Andrea Melchior

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.

Energetics and structure of uranium(VI)-acetate complexes in dimethyl sulfoxide.

The thermodynamics of the complexation between uranium(VI) and acetate in dimethyl sulfoxide (DMSO) was studied at 298 K in an ionic medium of 0.1 mol dm(-3) tetrabutyl ammonium perchlorate. The results show that the uranyl ion forms three strong successive mononuclear complexes with acetate. The complexes, both enthalpically and entropically stabilized, are significantly more stable in DMSO than in water. This feature can be ascribed to the weak solvation of acetate in DMSO. The thermodynamic parameters for the formation of the uranium(VI) complexes with acetate in DMSO are compared with those with ethylenediamine in the same solvent. The difference between the two ligand systems reveals that, for the complexation reactions involving charge neutralization, the reorganization of the solvent gives a very important contribution to the overall complexation energetics. The coordination mode of acetate in the uranyl complexes and the changes of the solvation sphere of UO(2)(2+) upon complexation were investigated by FT-IR spectroscopy in DMSO and in acetonitrile/DMSO mixtures. In addition, DFT calculations were performed to provide an accurate description of the complexation at the molecular level. The experimental and calculated results suggest that acetate is solely bidentate to UO(2)(2+) in the 1:1 and 1:3 complexes but mono- and bidentate in the 1:2 complexes. The DFT calculations also indicate that the medium effects must always be taken into account in order to gain accurate information on the complex formation in solution. In fact, the relative stability of the reaction products changes markedly when the DFT calculations are carried out in vacuum or in DMSO solution.

Oxygenation reaction of Co(trien)2+ complex in dimethylsulfoxide and the aerobic oxidation of 2,6-di-tert-butylphenol catalyzed by Co(II)–amine complexes

Abstract The kinetics and the thermodynamics of the oxygenation reaction of Co(trien)2+ complex (trien=1,4,7,10-tetraazadecane) have been studied in dimethylsulfoxide (dmso) at 298 K and in a medium adjusted to 0.1 mol dm−3 with Et4NClO4 by means of UV–Vis spectrophotometric technique. The reaction mechanism is consistent with the fast formation of an initial CoL–O2 species, followed by a rate determining step which gives the final (CoL)2O2 adduct. The results are compared with similar data in water and discussed considering the effects of ligand/solvent substitution. The catalytic activity of Co(trien)2+ as well as that of other diamine complexes, CoL2 (L=ethylenediamine (en), N,N′-dimethylethylenediamine) has also been tested towards the aerobic oxidation of 2,6-di-tert-butylphenol in mild conditions and the results are discussed in term of both catalytic ability and selectivity.

Nickel(II) Complexation with Nitrate in Dry [C4mim][Tf2N] Ionic Liquid: A Spectroscopic, Microcalorimetric, and Molecular Dynamics Study

The complex formation of nitrate ions with nickel(II) in dry [C4mim][Tf2N] ionic liquid (IL) was investigated by means of UV-visible spectrophotometry, isothermal titration calorimetry (ITC), extended X-ray absorption fine structure spectroscopy (EXAFS), and molecular dynamics (MD) simulations. EXAFS spectroscopy and MD simulations show that the solvated Ni(II) cation is initially coordinated by the oxygens of the [Tf2N](-) anion of IL, which can behave either as mono- or bidentate. Spectroscopic and thermodynamic data show that Ni(II) is able to form up to three stable mononuclear complexes with nitrate in this solvent. The stability constants for Ni(NO3)j complexes (j = 1-3) calculated from spectrophotometry and ITC experiments decrease in the order log K1 > log K2 > log K3. The formation of the first two species is enthalpy-driven, while the third species is entropy-stabilized. The UV-vis spectra of solutions containing different nitrate/Ni(II) ratios show that the metal ion retains the six-coordinate geometry. Furthermore, the EXAFS evidences that nitrate is always bidentate. Molecular dynamics simulations show that the [Tf2N](-) anions bind Ni(II) through the sulfonyl oxygen atoms and can coordinate either as monodentate or chelate. The analysis of the MD data shows that introduction of nitrates in the first coordination sphere of the metal ion results in remarkable structural rearrangement of the ionic liquid.

