Synthesis,structure, and complexation properties of hydroxybenzyl analogs of diethylenetriaminepentaacetic acid

As part of continuous search for new chelating agents for transition metal ions, a practical method for the preparation of hydroxybenzyl derivatives of diethylenetriaminepentaacetic acid (DTPA) has been developed in this study. N,N″-bis(2-hydroxybenzyl)diethylenetriamine-N,N′,N″-triacetic acid (HBDTTA) and four other hydroxybenzyl derivatives of DTPA have been synthesized. The structures and chelating capacity of three ligands with Fe³⁺ have been predicted using density functional theory-based calculations at the BP86/TZVP level within a previously developed and tested computational procedure. The results show that like DTPA, HBDTTA can adopt several different six- and seven-coordinate complex structures. The good stability of Fe³⁺ complexes of two HBDTTA derivatives indicates that HBDTTA-type ligands can be modified by removing some of their functional groups without greatly affecting their metal-binding efficiency.     

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.     

Self-assembled via axial coordination magnesium porphyrin-imidazole appended fullerene dyad: spectroscopic, electrochemical, computational, and photochemical studies

Spectroscopic, redox, and electron transfer reactions of a self-assembled donor-acceptor dyad formed by axial coordination of magnesium meso-tetraphenylporphyrin (MgTPP) and fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) were investigated. Spectroscopic studies revealed the formation of a 1:1 C(60)Im:MgTPP supramolecular complex, and the anticipated 1:2 complex could not be observed because of the needed large amounts of the axial coordinating ligand. The formation constant, K(1), for the 1:1 complex was found to be (1.5 +/- 0.3) x 10(4) M(-1), suggesting fairly stable complex formation. The geometric and electronic structures of the dyads were probed by ab initio B3LYP/3-21G() methods. The majority of the highest occupied frontier molecular orbital (HOMO) was found to be located on the MgTPP entity, while the lowest unoccupied molecular orbital (LUMO) was on the fullerene entity, suggesting that the charge-separated state of the supramolecular complex is C(60)Im(*-):MgTPP(*+). Redox titrations involving MgTPP and C(60)Im allowed accurate determination of the oxidation and reduction potentials of the donor and acceptor entities in the supramolecular complex. These studies revealed more difficult oxidation, by about 100 mV, for MgTPP in the pentacoordinated C(60)Im:MgTPP compared to pristine MgTPP in o-dichlorobenzene. A total of six one-electron redox processes corresponding to the oxidation and reduction of the zinc porphyrin ring and the reduction of fullerene entities was observed within the accessible potential window of the solvent. The excited state events were monitored by both steady state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene, upon coordination of C(60)Im to MgTPP, the main quenching pathway involved electron transfer from the singlet excited MgTPP to the C(60)Im moiety. The rate of forward electron transfer, k(CS), calculated from the picosecond time-resolved emission studies was found to be 1.1 x 10(10) s(-1) with a quantum yield, Phi(CS), of 0.99, indicating fast and efficient charge separation. The rate of charge recombination, k(CR), evaluated from nanosecond transient absorption studies, was found to be 8.3 x 10(7) s(-1). A comparison between k(CS) and k(CR) suggested an excellent opportunity to utilize the charge-separated state for further electron-mediating processes.     

Complexation of uranium(VI) and samarium(III) with oxydiacetic acid: temperature effect and coordination modes

The complexation of uranium(VI) and samarium(III) with oxydiacetate (ODA) in 1.05 mol kg(-1) NaClO(4) is studied at variable temperatures (25-70 degrees C). Three U(VI)/ODA complexes (UO(2)L, UO(2)L(2)(2-), and UO(2)HL(2)(-)) and three Sm(III)/ODA complexes (SmL(j)((3-2)(j)+) with j = 1, 2, 3) are identified in this temperature range. The formation constants and the molar enthalpies of complexation are determined by potentiometry and calorimetry. The complexation of uranium(VI) and samarium(III) with oxydiacetate becomes more endothermic at higher temperatures. However, the complexes become stronger due to increasingly more positive entropy of complexation at higher temperatures that exceeds the increase in the enthalpy of complexation. The values of the heat capacity of complexation (Delta C(p) degrees in J K(-1) mol(-1)) are 95 +/- 6, 297 +/- 14, and 162 +/- 19 for UO(2)L, UO(2)L(2)(2-), and UO(2)HL(2)(-), and 142 +/- 6, 198 +/- 14, and 157 +/- 19 for SmL(+), SmL(2)(-), and SmL(3)(3-), respectively. The thermodynamic parameters, in conjunction with the structural information from spectroscopy, help to identify the coordination modes in the uranium oxydiacetate complexes. The effect of temperature on the thermodynamics of the complexation is discussed in terms of the electrostatic model and the change in the solvent structure.     

