentate ligands, and nitrate ions are coordinated monodentately. Hence the proposed general formula for the complexes suggests that the uranyl ion has a coordination number of eight in addition to the two oxygen atoms which have already been bonded to the U(VI) species.     

Preparation,properties, crystal and molecular structure of uranyl(VI) complexes with a new cyclic schiff base compartmental ligand

Some uranyl(VI) complexes with new acyclic and cyclic Schiff base compartmental ligands have been prepared and characterized. The ligands have been obtained by reaction of 4-chloro-2,6-diformylphenol and polyamines of the type NH₂(CH₂)₂X (CH₂)₂NH₂ (X= NH, S). The structure of the uranyl(VI) complex with the ligand 1,7,15,21-tetra- aza-4,18-dithia-11,25-dichloro 8,22-bis-metadiphenyl cyclophane-gb-7,14,21,28 has been determined by X-ray crystallography. The compound crystallizes in the orthorhombic space group Pbca with eight formula units in a cell of dimensions a = 26.654(3), b = 22.871(3), c = 8.875(5) Å. The structure was solved by standard methods and refined by full- matrix least squares to the conventional R index of 4.6% for 2678 independent observed reflexions. Five donor atoms (including sulphur) of the ligand are equatorially bonded to the uranyl group to form discrete monomeric molecules with the seven-coordinated metal in the usual distorted pentagonal bipyramidal coordination geometry. Selected bond distances are: UO (equatorial), 2.22(1) and 2.25(1) Å; UN, 2.60(1) and 2.59(1) Å; US, 3.018(4) Å.     

Complexes of Tungsten(VI) with Mucic Acid: a Spectrophotometric and Polarimetric Study in Aqueous Solution

Spectrophotometric work on mucic acid-W(VI) system shows the formation of three different oxoanion complexes in aqueous solution; their stability is dependent upon pH. One of the complexes is monomeric tungstodimucate and the other two are 2/2 species. An anomalous cryoscopic behaviour, similar to that of W(VI) tartaric system, has been observed for the dimeric complex formed at higher pH. The stoichiometries and conditional dissociation constants have been polarimetrically determined by means of competitive reactions between the mucic and tartaric ligands. ¹H and ¹³C NMR spectra have been interpreted for the similar complex species of both mucic and tartaric acids.     

Dioxouranium(VI) complexes with 2,6-acetylpyridinebenzoylhydrazones and -semicarbazones

Benzoylhydrazones and semicarbazones derived from 2,6-diacetylpyridine react with common dioxouranium(VI) compounds such as uranyl nitrate or [NBu₄]₂[UO₂Cl₄] to form air-stable complexes. 2,6-Diacetylpyridinebis(benzoylhydrazone) (H₂L¹), 2,6-diacetylpyridinebis(N4-phenylsemicarbazone) (H₂L²) and the asymmetric proligand 2,6-diacetylpyridine(benzoylhydrazone)(N4-phenylsemicarbazone) (H₂L³) give yellow products of the composition [UO₂(L)]. The neutral compounds contain doubly deprotonated ligands and possess uranium atoms with distorted pentagonal-bipyramidal coordination spheres. The equatorial coordination spheres of the metal atoms can be extended by the addition of a monodentate ligand such as pyridine or DMSO. The uranium atoms in the resulting complexes have hexagonal-bipyramidal coordination environments with the oxo ligands in axial positions.X-ray diffraction studies on [UO₂(L¹)(DMSO)], [UO₂(L²)], [UO₂(L²)(DMSO)] and [UO₂(L³)] show relatively short U-O bonds to the benzoylic oxygen atoms between 2.273(6) and 2.368(5) Å. This suggests a preference of these donor sites of the ligands over their imino and amine functionalities (U-N bond lengths: 2.502(7)-2.671(7) Å). The addition of a sixth ligand to the equatorial coordination sphere results in a lengthening of the metal-pyridine bonds.     

