Modeling of ion complexation and extraction using substructural molecular fragments

A substructural molecular fragment (SMF) method has been developed to model the relationships between the structure of organic molecules and their thermodynamic parameters of complexation or extraction. The method is based on the splitting of a molecule into fragments, and on calculations of their contributions to a given property. It uses two types of fragments: atom/bond sequences and "augmented atoms" (atoms with their nearest neighbors). The SMF approach is tested on physical properties of C2-C9 alkanes (boiling point, molar volume, molar refraction, heat of vaporization, surface tension, melting point, critical temperature, and critical pressures) and on octanol/water partition coefficients. Then, it is applied to the assessment of (i) complexation stability constants of alkali cations with crown ethers and phosphoryl-containing podands, and of beta-cyclodextrins with mono- and 1,4-disubstituted benzenes, and (ii) solvent extraction constants for the complexes of uranyl cation by phosphoryl-containing ligands.     

Synthesis of 1,3-bis(acetylacetonyloxy)- and 1,3-bis(benzoylacetonyloxy)benzene and their complexation with lanthanide ions

Claisen condensation of 1,3-bis(methoxycarbonylmethoxy)benzene with acetone and acetophenone afforded new chelating ligands consisting of two β-diketonate fragments, viz., 1,3-bis(acetylacetonyloxy)benzene and 1,3-bis(benzoylacetonyloxy)benzene, which are linked to each other through the resorcinol spacer. In the crystal, 1,3-bis(acetylacetonyloxy)benzene, unlike the starting ester, adopts a planar conformation and exists in the enol form. The acidities of these compounds and their complexation with lanthanide ions in aqueous ethanolic solutions were studied by pH-potentiometry. Depending on the concentration conditions and pH, the La³⁺, Gd³⁺, and Lu³⁺ ions form 1 : 1, 1 : 2, or 1 : 3 complexes with bis(β-diketones). The stability of the complexes increases as the atomic number of the lanthanide increases (La³⁺ < Gd³⁺ ≤ Lu³⁺). The complexation constants and selectivity of complexation substantially increase with increasing degree of deprotonation of the ligands, which indicates that both chelate groups of the ligands are simultaneously involved in coordination. The Ph substituents in bis(β-diketone) have a considerable effect on the composition and stability of complexes with lanthanide ions due to additional noncovalent inner-sphere interactions.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 614–622, March, 2005.     

Structure-property modelling of complex formation of strontium with organic ligands in water

The method of substructural molecular fragments based on representation of the molecular graph by ensembles of fragments and involving calculations of those contributions to the given property is applied to the modelling of stability constants of the complexes of strontium(II) with organic ligands in water. Reliability of predictions of developed structure-property models was examined using three different test sets of structurally diverse ligands. The obtained models have been used for generation and screening of combinatorial library of virtual ligands. Some hypothetical efficient Sr(II) binders were suggested.     

A Combined Experimental/Quantum-Chemical Study of Tetrel,Pnictogen, and Chalcogen Bonds of Linear Triatomic Molecules

Linear triatomic molecules (CO₂, N₂O, and OCS) are scrutinized for their propensity to form perpendicular tetrel (CO₂ and OCS) or pnictogen (N₂O) bonds with Lewis bases (dimethyl ether and trimethyl amine) as compared with their tendency to form end-on chalcogen bonds. Comparison of the IR spectra of the complexes with the corresponding monomers in cryogenic solutions in liquid argon enables to determine the stoichiometry and the nature of the complexes. In the present cases, perpendicular tetrel and pnictogen 1:1 complexes are identified mainly on the basis of the lifting of the degenerate ν 2 bending mode with the appearance of both a blue and a red shift. Van ′t Hoff plots of equilibrium constants as a function of temperature lead to complexation enthalpies that, when converted to complexation energies, form the first series of experimental complexation energies on sp¹ tetrel bonds in the literature, directly comparable to quantum-chemically obtained values. Their order of magnitude corresponds with what can be expected on the basis of experimental work on halogen and chalcogen bonds and previous computational work on tetrel bonds. Both the order of magnitude and sequence are in fair agreement with both CCSD(T) and DFA calculations, certainly when taking into account the small differences in complexation energies of the different complexes (often not more than a few kJ mol⁻¹) and the experimental error. It should, however, be noted that the OCS chalcogen complexes are not identified experimentally, most probably owing to entropic effects. For a given Lewis base, the stability sequence of the complexes is first successfully interpreted via a classical electrostatic quadrupole–dipole moment model, highlighting the importance of the magnitude and sign of the quadrupole moment of the Lewis acid. This approach is validated by a subsequent analysis of the molecular electrostatic potential, scrutinizing the σ and π holes, as well as the evolution in preference for chalcogen versus tetrel bonds when passing to “higher” chalcogens in agreement with the evolution of the quadrupole moment. The energy decomposition analysis gives further support to the importance/dominance of electrostatic effects, as it turns out to be the largest attractive term in all cases considered, followed by the orbital interaction and the dispersion term. The natural orbitals for chemical valence highlight the sequence of charge transfer in the orbital interaction term, which is dominated by an electron-donating effect of the N or O lone-pair(s) of the base to the central atom of the triatomics, with its value being lower than in the case of comparable halogen bonding situations. The effect is appreciably larger for TMA, in line with its much higher basicity than DME, explaining the comparable complexation energies for DME and TMA despite the much larger dipole moment for DME.     