Thermoanalytical and spectroscopic studies on hydrazinium lighter lanthanide complexes of 2-pyrazinecarboxylic acid

The new hydrazinium lanthanide metal complexes of 2-pyrazinecarboxylic acid (HpyzCOO) of the formulae (N₂H₅)₂[Ln(pyzCOO)₅] · 2H₂O (1), where Ln = La or Ce and (N₂H₅)₃[Ln(pyzCOO)₄(H₂O)] · 2NO₃ (2), where Ln = Pr, Nd, Sm or Dy have been synthesized and characterized by physico-chemical methods. The IR absorption bands of N–N stretching at 960 cm⁻¹ unambiguously prove the existence of N₂H₅ ⁺ ions. The bonding parameters β, b^1/2, % δ and η, have been calculated from the electronic spectroscopic (hypersensitive) bands of Pr(III) and Nd(III) complexes. All the complexes undergo endothermic followed by exothermic decomposition to leave the respective metal oxides as the end products. However, the DTA of the complexes 2 demonstrate rather sharp peak than the complexes 1, owing to overwhelming exothermicity, which may be due to the loss of both hydrazine and nitrate moieties in the same step. The X-ray powder diffraction studies reveal the existence of isomorphism among the member complexes.     

Thermodynamic studies on complexation of pervanadyl ion with aminopolycarboxylates

The solution equilibria of the vanadium(V) complexes formed by N-methyliminodiacetic acid (MIDA), nitrilotriacetic acid (NTA) and ethylenediamine-N,N′-diacetic acid (EDDA) were investigated spectrophotometrically. In acidic media all three complexes have the same formula as VO₂Y, where Y represents aminopolycarboxylate anion. In alkaline media these complexes dissociate yielding HVO_4²⁻ and Y. The formation constants of 1:1 complexes (log K_VO2Y: 10·16 ± 0·12 for MIDA; 13·8 ± 0·4 for NTA; 14·5 ± 0·3 for EDDA at 25°C and I = 1·0 M) point to a cis configuration of the two oxygen atoms in the VO₂ unit. The protonation constants of aminopolycarboxylates were also determined potentiometrically.     

Solventothermal syntheses, structures and photoluminescence of two-dimensional lanthanide coordination polymers with adipic acid

Four lanthanide coordination polymers formulated as [Ln₂(Ad)₃(H₂O)₄] · 0.25H₂O ( Ln = Tb (I), Pr (II), Ho (III), Dy (IV); H₂Ad = adipic acid), have been solventothermally synthesized from the self-assembly of the lanthanide ions (Ln³⁺) with the exible adipic dicarboxylate ligand. All of them were characterized by IR spectroscopy and single-crystal X-ray diffraction. Structural analyses revealed that these complexes had intricate two-dimensional interpenetrated metal-organic networks. In addition, the photoluminescent properties of complex I was discussed in detail, which shows strong green emission, corresponds to ⁵ D ₄ → ⁷ F ₅ transition of Tb³⁺ ions.     

Syntheses, structures, and photoluminescence of three-dimensional lanthanide coordination polymers with 2,5-pyridinedicarboxylic acid

Four new open-framework coordination polymers of lanthanide 2,5-pyridinedicarboxylates, with the formulas Pr2(pydc)₃(H₂O)₂ (1), Ln(pydc)(Hpydc) (Ln=Tb (2), Er (3), Eu (5)), and Gd(pydc)(nic)(H₂O) (4) (H₂pydc=2,5-pyridinedicarboxylic acid, Hnic=nicotinic acid), have been hydrothermally synthesized and four of them (except Eu (5)) have been structurally characterized. Complex 1 consists of two types of ligand-binding modes contributing to link the PrO₇N(H₂O) polyhedral chains to three-dimensional (3D) open-framework architecture. Complexes 2 and 3 are isostructural and feature unique 3D cage-like supramolecular frameworks remarkably different from that of 1, owing to the different ligand-bridging pattern. Complex 4, however, has the distinct 3D open-framework architecture due to the presence of unexpected nicotinate ligands, which may be derived from pydc ligands via in-situ decarboxylation under the hydrothermal condition.     

