Ion association of lithium and sodium picrate in 2-propanol at 25°C. II. Spectrophotometric measurements

The molar absorptivities of lithium picrate and of sodium picrate have been measured as a function of salt concentration (in the 0.1–1 mmol-dm^–3 concentration range) in 2-propanol at 25°C. Values of the molar absorptivities of the free picrate ion _i, of the ion pair _p, and of the ion pair association constant K_a have been calculated for each salt. The K_a values for these two salts in this solvent calculated from spectrophotometric measurements were found to be the same within experimental uncertainty as the values of K_a calculated from conductance measurements.     

Solvent extraction of sodium picrate with 15-crown-5 into carbon tetrachloride. The determination of the dimer-formation constant of a crown ether-univalent metal salt 1∶1∶1 complex

The theory is derived to determine the dimer-formation constant,K ₂, of a crown ethermetal salt 111 complex in organic solvents of low dielectric constant by solvent extraction. The theoretical predictions are verified experimentally by extraction of sodium picrate (NaA) with 15-crown-5 (15C5) into carbon tetrachloride. All the experiments were conducted at 25°C. The logK ₂ value of the Na(15C5)A complex in carbon tetrachloride has been determined to be 4.05±0.11. Moreover, the partition constant of the complex is calculated.     

Extraction of sodium picrate into nitrobenzene in the presence of dicyclohexyl-18-crown-6

From extraction experiments and -activity measurements, the extraction constant corresponding to the equilibrium Na⁺(aq)+A^–(aq)+L(nb)NaL⁺(nb)+A^–(nb) taking place in the two-phase water-nitrobenzene system (A^–=picrate, L= dicyclohexyl-18-crown-6; aq-aqueous phase, nb=nitrobenzene phase) was evaluated as logK _ex(NaL⁺, A^–)=2.6.Further, the stability constant of the dicyclohexyl-18-crown-6-sodium complex in nitrobenzene saturated with water was calculated: _nb(NaL⁺)=7.8.     

Solvent effects on extraction of potassium picrate with benzo-18-crown-6. A new equation for the determination of ion-pair formation of crown ether-metal salt complexes in water

In order to determine the ion-pair formation constant of a crown ether-metal salt 1:1:1 complex in water, an equation is derived from regular solution theory and its predictions are verified experimentally by the solvent extraction method using benzo-18-crown-6 (B18C6), potassium picrate (KA), and various diluents of low dielectric constant. The distribution constants of B18C6 itself and the overall extraction constants of KA with B18C6 were determined at 25±0.2°C. The distribution constants of the neutral K(B18C6)A complex were calculated from these data. The literature value for the complex-formation constant of K(B18C6)⁺ in water and the ion-pair formation constant (K _K(B18C6)A ) for K(B18C6)A in water determined in this study were log K _K(B18C6)A =3.12±0.23 at 25°C). The distribution behavior of B18C6 and K(B18C6)A is explained in terms of regular solution theory. The molar volumes V (cm³·mol^–1) and solubility parameters (cal^1/2-cm^–3/2) are as follows: V _B18C6 =249±36; V _K(B18C6)A =407±56; _B18C6 = 11.5 ± 0.5; and _K(B18C6)A = 11.5 ± 0.5.     

Anion dependence of crown ether selectivities for alkali picrates and sulfonates in dioxane and toluene. A synergistic effect

The binding constants,K _N, of sodium and potassium 8-anilinonaphthalene-1-sulfonate (ANS) and of sodium 5-dimethylamino-1-naphthalenesulfonate (DNS) to benzo-18-crown-6 bound to a 2% cross-linked polystyrene network (RN18C6) were measured spectrophotometrically in dioxane and the results compared with those obtained for picrate salts. The network RN18C6 was then used to measure in dioxane and toluene by a competition method the equilibrium constant,K, of the reaction A^–M⁺N+CrA^–M⁺Cr+N.A^–M⁺N denotes the ionic solute (ANS, DNS, methyl orange or picrate salt) bound to the network RN18C6 (N) and A^–M⁺Cr is the solute bound to a soluble ligand Cr, where Cr represents a series of 18-crown-6 and 15-crown-5 compounds. Combining theK _N andK values the formation constants,K _L, of the crown ether complexes of the respective salts were obtained in dioxane. The data show a reversal in the complexation strength of the 18-crown-6 compounds in dioxane when sodium picrate is replaced by sodium ANS. The results were rationalized in terms of a synergistic effect exerted by dioxane, with dioxane forming a 1:1 dioxanate with the crown ion pair complex. This effect is especially strong with ANS and with a rigid planar crown ether like dibenzo-18-crown-6. The binding constants,K _N, of NaANS and NaDNS to RN18C6 in dioxane are nearly three times larger than for sodium picrate, and the same holds for the potassium salts. Differences in anion interactions with the network appear to be a plausible cause for the anion dependence ofK _N.     

