A Polarographic Study of Tl+, Pb2+ and Cd2+ Complexes with Aza-18-crown-6 and Dibenzopyridino-18-crown-6 in some Binary Mixed Non-aqueous Solvents

The complexation of Tl⁺, Pb²⁺and Cd²⁺ cations by macrocyclic ligands, aza-18-crown-6 (L1) and dibenzopyridino-18-crown-6 (L2) was studied in some binary mixtures of methanol (MeOH), n-propanol (n-PrOH), nitromethane (NM) and acetonitrile (AN) with dimethylformamide (DMF) at 22 °C using DC (direct current) and differential pulse polarographic techniques (DPP). The stoichiometry and stability constants of the complexes were determined by monitoring the shifts in half-waves or peak potentials of the polarographic waves of metal ions against the ligand concentration. In all of the solvent systems, the stability of the resulting 1:1 complexes was found to be L1 > L2. The selectivity order of the L2 ligand for the cations was found to be Pb²⁺ > Tl⁺ > Cd²⁺ and the selectivity of the L1 ligand for Pb²⁺ ion was greater than that of Tl⁺ ion. The results show that the stability of the complexes depends on the nature and composition of the mixed solvents. There is an inverse relationship between the stability constants of the complexes and the amount of dimethylformamide in the mixed solvent systems.     

A Polarographic Study of Tl + , Pb 2 and Cd 2+ Complexes with Dicyclohexano-18-Crown-6 in Some Binary Mixed Solvents

The complexes of Tl⁺, Pb²⁺ and Cd²⁺ cations with the macrocyclic ligand, dicyclohexano-18-crown-6\linebreak(DC18C6) were studied in water/methanol (H₂₊O/MeOH), water/1-propanol (H₂₊O/1-PrOH), water/acetonitrile (H₂₊O/AN), water/dimethylformamide (H₂₊O/DMF), dimethylformamide/acetonitrile (DMF/AN), dimethylformamide/methanol (DMF/MeOH), dimethylformamide/1-propanol (DMF/1-PrOH) and dimethylformamide/nitromethane (DMF/NM) mixed solvents at 22 °C using differential pulse polarography (DPP), square wave polarography and conductometry. In general, the stability of the complexes was found to decrease with increasing concentration of water in aqueous/non-aqueous mixed solvents with an inverse relationship between the stability constants of the complexes and the concentration of DMF in non-aqueous mixed solvents. The results show that the change in stability of DC18C6.Tl⁺, vs the composition of solvent in DMF/AN and DMF/NM mixed solvents is apparently different from that in DMF/MeOH and DMF/1-PrOH mixed solvents. While the variation of stability constants of the DC18C6.Tl⁺ and DC18C6.Pb²⁺ complexes vs the composition of H₂₊O/AN mixed solvents is monotonic, an anomalous behavior was observed for variations of log K_f vs the composition of H₂₊O/1-PrOH and H₂₊O/MeOH mixed solvents. The selectivity order of the DC18C6 ligand for the cations was found to be Pb²⁺ > Tl⁺ > Cd²⁺.     

Study of Complex Formation Between 18-Crown-6 and Pb2+, Tl+ and Cd2+ Cations in Binary Aqueous/Non-aqueous Solvents Using Polarographic Techniques (DPP and SWP)

The complexation of Pb²⁺, Tl⁺ and Cd²⁺ cationsby 18-crown-6 was studied in water/propanol (H₂O/PrOH),water/acetonitrile (H₂O/AN) and water/dimethylformamide(H₂O/DMF) binary systems at 20 °C using squarewave polarography (SWP) and differential pulse polarography (DPP).It was confirmed that the stoichiometry of each of the complexes formed between 18C6 and the respective cations is 1 : 1. The formation constants of the complexes were found to increase with increasing concentration of the non-aqueous solvent. In all cases, a stability order of Pb²⁺ > Tl⁺ > Cd²⁺ was observed. In general,the stabilities of individual complexes were found to decrease as the binary solvent mixture varied from H₂O/AN to H₂O/PrOH to H₂O/DMF.     