Hydrothermal syntheses,and crystal structures of new lanthanide coordination polymers with nitrilotriacetic acid

Hydrothermal reactions of Nd(ClO₄)₃·6H₂O, Gd(ClO₄)₃·6H₂O and Er₂O₃ with H₃NTA (nitrilotriacetic acid) afford three new lanthanide coordination polymers, {[Nd(NTA)(H₂O)]· 2H₂O} _n (1), {[Gd(NTA)(H₂O)]·2H₂O} _n (2) and {[Er(NTA)(H₂O)]·H₂O} _n (3), characterized by elemental analysis and IR spectroscopy. X-ray single crystal structural analyses showed that 1 and 2 are an isomorphous 2D-layered framework containing the nine-coordinated Nd(III) (or Gd(III)), and woven into a 3D suprastructure by interlayer hydrogen bonding while 3 is a 3D structure with eight-coordinate Er(III).     

VO2+ complexation by bioligands showing keto-enol tautomerism: a potentiometric, spectroscopic, and computational study

The interaction of VO(2+) ion with ligands of biological interest that are present in important metabolic pathways--2-oxopropanoic acid (pyruvic acid, pyrH), 3-hydroxy-2-oxopropanoic acid (3-hydroxypyruvic acid, hydpyrH), oxobutanedioic acid (oxalacetic acid, oxalH(2)), (S)-hydroxybutanedioic acid (l-malic acid, malH(2)), and 2,3-dihydroxy-(E)-butanedioic acid (dihydroxyfumaric acid, dhfH(2))--was described. Their complexing capability was compared with that of similar ligands: 3-hydroxy-2-butanone (hydbut) and 3,4-dihydroxy-3-cyclobutene-1,2-dione (squaric acid, squarH(2)). All of these ligands (except l-malic acid) exhibit keto-enol tautomerism, and the presence of a metal ion can influence such an equilibrium. The different systems were studied with electron paramagnetic resonance (EPR) and UV-vis spectroscopies and with pH potentiometry. Density functional theory (DFT) methods provide valuable information on the relative energy of the enol and keto forms of the ligands both in the gas phase and in aqueous solution, on the geometry of the complexes, and on EPR and electronic absorption parameters. The results show that most of the ligands behave like α-hydroxycarboxylates, forming mono- and bis-chelated species with (COO(-), O(-)) coordination, demonstrating that the metal ion is able to stabilize the enolate form of some ligands. With dihydroxyfumaric acid, the formation of a non-oxidovanadium(IV) complex, because of rearrangement of dihydroxyfumaric to dihydroxymaleic acid (dhmH(2)), can be observed. With 3-hydroxy-2-butanone and 3,4-dihydroxy-3-cyclobutene-1,2-dione, complexation of VO(2+) does not take place and the reason for this behavior is explained by chemical considerations and computational calculations.     

Three lanthanide coordination polymers with nitrilotriacetic acid: hydrothermal syntheses,crystal structures and fluorescent property

Hydrothermal reactions of Sm₂O₃, Gd(ClO₄)₃?·?6H₂O and Tb(ClO₄)₃?·?6H₂O with nitrilotriacetic acid, give rise to three lanthanide coordination polymers, {[Sm(NTA)(H₂O)₂]?·?H₂O} _n (1), {[Gd(NTA)(H₂O)]?·?H₂O} _n (2) and {[Tb(NTA)(H₂O)]?·?H₂O} _n (3). Their solid-state structures have been characterized by elemental analysis, and IR spectroscopy. X-ray single-crystal diffraction analyses indicated that 2 and 3 are isomorphous three-dimensional coordination polymers with eight-coordinate Gd(III) (or Tb(III)), while 1 forms a two-dimensional coordination polymer containing nine-coordinate Sm(III). The photophysical properties of 3 have been studied with excitation and emission spectra, which exhibit strong green emission.     

A novel one‐dimensional coordination polymer with Cd2+ and diethylenetriaminepentaacetic acid

catena‐Poly­[[[bis­[di­aqua(4,4′‐bi­pyridine)­cadmium(II)]‐bis­[μ‐(N′′‐carboxy­methyl­diethyl­enetri­amine‐N,N,N′,N′′‐tetra­ace­ta­to)­cadmium(II)]]‐μ‐4,4′‐bi­pyridine] tetradecahydrate], [Cd₄­(C₁₄H₁₉N₃O₁₀)₂(C₁₀H₈N₂)₃(H₂O)₄]·14H₂O or [Cd₄(HD­TPA)₂(BPY)₃(H₂O)₄]·14H₂O, where BPY is 4,4′‐bi­pyridine and HDTPA^4? is N′′‐carboxy­methyl­diethyl­enetri­amine‐N,N,N′,N′′‐tetra­acetate, consists of a one‐dimensional coordination polymer formed from a secondary building unit which comprises four Cd centres. The chain structure of the title compound was obtained by the use of a multidentate organic ligand, N,N,N′,N′′,N′′‐diethyl­enetri­amine­penta­acetic acid (H₅DTPA), which forms multiple chelate rings with the Cd metal centres. An extended network is formed via hydrogen bonds.     