Hydration of Cisplatin Studied by an Effective Ab Initio Pair Potential Including Solute-Solvent Polarization.

The hydration of cis-[PtCl2(NH3)2] (cisplatin) has been studied by means of classical molecular dynamics simulations using a new interaction potential obtained by fitting about 4000 ab initio interaction energies calculated at the MP2 level. The functional form included several r(-n) terms (n = 4, 6, 8, 12) to achieve an accurate description of the interactions in the different regions around the cisplatin. Bulk solvent effects on the cisplatin-water molecule interactions have been included by means of a continuum model. Radial Distribution Function (RDF) analysis does not provide a clear enough description of the hydration pattern due to the intricate solvent arrangement around the solute. Angle-solved RDFs and spatial distribution functions have been used to provide more detailed pictures of the local hydration structure around the two ligands, chloride and ammine groups, and the axial region. Based on this information, it is shown a more convenient way to compute the running coordination number for the first hydration shell by simultaneously considering angle-solved RDFs centered on the ligand representative atoms of the complex: ammino N, Cl, and Pt atoms. This way, the hydration number is obtained by integrating over an interlocking-sphere volume built by the spheres centered on the cation and the main atoms of each ligand. Compared to previous works dealing with cisplatin hydration, the global hydration number for the first coordination shell is now higher and involves about 27 water molecules. The importance of the structural sampling, the computational level, as well as the functional form adopted for the interaction potential are thoroughly discussed with respect to the previous proposed intermolecular potential.

Hydration of Two Cisplatin Aqua-Derivatives Studied by Quantum Mechanics and Molecular Dynamics Simulations

The hydration of the cisplatin aqua-derivatives, cis-[PtCl(H2O)(NH3)2](+) (w-cisplatin) and cis-[Pt(H2O)2(NH3)2](2+) (w2-cisplatin), has been studied by means of classical molecular dynamics simulations. The new platinum complex-water interaction potential, w-cisplatin-W, has been built on the basis of the already obtained cisplatin-water interaction potential (cisplatin-W) [J. Chem. Theory Comput. 2013 9, 4562]. That potential has been then transferred to the w2-cisplatin-W potential. The w-cisplatin and w2-cisplatin atomic charges were specifically derived from their solutes wave functions. Bulk solvent effects on the complex-water interactions have been included by means of a continuum model. Classical MD simulations with 1 platinum complex and 1000 SPC/E water molecules have been carried out. Angle-solved radial distribution functions and spatial distribution functions have been used to provide detailed pictures of the local hydration structure around the ligands (water, chloride, and ammine) and the axial region. A novel definition of a multisite cavity has been employed to compute the hydration number of complexes in order to provide a consistent definition of their first-hydration shell. Interestingly, the hydration number decreases with the increase of the complex net charge from 27 for cisplatin to 23 and 18 for w-cisplatin and w2-cisplatin, respectively. In parallel to this hydration number behavior, the compactness of the hydration shell increases when going from the neutral complex, i.e. cisplatin, to the doubly charged complex, w2-cisplatin. Quantum mechanics estimation of the hydration energies for the platinum complexes allows the computation of the reaction energy for the first- and second-hydrolysis of cisplatin in water. The agreement with experimental data is satisfactory.

Efficient fluoride adsorption by mesoporous hierarchical alumina microspheres

Mesoporous Hierarchical Alumina Microspheres (HAM) with high efficiency for fluoride removal have been synthesized and characterized. Two types of HAM, differing mostly in crystallinity, surface area and pore size have been obtained. Fluoride adsorption studies have been carried out by means of potentiometry and Isothermal Titration Calorimetry (ITC). The latter method has been applied for the first time to obtain direct determination of the adsorption enthalpy (ΔHads) of F− ion on HAM. The kinetics of the reaction revealed a two-step process for fluoride adsorption on the adsorbent material. The ΔHads values obtained are clearly negative for the different samples investigated. Experimental adsorption data are well fitted by a Langmuir isotherm. The adsorption constant obtained for type A is 1 order of magnitude higher than for type B, showing that the synthetic protocol has a remarkable effect on this parameter. The highest defluoridation capacity reaches 26 mmol g−1 after 1 hour of equilibration for the amorphous HAM, which is higher than for other adsorbents reported in the literature.