COMPETITIVE NMR STUDY OF THE COMPLEXATION OF SOME ALKALINE EARTH AND TRANSITION METAL IONS WITH 12-CROWN-4, 15-CROWN-5 AND BENZO-15-CROWN-5 IN ACETONITRILE SOLUTION USING THE LITHIUM-7 NUCLEUS AS A PROBE

Abstract

⁷Lithium NMR measurements were used to determine the stoichiometry and stability of Li⁺ complexes with 12-crown-4, 15-crown-5 and benzo-15-crown-5 in acetonitrile solution. A competitive ⁷Li NMR technique was also employed to probe the complexation of Mg²⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ ions with the same crown ethers. In all cases, the stability of the resulting 1:1 complexes was found to decrease in the order 15-crown-5 > benzo-15-crown-5 > 12-crown-4. Ca²⁺ and Cd²⁺ ions formed the most stable complexes in the series.     

Complex Formation of Potassium Hexachloroplatinate with 18-Crown-6 and Dibenzo-18-Crown-6 in Acetonitrile

The products of the reactions between potassium hexachloroplatinate {K₂PtCl₆} and 18-crown-6 or dibenzo-18-crown-6 in acetonitrile were studied. Pure crystalline compounds [2K·2(18-crown-6)· 2CH₃CN]²⁺·[PtCl₆]^2-·2H₂O, [2K·dibenzo-18-crown-6·CH₃CN]^{2 +}·[PtCl₆]^{2 -}, and [2K·dibenzo-18-crown-6·CH₃CN]^{2 +}·[Pt₂Cl₁₀]^{2 -} were obtained. Physicochemical properties of these compounds were studied, and their near- and far-IR IR spectra and thermogravimetric curves were considered. The composition of the complexes is determined by metal:ligand molar ratio and crown ether nature. It was found that acetonitrile is coordinated via the nitrogen atom.     

Competitive 7Li NMR Study on the Mn2+, Zn2+ and Cd2+ Complexes of Two New Branched Hexadentate (N6) Amines Containing the Pyridine Moiety in Nitromethane and Acetonitrile Solutions

Lithium-7 NMR spectroscopy was used to investigate the stoichiometry and stability of a Li⁺ complex with two new branched amines, 4,7-bis(2-pyridylmethyl)-4,7-diazadecane-1,10-diamine (L¹) and 4,8-bis(2-pyridylmethyl)-4,8-diazaundecane-1,11-diamine (L²), in acetonitrile and nitromethane. A competitive ⁷Li NMR method was also employed to probe the complexation of Mn²⁺, Zn²⁺ and Cd²⁺ ions with L¹ and L² in the same solvent systems. The formation constants of the resulting complexes were evaluated from computer fitting of the mole ratio data with an equation that relates the observed chemical shifts to the formation constant. In both solvents, the stability of the resulting 1:1 complexes was found to vary in the order Cd²⁺ > Zn²⁺ > Mn²⁺ > Li⁺.     

Conductometric studies of the thermodynamics complexation of Li+, Na+, K+, Mg2+, and Ba2+ ions with 4′,4″(5″)-Di-tert-butyldibenzo-18-crown-6 ligand in acetonitrile, ethanol and methanol solutions

The complexation reaction between 4′,4″(5″)-di-tert-butyldibenzo-18-crown-6, ligand and Li⁺, Na⁺, K⁺, Mg²⁺, and Ba²⁺ ions were studied conductometrically in acetonitrile, ethanol, and methanol solutions. The formation constants of the 1:1 and 2:1 complexes (metal to ligand) were calculated from the computer fitting of the molar conductance in various mole ratios at 10, 20, 30, and 40 °C. The enthalpy and entropy changes of the complexation reactions in acetonitrile, ethanol, and methanol were estimated at four different temperatures.     

Metal Ion Interaction of Two New Tritopic N2O13 Macrobicycles with B15C5 Moieties in Acetonitrile Solution