Thermal and spectroscopic investigations of new lanthanide complexes with 1,2,4-benzenetricarboxylic acid

The new 1,2,4-benzenetricarboxylates of lanthanide(III) of the formula Ln(btc)·nH₂O, where btc is 1,2,4-benzenetricarboxylate; Ln is La-Lu, and n=2 for Ce; n=3 for La, Yb, Lu; and n=4 for Pr-Tm were prepared and characterized by elemental analysis, infrared spectra and X-ray diffraction patterns. Polycrystalline complexes are isotructural in the two groups: La-Tm and Yb, Lu. IR spectra of the complexes show that all carboxylate groups from 1,2,4-benzentricarboxylate ligands are engaged in coordination of lanthanide atoms. The thermal analysis of the investigated complexes in air atmosphere was carried out by means of simultaneous TG-DTA technique. The complexes are stable up to about 30°C but further heating leads to stepwise dehydration. Next, anhydrous complexes decompose to corresponding oxides. The combined TG-FTIR technique was employed to study of decomposition pathway of the investigated complexes.     

Non-aqueous solvation of n-octanol and ethanol: spectroscopic and computational studies

Raman spectroscopy was used to examine the interactions of the free O-H bonds in n-octanol and ethanol with the organic solvents carbon tetrachloride (CCl(4)), cyclohexane, and benzene. These spectra reveal that the solvents CCl(4) and cyclohexane have a small effect on the free O-H peak of alcohols, whereas benzene as a solvent significantly red-shifts the free O-H band. Calculated spectra were generated via MP2/6-31G* calculations and the B3LYP/6-31+G**//MP2/6-31G*-derived Boltzmann populations of each ethanol complex and are consistent with the experimental results. Additional spectra were calculated using Boltzmann populations derived from single-point energies at the polarizable continuum model (PCM) level with the B3LYP/6-31+G** level of theory to take overall solvent effects into account, and these simulated spectra are also largely consistent with the experimental results. Analysis of the computational results reveals a lengthening of the O-H bond from the O-H interaction with the delocalized electronic structure of benzene as well as a bimodal distribution of the free O-H peak of the alcohol/benzene mixtures due to two distinctly different types of alcohol/benzene complexes.     

Effect of temperature on the complexation of NpO2+ with benzoic acid: Spectrophotometric and calorimetric studies

The equilibrium constants of the 1:1 NpO₂⁺/benzoate complex were determined by spectrophotometric titrations at variable temperatures (T = 283 to 343 K) and the ionic strength of 1.05 mol · kg⁻¹. The enthalpy of complexation at T = 298 K was determined by microcalorimetric titrations. Similar to other monocarboxylates, benzoate forms a weak complex with NpO₂⁺ and the complexation is strengthened as the temperature is increased. The complexation is endothermic and is entropy-driven. The enhancement of the complexation at elevated temperatures is primarily attributed to the increasingly larger entropy gain when the water molecules are released from the highly-ordered solvation spheres of NpO₂⁺ and benzoate to the bulk solvent where the degree of disorder is higher at higher temperatures. The spectroscopic features of the Np(V)/benzoate system, including the effect of temperature on the absorption bands, are discussed in terms of ligand field splitting and a thermal expansion mechanism.     

Thermodynamic and structural study of complexation of phenylboronic acid with salicylhydroxamic acid and related ligands

Stability constants of boronate complexes with a highly efficient bioconjugation ligand salicylhydroxamic acid, its derivatives and some structurally related compounds were determined by potentiometric and spectroscopic titrations at variable pH allowing one to obtain detailed stability – pH profiles and to identify the optimum pH for complexation with each ligand. The N,O‐binding of salicylhydroxamic acid via condensation of boronic acid with phenolic OH and hydroxamic NH groups was established by crystal structure determination of isolated complexes with phenylboronic and 4‐nitrophenylboronic acids. Although this type of binding is impossible for N‐methylated salicylhydroxamic acid it still forms stable boronate complexes supposedly involving unusual 7‐membered –O‐B‐O‐ cycle supported by ¹H NMR studies. Hydroxamic acids lacking ortho‐OH group and salicyloyl hydrazide form less stable boronate complexes, which nevertheless possess stabilities similar to those of catechole complexes and may be useful for conjugation applications. In contrast to other ligands, which form tetrahedral anionic complexes, salicylamidoxime forms tetrahedral, but neutral boronate complex with high stability in weakly acid solutions. The highest affinity in neutral and acid solutions surpassing that of salicylhydroxamic acid is observed with 2,6‐dihydroxybenzhydroxamic acid (K_obs = 5.2 × 10⁴ at pH 7.4). Fairly stable mono‐ and bisboronate complexes are formed with 2,5‐dihydroxy‐1,4‐benzdihydroxamic acid, which also possesses intense fluorescence and may serve as a boronic acid sensor with detection limit 4 μM. Results presented in this study provide quantitative basis for rational applications of hydroxamic acid derivatives in bioconjugation and sensing.     

