Extraction of sodium and potassium picrates with 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (lariat 16C5) into various organic solvents. Elucidation of fundamental equilibria which determine the extraction ability for Na(+) and K(+) and the selectivity

To quantitatively elucidate the effects of the side chains and diluents on the extraction selectivity for sodium and potassium picrates of 15-(2,5-dioxahexyl)-15-methyl-16-crown-5 (L16C5) from the viewpoint of equilibrium, the constants for the overall extraction (K(ex)), the partition for various diluents of low dielectric constants (K(D,MLA)), and the aqueous ion-pair formation (K(MLA)) of L16C5-sodium and -potassium picrate 1:1:1 complexes were determined at 25 degrees C; the distribution constants of L16C5 were also measured at 25 degrees C. The log K(MLA) values for Na(+) and K(+) are 2.74+/-0.29 and 1.70+/-0.36, respectively. In going from 16-crown-5 (16C5) to L16C5, the side chains decrease the K(MLA) value, but do not increase the difference in K(MLA) between Na(+) and K(+). The distribution behavior of L16C5 and its 1:1:1 complexes with the alkali metal picrates closely obeys regular solution theory, except for chloroform. Molar volumes and solubility parameters of L16C5 and the 1:1:1 complexes were determined. The magnitude of K(ex) is mainly governed by the K(M(L16C5)A) value. For every diluent, L16C5 shows Na(+) extraction selectivity over K(+). The Na(+) extraction selectivity of L16C5 is determined completely by K(M(L16C5)A). The extraction ability and selectivity for sodium and potassium picrates by L16C5 are compared with those of 16C5 on the basis of the fundamental equilibrium constants.     

Extraction of sodium and potassium picrates with 16-crown-5 into various diluents. Elucidation of fundamental equilibria determining the extraction selectivity for Na(+) over K(+)

The constants of the overall extraction equilibrium (K(ex)), the partition for various diluents having low dielectric constants (K(D,MLA)), the aqueous ion-pair formation (K(MLA)), and the dimer formation in CCl(4) of 16-crown-5 (16C5)-alkali metal (Na, K) picrate 1:1:1 complexes were determined at 25 degrees C; the distribution constants of 16C5 were also measured at 25 degrees C. The logK(MLA) of Na and K are 4.14+/-0.19 and 3.05+/-0.28, respectively. The partition behavior of 16C5 and its 1:1:1 complexes with the alkalimetal picrates can be explained by regular solution theory, except for CHCl(3); the molar volumes and solubility parameters of 16C5 and the 1:1:1 complexes were determined. The magnitude of K(ex) largely depends on that of K(MLA). For every diluent, 16C5 always shows Na(+) extraction-selectivity over K(+). The K(MLA) value most contributes to the extraction selectivity of 16C5 for Na(+) over for K(+) among the three fundamental equilibrium constants, the aqueous 1:1 complex-formation constant of 16C5 with the alkali metal ion, K(MLA), and K(D,MLA). Furthermore, correct contributions of a methylene group to distribution constants of organic compounds between diluents of low dielectric constants and water were determined by the distribution constants of 16C5 and 15-crown-5; the additivity of the contributions of functional groups to the partition constant of a crown ether was verified.     

Evaluation of the Hydrophilic Property of Sodium Ion-pair Complexes with 3m-Crown-m Ethers (m = 5, 6) and Their Benzo-derivatives by Solvent Extraction

The ion-pair formation constants {K(j)(0): j = MA (metal salt), MLA} of NaO(2)CCF(3) (Na(+)tfa(-)) and its ion-pair complexes (MLA) in water (w) were determined potentiometrically at 25 degrees C and an ionic strength (I) of zero. 15-Crown-5 (15C5), 18-crown-6 ethers (18C6), and their mono-benzo derivatives were used as crown ethers (L). The extraction of Natfa by these four L from w into 1,2-dichloroethane was done at 25 degrees C, and then the extraction constants (K(ex)) for NaLtfa were calculated by using the K(j) values, which were estimated from the corresponding K(j)(0) ones at I of the w-phases, and other equilibrium constants. Also, the distribution constants (K(D,MLA)) of NaLtfa between the two phases were obtained from a thermodynamic cycle expressing K(ex). An interaction of w-molecules with NaLA was considered using a relation of log K(D,MLA) with log K(D,L), derived from the Scatchard-Hildebrand equation, where K(D,L) denotes the distribution constant of L between the two phases. The interaction increased in the order of NaL (picrate) < free L 相似文献   