The synthesis of two new tritopic crown ligands (L1 and L2) bearing two benzo-15-crown-5 lateral moieties linked through a dibenzo-trioxa chain together with their interaction with metal ions, in acetonitrile and acetonitrile–water (50%, v/v) solutions is reported. The influence of K⁺, Na⁺, Li⁺, Ca²⁺, Ba²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Pb²⁺ and Al³⁺, on the spectroscopic properties of these diaza-polyoxa ligands was investigated by absorption spectrophotometry and in some cases by fluorescence emission spectroscopy. Coordination with alkaline (Na⁺, K⁺ and Li⁺) and alkaline earth (Ca²⁺and Ba²⁺) metal ions is assumed to be weak with both macrobicyclic ligands, while the interaction with both imine and amine derivatives causes a minor effect in the absorption spectra. Coordination with Cu²⁺, Zn²⁺ and Pb²⁺ in acetonitrile solution causes a major change in the absorption spectra of the chromophores. In the case of Cu²⁺, addition of the metal to L1 or L2 leads to a blue–violet complex in solution with an absorbance maximum centred at 590 nm. Interaction of the Schiff-base L1 with Pb²⁺ leads to a short wavelength shift in the absorption bands, comparable with the ZnL1 complex. Presence of transition metal ions such as Co²⁺, Ni²⁺and Cd²⁺ do not remarkably affect the absorption spectra of L1 and L2 in solution. Trivalent aluminium has a modest effect in the absorption bands of both N₂O₁₃ donor set bismacrocyclic ligands. The fluorescence study of L2 in the presence of Na⁺, K⁺, Ca²⁺, Ba²⁺, Co²⁺, Cu²⁺, Ni²⁺, Pb²⁺ and Al³⁺shows that Cu²⁺, Pb²⁺ and Al³⁺ complexes form non-fluorescent complexes.     

Cadmium-113 NMR and Theoretical Studies of Complexation of Cadmium Ion with 15-Crown-5 and Benzo-15-Crown-5 in Acetonitrile and Its Binary Mixtures with Water and Nitromethane

Cadmium-113 nuclear magnetic resonance was used as a sensitive probe to study the interaction of Cd²⁺ ion with 15-crown-5 and benzo-15-crown-5 in acetonitrile and its binary mixtures with water and nitromethane. The observed ¹¹³Cd-NMR chemical shift changes at a constant Cd²⁺ ion concentration and varying crown concentrations were found to be consistent with a fast exchange model. The formation constants of the resulting 1:1 complexes were evaluated from computer fitting of the chemical shift-mole ration data to an equation which relates the observed chemical shifts to the formation constant. The geometries of the macrocyclic ligands and their cadmium complexes were optimized by an ab initio method, and the calculated binding energies of the resulting complexes were compared. Both the ¹¹³Cd-NMR and ab initio studies revealed that, in all cases, 15-crown-5 forms a more stable Cd²⁺ complex than benzo-15-crown-5. In the case of the both crown ethers, there is an inverse relationship between the stability of complexes and solvating ability of the solvent systems used.     

Conductance Studies on Complex Formation Between Aza-18-Crown-6 with Ag+, Hg2+ and Pb2+ Cations in DMSO–H2O Binary Solutions

The complexation reactions between Ag⁺, Hg²⁺ and Pb²⁺ metal cations with aza-18-crown-6 (A18C6) were studied in dimethylsulfoxide (DMSO)–water (H₂O) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes in most cases is 1:1(ML), but in some cases 1:2 (ML₂) complexes are formed in solutions. A non-linear behaviour was observed for the variation of log K _f of the complexes vs. the composition of the binary mixed solvents. Selectivity of A18C6 for Ag⁺, Hg²⁺ and Pb²⁺ cations is sensitive to the solvent composition and in some cases and in certain compositions of the mixed solvent systems, the selectivity order is changed. The values of thermodynamic parameters (ΔH _c^o, ΔS _c^o) for formation of A18C6–Ag⁺, A18C6–Hg²⁺ and A18C6–Pb²⁺ complexes in DMSO–H₂O binary systems were obtained from temperature dependence of stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents.     

Study of Complex Formation Between Dicyclohexano-18-Crown-6 (DCH18C6) with Mg 2+, Ca2+, Sr 2+, and Ba2+ Cations in Methanol–Water Binary Mixtures Using Conductometric Method