Specific interactions including imidazolium andN-methylimidazolium lons. I. Interactions with the picrate anion

The conductances of solutions of methylimidazolium and imidazolium picrate (MeImHPic and ImHPic) in nitrobenzene-benzene mixtures (27.2–. These triple ions are highly stabilized by the hydrogen bond between the second NH group of the ion pair and the second picrate ion. Values of the formation constants for the ion pair ImHPic and for the triple ion PicImHPic^– have been calculated and are discussed.     

Ionic Association and Mobility IV. Ionophores in Dimethylsulfoxide at 25°C

The results of conductance measurements on pyridinium picrate, tetraphenylo-sonium picrate, potassium picrate, tetraphenylantimony picrate, tetrapropylam-monium, tetrafluoroborate, tetramethylammonium hexafluorophosphate ion association noncoulombic interaction in dimethyl sulfoxide (DMSO) at 25°C in the concentration range 1–15×10^–4 M are reported. The data were analyzed by the Justice modification of the Fuoss–Hsia equation. Except for pyridinium picrate all salts studied were found to be associated.Application of the Justice Barthel–Bjerrum model of ion association permitted calculation of the noncoulombic portion of the potential of mean force, W _±. Ionic limiting conductances were calculated for six ions using known values of previously determined transport numbers. A table of most current limiting ionic conductances for a variety of ions in DMSO at 25°C has been established.     

Stabilization of a K+-(bis-Cage-Annulated 20-Crown-6) Complex by Bidentate Picrate

Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K⁺ and Rb⁺. A stable host–guest complex was prepared by slow evaporation of a CH₂Cl₂–hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 K⁺ complex and picrate anion was calculated to be ca. –64.9 kcal mol^–1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10.     

Tetraalkylammonium Picrates in the Dichloromethane–Water System: Ion-Pair Formation and Liquid–Liquid Distribution of Free Ions and Ion Pairs

Equilibria concerning picrates of tetraalkylammonium ions (Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, Bu₃MeN⁺) in a dichloromethane−water system have been investigated at 25 ^∘C. The 1:1 ion-pair formation constants (K _IP,o ^o) in dichloromethane at infinite dilution were conductometrically determined. The distribution constants (K _D ^o) of the ion pairs and the free cations between the solvents were determined by a batch-extraction method. The K _IP,o ^o value varies in the cation sequence, Bu₄N⁺ ≈ Pr₄N⁺ ≈ Et₄N⁺ < Bu₃MeN⁺ < < Me₄N⁺; this trend is explained by the electrostatic cation−anion interaction taking into account the structures of the ion pairs determined by density functional theory calculations. For the ion pairs of the symmetric R₄N⁺ cations, there is a linear positive relationship between log₁₀ K _D ^o and the number of methylene groups in the cation (N _{CH 2}). The ion pair of asymmetric Bu₃MeN⁺ has a higher distribution constant than that expected from the above log₁₀ K _D ^o versus N _{CH 2} relationship. These cation dependencies of log₁₀ K _D ^o for the ion pairs are explained theoretically by using the Hildebrand-Scatchard equation. For all the cations, the log₁₀ K _D ^o value of the free cation increases linearly with N _{CH 2}; the variation of log₁₀ K _D ^o is discussed by decomposing the distribution constant into the Born-type electrostatic contribution and the non-Born one, and attributed to the latter that is governed by the differences in the molar volumes of the cations. The cation dependencies of the ion-pair extractability and ion pairing in water are also discussed. An erratum to this article can be found at     

Vibrational spectral studies of ion-ion and ion-solvent interactions. II. Zinc nitrate in water/dioxane mixtures

Raman and infrared spectral data have been collected forp-dioxane and solvated and bound nitrate ions for Zn(NO₃)₂/p-dioxane/water systems. It is concluded that Zn²⁺ is preferentially solvated by water and that this aquation is also responsible for a lower concentration of ion pairs than is found for methanol/Zn(NO₃)₂ solutions for which the dielectric constant of the bulk solvent is similar. Values of K₁ (M^–1), the association constant for Zn(NO₃)⁺, are 0.22±0.02 (2/1 solvent,D=12.6) and 0.071±0.01 (4/1 solvent,D=33.0). The log K₁ against 1/D plot is not linear.     

