M. Mohajeri

Complexation process between La3+ cation and 4′-nitrobenzo-15-crown-5 in some binary mixed non-aqueous solvent

The complexation reaction between 4′-nitrobenzo-15-crown-5 (4′-NB15C5) with La3+ cation was studied in ethanol-acetonitrile (EtOH-AN), ethanol-methylacetate (EtOH-MeOAc), ethanol-1,2 dichloroethane (EtOH-DCE), and ethanol-nitrobenzene (EtOH-NB) binary mixed solvent solutions at different temperatures using the conductometric method. The results show that in most cases, the stoichiometry of the complex formed between 4′-NB15C5 and La3+ cation is 1: 1 [ML]. The stability constant of (4′-nitrobenzo-15C5La)3+ complex changes by the nature and composition of the solvents systems. The stability order of (4′-nitrobenzo-15C5La)3+ complex in the binary mixed solvents with 10 mol % of ethanol at 25°C was found to be: EtOH-MeOAc > EtOH-NB > EtOH-AN ~ EtOH-DCE. A non-linear behavior was observed for variations of logKf of the complex in terms of the solvent composition. The standard values of the thermodynamic parameters (ΔHc° ΔSc°) for complexation reaction were obtained from temperature dependence of the stability constant of the complex and the results show that, in most solvent systems, the complexation reaction between La3+ and 4′-NB15C5 is entropy stabilized, but from enthalpy view point, except in a few cases, it is athermic.

Effect of solvent on complexation between Y3+ cation and 4,13-diaza-18-crown-6 in some binary mixed non-aqueous solvents

The complexation reaction of 4,13-diaza-18-crown-6 (DA18C6) with Y3+ cation was studied in some binary mixed solvent solutions of acetonitrile (AN) with methanol (MeOH), ethanol (EtOH), 2-propanol (2-PrOH) and methyl acetate (MeOAc) at different temperatures by conductometric method. The obtained data show that in all studied solutions the stoichiometry of the complex formed between DA18C6 and Y3+ cation is 1: 1 [ML], but in the case of pure MeOAc, a 2: 1 [ML2] complex is formed in solution upon addition of the ligand to the metal salt solution, and further addition of the ligand results in formation of a M2L2 complex in solution. This results show that the stoichiometry of the composition of the macrocyclic complexes may be affected by the nature of the solvent system. The results obtained in this study show that the stability constant of the resulting 1: 1 [ML] complex in the binary solvent solutions decreases in the order: AN-MeOAc > AN-2PrOH > AN-MeOH > AN-EtOH. A non-linear relationship was observed between the stability constant (logKf) of [Y(DA18C6)]3+ complex with the composition of the binary mixed solvent solutions. The corresponding standard thermodynamic parameters (H°c, Δ S°c) for 1: 1 [ML] complexation reaction between DA18C6 and Y3+ cation were obtained from temperature dependence of the stability constant of the complex. The results show that, in all solvent systems, the (DAI8C6.Y)3+ complex is entropy stabilized, but from enthalpy point of view, depending on the solvent system, it is stabilized or destabilized and the result show that the values of both thermodynamic quantities change with the nature and composition of the binary mixed solvent solutions.

A new cerium(III) ion-selective electrode based on dicyclohexano-18-crown-6 as ionophore

A coated membrane electrode has been designed and constructed for measuring the cerium(III) cation activity in solution by using dicyclohexano-18-crown-6 (DCH18C6) as a membrane carrier (ionophore), dibuthyl phthalate (DBP) as plasticizer, sodium tetraphenylborate (NaTPB), poly(vinyl chloride) (PVC) as matrix, and tetrahydrofuran (THF) as solvent. A graphite rod was then coated with the membrane by the dip-dry method. The calibration curve for the constructed electrode under the optimized conditions exhibit a Nernstian response of 19.8±0.5 mV/decade for Ce3+ cation over a relatively wide concentration range (10−2 to 10−5 M) at 25°C and pH 4–8. The detection limit for the electrode was found to be 1.44 × 10−6 M. The response time of the cerium(III)-selective electrode was less than 25 s. The effect of interfering ions and the selectivity of the electrode were also studied. This electrode was successfully used as an indicator electrode for potentiometric titration of cerium(III) nitrate with a standard solution of NaF. The electrode could be used for at least 30 days without any appreciable change in its sensitivity.

A complexation study of 15-crown-5 with Co2+ cation in some pure and mixed organic solvents

The stability constant (log Kf) and the thermodynamic parameters (free energies, enthalpies, and entropies) of the complexation of Co2+ cation with 15-crown-5 (15C5) in acetonitrile-methanol (AN/MeOH), acetonitrile-nitrobenzene (AN/NB), acetonitrile-dichloromethane (AN/DCM) and acetonitrile-1,2-dichloroethane (AN/DCE) binary solvent solutions were calculated from the experimental conductance data at different temperatures. The complexation behavior of the crown ether used in these media was discussed in view of the estimated parameters. In all solvent systems, 15-crown-5 formed a 1: 1 complex with Co2+ cation. The stability order of (Co-15C5)2+ complex in the binary mixed solvents at 25°C was found to be: AN/NB > AN/DCM ≈ AN/DCE > AN/MeOH. In most cases, a non-linear relationship was observed for changes of log Kf of (Co-15C5)2+ complex versus the composition of the binary mixed solvent systems. The experimental results show that the standard thermodynamic parameters of the complexation process change with the nature and composition of the binary solvent solutions.

Complexation of Cd2+, Ni2+, and Ag+ metal ions with 4,13-didecyl-l,7,10,16-tetraoxa-4,13-diazacyclooctadecane in acetonitrile-ethylacetate binary mixtures

Conductometric titrations have been performed in acetonitrile-ethylacetate (AN-EtOAc) binary solutions at 288, 298, 308, and 318 K to obtain the stoichiometry, the complex stability constants and the standard thermodynamic parameters for the complexation of Cd2+, Ni2+, and Ag+ cations with 4,13-didecyl-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (cryptand 22DD). The stability constants of the resulting 1: 1 complexes formed between the metal cations and the ligand were determined by computer fitting of the conductance-mole ratio data. There is a non-linear relationship between the logKf values of complexes and the mole fraction of ethylacetate in the mixed solvent system. In addition, the conductometric data show that the stoichiometry of the complexes formed between the Cd2+, Ni2+, and Ag+ cations with the ligand changes with the nature of the solvent. The standard enthalpy and entropy values for the 1: 1 [ML] complexation reactions were evaluated from the temperature dependence of the formation constants. Thermodynamically, the complexation processes of the metal cations with the C22DD, is mainly entropy governed and the values of thermodynamic parameters are influenced by the nature and composition of the binary mixed solvent solutions.