The complexation reactions between Mg²⁺,Ca²⁺,Sr²⁺ and Ba²⁺ metal cations with macrocyclic ligand, dicyclohexano-18-crown-6 (DCH18C6) were studied in methanol (MeOH)–water (H₂O) binary mixtures at different temperatures using conductometric method . In all cases, DCH18C6 forms 1:1 complexes with these metal cations. The values of stability constants of complexes which were obtained from conductometric data show that the stability of complexes is affected by the nature and composition of the mixed solvents. While the variation of stability constants of DCH18C6-Sr ²⁺ and DCH18C6-Ba²⁺versus the composition of MeOH–H₂O mixed solvents is monotonic, an anomalous behavior was observed for variations of stability constants of DCH18C6-Mg²⁺ and DCH18C6-Ca²⁺ versus the composition of the mixed solvents. The values of thermodynamic parameters (ΔH_c°, ΔS_c°) for complexation reactions were obtained from temperature dependence of formation constants of complexes using the van’t Hoff plots. The results show that in most cases, the complexation reactions are enthalpy stabilized but entropy destabilized and the values of thermodynamic parameters are influenced by the nature and composition of the mixed solvents. The obtained results show that the order of selectivity of DCH18C6 ligand for metal cations in different concentrations of methanol in MeOH–H₂O binary system is: Ba²⁺>Sr²⁺>Ca²⁺> Mg²⁺.     

Complexation studies of Mn2+, Zn2+ and Cd2+ ions with a series of tetradentate (N4) Schiff base ligands containing pyridine moiety in acetonitrile and nitromethane solutions by a competitive NMR technique using 7Li nucleus as a probe

Lithium-7 NMR spectroscopy was used to investigate the stoichiometry and stability of a Li⁺ complex with N¹,N²-bis(pyridin-2-ylmethylene)ethane-1,2-diamine (L¹), N¹,N³-bis(pyridin-2-ylmethylene)propane-1,3-diamine (L²) and N¹,N⁴-bis(pyridin-2-ylmethylene) butane-1,4-diamine (L³) in acetonitrile (AN) and nitromethane (NM) solutions. A competitive ⁷Li NMR method was also employed to probe the complexation of Mn²⁺, Cd²⁺ and Zn²⁺ ions with L¹, L² and L³ in the same solvents. The formation constants of the resulting complexes were evaluated from computer fitting of the mole ratio data to an equation that relates the observed chemical shifts to the formation constant. In both solvents, the stability of the resulting 1:1 complexes were found to vary in the order Zn²⁺>Cd²⁺>Mn²⁺>Li⁺. In addition, the stability of M–L complexes of M²⁺ with the Schiff base ligands found to vary in the order M²⁺–L¹ > M²⁺–L² > M²⁺–L³.     

Crown-containing butadienyl dyes 8. Structures and complexation of chromogenic dithia-15(18)-crown-5(6) ethers

The structures of new butadienyl dyes of the benzothiazole series containing the dithia-15-crown-5 (2a) or dithia-18-crown-6 (2b) fragments were established by X-ray diffraction. Complexation of dyes 2a,b with Hg²⁺, Pb²⁺, Cd²⁺, Ag⁺, Zn²⁺, and alkaline-earth cations in aqueous-acetonitrile solutions was studied by spectrophotometry. At a high percentage of water in solutions (P _w ≈ 50%), these dyes have a very low ability to bind Pb²⁺ cations (logK < 2) and virtually do not bind Cd²⁺, Zn²⁺, and alkaline-earth cations. At the same time, these dyes form stable 1: 1 complexes with Hg²⁺ and Ag⁺ cations at all P _w. The stability constants of complexes with the Ag⁺ cation increase with increasing P _w because the free energy of hydration of this cation is much lower than the free energy of solvation in acetonitrile. In the P _w range from 0 to 75%, the stability constants of the complexes of dyes 2a,b with the Hg²⁺ cation are larger than those of the corresponding complexes with the Ag⁺ cation by more than four orders of magnitude. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 90–96, January, 2006.     

NMR study of the stoichiometry and stability of complexation reaction between Mg2+, Ca2+, Sr2+ and Ba2+ ions and 60-crown-20 in binary acetonitrile solvent mixtures

Proton nuclear magnetic resonance (¹H-NMR) spectroscopy was used to study the complexation reaction between Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ ions and 60-crown-20 in a series of binary mixtures of deuterated acetonitrile (AN), nitromethane (NM) and D₂O at 27?°C. Formation constants of the 1:1 complexes were determined through computer fitting of the chemical shift/mol ratio data and found to vary in the order of Ba²⁺?>?Sr²⁺?>?Mg²⁺?≈?Ca²⁺. The influence of the solvent composition on the stability of the resulting complexes was also discussed. In all cases, the changes in the stability constants with the solvent composition were monotonic and showed a good correlation with the inherent solvation ability of the pure solvents which form the mixture.     