Potentiometric studies on the complexation of copper(II) by phenolic acids as discrete ligand models of humic substances

Studies on the complexation of copper(II) by phenolic acids, as ligand models of humic substances were done by potentiometry. The acids under study were: 3,4-dihydroxyhydrocinnamic acid or hydrocaffeic acid (1), 3,4-dihydroxyphenylacetic acid (2) and 3,4-dihydroxybenzoic acid or protocatechuic acid (3). Acidity constants of the ligands and the formation constants of metal-ligand complexes were evaluated by computer programs. The carboxylic group of the phenolic acids has different pK_a1 values, being the dissociation constants intrinsically related with the distance between the function and the aromatic nucleus. The results obtained allow concluding that acidity constants of the catechol moiety of the compounds are similar with pK_a2 and pK_a3 values between 9.47-9.41 and 11.55-11.70. The complexation properties of the three ligands towards copper(II) ion are quite similar, being the species found not different either in nature or stability. Although the model ligands have some structural differences no significant differences were found in their complexation properties towards copper(II). So, it can be postulated that complexation process is intrinsically related with the presence of a catechol group.     

Potentiometric and Conductometric Studies on the Complexes of Some Rare Earth Metals with Rhodanine Azosulfonamide Derivatives, XII

Stability constants for La³⁺, Ce³⁺, UO₂ ²⁺, and Th⁴⁺ metal ion complexes with rhodanine azosulfonamide derivatives have been determined potentiometrically in 0.1 M KCl in a 30% (v/v) ethanol–water mixture. The order of the stability constants of the complexes found was to La³⁺ < Ce ²⁺ < UO ₂ ²⁺ < Th ⁴⁺. The influence of substituents on the stability of the complexes was examined on the basis of electron-repelling property of the substituent. The effect of temperature on the stability constants was studied and the Gibbs energy, the enthalpy, and entropy of complexation thermodynamic parameters were derived and are discussed. The stoichiometries of these complexes were determined conductometrically and the results indicated the formation of 1:1 and 1:2 (metal:ligand) complexes.     

Study of pH-dependent charges of soils by surface acid-base properties

The pH-dependent charges of Hungarian soils have been studied via surface acid-base properties. The intrinsic stability constants of protonation and deprotonation processes, as well as the concentration of surface sites, have been determined by surface complexation modeling. The protonation and deprotonation constants have been nearly the same for most soils. There is a relation between the concentration of surface sites and composition, expect for the freshly deposited soils with high primary silicate content. The results show that the concentrations of silanol and aluminol sites are different for each soil, the intrinsic stability constants of protonation and deprotonation processes, however, are nearly the same within experimental error. This can only be explained if these stability constants are real thermodynamic equilibrium constants. The fact that these constants are nearly the same supports the conclusion that we succeeded in excluding all processes that would disturb the measurements. The parameters characteristic of the edge sites of the soils are of two types: (a) the parameters depending on the quality and composition of the soils, (i.e.), the concentration of surface sites; (b) the parameters depending on the thermodynamically well-defined acid-base processes, independent of soil composition.     

The stability and thermodynamic functions of 4-amino-1,6-dihydro-1-methyl-2-methylthio-5-nitroso-6-oxo-pyrimidine complexes with Zn(II), Cd(II) and Hg(II)

The stability constants of the complexes formed by 4-amino-1,6-dihydro-1-methyl-2-methylthio-5-nitroso-6-oxo-pyrimidine with Zn(II), Cd(II) and Hg(II) ions, in aqueous media at different ionic strengths and temperatures, have been calculated by Bjerrum's method.From the data obtained, the thermodynamic functions of complexation processes are reported.     