Extraction of Alkali Metal (Na—Cs) Picrates with Dibenzo-18-crown-6 into Various Organic Solvents. Elucidation of Fundamental Equilibria which Govern the Extraction-Ability and -Selectivity

In order to quantitatively investigate effects of the size, the structuralrigidity, and the lipophilicity of dibenzo-18-crown-6 (DB18C6) on itsextraction-ability and -selectivity for alkali metal ions, constants of theoverall extraction (Kex), the distribution for various diluents of lowdielectric constants (K_D,MLA), and the aqueous ion-pairformation (K_MLA) of DB18C6-alkali metal (Na-—Cs) picrate 1:1:1 complexes were determined at 25°C; the partition constants of DB18C6 itself were also measured at 25°C. The log K_MLA of Na, K, Rb, and Cs are -0.14 ± 0.11, 1.30 ± 0.10, 1.00 ± 0.09, and 0.24 ± 0.11, respectively. The partition behavior of DB18C6 and its1:1:1 complexes with the alkali metal picrates can be clearly explained byregular solution theory, except for chloroform. The molar volumes andsolubility parameters of DB18C6 and the 1:1:1 complexes were determined.A relation between molar volumes of the complexes and K_MLAis discussed. The magnitude of Kex is largely determined by that ofK_D,MLA. For every diluent, the extraction selectivity of DB18C6increases in the order Na > Cs > Rb > K. The K extraction-selectivity of DB18C6 over Na is the highest among all the combinations of the two neighboring alkali metals in the periodic table. The extraction-ability and -selectivity for the alkalimetal picrates and their change with the diluent of DB18C6 were completely elucidated by the four fundamental equilibria and regular solution theory.     

Extraction equilibria of various metal picrates with 19-crown-6 between benzene and water. Effect of the extra methylene group on extraction ability and selectivity

The overall extraction equilibrium constants (K(ex)) of picrates of Li(+), Na(+), K(+), Rb(+), Cs(+), Ag(+), Tl(+), and Sr(2+)with 19-crown-6 (19C6) were determined between benzene and water at 25 degrees C. The K(ex) values were analyzed into the constituent equilibrium constants, i.e. the extraction constant of picric acid, the distribution constant of the crown ether, the formation constant of the metal ion-crown ether complex in water, and the ion-pair extraction constant of the complex cation with the picrate anion. The effects of an extra methylene group of 19C6 on the extraction ability and selectivity are discussed in detail by comparing the constituent equilibrium constants of 19C6 with those of 18-crown-6 (18C6). The K(ex) value of 19C6 for each metal ion is lower than that of 18C6, which is mostly attributed to the higher lipophilicity of 19C6. The extraction ability of 19C6 for the univalent metal ions decreases in the order Tl(+)>K(+)>Rb(+)>Ag(+)>Cs(+)>Na(+)Li(+), which is the same as that observed for 18C6. The difference in logK(ex) between the univalent metals is generally smaller for 19C6 than for 18C6. The extraction selectivity of 19C6 is governed by the selectivity in the ion-pair extraction, whereas that of 18C6 depends on both the selectivities in the ion-pair extraction and in the complexation in water.     

Hydration effect on the ion-pair extraction of lithium picrate by hydrophobic benzo-15-crown-5 ether into various less-polar diluents.

