Synthesis and characterization of 1,3,4-thiadiazole-2,5-dithio crown ethers

Synthesis

1,3,4-thiadiazole-2,5-dithio crown ethers I–IV were synthesized by the reactions shown in Scheme 1. The complexation studies performed with these compounds are also shown.

When bismuthiol (1,3,4-thiadiazole-2,5-dithiol) dipotassium salt was allowed to react with a dichloride derivative (1,2-dibromoethane, diethylene glycol dichloride, triethylene glycol dichloride, or tetraethylene glycol dichloride) under a nitrogen atmosphere in acetonitrile, the product of a heterocyclization was obtained.

Ion-pair extraction

Ion-pair extraction involves selective transfer of an inorganic reagent from one phase (e.g. water) to an immiscible phase (e.g. chloroform or dichloromethane). With crown ethers, this type of extraction involves the formation of an ion pair between the cation complexed in the crown ether and the counter ion. The extraction equilibrium constant (K_ex) can be calculated using the following equations 1–3 [16], [17], [18], [19]:

where M^m+ is the metal ion (m=1–3), A⁻ is the counter ion, L is the crown ether, and MLA_m is the ion pair that contains M^m+, A⁻ and L.

The calculated selectivity factors, S_f (equation 4), of the heterocyclic ligands show that ligand II is selective toward Ag⁺, Cd²⁺ and Pb²⁺ among the investigated metals in the competitive extraction solution. Ligands I, III and IV are selective toward Ag⁺, Cd²⁺, Cu²⁺ and Pb²⁺. When the metal ion concentration is reduced to 10⁻⁴ mol/L, ligand I becomes selective toward Ag⁺ and Fe³⁺, ligand II toward Cd²⁺, Pb²⁺, Fe³⁺ and Na⁺, ligand III toward Fe³⁺ and Cd²⁺ and ligand IV toward Fe³⁺. The proportions of the metal ions extracted by each ligand were also calculated. For all ligands, the proportion extracted for each metal ion increases with increasing radius of the ion in the following order: Mg²⁺, Ni²⁺, Co²⁺, Pb²⁺, Fe³⁺, Cd²⁺, Ag⁺, Cr³⁺, Na⁺, Ca²⁺, Mn²⁺, Cu²⁺ and K⁺ [20], [21]. These results show that the ligands are more selective toward Cu²⁺, Fe³⁺, Cd²⁺ and Ag⁺ at 10⁻³ mol/L and Fe³⁺, Cd²⁺ and toward Ag⁺ at 10⁻⁴ mol/L (see Tables in Supporting Information).

Synthesis of 1,3,4-thiadiazole-2,5-dithio crown ethers I–IV

Bismuthiol dipotassium salt (2.26 g; 10 mmol) was added to acetonitrile (400 mL) and the mixture was heated under reflux for 2 h in order to ensure a homogeneous solution. This solution was treated dropwise under reflux for 7 h with a solution of 1,2-dibromoethane, diethylene glycol dichloride, triethylene glycol dichloride, or tetraethylene glycol dichloride (10 mmol) in acetonitrile (100 mL). After the addition was completed, the mixture was heated under reflux under a nitrogen atmosphere for an additional 5 days and then cooled. The resultant precipitate of I–IV was filtered. The mother liquor was concentrated under reduced pressure, treated with distilled water (40 mL) and the mixture was extracted with chloroform (3×10 mL). The extract was dried with anhydrous K₂SO₄, filtered and concentrated under reduced pressure. An additional amount of I–IV crystallized after the solution was cooled in the refrigerator for 2 days. The combined product was crystallized from ethanol.

Metal extraction studies

Ion-pair extraction was performed to evaluate the selectivity of the bismuthiol crown ethers for various metal ions. Solutions of the ligands were prepared in organic solvents (I in CH₂Cl₂, and II–IV in CHCl₃). In the first experiment, a metal salt, (Mn(NO₃)₂·4H₂O, Zn(NO₃)₂·6H₂O, Cr(NO₃)₃·9H₂O, Fe(NO₃)₃·9H₂O, KNO₃, Ca(NO₃)₂·4H₂O, NaNO₃, Cu(NO)₃·2H₂O, Mg(NO₃)₂·6H₂O, Cd(NO₃)₂·4H₂O, Pb(NO₃)₂, Ni(NO₃)₂·6H₂O, Co(NO₃)₂·6H₂O, AgNO₃, pH=4.57, 4.30, 3.00, 3.60, 3.95, 4.40, 4.90, 4.10, 4.75, 4.40, 3.60, 4.60, 3.95, 4.00, respectively, in the aqueous solution), and the synthesized crown ether were put together in a chloroform/water (1:1) or dichloromethane/water (1:1) mixture at 25°C. In the second experiment, a competitive extraction was performed with an aqueous solution containing all 14 metal ions (pH=4.20–4.30). The prepared mixtures were shaken at 25°C for 1 h. The process was stopped after incubation for 30 min to equilibrate the solution at 25°C. After each extraction, the concentration of the metal ion remaining in the aqueous solution was measured by atomic absorption spectrophotometry and inductively coupled plasma atomic emission spectrophotometry. Relative selectivity factors (log S_f) were calculated from the data. The results for competitive metal extractions with the 10⁻³ and 10⁻⁴ mol/L metal ion solutions and synthesized bismuthiol crown ethers are tabulated in Supporting Information.