Study of Complex Formation between N-Phenylaza-15-Crown-5 with Mg2+, Ca2+, Ag+ and Cd2+ Metal Cations in Some Binary Mixed Aqueous and Non-aqueous Solvents using the Conductometric Method

The complexation reactions between Mg²⁺, Ca²⁺, Ag⁺ and Cd²⁺ metal cations with N-phenylaza-15-crown-5 (Ph-N15C5) were studied in acetonitrile (AN)–methanol (MeOH), methanol (MeOH)–water (H₂O) and propanol (PrOH)–water (H₂O) binary mixtures at different temperatures using the conductometric method. The conductance data show that the stochiometry of all of the complexes with Mg²⁺, Ca²⁺, Ag⁺ and Cd²⁺ cations is 1:1 (L:M). The stability of the complexes is sensitive to the solvent composition and a non-linear behaviour was observed for variation of log K _f of the complexes versus the composition of the binary mixed solvents. The selectivity order of Ph-N15C5 for the metal cations in neat MeOH is Ag⁺>Cd²⁺>Ca²⁺>Mg²⁺, but in the case of neat AN is Ca²⁺>Cd²⁺>Mg²⁺>Ag⁺. The values of thermodynamic parameters (ΔH _c ^o , ΔS _c ^o ) for formation of Ph-N15C5–Mg²⁺, Ph-N15C5–Ca²⁺, Ph-N15C5–Ag⁺ and Ph-N15C5–Cd²⁺ complexes were obtained from temperature dependence of stability constants and the results show that the thermodynamics of complexation reactions is affected by the nature and composition of the mixed solvents.     

Study of complex formation between 18-crown-6 and diaza-18-crown-6 with uranyl cation (UO2 2+) in some binary mixed aqueous and non-aqueous solvents

The complexation reaction between UO₂ ²⁺ cation with macrocyclic ligand, 18-crown-6 (18C6), was studied in acetonitrile–methanol (AN–MeOH), nitromethane–methanol (NM–MeOH) and propylencarbonate–ethanol (PC–EtOH) binary mixed systems at 25 °C. In addition, the complexation process between UO₂ ²⁺ cation with diaza-18-crown-6 (DA18C6) was studied in acetonitrile–methanol (AN–MeOH), acetonitrile–ethanol (AN–EtOH), acetonitrile–ethylacetate (AN–EtOAc), methanol–water (MeOH–H₂O), ethanol–water (EtOH–H₂O), acetonitrile–water (AN–H₂O), dimethylformamide–methanol (DMF–MeOH), dimethylformamide–ethanol (DMF–EtOH), and dimethylformamide–ethylacetate (DMF–EtOAc) binary solutions at 25 °C using the conductometric method. The conductance data show that the stoichiometry of the complexes formed between (18C6) and (DA18C6) with UO₂ ²⁺ cation in most cases is 1:1 [M:L], but in some solvent 1:2 [M:L₂] complex is formed in solutions. The values of stability constants (log K_f) of (18C6 · UO₂ ²⁺) and (DA18C6 · UO₂ ²⁺) complexes which were obtained from conductometric data, show that the nature and also the composition of the solvent systems are important factors that are effective on the stability and even the stoichiometry of the complexes formed in solutions. In all cases, a non-linear relationship is observed for the changes of stability constants (log K_f) of the (18C6 · UO₂ ²⁺) and (DA18C6 · UO₂ ²⁺) complexes versus the composition of the binary mixed solvents. The stability order of (18C6 · UO₂ ²⁺) complex in pure studied solvents was found to be: EtOH > AN ≈ NM > PC ≈ MeOH, but in the case of (DA18C6 · UO₂ ²⁺) complex it was : H₂O > MeOH > EtOH.     