The ion-pair formation constant (K(MLA)(0) in mol(-1) dm(3)) for Li(B15C5)(+) with a picrate ion (Pic(-)) in water was determined by potentiometry with a K(+)-selective electrode at 25 degrees C and an ionic strength of 0, where B15C5 denotes benzo-15-crown-5 ether. Using the concentration equilibrium constants, K(MLA), estimated from this value, the extraction constants (mol(-2) dm(6) unit) of about ten diluents were re-calculated from previously reported extraction data. Also, the distribution constants of an ion-pair complex, Li(B15C5)Pic, between water and the diluents were re-estimated. A disagreement in the determined K(MLA) value between a solvent-extraction method and potentiometry was explained in terms of the Scatchard-Hildebrand equation; it came from the fact that the hydration of Li(I) in Li(B15C5)Pic was larger than that of free B15C5 in water. Then, the previously determined value by the former method was re-estimated using the potentiometric K(MLA) value.     

Solvent effects on extraction of alkali metal picrates with 15-crown-5 into various organic solvents. Elucidation of fundamental equilibria which govern extraction-efficiency and -selectivity

Extractions of alkali metal (Na-Cs) picrates (MA) with 15-crown-5 (15C5) into various diluents of low dielectric constant were conducted at 25 degrees C. Using the extraction data, the ion-pair formation constants (K(MLA)) in water of 15C5-MA 1:1:1 complexes were determined by an equation derived from the regular solution theory (logK(MLA)=4.43+/-0.27 for Na, 3.27+/-0.42 for K, 3.58+/-0.35 for Rb, and 2.78+/-0.41 for Cs). The actual overall extraction equilibrium constants were obtained by considering the concentrations of the 1:1:1 15C5 complexes and the ion-pair formation between uncomplexed alkali metal and picrate ions in the aqueous phase. The distribution constants of the 15C5 complexes were calculated and their partition behavior is explained by the regular solution theory. Molar volumes and solubility parameters of 15C5 itself and the complexes were determined. Extraction-efficiency and -selectivity of 15C5 for alkali metal picrates were completely elucidated from the standpoint of equilibrium.     

Extraction of Alkali Metal (Li, Na, K) Picrates with Benzo-15-crown-5 into Various Organic Solvents. Elucidation of Fundamental Equilibria Determining the Extraction-ability and -selectivity

To quantitatively elucidate the effects of the benzo group on the extraction-selectively and -ability of benzo-15-crown-5 (B15C5)for alkali metal ions, the constants of the overall extraction (K_ex), thedistribution for various diluents having low dielectric constants (K_D,MLA), and the aqueousion-pair formation (K_MLA) of B15C5-alkali metal (Li, Na, K) picrate 1:1:1 complexes (MLA) weredetermined at 25 °C. The partition constants of B15C5were also measured at 25 °C. The log K_MLA values for Li⁺, Na⁺, and K⁺ are -0.32 ± 0.22, 2.66 ± 0.19, and 0.71 ± 0.47, respectively. In going from 15-crown-5 (15C5) to B15C5, the benzo group considerably decreasesthe K_MLA value for the same alkali metal ion. The distributionbehavior of B15C5 and its 1:1:1 complexes with the alkali metal picrates closely obeys regularsolution theory, omitting chloroform. Molar volumes and solubility parameters of B15C5and the 1:1:1 complexes were determined. For every diluent, the K_ex valuefor B15C5 increases in the order Li⁺ < K⁺ < Na⁺. K_D,MLA makes anunfavorable contribution to the Na⁺ extraction-selectivity of B15C5 because of the smallest molar volume of the Na(B15C5)A complex. The Na⁺ extraction-selectivity of B15C5 is determined completely by much the highest K_Na(B15C5)A value.The extraction-ability and -selectivity of B15C5 for the alkali metal picrates are compared with those of 15C5on the basis of the underlying equilibrium constants.     

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.     