Cobalt(II) and cadmium(II) chelates with nitrogen donors and O2 bonding to Co(II) derivatives

The formation of Cd(II) and Co(II) complexes with N-methylethylenediamine (men) has been studied at 298 K in dimethylsulfoxide (dmso) in an ionic medium set to 0.1 mol dm⁻³ with Et₄NClO₄ in anaerobic conditions by means of potentiometric, UV-Vis, calorimetric and FT-IR technique. Mononuclear ML_j (M=Cd, Co; j=1-3) complexes are formed in exothermic reactions, whereas the entropy changes oppose the complexes formation. The results are discussed in terms of different basicities and steric requirements and the whole of the thermodynamic data reported till now for the two ions with a number of diamines are summarized to visualize the selectivity of the ligands. The dioxygen uptake of Co(men)₂ species has also been studied by means of UV-Vis and EPR techniques. The kinetic parameters and stability constants obtained for the formation of the superoxo and μ-peroxo species are discussed in terms of solvent effect and steric hindrance due to methyl group.Cyclic voltammetry was used to confirm the stability constant for the Co(dmen)₂ (dmen=N,N^′-dimethylethylenediamine) superoxo adduct formation but was not successful to investigate this Co(men)₂-O₂ system.     

Role of aliphatic and phenolic hydroxyl groups in uranyl complexation by humic substances

Relativistic density functional calculations of uranyl complexes with alcohols were carried out to study how phenolic and aliphatic hydroxyl groups of humic substances may contribute to uranyl complexation by humic substances. According to recent experimental work, blocking of phenolic OH groups decreases the loading capacity, but has no effect on the key geometric parameters of uranyl humate complexes. This can be understood on the basis of our calculations which showed uranium-oxygen distances to be very similar for complexes with rather different types of O-donor ligands, with average U-O_eq ∼ 237 pm for fivefold coordinated uranyl (VI) complexes, both for O⁻ and OH functional groups. Uranyl complexation by alcohol moieties seems unlikely at environmental conditions as a high pH is required for the deprotonation of these groups; we confirm an alternative complexation mechanism that overcomes the ligand deprotonation problem. Similarities in structures and energetic suggest that complexes of both aliphatic and phenolic alcoholates may well contribute in comparable fashion to the complexation of uranyl by humic acids.     

Complex formation and stability of westiellamide derivatives with copper(II)

The Cu^II coordination chemistry of three synthetic analogues of westiellamide (H₃L^wa) with an [18]azacrown-6 macrocyclic structure and imidazole (H₃L¹), oxazole (H₃L²), or thiazole (H₃L³) heterocyclic donors in addition to the peptide groups, is reported. The N_heterocycle–N_peptide–N_heterocycle binding sites are highly preorganized for the coordination to Cu^II ions. The stability constants of mono- and dinuclear Cu^II complexes of H₃L¹, H₃L², and H₃L³, obtained by isothermal titration microcalorimetry, are reported. EPR and NMR spectroscopy as well as electrospray ionization mass spectrometry (ESI-MS) were used to characterize the complexes formed in solution. The stabilities of the mononuclear and dinuclear Cu^II complexes of the three ligands are in the range of 10⁵ M⁻¹, but there are subtle differences; specifically the oxazole-derived ligand has, in contrast to the other two macrocycles, a negative formation entropy for coordination to the first Cu^II ion and a higher stability for complexation to a second Cu^II center in comparison with the first Cu^II center (cooperativity). Differences between the three ligands are also apparent in terms of the formation mechanism. With the oxazole-based ligand H₃L², NMR spectroscopy, EPR spectroscopy, and ESI-MS indicate the formation of a ligand–Cu^II 2:1 intermediate, and this may explain the differences in the formation entropy as well as the cooperativity.     

Quantitative 27Al NMR spectroscopic studies of Al (III) complexes with organic acid ligands and their comparison with GEOCHEM predicted values

The toxic inorganic monomeric forms of aluminium (Al) that limit plant growth have been shown to be effectively detoxified by complexation with organic acid ligands released by breakdown of added organic materials. The binding capacity of these acids is dependent on the degree of dissociation of their carboxyl groups and their ability to form bonds with Al. ²⁷Al NMR spectroscopy provides a non-invasive technique to study the bonding of Al with potential ligands without disturbing the equilibrium of the system. In single ligand systems containing oxalic acid, three ²⁷Al resonance peaks were observed at 6.4, 11.4 and 16.0 ppm downfield from the Al³⁺ reference peak at 0 ppm. These were assigned to Alox, Alox₂ and Alox₃ complexes respectively and were observable at pH values down to 3.5. In the presence of the citrate ligand, two ²⁷Al resonance peaks at 6.1 and 11.3 ppm, assigned respectively to the Alcit and Alcit₂ complexes, were observed at pH 3.4. At pH 4.3 and an Al:citrate molar ratio of 1:2, the 6.1 ppm peak was not visible, and the second peak further downfield was split into two unresolved peaks at 10.8 and 12.4 ppm indicating the presence of two forms of the Alcit₂ complex. Distribution of Al between the various species, based on integration of the resonance peaks and equilibrium calculations carried out using GEOCHEM, is discussed in light of the stability constants present in the database of GEOCHEM version (v.) 1.23 and GEOCHEM-PC v. 2.0. Large discrepancies between the computed values and the NMR measured values indicate the need to incorporate more recent literature values in the database for realistic equilibrium calculations in systems containing organic acid ligands. The potential of using quantitative ²⁷Al NMR measurements to calculate stability constants is discussed.     