Thermodynamic study of the complexation of trivalent actinide and lanthanide cations by ADPTZ, a tridentate N-donor ligand

To better understand the bonding in complexes of f-elements by polydentate N-donor ligands, the complexation of americium(III) and lanthanide(III) cations by 2-amino-4,6-di-(pyridin-2-yl)-1,3,5-triazine (ADPTZ) was studied using a thermodynamic approach. The stability constants of the 1:1 complexes in a methanol/water mixture (75/25 vol %) were determined by UV-visible spectrophotometry for every lanthanide(III) ion (except promethium), and yttrium(III) and americium(III) cations. The thermodynamic parameters (DeltaH degrees , DeltaS degrees) of complexation were determined from the temperature dependence of the stability constants and by microcalorimetry. The trends of the variations of DeltaG degrees , DeltaH degrees , and DeltaS degrees across the lanthanide series are compared with published results for other tridentate ligands and confirm strongly ionic bonding in the lanthanide-ADPTZ complexes. Comparison of the thermodynamic properties between the Am- and Ln-ADPTZ complexes highlights an increase in stability of the complexes by a factor of 20 in favor of the americium cation. This difference arises from a more exothermic reaction enthalpy in the case of Am, which is correlated with a greater degree of covalency in the americium-nitrogen bonds. Quantum chemistry calculations performed on a series of trivalent actinide and lanthanide-ADPTZ complexes support the experimental results, showing a slightly greater covalence in the actinide-ligand bonds that originates from a charge transfer from the ligand sigma orbitals to the 5f and 6d orbitals of the actinide ion.     

First dicyanamide-bridged spin-crossover coordination polymer: synthesis, structural, magnetic, and spectroscopic studies

We report here on the synthesis and characterisation of a first iron(II) spin-crossover coordination polymer with the dca spacer ligand, having the formula [Fe(aqin)2(dca)]ClO4.MeOH (aqin=8-aminoquinoline, dca=dicyanamide), which displays a two-step complete spin transition. Variable-temperature magnetic susceptibility measurements and M?ssbauer spectroscopy have revealed that the two relatively gradual steps are centred at 215 and 186 K and are separated by an inflection point at about 201 K, at which 50 % of the complex molecules undergo a spin transition. The two steps are related to the existence of two crystallographically inequivalent metal sites, as confirmed by the structural and M?ssbauer studies. The crystal structure was resolved at 293 K (HS form) and 130 K (LS form). Both spin-state structures belong to the triclinic P1 space group (Z=2). The complex assumes a linear chain structure, in which the active iron(II) sites are linked to each other by anionic dicyanamide ligands acting as chemical bridges. The Fe-Fe distances through the dca ligand are 8.119(1) and 7.835(1) A in the high-spin and low-spin structures, respectively. The polymeric chains extend along a (1, 0, -1) axis and are packed in sheets, between which the perchlorate anions and methanol molecules are inserted. The complex molecules are linked together by pi-stacking interactions and H-bonding between the H-donor aqin ligands and the perchlorate ions. These structural features provide a basis for cooperative interactions in the crystal lattice. Analysis of the two-step spin-crossover character in this compound suggests that covalent interactions through the spacer ligand do not provide the main mechanism of cooperativity.     

Infrared spectroscopic studies of the effect of elevated temperature on the association of pyroglutamic acid with clay and other minerals

Fourier transform i.r. measurements of L-pyroglutamic acid dispersed in a matrix of a clay, silica or alumina have been obtained at various temperatures between 25 and 220 degrees C. The i.r. spectrum of L-pyroglutamic acid varies in a manner dependent upon the matrix material and shows considerable change as the temperature of the mixtures is increased. The differences in the spectrum at elevated temperatures are explained in terms of a chemical reaction between hydroxyl groups in the matrix and the carboxylic acid. The i.r. spectra of trimethylsilyl derivatives of L-pyroglutamic acid and aluminum pyroglutamate were also measured to assist the understanding of spectra and interpretation of the spectral changes dependent upon increasing temperature.