Equilibrium studies on complexation in water and solvent extraction of zinc(II) and cadmium(II) with benzo-18-crown-6

The formation constants (K(ML)) in water of 1:1 complexes of benzo-18-crown-6 (B18C6) and 18-crown-6 (18C6) with Zn(2+) and Cd(2+), the sizes of which are much smaller than the ligand cavities, were determined at 25 degrees C by conductometry. Compared with Cd(2+), the crown ethers form more stable complexes with Zn(2+) although the size of Zn(2+) is less suited for the cavities. B18C6 forms a more stable complex with each metal ion than 18C6. Moreover, the extraction equilibria of these metal ions (M(2+)) with B18C6 (L) for the benzene/water system in the presence of picric acid (HA) were investigated at 25 degrees C. The association between L and HA in benzene was examined for evaluating the intrinsic extraction equilibria of M(2+) with B18C6. The extracted species were found to be MLA(2) and ML(2)A(2), and the overall extraction constants (K(ex,1) and K(ex,2), respectively) were obtained. The values of K(ex,1) for these metal ions are almost the same, but the K(ex,2) is larger for Zn(2+) than for Cd(2+). The extraction selectivity was interpreted quantitatively by the constituent equilibrium constants, i.e. K(ML), the ion-pair extraction constant of ML(2+) with A(-), and the adduct formation constant of MLA(2) with L in benzene.     

The solvent extraction of alkali metal picrates with 4,13-N,N'-dibenzyl-4,13-diaza-18-crown-6

Extraction of alkali metal picrates with N,N'-dibenzyl-18-crown-6 was carried out, with dichloromethane as water-immiscible solvent, as a function [ligand]/[metal cation]. The extractability of metal picrates (Li(+), Na(+), K(+), Rb(+), Cs(+)) was evaluated as a function of [L]/[M(+)]. The extractability of complex cation-picrate ion pairs decreases in this sequence: Li(+)>Rb(+)>Cs(+)>K(+)>Na(+). The overall extraction equilibrium constants (K(ex)) for complexes of N,N'-dibenzyl-18-crown-6 with alkali metal picrates between dichloromethane and water have been determined at 25 degrees C. The values of the extraction constants (logK(ex)) were determined to be 10.05, 6.83, 7.12, 7.83, 6.73 for Li(+), Na(+), K(+), Rb(+) and Cs(+) compounds, respectively. DB186 shows almost 2-fold extractability against Li(+) compared to the other metal picrates, whereas it shows no obvious extractability difference amongst the other metal cations when [L]/[M(+)] is 0.2-1. However, an increasing extractability is observed for Cs(+) when [L]/[M(+)] [1].     

Liquid-liquid distribution of ion associates of hexachloroplatinate(IV) with quaternary ammonium counter ions

The distribution behavior of ion associates of PtCl(6)(2-) with quaternary ammonium cations (Q(+)) between aqueous phase and two organic phases (chloroform and carbon tetrachloride) was examined, and the extraction constants (log K(ex)) were determined. A linear relationship was found between log K(ex) and the total number of carbon atoms in Q(+); from the slope of the line, the contribution of a methylene group to log K(ex) was calculated to be 0.6. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetraalkylammonium cations. From the extraction constants obtained, an extraction method based on the ion association of PtCl(6)(2-) with Q(+) can be used for the separation and determination of platinum at the 10(-6) M level.     

Spectral Properties of Extracted Complexes of MetalPicrates with Crown Ethers

The investigation on UV-visible spectra of species formed by extracting some metal picrates with benzo-15-crown-5(B15C5) and dibenzo-18-crown-6(DB18C6) verified that there are some interactions of picrate anion with K , Na and rare earth ions in loaded organic phase. By the study of the charge transfer band and absorption spectra of picrate anion, it can be determined whether an ion pair has been formed and either a 1 : 1 contact ion pair or a 1 : 2 crown-separated ion pair involved in organic phase can be distinguished for an ion-pair extraction.     

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.     

