Xian Xin Zhang

Approaches to improvement of metal ion selectivity by cryptands

Abstract Several approaches to improvement of metal ion selectivity by mixed oxygen–nitrogen donor cryptands are discussed. Because of the relatively rigid structures of cryptands, thermodynamic stabilities of the cryptate complexes strongly depend on the match of the cation size and cryptand cavity diameters. Symmetry is an important factor influencing the properties of cryptand complexation. One rule to improve metal ion selectivity by oxygen–nitrogen donor cryptands is to obtain the ligand that has the best size-match cavity with the cation while keeping a symmetric spherical-coordination array. High selectivity for a small cation can be obtained when the cryptand is able to form a number of six-membered chelate rings with the metal ion while the requirements of the high-symmetric donor atom array and size-matched cavity are met. On the other hand, introduction of benzene rings and other groups to cryptands usually decreases metal ion binding and selectivity.

Enantiomeric recognition of chiral ammonium salts by chiral pyridino- and pyrimidino-18-crown-6 ligands: Effect of structure and solvents

Chiral pyridino- 18-crown-6 ligands interact with chiral primary organic ammonium salts by hydrogen bonding from the ammonium cation to the pyridino nitrogen and two alternate ring oxygen atoms. Enantiomeric recognition in these interactions are caused by the steric bulk of the substituents at chiral macrocycle ring positions. Recognition is best for the interaction of chiral pyridino-18-crown6 hosts with the enantiomers of a-( 1-naphthylethy1)ammonium perchlorate (NapEtHC10,) over (a-phenylethy1)ammonium perchlorate (PhEtHClO,) possibly because of a greater x-x overlap between the naphthalene ring of the guest and pyridine ring of the host. Solvents play an important role in the degree of recognition. A binary solvent composed of 7/3 &H4C12/CH,0H (v/v) gave an enhanced degree of recognition. A new chiral pyrimidino- 18-crown-6 ligand exhibited recognition for the enantiomers of NapEtHC10,.

A thermodynamic study of complexion of alkali and alkaline-earth metal ions with low-symmetry cryptands

Abstract Log K, ΔH and TΔS values for interactions of two low-symmetry cryptands, [3.2.1] and [4.2.2], with alkali and alkaline-earth metal ions have been determined at 25°C by calorimetric titration in a 95:5 (vol./vol.) methanol/water (95% M/W) solvent and an aqueous solution. Cryptan [3.2.1] forms very stable complexes with alkali metal ions (except Li+), Sr2+ and Ba2+ in 95% M/W. The log K values have the sequences of K+>Rb+>Na+>Cs+>Li+ and Ba2+>Sr2+. Because of its large size, [4.2.2] forms less stable complexes than [2.2.2], [3.2.1] and [3.2.2] with Na+, K+ and Ba2+ but a highly stable complex with Cs+ is formed. Cryptand [3.2.1] is an isomer of cryptand [2.2.2], but it shows cation-binding properties different from those of [2.2.2] due to its low symmetry, [3.2.1] binds Li+ and Cs+ more strongly but Na+, K+, Rb+, Sr2+ and Ba2+ more weakly than does cryptand [2.2.2]. The decrease in log K values for [3.2.1] interactions with Na+, K+, Rb2+ and Ba2+ results from decreased enthalpic gains suggesting a larger steric strain for the [3.2.1] complexes than for the [2.2.2] ones. The K+/Na+ selectivity factors of [3.2.1] and [4.2.2] are lower than that of [2.2.2]. The study shows that symmetry is an important factor in cryptand complexation and selectivity. Cryptands of high symmetry usually show better complexation and selectivity properties than those of low symmetry. Thermodynamic quantities give some insight into cryptand complexation with the metal ions. The log K values for interaction of [3.2.1] with the cations decrease dramatically in aqueous solution, which is caused by large decreases in enthalpic gains.