New ditopic receptors for alkylammonium ion complexation

New receptor molecules have been synthesized in which ,-bis-(4-hydroxyphenyl)-1,4-diisopropylbenzene is linked to 1,10-diaza-18-crown-6, 1,10-diaza-21-crown-7 or 1,13-diaza-24-crown-8 units by ethylene or 1,4-butylene bridges. Binding abilities of the new receptors and the model compoundN,N-didecyl-1,10-diaza-18-crown-6 toward alkali metal cations and alkylammonium ions were assessed by picrate extraction. Spectral evidence for inclusion of alkylammonium ions within the receptor cavity was obtained by¹H NMR spectroscopy. From a¹H NMR titration experiment conducted in CDCL₃–CD₃OD (91), a relatively strong inclusion complex (K _a 900M^–1) of the receptor having a 1,10-diaza-18-crown-6 subunit and ethylene spacers with propylammonium picrate was observed.     

Coordination chemistry of alkali and alkaline earth cations: Synthesis and X-ray crystal structure of cesium (picrate) (benzo-15-crown-5) Cs+C6H2N3O 7 − (C14H20O5)

Crystals of the Cs⁺ Pic^– (B15C5) complex (Pic=picrate; B15C5=benzo-15-crown-5) (M _r =629.3) are yellow prisms which belong to the triclinic space groupP witha=7.377(4),b=11.372(2),c=14.806(2) , =90.31(1), =91.06(2), =108.32(2)⁰,Z=2,D _x =1.77, andD _m =1.77 g cm^–3. FinalR=0.055 for 3575 observed reflections out of a total of 4004 measured reflections. Cesium forms a 1:1 anion paired complex with B15C5 like sodium rather than a charge separated sandwich structure as found for potassium and expected for cesium in view of the ion-cavity radius concept. The Cs cation is 9-coordinate involving the five crown oxygens (Cs...O, 3.00(1) to 3.24(1) ), the phenoxide (Cs...O^–, 3.03(1) ) and anortho nitro group oxygen (Cs...O, 3.01(1) ) of the picrate counteranion and, uniquely, with two additional oxygens (Cs...O, 3.17(1) and 3.40(1) ) from apara nitro group of the picrate belonging to the adjacent molecule in the lattice. The Cs⁺ ion lies 2.07 out of the mean plane formed by the crown oxygens. This system provides the first structural evidence that the interaction stoichiometry of an alkali cation with a cyclic multidentate ligand is not a function of the ion and cavity size alone but also of its Lewis acid strength as modified by the charge neutralizing anion. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82027.     

Equilibrium Study on Ion-Pair Formation in Water and Distribution Between Water and m-Xylene of Tetraalkylammonium Picrates

The 1:1 ion-pair formation constants (K _IP) of tetraalkylammonium (Me₄N⁺, Et₄N⁺, Pr₄N⁺, Bu₄N⁺, and Bu₃MeN⁺) picrates in water were determined by capillary electrophoresis at 25°C. The ion-pair extraction constants (K _ex,ip) of the picrates from water to m-xylene were determined by a batch-extraction method at 25°C, and the distribution constants (K _D) of the neutral ion-pairs were calculated from the relationship K _D = K_ex,ip/K _IP. The tetraalkylammonium ion having more methylene groups generally forms a slightly more stable ion-pair with the picrate ion in water, which is attributed to the lower hydration of the cation. For Me₄N⁺, Et₄N⁺, Pr₄N⁺, and Bu₄N⁺, the distribution of the ion pair into m-xylene increases in that order, and a linear relationship was found between log K _D and the number of methylene groups in the cation. This is consistently explained by the regular solution theory. It was also revealed that the ion pairs have a strong specific interaction with water. The ion pair of Bu₃MeN⁺ has a higher distribution constant than that expected from the relationship between log K _D and the number of methylene groups for the symmetrical tetraalkylammonium ions. The cation dependence of the ion pair extractability is mostly governed by that of the distribution of the ion pair.     