Fluoroquinolones: A micro-species equilibrium in the protonation of amphoteric compounds

The knowledge of the speciation of fluoroquinolones is of great actuality for the implications on the activity, bioavailability and pharmacokinetics. Literature reports a number of contrasting evaluations on the existence of tautomeric forms of mono-protonated species, described by a set of protonation micro-constants. Here the protonation sequence and the related protonation constants of four representative molecules are evaluated by a combined potentiometric-spectrophotometric method. The experimental observations necessary to differentiate between a protonation scheme represented by macro-constants alone, and the one that requires the introduction of a micro-protonation scheme, are clearly delineated based on a careful analysis of experimental data and of simulated models. The role of the medium was investigated and UV-vis spectra in water- methanol solution were analyzed. The existence of the zwitterionic species alone at physiological pH in water, and an increase of the relative amount of the neutral species with the lipophilicity of the medium were remarked. This surely affects the bioavailability of FQs, with the increase of the neutral species when the molecules approach the local lipophilic environment close to the cellular membranes. NMR studies allowed the attribution of the protonation sites of the different forms. Quantum chemical evaluation of all the possible existent forms with different protonation degrees and in different sites strongly substantiates the experimental results. The study of the relevant frontier molecular orbitals completed the detailed theoretical characterization of the species.

Co(II) and Cd(II) complexation with two dipyridine-containing macrocyclic polyamines in water and dimethyl sulfoxide

Solvent effects in the formation of Co(II) and Cd(II) complexes with the two macrocyclic ligands 2,5,8,11,14-pentaaza[15]-[15](2,2′)[1,15]-bipyridylophane (L1) and trimethyl-5,8,11-2,5,8,11,14-pentaaza[15]-[15](2,2′)[1,15]-bipyridylophane (L2), both containing dipyridine units, were analysed by determining the thermodynamic parameters (logK, ΔH°, TΔS°) for the complexation reactions in water and dimethyl sulfoxide (DMSO) by means of potentiometric, spectrophotometric and calorimetric techniques. N-Methylation leads to different solvation properties of the ligands and to different abilities in stabilizing metal complexes via the formation of M–NH⋯S hydrogen bonds to solvent molecules (S) which enhances the σ-donating properties of the donor atom. In contrast to expectations based on the stronger solvation of Cd(II) and Co(II) in DMSO than in water, complexes with L1 display higher stability in DMSO than in water, indicating that ligand solvation, instead of metal ion solvation, plays a major role in determining the stability of L1 complexes in the two solvents. On the other hand, for ligand L2, in which tertiary amino groups are present, an almost opposite trend of stability is observed. Because L2 is less solvated than L1 in water and tertiary nitrogens are weaker bases in DMSO than in water, the stronger solvation of metal ions in DMSO prevails in determining the stability of L2 complexes. Semi-empirical calculations were also performed to obtain some structural information in the gas phase.

Thermodynamics of complex formation of silver(I) with N-donor ligands in non-aqueous solvents

The results of a potentiometric and calorimetric study on the complexation reactions of neutral N-donor ligands with silver(I) in propylenecarbonate (PC) and dimethylformamide (DMF) are reported. The ligands concerned in DMF are butylamine (n-but), 1,2-diaminoethane (EN), bis(2-aminoethyl)amine (DIEN) and N,N’-bis(2-aminoethyl)ethane-1,2-diamine (TRIEN) whereas in PC results are provided for EN and DIEN, because of side reactions occurring for n-but and TRIEN. The data are compared to those previously reported in dimethylsulfoxide (DMSO), acetonitrile (AN) and water solvent media which present quite different dielectric constants (ε) and donor numbers (Dn). The trend of stabilities of the mononuclear AgL and AgL2 formed is discussed in terms of different cation and amines solvation in the different solvents. TRIEN can form bimetallic species in DMF, but not in DMSO. Given the lower ε value for DMF than for DMSO, Ag2TRIEN formation is evidently more influenced by the lower solvation of Ag(I) ion in DMF, rather than by difference in dielectric constants of these two solvents. In PC in addition to mononuclear complexes of higher stability with respect to the former solvents, also polynuclear Ag2L and Ag3L2 species are found.