Influence of metal cations on spectral characteristics of crown ether vinylogs with different terminal polar groups

Complexation of crown ether vinylogs containing different terminal polar groups with alkali and alkali-earth metal ions in solutions was studied by spectrophotometry. The hypsochromic shift of absorption band maxima in the UV-Vis absorption spectra indicates that the ligands containing the monobenzocrown ether fragments interact with metal ions. The scheme of complexation was proposed, and the stability constants of the complexes were determined. The efficiency of complexation depends on the metal cation size and the structure of the ionophoric fragment.     

Conductance and Thermodynamic Study of Thallium and Silver Ion Complexes with Crown Ethers in Different Binary Acetonitrile–Water Solvent Mixtures

The complexation reactions between Ag⁺ andTl⁺ ions with 15-crown-5 (15C5) and phenyl-aza-15-crown-5(PhA15C5) have been studied conductometrically in 90%acetonitrile-water and 50% acetonitrile - water mixed solvents attemperatures of 293, 298, 303 and 308 K. The stability constants of theresulting 1 : 1 complexes were determined, indicating that theTl⁺ complexes are more stable than the Ag⁺complexes. The enthalpy and entropy of crown complexation reactions were determined from the temperature dependence of the complexation constants.The enthalpy and entropy changes depend on solvent composition and the T S⁰ _o–H⁰ plotshows a good linear correlation, indicating the existence of entropy –enthalpy compensation in the crown complexation reactions.     

Complexation of iodine with macrocyclic polyethers and sulfides

The complexation of iodine with 12-, 15-, and 18-membered crown ethers containing two sulfur atoms in the macrocyclic ring was studied by the spectrophotometric method. It was shown that, in CCl₄, the stability constants of complexes with a 11 composition have a value of the order of magnitude of 10². The enthalpies of complexation are (22–34) kJ/mole, which is practically not different from the parameters of complexes of iodine with simple sulfides.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 6, pp. 747–750, November–December, 1988.     

Stability constants: comparative study of fitting methods. Determination of second-order complexation constants by (23)Na and (7)Li NMR chemical shift titration

NMR chemical shift titration has been widely used as a method for the determination of stability constants. Systems involving metal-ligand complexation have been investigated using a number of methodologies. There are significant differences in the values reported for stability constants obtained by different experimental methods, such as calorimetry and ion selective electrode (ISE) titrations; nor has NMR chemical shift titration always yielded consistent results. Different researchers have obtained different results for the same system with results differing by as much as an order of magnitude. The chemical shift data are generally plotted against the concentration ratio of the metal and ligand for a set of solutions. A nonlinear least squares fitting method using an analytical solution of the cubic equation for the equilibrium concentration of the free ligand is used in this study and compared with methods used in the literature. Second-order association constants for the LiClO(4):12-crown-4 system in acetonitrile and the NaClO(4):12-crown-4 system in methanol are reported. Formation of both 1:1 and 1:2 metal-ligand complexes are considered. The LiClO(4):12-crown-4 acetonitrile system had been investigated previously by NMR titration but only 1:1 complexation was considered in that study. This study provides convincing evidence that both 1:1 and 1:2 complexes are important, at least, in the lithium system. A Monte Carlo investigation of the propagation of errors from the chemical shifts to the stability constants shows that the choice of data analysis methods may, in part, contribute to discrepancies and that the nonlinear nature of the model can dramatically affect the error limits on the stability constants.     

Intramolecular Sandwich Complexation of Light Lanthanoid Nitrates with Bis(benzo-12-crown-4)s: Enhanced Selectivity for Eu

Spectrophotometric titrations performed in anhydrous acetonitrile at 25°C give the complex stability constants (log Ks) and the Gibbs free energy changes (-G° for the stoichiometric 1 : 1 intramolecular sandwich complexation of light lanthanoid (III) nitrates (La Gd) with the polymethylene-bridged bis(benzo-12-crown-4)s 1, the corresponding dioxo derivative 2 and its dihydroxy analogue 3. The complex stability sequence as a function of reciprocal ionic radius of lanthanoids showed similar profiles in stability constants (log Ks) and maximum stabilities were obtained at Eu3+ for the complexation of light lanthanoids with the three bis(benzo-12-crown-4)s 1–3. The cation binding abilities and relative selectivities for the trivalent lanthanoid ions of these structurally related bis(benzo-12-crown-4)s 1–3 are discussed according to the derivatization of the bridging.     