A thermodynamic study of the association of alkali metal cations with dicyclohexano-18-crown-6

A thermodynamic study of the association of Na⁺, K⁺, Rb⁺, and Cs⁺ with dicyclohexano-18-crown-6 in acetonitrile has been carried out at 308, 303, 298, 293, and 288 K using a conductometric technique. The observed molar conductivities, A, were found to decrease significantly for mole ratios less than unity. A model involving 11 stoichiometry has been used to analyze the conductivity data. The stability constant,K, and the limiting molar conductivity, A _c , for each 11 complex were determined from the conductivity data by using a nonlinear least squares curve fitting procedure. The binding sequence, based on the value of logK at 298 K, as derived from this study is K⁺>Na⁺>Rb⁺>Cs⁺. Values of H ^o and S ^o are reported and their significance is discussed.     

Interaction between the buffer Tris and the eu(III) ion: Luminescence and potentiometric investigation

The interaction between the buffer 2-amino-2-(hydroxymethyl)propane-1,3-diol (Tris) and the Eu(III) ion has been studied by luminescence spectroscopy in D₂O. Emission and excitation spectra (⁵D₀←⁷F₀ transition) indicate an interaction with both [TrisH]⁺ and neutral Tris species. The former is weak and probably of the outer-sphere type. The latter is of inner-sphere type and corresponds to the formation of the [Eu(Tris)]³⁺ species (estimated logK₁ = 2.3 ± 0.3). Buffer Tris is also demonstrated to prevent the formation of an Eu-hydroxo species in the pD range of 7–8. Potentiometric measurements in H₂O allowed a more precise calculation of the stability constant: logK₁ = 2.44 ± 0.07. Comparison with the data for aliphatic amines and other metal ions lead to the conclusion that the Eu/Tris interaction is mainly achieved through the amino group. ¹H-NMR spectra in presence of Tb(III) ions confirmed both this assumption and the presence of a weak interaction with TrisH⁺. Quantitative determinations of association constants between lanthanide ions and macromolecules of biological interest performed in presence of Tris should, therefore, be corrected for the Eu/Tris interaction.     

Diffusion coefficients of ions through ion excange membrane in Donnan dialysis using ions of different valence

Donnan dialysis with an ion exchange membrane was investigated for ions of different valence. The effective diffusion coefficients (D_e) of various kinds of ions in the membrane were obtained by fitting of the equation derived from the Nernst–Planck equation to three or more sets of experimental data for Donnan dialysis. It became apparent that the value of D_e/D_s of monovalent ions (e.g., K⁺ or Na⁺ ions) at z_A=1 and z_B=2 (feed ions are monovalent ones and driving ions are bivalent ones) remained constant at ca. 1/210 and that of bivalent ions (e.g., Ca²⁺, Cu²⁺, or Mg²⁺ ions) remained constant at ca. 1/526 where D_s denotes the diffusion coefficient of ions at infinite dilution in water calculated from the Nernst–Einstein equation, and z_A and z_B represent the valences of the feed and driving ions, respectively. D_e/D_s of monovalent ions (e.g., H⁺, K⁺, or Na⁺ ions) at z_A=2 and z_B=1 (feed ions are bivalent ones and driving ions are monovalent ones) was constant at ca. 1/23.3 and that of bivalent ions remained constant at ca. 1/58.4. It was proved that D_e/D using D_e at z_A=1 and z_B=2 was constant at 1/3.0 and that at z_A=2 and z_B=1 remained constant at 3.0 where D represents the diffusion coefficient of ions in the membrane at z_A=z_B (the valences of both feed and driving ions are equal). Therefore, it was found that a large flux of ions could be obtained using the monovalent driving ions in Donnan dialysis. On the other hand, the small flux can be obtained using bi- or higher-valent driving ions.     

Transfer activity coefficients of crown ethers and their complexes with univalent metal ions between pairs of polar organic solvents

From a comparison of transfer activity coefficients, [(LM⁺)]_PC,2 between propylene carbonate and solvent S₂ of alkali or silver ions complexed with dibenzo-substituted crown ethers (L=DB-18-cr-6, DB-21-cr-7, DB-24-cr-8, DB-30-cr-10) it can be concluded that in the complex LM⁺ both L and M⁺ are solvated, particularly in solvents of high donicity, e.g., N,N-dimethylformamide. From the abnormally low ionic mobility of DB-30-cr-10K⁺ in acetonitrile and the high value of the association constant of the ion pair DB-30-cr-10KBr it is concluded that the outer solvent shell is stripped upon formation of the ligand separated ion pair. A linear relation is found between [log (LM⁺)]_PC,2 and [log(M⁺)]_PC,2 only when L is 18-cr-6, B-18-cr-6, or DB-18-cr-6. Deviation from the linearity of complexes of the larger dibenzo crown ethers is attributed to the flexibility of L. It is shown that solution of 18-cr-6, DB-18-cr-6 and DB-30-cr-10 is enthalpy assisted to a greater extent in acetonitrile than in methanol, while the entropy of solution is more favorable in the latter.     