Synthesis, spectroscopic characterization and biological activity on newly synthesized copper(II) and nickel(II) complexes incorporating bidentate oxygen-nitrogen hydrazone ligands

We report the synthesis of the hydrazone ligands, 1-(phenyl-hydrazono)-propan-2-one (PHP), 1-(p-tolyl-hydrazono)-propan-2-one (THP), 1-[(4-chloro-hydrazono)]-propan-2-one (CHP), and their Ni(II) and Cu(II) metal complexes. The structure of the ligands and their complexes were investigated using elemental analysis, magnetic susceptibility, molar conductance and spectral (IR, UV, and EPR) measurements. IR spectra indicate that the free ligands exist in the hydrazo-ketone rather than azo-enol form in the solid state. Also, the hydrazo-NH exists as hydrogen bonded to the keto-oxygen either as intra or as intermolecular hydrogen bonding. In all the studied complexes, all ligands behave as a neutral bidentate ligands with coordination involving the hydrazone-nitrogen and the keto-oxygen atoms. The magnetic and spectral data indicate a square planar geometry for Cu²⁺ complexes and an octahedral geometry for Ni²⁺ complexes. The ligands and their metal chelates have been screened for their antimicrobial activities using the disc diffusion method against the selected bacteria and fungi. They were found to be more active against Gram-positive than Gram-negative bacteria. It may be concluded that the antimicrobial activity of the compounds is related to cell wall structure of bacteria.Protonation constant of (PHP) ligand and stability constants of its Cu²⁺ and Ni²⁺ complexes were determined by potentiometric titration method in aqueous solution at ionic strength of 0.1 M sodium nitrate. It has been observed that the hydrazone ligand (PHP) titrated here has one protonation constant. The divalent metal ions Cu²⁺ and Ni²⁺ form with (PHP) 1:1 and 1:2 complexes. The insolubility of (THP) and (CHP) ligands in aqueous medium does not permit the determination of their protonation constants and formation constants of the corresponding complexes in aqueous solution.     

Complexation thermodynamics and the structure of the binary and the ternary complexes of Am, Cm and Eu with CDTA and CDTA + IDA

The stability constants and the thermodynamic parameters of the formation of the binary complexes of trivalent Am³⁺, Cm³⁺ and Eu³⁺ with CDTA and of their ternary complexes with CDTA + IDA were determined by solvent extraction measurements in aqueous solutions of I = 6.60 m (NaClO₄) at temperatures of 0-60 °C. The endothermic enthalpy and the positive entropy values reflect the significant effects of cation dehydration and of the rigidity of the ligand structure in the formation of these complexes. TRLFS and NMR (¹H and ¹³C) data provided information on the structure of the ternary complexes in solution. The size and rigidity of CDTA affect the binding mode of IDA in the complexation of M(CDTA)(IDA)(H₂O)³⁻ and M(CDTA)(IDA)³⁻ in which IDA has a bidentate coordination mode in the former and a tridentate coordination mode in the latter.     

Speciation studies of ternary complexes of some essential divalent metal ions with L-histidine and L-glutamic acid in DMSO-water mixtures

Abstract

Chemical speciation of ternary complexes of Ca(II), Mg(II) and Zn(II) ions with L-histidine as the primary ligand (L) and L-glutamic acid as the secondary ligand (X) has been studied pH metrically in the concentration range of 0.0-60.0% v/v DMSO-water mixtures maintaining an ionic strength of 0.16 mol L^-1 using sodium chloride at 303.0 K. Titrations were carried out in different relative concentrations (M:L:X = 1.0:2.5:2.5, 1.0:2.5:5.0, 1.0:5.0:2.5) of metal (M) to L-histidine to L-glutamic acid with sodium hydroxide. Stability constants of ternary complexes were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The predominant species detected for Ca(II), Mg(II) and Zn(II) are ML₂XH₂, MLXH₂ and MLX₂. Extra stability of ternary complexes compared to their binary complexes was explained to be due to electrostatic interactions of the side chains of ligands, charge neutralisation, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of DMSO and the plausible equilibria for the formation of species are discussed.     