Liquid-liquid distribution of ion-associates of acidic dyes with quaternary ammonium counter-ions

Diprotic acid dyes [H(2)A: Bromophenol Blue (BPB), Bromochlorophenol Blue (BCPB), Bromocresol Purple (BCP), Bromocresol Green (BCG), Bromothymol Blue (BTB)] can be extracted as 1:1 ion-associates Q(+)HA(-) and 1:2 ion-associates (Q(+))(2)A(2-) with quaternary ammonium cations (Q(+)) into chloroform. The extraction constants (log K(ex)) of the 1:1 and 1:2 ion-associates have been determined. Linear relationships between log K(ex) and the number of methylene groups in the quaternary ammonium ions were observed; from the slope of the line, the contribution of a methylene group to log K(ex) was found to be 0.43-0.65. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetra-alkylammonium cations, for both the 1:1 and 1:2 ion-associates. From the extraction constants obtained, the extractability of acidic dyes was in the order BTB > BCG > BPB > BCPB > BCP.     

Second-Sphere Coordination of Ferrioxamine B and Association of Deferriferrioxamine B, CH(3)(CH(2))(4)NH(3)(+), NH(4)(+), K(+), and Mg(2+) with Synthetic Crown Ethers and the Natural Ionophores Valinomycin and Nonactin in Chloroform

The interaction of ferrioxamine B, FeHDFB(+), through a protonated amine side chain, with various host ionophore structures to form a host-guest complex in the second coordination shell has been investigated. Host-guest association constants (K(a)) in water saturated chloroform are reported for synthetic crown ethers with different cavity size and substituents (18-crown-6 and its dicyclohexano, benzo, and dibenzo derivatives; dibenzo and dicyclohexano derivatives of 24-crown-8; and dibenzo-30-crown-10). The natural ionophores valinomycin and nonactin were also found to form stable second-sphere complexes with ferrioxamine B in wet chloroform. Results are reported for both picrate and perchlorate salts of FeHDFB(+). Since the protonated amine side chain of ferrioxamine B may be viewed as a substituted amine, the host-guest association constants for FeHDFB(+) are compared to the interaction of Mg(2+), K(+), NH(4)(+), CH(3)(CH(2))(4)NH(3)(+), and H(4)DFB(+) with the same ionophores. This is the first report of nonactin complexation of this series of cations in an organic medium of low polarity and one of the few reports of valinomycin complexation. To the best of our knowledge these are the first reported stability constants for the association of (Mg(2+),2pic(-)) with natural and synthetic ionophores in chloroform. K(a) values for ferrioxamine B complexation by the synthetic crown ethers are influenced by ring size and substituent. Despite significant preorganization capabilities, the large cavities of valinomycin, nonactin and benzo-30-crown-10 do not form as stable host-guest assemblies with bulky substituted amine cations such as ferrioxamine B as does cis-dicyclohexano-18-crown-6.     

Ion-pair formation between Cd(II), Na(I), and Ag(I) complex ions with 18-crown-6 ether derivatives and various pairing anions in water: an understanding of the ion-pair formation based on the HSAB principle

The ion-pair formation constants (K(MLX)(0)/mol(-1) dm(3)) of CdL(2+) with Br(-) or NaL(+) with N,N-diethyldithiocarbamate ion (DDTC(-)) in water were determined potentiometrically at 25°C; ionic strength (I)→0: L denotes 18-crown-6 ether (18C6) and its mono-benzo derivative for the CdBr(2)-L system and 15-crown-5 ether and 18C6 for the NaDDTC-L one. The formation constant corresponding to the simple salt, NaDDTC, in water was also determined at I→0. Using the log K(CdLX)(0) values of CdLCl(+), CdLBr(+), CdLPic(+), and CdLSO(4), then CdL(2+) and picrate ion (Pic(-)) in water have been classified with the hard and soft acids and bases principle, where the values were available in the literature, except for CdLBr(+). The same classification was examined in NaX-L systems with X(-) = DDTC(-), trifluoroacetate ion, MnO(4)(-), ReO(4)(-), Pic(-), and BPh(4)(-) and the AgPic-L one. Consequently, CdL(2+), NaL(+), and AgL(+) were classified as the hard acids, while Pic(-) and BPh(4)(-) as the hard bases. These results reflected the reactivities of the complex ions in ion-pair formation with X(-) and SO(4)(2-) in water.     