Thermodynamics of the interaction of 18-crown-6 with K+, Ti+, Ba2+, Sr2+ and Pb2+ from 323.15 to 398.15 K

The interaction of 18-crown-6 (18C6) with Tl+, K+, Sr2+, Pb2+ and Ba2+ in aqueous solution at 323.15, 348.15, 373.15 and 398.15 K, and at 1.52 MPa has been investigated using an isothermal flow calorimetry technique. Equilibrium constant (K), enthalpy change (ΔH°), entropy change (ΔS°) and heat capacity change (ΔC°p) values were determined for the interaction 18C6 + Mn+= 18C6Mn+. This reaction is exothermic for the complexation of each of the cations in the temperature range studied. The ΔH° and ΔS° values decrease (become more negative) with increasing temperature in most cases. The complexes become less stable at higher temperatures as indicated by the decrease in log K values with temperature. The stability sequences for the metal ions reacting with 18C6 are the same at all temperatures, i.e. Pb2+ > Ba2+ > Sr2+ > Tl+ > K+. The association of 18C6 with these cations is enthalpy driven. The decrease in ΔH° and ΔS° values with temperature is the result of the formation of a highly organized water structure around the complexes due to ion–dipole, dipole–dipole and hydrophobic interactions. The charge densities of the cations are important in the explanation of the trends of solvation/desolvation, and the trends in ΔH° and ΔS° with temperature. Cations with higher charge densities have larger desolvation effects as complexation occurs. These effects may result in increased ΔH° and ΔS° values with increasing temperature. The ΔH° and ΔS° values go through a maximum as the temperature increases for the complexation of 18C6 with Ba2+ and Pb2+, suggesting the trade off between complexation and desolvation in determining the magnitude of the ΔH° and ΔS° values.

A thermodynamic study of enantiomeric recognition of organic ammonium cations by pyridino-18-crown-6 type ligands in methanol and a 1: 1 methanol-1,2-dichloroethane mixture at 25.0°C

LogK, ΔH, andTΔS values for interactions of (R) and (S) enantiomers of α-(1-naphthyl)ethylammonium perchlorate (NapEt), α-phenylethylammonium perchlorate (PhEt), and the hydrogen perchlorate salt of 2-amino-2-phenylethanol (PhEtOH) with a series of chiral and achiral pyridino-18-crown-6 type ligands and 18-crown-6 (18C6) were determined from calorimetric titration data valid in methanol and in a 1: 1 (v/v) methanol-1,2-dichloroethane (MeOH-1,2-DCE) mixture at 25.0°C. In the MeOH-1,2-DCE solvent mixture, the chiral macrocyclic ligands exhibit excellent recognition for enantiomers of the three organic ammonium cations as shown by large differences in logK values (Δ logK) which range from 0.4 to 0.6 (2.5- to 4.0-fold difference in binding constants). The Δ logK values in the solvent mixture MeOH-1,2-DCE are increased by 0.1–0.5 logK units over those in absolute methanol, indicating a favorable effect of 1,2-dichloroethane on enantiomeric recognition. In 1,2-dichloroethane, however, the interactions are too strong (logK>6) to observe the degree of recognition by a direct calorimetric method. Complexation of organic ammonium cations by these macrocyclic ligands is driven by favorable enthalpy changes. The entropy changes ure unfavorable in all cases. The thermodynamic origin of enantiomeric recognition for NapEt in 1:1 (v/v) MeOH-1,2-DCE is enthalpic, but those for PhEt and PhEtOH are entropic. Effects of the ligand structure and flexibility and of the organic cation structure on recognition and complex stability are discussed on the basis of the thermodynamic quantities. Different thermodynamic behaviors of achiral 5 and 18C6 from those of chiral macrocyclic ligands indicate a difference between chiral and achiral macrocycle interactions with the chiral organic ammonium cations. The different thermodynamic behavior of NapEt enantiomers compared to those of PhEt and PhEtOH enantiomers supports the idea that the solution complex conformation of NapEt is different from those of PhEt and PhEtOH. π-π interaction is absent for the PhEt and PhEtOH complexes with diesterpyridino-18-crown-6 ligands in solution. Therefore, the higher degree of enantiomeric recognition for NapEt than for either PhEt or PhEtOH by these chiral macrocyclic ligands is a result of the presence of π-π interaction in the NapEt system.