Thermodynamics of the complex formation in pyridine between gold(i) and ligands coordinating via N,P, As or Sb

The thermodynamics of complex formation between gold(I) and the ligands tricyclohexylphosphine, triphenylamine, -phosphine, -arsine, and -stibine has been determined in pyridine solution by potentiometric and calorimetric measurements. As expected, the very soft gold(I) displays a more marked stability (b)-sequence N ⪡ P > As > Sb than its lighter, and less soft, congener silver(I). Like all complexes of these ligands so far studied, the present ones are strongly enthalpy stabilized while the entropy changes are generally unfavourable. The stepwise entropy changes show quite peculiar differences between the various ligands, however. The thermodynamics of these complexes is in striking contrast to that of the gold(I) halido complexes in pyridine which are strongly entropy stabilized, while the enthalpy changes are small.     

Kinetics and mechanism of oxidation of d-glucopyranose 6-phosphate and d-ribofuranose 5-phosphate by vanadium(V) in perchloric acid media

The second-order rate constants for the oxidations of d-glucopyranose 6-phosphate and d-ribofuranose 5-phosphate by vanadium(V) in perchloric acid media have been measured spectrophotometrically in the visible region. The order with respect to [H⁺] is less than unity for each reaction. The enthalpy and entropy of activations of the reactions are higher than those for the corresponding two-electron oxidations by chromium(VI). The oxidation involves free-radical intermediates.     

Thermodynamic Parameters of Opioid Binding in the Presence and Absence of G-Protein Coupling

Abstract

We have investigated the thermodynamic parameters of various opioid ligands interacting with their receptors in rat brain membranes. Affinity constants (K_a), enthalpy and entropy values were obtained from homologous displacement experiments performed at 0, 24 and 33°C. It was found that all the opioid agonists tested ([³H]dihydromorphine (DHM) μ alkaloid; [³H]DAMGO μ peptide; [³H]deltorphin-B δ peptide) display endothermic binding accompanied with a large entropy increase, regardless of their chemical structure (alkaloid or peptide), or of their μ or δ receptor selectivity. In contrast, binding of the antagonist naloxone is exothermic, mainly enthalpy driven. Na⁺ or Mg²⁺ results only in quantitative changes of the thermodynamic parameters. In the presence of the GTP-analog Gpp(NH)p; or Gpp(NH)p + Na⁺; or Gpp(NH)p + Na⁺ + Mg²⁺ the affinity of DHM binding dramatically decreases which might reflect functional uncoupling of the receptor-ligand complex and G-proteins. This altered molecular interactions are also indicated by curvilinear van't Hoff plot and entropy increase. It is concluded that the thermodynamic analysis provides means of determining the underlying driving forces of ligand binding and helps to delineate its mechanism.     

The thermodynamics of complex formation of cyclic tetra-aza-tetracetic acids

Enthalpy changes for the complexation of alkaline-earth and transition metals with three cyclic tetra-aza-tetracetic acids (cDOTA, cTRITA and cTETA) were obtained by continuous titration calorimetry. From these values and free energy data, the entropy changes for the same reactions were derived. The results show that these complexes are stabilised by both favourable enthalpy and entropy changes, except those of Mg²⁺ and those of Sr²⁺ and Ba²⁺ with cTETA. Generally, the entropy changes for the reactions of the alkaline-earth metals are higher than for the reactions of the non cyclic polyaminocarboxylic acids, but for the reactions of the transition metals the entropy changes are comparable for the cyclic and non cyclic ligands. These results are discussed in terms of a model of ‘cage’ coordination of the metals.The enthalpy changes decrease with the increase in size of the tetra-aza ring (except in the case of Cu²⁺) but no specific cavity size effect is noticeable. Consideration of the temperature-dependent and temperature-independent contributions to ΔH supports the idea that the number of coordinated nitrogen atoms and carboxylate groups vary along the series.