Extraction of Sodium and Potassium Perchlorates with Dibenzo-18-crown-6 into Various Organic Solvents. Quantitative Elucidation of Anion Effects on the Extraction-Ability and -Selectivity

The constants for overall extraction into various diluents of low dielectric constants (K_ex) and aqueous ion-pair formation (K_MLA) of dibenzo-18-crown-6 (DB18C6)–sodium and potassium perchlorate 1:1:1 complexes (MLA) were determined at 25°C. The K_ex value was analyzed by the four underlying equilibrium constants. The K_MLA values were determined by applying our established method to this DB18C6/alkali metal perchlorate extraction system. The K_M(DB18C6)A value of the perchlorate is much greater for K⁺ than for Na⁺, and is much smaller than that of the picrate. The K_MLA value makes a negative contribution to the extractability of DB18C6 for MClO₄, whereas the value of the MLA distribution-constant does a major one. The partition behavior of M(DB18C6)ClO₄ obeys the regular solution theory. However, the M(DB18C6)ClO₄ complexes in the diluent of high dipole moment somewhat undergo the dipole–dipole interaction. DB18C6 always shows high extraction selectivity for KClO₄ over NaClO₄, which is governed largely by the much greater K_MLA value for K⁺ than for Na⁺. The K⁺ extraction-selectivity of DB18C6 over Na⁺ for perchlorate ions is comparable to that for picrate ions. By comparing this perchlorate system with the picrate one, the anion effects on the extraction-efficiency and -selectivity of DB18C6 for Na⁺ and K⁺ was discussed in terms of the fundamental equilibrium constants.     

Synthesis and crystal structure of ammonium (picrate) (dibenzo-18-crown-6)

NH₄(Pic)(DB18C6) (Pic=picrate and DB18C6=dibenzo-18-crown-6), (C₂₆H₃₀N₄O₁₃) FW 606.56, arthorhombic,Pmn2₁,a=26.045(5),b=12.055(3),c=8.982(3) Å,V=2820(1) ų,Z=4,D _c =1.429 g/cm³, CuK, =1.54184 Å, (CuK)=9.5 cm^–1,F(000)=1272,T=298 K. The structure has been refined toR=0.0475 for 2617 unique observed reflections. In the lattice the 1:1 complex exists as a 2:2 dimer in which the crown are coupled through the Pic anions and NH₄ ⁺ cations. The asymmetric unit consists of two independent half crown ethers of which two opposite O atoms are on the mirror plane, two half ammonium cations of which the N and two H atoms are also on the mirror plane while the Pic anion is in a general position. Relative to each other, the corwn ethers are shifted by about 7.3 Å alongb and 1 Å alongc. The 1:1 sandwich of NH₄ with DB18C6 and Pic on dimerisation becomes a club pseudo-sandwich with three phenyl rings on either side of the mirror plane, thus forming a nearly parallel stack with a 3.6 Å inter-ring distance. The NH₄ ions hold the structure; two H atoms on the mirror plane are hydrogen-bonded to the opposite oxygens of the crown located on the purely aliphatic part of the ring (2.10(1), 2.06(3) and 2.26(3), 2.05(1) Å) for the two independent crowns, respectively, while the other two H atoms form mirror-related bifurcated hydrogen bonds with the phenoxide oxygen (1.99(1) and 2.01(1) Å) and theo-nitrogen oxygen (2.44(2) and 2.34(1) Å) of the picrates. Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82037 (29 pages)     

Interactions of ligands with lithium picrate in dioxane

The interaction of several cyclic ethers, poly(ethylene glycol dimethyl ether)s, nitriles, esters and ketones with lithium picrate in dioxane at 25°C was investigated by means of a competition method using cross-linked poly(ethylene oxide) (PEO) as the insoluble ligand. The Langmuir-Klotz isotherm of LiPi binding to the PEO gel gave an apparent binding constant, K, as a function of the concentration of added ligand. Plots of 1/K vs. the ligand concentration then yielded the formation constant, K _L , of the ligand complex with LiPi. The results demonstrate that in dioxane one ligand molecule binds to LiPi. While there is some dependence on the Gutmann donicity number of the respective ligands, steric factors often play a dominant role in determining the value of K _L . The results were compared with complexation data obtained on similar systems but by different methods.