Thermodynamic and NMR Studies of Solvent Effect on Enantiomeric Recognition for a Chiral Organiz Ammonium Cation by Chiral Diketopyridino-18- Crown-6 Type Ligands at 25.0 deg C.

Abstract Three chiral diketopyridino-18-crown-6 type macrocycles have been shown to exhibit a high degree of enantiomeric recognition toward α-(1-naphthyl)ethylammonium perchlorate (NapEt) in various ratios of chloroform/methanol (CDCl3/CD3OD) and 1,2-dichloroethane/methanol (C2H4Cl2/CH3OH) solvent mixtures (from 100% to 10% methanol component). In most cases, differences in log K values (Δlog K) for (R)− and (S)-NapEt complexation with the chiral macrocycles are larger than 0.5. The degree of the enantiomeric recognition indicated by the Δlog K value changes noticeably with the binary solvent components. The recognition is better in the solvents having a moderate methanol component than in the binary solvents having either a high or a low methanol component. The highest degree of recognition is observed in 6/4 (v/v) CDCl3/CD3OD and C2H4Cl2/CH3OH solvent mixtures and in a 7/3 (v/v) C2H4Cl2/CH3OH mixture for chiral (S,S)-1 macrocycle. Thermodynamic parameters determined in solvent mixtures of C2H4Cl2/CH3OH...

New 2-methylenepropylene-bridged cryptands with high sodium ion selectivity: A thermodynamic study of complexation

Thermodynamic quantities for the interactions of mono- and tri(2-methylenepropylene)-bridged cryptands, cryptand [3.3.1], cryptand [2.2.2], and 18-crown-6-with Na+, K+, Rb+, and Cs+ have been determined by calorimetric titration in an 80:20 (v/v) methanol: water solution at 25°C. Incorporation of the 2-methylenepropylene (−CH2C(=CH2)CH2−) bridge(s) into cryptand [2.2.2] results in a large change in the ligand-cation binding properties. Tri(2-methylenepropylene)-bridged cryptand [2.2.2] (2) shows high selectivity factors for Na+ over K+ and other alkali cations, while 2-methylenepropylene-bridged cryptand [2.2.2.] (1) selects K+ over Na+, as does cryptand [2.2.2]. The K+/Na+ selectivity is reversed with increasing number of 2-methylenepropylene bridges. This observation indicates that increasing the number of 2-methylenepropylene bridges on cryptand [2.2.2] favors complexation of a small cation over a large one. The logK values for the formation of 1 and 2 complexes (except 1-Cs+ and 2-Na+) decrease as compared with those for the corresponding [2.2.2] complexes. Formation of six-membered chelate ring(s) by the propyleneoxy unit(s) of 1 and 2 with a cation stabilizes the cryptate complexes of the small Na+ and destabilizes the complexes of large alkali metal cations. Thermodynamic data indicate that the stabilities of the cryptate complexes studied are dominated mostly by the enthalpy change. In most cases, both stabilization of Na+ complexes and destabilization of the complexes of large alkali metal cations by six-membered chelate ring(s) also result from an enthalpic effect. Cryptand [3.3.1] shows a selectivity for K+ over Cs+, despite its two long CH2(CH2OCH2)3CH2 bridges. The [3.1] macroring portion of [3.3.1]may be too small to effectively bind the Cs+, resulting in the low stability of the Cs+ complex.

Complexation of Metal Ions with Azacrown Ethers Bearing an 8-Hydroxyquinoline Side Arm