Extraction of some univalent salts into 1,2-dichloroethane and nitrobenzene: analysis of overall extraction equilibrium based on elucidating ion-pair formation and evaluation of standard potentials for ion transfers at the interfaces between their diluents and water

Sodium permanganate, sodium picrate (NaPic), Bu(4)NPic, Me(4)NPic, and Et(4)NPic were extracted at an ionic strength of 2 × 10(-5) to 0.08 mol dm(-3) and 25°C from water (w)-phases into the organic (o)-ones, 1,2-dichloroethane (DCE) and nitrobenzene (NB). Thereby, apparent distribution constants (K(D,±)) of the anions (A(-)) or the cations (M(+)) and ion-pair formation ones (K(MA)(org)) of the univalent salts (MA) in the o-phases were determined at 25 °C, where K(D,±) = ([A(-)](o)[M(+)](o)/[A(-)][M(+)])(1/2) = (K(D,A)K(D,M))(1/2) and K(MA)(org) = [MA](o)/[M(+)](o)[A(-)](o). Also, the K(ex) and K(D,MA) values with A(-) = Pic(-), MnO(4)(-) were estimated from the relations K(ex) (= [MA](o)/[M(+)][A(-)]) = K(MA)(org)(K(D,±))(2) and = K(MA)K(D,MA), respectively. Standard potentials (Δψ(tr)(0)) for ion transfers at the w/DCE and w/NB interfaces were evaluated from the log K(D,A) or log K(D,M) values by assuming the relations K(D,Pic) = K(D,Et4N) and = K(D,Me4N), respectively. The thus-obtained Δψ(tr)(0) values, especially for the w/DCE system, were in good agreement with the values based on the extra-thermodynamic assumption for Ph(4)As(+) and BPh(4)(-) transfers at the interfaces. In the present extraction systems, the ion-pair formation of MA in the w- and o-phases was less effective in the determination of their distribution constants into the two o-phases.     

Liquid-liquid distribution of ion-associates of dichlorocuprate(I) with quaternary ammonium counter-ions

The dichlorocuprate(I) anion CuCl(-)(2) can be extracted as its ion-associates Q(+).CuCl(-)(2) with quaternary ammonium cations (Q(+)) into chloroform. The extraction constants K(ex) have been determined, and the log K(ex) values found for the various counter-ions used are 1.93 for (C(3)H(7))(4)N(+), 4.10 for (C(4)H(9))(4)N(+), 6.57 for (C(5)H(11))(4)N(+), 1.57 for C(8)H(17)N(+) (CH(3))(3), 2.83 for C(10)H(21)N(+) (CH(3))(3) 4.12 for C(12)H(25)N(+) (CH(3))(3) and 5.21 for C(14)H(29)N(+)(CH(3))(3), respectively. A linear relationship was found between log K(ex) and the total number of carbon atoms in Q(+); from the slope of the line, the contribution of a methylene group to log K(ex) was calculated to be 0.59. The extractability with alkyltrimethylammonium cations was larger than that with symmetrical tetra-alkylammonium cations and the difference in log K(ex) for two cations (one of each type) with the same number of carbon atoms was about 0.4. From the extraction constants obtained, the extractability of CuCl(-)(2) was found to lie between that of ReO(-)(4) and ClO(-)(4).     

Role of alkali metal cation size in the energy and rate of electron transfer to solvent-separated 1:1 [(M+)(acceptor)] (M+ = Li+, Na+, K+) ion pairs.