Solvent extraction of Er(III) and Lu(III) with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester in presence of some reagents

The extraction of Er(III) and Lu(III) from thiocyanate media with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHEHPA) and also with mixtures of EHEHPA and thenoltrifluoroacetone (HTTA) or tri-n-octylphosphine oxide (TOPO) or bis 2-ethylhexyl sulfoxide (B2EHSO) in benzene has been studied systematically. Synergistic effects have been observed with mixtures of EHEHPA+HTTA or TOPO. On the other hand, antagonistic effects have been observed with mixtures of EHEHPA+B2EHSO. These extraction data have been analyzed theoretically with the aid of a computer by taking into account complexation of the metal in the aqueous phase by SCN^– and plausible complexation in the organic phase. The extraction constants of the various product species have been deduced by non-linear regression analysis. The stability constants for the thiocyanate complexes of the metal ions have also been determined.     

Stability of Ag Cryptate in Mixed Solvent with Negative Deviations from Additivity

Coordination model of solvation was used to analyze the change in stability constants of Ag(I) cryptate in water–DMSO solvent. The resolvation constants, distribution of solvatocomplexes, and composition of solvation shells of Ag⁺ and 2.2.2-cryptand in a mixed solvent were determined and the effect of solvation of coordinating (reaction) centers of reagents on the stability of a complex was estimated. It was shown that specific intermolecular interactions of water and organic component of a mixed solvent should be taken into consideration when studying the equilibria of complexation in such systems.     

Stability of Ag<Superscript>+</Superscript> Complexes with Cryptand 222 in Ionic Liquids

Stability constants of silver(I) complexes with cryptand 222 were measured in a number of ionic liquids, applying potentiometric titration. The ionic liquids were based on 1-butyl-3-methylimidazolium, 1-ethyl-3-methylimidazolium, 1-butyl-1-methyl-pyrrolidinium and 1-methyl-1-propyl-pyrrolidinium cations, as well as on tetrafluoroborate, triflate and bis(trifluoromethane sulfonyl) imide. The stability constants, expressed in log K scale, were within the broad range of 8.4–17.2. The formation of the Ag⁺222 cryptates was not detected in ionic liquids based on halide anions. Free enthalpy of silver(I) transfer from dimethylsulfoxide as a reference molecular solvent to ionic liquids was calculated applying the cryptate assumption. The results were discussed in terms of the competition between silver(I) complexation by ion forming ionic liquid and its complexation by cryptand 222.in final form: 6 December 2004This revised version was published online in July 2005 with a corrected issue number.     

冠醚与金属离子配位作用的扩环效应

在冠醚与阳离子配位作用的热力学性质研究中，含苯并冠醚与金属离子配伍作用的能力低于母体冠醚[1]．一般认为，这是由于苯环的吸电子效应降低了邻近二个供电氧原子的电荷密度所致，而很少注意引入亚甲基降低苯环的吸电子效应对冠醚配伍作用的影响．我们近来的研究表明，在较阶队骨格冠醚分子中引入额外的亚甲基，与母体冠醚相比，对于键和钠离子具有高度的选择性．这对于设计和合成具有高选择性的功能冠醚，可作为一个较有力的。具［2-4］．为了进一步探索在给定供电氧原子的情况下，冠醚的扩环效应，我们研究了含二苯并冠醚（化合物I-m）…     

Experimental evidence for a general model of modulated MOF nanoparticle growth

Nanoparticles of metal–organic frameworks (nanoMOFs) boast superior properties compared to their bulk analogs, yet little is known about how common synthetic parameters dictate particle sizes. Here, we provide experimental evidence for the “seesaw” model of nanoMOF growth. Solution acidity, ligand excess, and reactant concentrations are decoupled and shown to form the key independent determinants of nanoMOF sizes, thereby validating the proposal that nanoMOFs arise from coupled equilibria involving ligand deprotonation and metal–ligand complexation. By achieving the first demonstration of a seesaw relationship between nanoMOF sizes and ligand excess, these results provide further proof of the model, as they required deliberate manipulation of relationships outlined by the model. Exploring the relative impacts of these parameters reveals that ligand excess has the greatest ability to decrease sizes, although low acidity and high concentrations can exhibit similar effects. As a complement to existing models of polymer formation and crystal growth, the seesaw model therefore offers a powerful tool for reliable control over nanoMOF sizes.

Nanoparticles of metal–organic frameworks (nanoMOFs) boast superior properties compared to their bulk analogs, yet little is known about how common synthetic parameters dictate particle sizes.