Thermodynamic quantities (log, K, ΔH, and TΔS) for theinteractions of six azacrown ethers each bearing an 8-hydroxyquinoline (CHQ)side arm (1-6) with Na+, K+, Ba2+, and Cu2+ were determined by calorimetrictitration in methanol solution at 25°C. The results indicate that theseligands form stable complexes with the cations studied. Ligands 1 and 3 thathave CHQ attached through position 7 (next to the OH group) show highselectivity for Cu2+ (log K values of 8.12 and 9.44, respectively) over Na+,K+, and Ba2+ by more than four orders of magnitude. On the other hand,ligands 2 and 4 that have CHQ attached through position 2 (next to thequinoline nitrogen group) form more stable complexes with Na+, K+, and Ba2+,but less stable complexes with Cu2+, than ligands 1 and 3. All ligandsinteract more strongly with K+ than with Na+. The K+/Na+ selectivity forligands 4 and 5 is about 1.5 log K units. All complexation reactions displaynegative enthalpy changes. In most cases the entropy changes are alsonegative, indicating that formation of the complexes is enthalpy driven. 1HNMR spectral experiments demonstrate coordination of the cations by alldonor atoms of the ligands including those of the CHQ arm. In all cases, theOH signal is observed in the 1H NMR spectra, suggesting that thecomplexation with the cations does not involve deprotonation of the CHQgroups in the ligands.

THERMODYNAMIC STUDY OF COMPLEXATION OF ALKALI AND ALKALINE-EARTH METAL IONS WITH BENZENE-CONTAINING CRYPTANDS

Abstract Log K , Δ H , and Δ S values for interactions of four benzene-containing cryptands ( 1–4 ) with alkali metal ions, Sr 2+ , and Ba 2+ were determined at 25°C by calorimetric titration in absolute methanol. The cryptands form stable complexes with the metal ions (except Li + ). Log K values range from 2.90 (2-Cs + interaction) to 5.10 (4-K + interaction). Very low reaction heats for Li + complexation with 1–4 indicate that the cavity sizes of these cryptands are too large to match the small Li + . Different structures of the benzo bridges of the cryptands have an effect on cation complexation. All cryptands studied selectively bind K + over the other cations. Cryptand 4 selects K + over Ba 2+ by a factor of 10, which is a reversal of the common Ba 2+ over K + selectivity order demonstrated by most [2,2,2]-type cryptands. Cryptands 1 and 2 show more negative (favorable) Δ H values and more negative or less positive (unfavorable) Δ S values for the interactions with the metal ions (except Li + and 2-Cs + ) than cryptands 3 and 4 .

Crystal Structures of Cs+-Crown Ether Complexes Containing Polynuclear Mercury Iodide Anions

Reactions of CsI and HgI2 with benzo-15-crown-5 (B15C5) and 15-crown-5 (15C5) in an ethanol-acetone mixture produced [Cs(B15C5)2]2[Hg2I6] (1) and {[Cs(15C5)]2[Hg2I6]}n (2), respectively. The structures of the two complexes are quite different. Molar ratios of Cs+ : crown ether are 1 : 2 in 1 and 1 : 1 in 2. Complex 1 consists of two Cs(B15C5)2+ cations and a Hg2I62- anion. Cs+ lies between the two crown-5 ligands, resulting in a sandwich-type cation. Cationic Cs(B15C5)2+ and anionic Hg2I62- are linked together by electrostatic interactions and the complex 1 is an ion pair compound. Complex 2 consists of infinite [Cs(15C5)]2[Hg2I6] units. Each structural unit contains two Cs(15C5)+ cations and a Hg2I62- anion. Cs+ is coordinated by five oxygen atoms of 15C5, three iodine atoms of Hg2I62-, and an iodine atom of Hg2I62- in an adjacent structural unit. The interactions between the Cs+ of Cs(15C5)+ and an I− in Hg2I62- from adjacent structural units polymerize the complex 2, resulting in a one-dimensional network structure. The anions of Hg2I62- in both complexes are similar. The two mercury atoms are linked through two bridging iodine atoms and each mercury is also coordinated by two terminal iodines. Crystal data for 1: space group P21/c (No. 14), a = 12.253(4), b = 20.945(7), c = 16.110(6) Å, β = 111.0(1)°, V = 3860 Å3, Z = 4, R = 0.082 (Rw = 0.089). Crystal data for 2: space group P21/c (No. 14), a = 12.157(4), b = 8.546(4), c = 20.666(6) Å, β = 91.54(3)°, V = 2146 Å3, Z = 4, R = 0.034 (Rw = 0.048).