The effect of cation size on the rate and energy of electron transfer to [(M(+))(acceptor)] ion pairs is addressed by assigning key physicochemical properties (reactivity, relative energy, structure, and size) to an isoelectronic series of well-defined M(+)-acceptor pairs, M(+) = Li(+), Na(+), K(+). A 1e(-) acceptor anion, alpha-SiV(V)W(11)O(40)(5-) (1, a polyoxometalate of the Keggin structural class), was used in the 2e(-) oxidation of an organic electron donor, 3,3',5,5'-tetra-tert-butylbiphenyl-4,4'-diol (BPH(2)), to 3,3',5,5'-tetra-tert-butyldiphenoquinone (DPQ) in acetate-buffered 2:3 (v/v) H(2)O/t-BuOH at 60 degrees C (2 equiv of 1 are reduced by 1e(-) each to 1(red), alpha-SiV(IV)W(11)O(40)(6-)). Before an attempt was made to address the role of cation size, the mechanism and conditions necessary for kinetically well behaved electron transfer from BPH(2) to 1 were rigorously established by using GC-MS, (1)H, (7)Li, and (51)V NMR, and UV-vis spectroscopy. At constant [Li(+)] and [H(+)], the reaction rate is first order in [BPH(2)] and in [1] and zeroth order in [1(red)] and in [acetate] (base) and is independent of ionic strength, mu. The dependence of the reaction rate on [H(+)] is a function of the constant, K(a)1, for acid dissociation of BPH(2) to BPH(-) and H(+). Temperature dependence data provided activation parameters of DeltaH = 8.5 +/- 1.4 kcal mol(-1) and DeltaS = -39 +/- 5 cal mol(-1) K(-1). No evidence of preassociation between BPH(2) and 1 was observed by combined (1)H and (51)V NMR studies, while pH (pD)-dependent deuterium kinetic isotope data indicated that the O-H bond in BPH(2) remains intact during rate-limiting electron transfer from BPH(2) and 1. The formation of 1:1 ion pairs [(M(+))(SiVW(11)O(40)(5-))](4-) (M(+)1, M(+) = Li(+), Na(+), K(+)) was demonstrated, and the thermodynamic constants, K(M)(1), and rate constants, k(M)(1), associated with the formation and reactivity of each M(+)1 ion pair with BPH(2) were calculated by simultaneous nonlinear fitting of kinetic data (obtained by using all three cations) to an equation describing the rectangular hyperbolic functional dependence of k(obs) values on [M(+)]. Constants, K(M)(1)red, associated with the formation of 1:1 ion pairs between M(+) and 1(red) were obtained by using K(M)(1) values (from k(obs) data) to simultaneously fit reduction potential (E(1/2)) values (from cyclic voltammetry) of solutions of 1 containing varying concentrations of all three cations to a Nernstian equation describing the dependence of E(1/2) values on the ratio of thermodynamic constants K(M)(1) and K(M)(1)red. Formation constants, K(M)(1), and K(M)(1)red, and rate constants, k(M)(1), all increase with the size of M(+) in the order K(Li)(1) = 21 < K(Na)(1) = 54 < K(K)(1) = 65 M(-1), K(Li)(1)red = 130 < K(Na)(1)red = 570 < K(K)(1)red = 2000 M(-1), and k(Li)(1) = 0.065 < k(Na)(1) = 0.137 < k(K)(1) = 0.225 M(-1) s(-1). Changes in the chemical shifts of (7)Li NMR signals as functions of [Li(5)1] and [Li(6)1(red)] were used to establish that the complexes M(+)1 and M(+)1(red) exist as solvent-separated ion pairs. Finally, correlation between cation size and the rate and energy of electron transfer was established by consideration of K(M)(1), k(M)(1), and K(M)(1)red values along with the relative sizes of the three M(+)1 pairs (effective hydrodynamic radii, r(eff), obtained by single-potential step chronoamperometry). As M(+) increases in size, association constants, K(M)(1), become larger as smaller, more intimate solvent-separated ion pairs, M(+)1, possessing larger electron affinities (q/r), and associated with larger k(M)(1)() values, are formed. Moreover, as M(+)1 pairs are reduced to M(+)1(red) during electron transfer in the activated complexes, [BPH(2), M(+)1], contributions of ion pairing energy (proportional to -RT ln(K(M)(1)red/K(M)(1)) to the standard free energy change associated with electron transfer, DeltaG degrees (et), increase with cation size: -RT ln(K(M)(1)red/K(M)(1)) (in kcal mol(-1)) = -1.2 for Li(+), -1.5 for Na(+), and -2.3 for K(+).