Thermodynamics of Solvation of 18-Crown-6 and Its Alkali-Metal Complexes in Various Solvents

Heats of solution of 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) in acetonitrile, 1,2-dichloroethane, N,N-dimethylformamide, dimethyl sulfoxide, nitromethane, propylene carbonate, pyridine and water were measured at 25 °C and the enthalpies of the transfer of 18-crown-6 from waterto the aprotic solvents were derived. The thermodynamic quantities, G₁°, H₁° and T S₁°, for the formation of the[M(18-crown-6)]⁺ (M⁺ = Na⁺, K⁺, Rb⁺, Cs⁺, NH₄ ⁺) complexeswere determined by titration calorimetry in dimethyl sulfoxide containing0.1 mol dm^-3 (C₂H₅)₄NClO₄ as a constant ionic medium at 25 °C. These thermodynamic quantities suggest that the complexationof 18-crown-6 with the alkali-metal ions mainly reflects the different solvationof 18-crown-6 and also the different degree of solvent structure.     

Study of complexation of phenylaza-15-crwon-5, 4-nitrobenzo- 15-crown-5, and benzo-15-crown-5 with Ag+, Tl+ and Pb2+ ions in methanol by competitive potentiometry

The complexation reaction of phenylaza-15-crwon-5, 4- nitrobenzo- 15-crown-5, and benzo-15-crown-5 with Ag⁺, Tl⁺ and Pb²⁺ ions in methanol solution have been studied by a competitive potentiometric method. The Ag⁺/Ag electrode used both as an indicator and reference electrode in a concentration cell. The emf of cell monitored as the crown ethers concentration varies through the titration. The stoichiometry and stability constants of resulting complexes have been evaluated by MINIQUAD. The stoichiometry for all resulting complexes was 1:1. The stability of these metal ions with derivatives of 15-crown-5 are in order phenylaza-15-crown-5 > Benzo-15-crown-5 > 4-nitrobenzo-15-crown-5, and for the each used crown ethers are as Pb²⁺ > Ag⁺ > Tl⁺. The effect of the substituted group on the stability of resulting complexes was considered. The obtained results are novel and interesting.     

A Conductometric Study of Complexation Reactions Between Dibenzo-18-Crown-6 (DB18C6) with Cu2+, Zn2+, Tl+ and Cd2+ Metal Cations in Dimethylsulfoxide–Ethylacetate Binary Mixtures

The complex formation between Cu²⁺, Zn²⁺, Tl⁺ and Cd²⁺ metal cations with macrocyclic ligand, dibenzo- 18-crown-6 (DB18C6) was studied in dimethylsulfoxide (DMSO)–ethylacetate (EtOAc) binary systems at different temperatures using conductometric method. In all cases, DB18C6 forms 1:1 complexes with these metal cations. The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, Genplot. The non-linear behaviour which was observed for variations of log K _f of the complexes versus the composition of the mixed solvent was discussed in terms of changing the chemical and physical properties of the constituent solvents when they mix with one another and, therefore, changing the solvation capacities of the metal cations, crown ether molecules and even the resulting complexes with changing the mixed solvent composition. The results show that the selectivity order of DB18C6 for the metal cations in pure ethylacetate and pure dimethylsulfoxide is: Tl⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺ but the selectivity order is changed with the composition of the mixed solvents. The values of enthalpy changes (ΔH°_C) for complexation reactions were obtained from the slope of the van’t Hoff plots and the changes in standard enthalpy (ΔS°_C) were calculated from the relationship: ΔG°_C,298.15=ΔH°_C − 298.15 ΔS°_C. The obtained results show that in most cases, the complexes are enthalpy stabilized, but entropy destabilized and the values of ΔH°_C and ΔS°_C depend strongly on the nature of the medium.     

Stability of Complexes of NH4 + with 18-Crown-6, Dicyclohexyl-18-Crown-6, Dibenzo-18-Crown-6 and Dibenzo-24-Crown-8 in Water Saturated Nitrobenzene

From extraction experiments with ²²Na tracer, the exchange extraction constants corresponding to the NH₄ ⁺(aq) + NaL⁺ (nb)NH₄L⁺(nb) + Na⁺ (aq) equilibrium taking place in the two-phase water-nitrobenzene system (L = 18-crown-6, dicyclohexyl-18-crown-6, dibenzo-18-crown-6 and dibenzo-24-crown-8; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Furthermore, the stability constants of the NH₄L⁺ complexes in nitrobenzene saturated with water were calculated; they were found to increase in the order dibenzo-24-crown-8 (DB24C8) < dibenzo-18-crown-6 (DB18C6) < dicyclohexyl-18-crown-6 (DCH18C6) < 18-crown-6 (18C6).