The Influence of Alkyl Chain Length and Steric Effect on the Stability Constants and Extractability of Co(II) Complexes with 1-Alkyl-2-Methylimidazoles

Abstract

Using the liquid-liquid partition method, the formation of Co(II) complexes with 1-alkyl-2-methylimidazoles (where alkyl = C 4 H 9 trough C 12 H 25 ) at 25 • C and at a fixed ionic strength of the aqueous phase (I = 0.5, (HL)NO 3 , KNO 3 ) were studied. Dichloromethane, trichloromethane, and 2-ethylhexanol were used as diluents. The tetrahedral and octahedral complexes were formed. Stability constants (β n ) of the complexes as well partition ratios (P n ) of the extracted species were determined. It was shown that both β n and P n increased with an increasing 1-alkyl chain length. Tetrahedral complexes are more readily extractable by organic solvent. Their P n values are the highest. INTRODUCTIONImidazole forms a coordination bond with metal cations by electron pair pyridinic nitrogen atom, forming more stable complexes than it appears from its basicity (1).The presence of alkyl group in position 1 of the imidazole ring slightly increases the basicity of the molecules (2) and their complexing properties by increasing the inductive effect (3). The substitution of the methyl group in position 2, 4, or 5 of the imidazole ring increases the basicity of the pyridinic nitrogen atom (2) by an order of magnitude. At the same time, the subsituent hinders the formation of the 2-methylimidazole complexes with metal ions due to steric hindrance. The steric effect complicates the formation of the octahedral species and simultaneously favours the tetrahedral complexes. The complexes of Co(II), Zn(II), and Cd(II) with derivatives of alkylimidazole having steric effect in the aqueous solution reveal a tendency to change the coordination structure from octahedral to tetrahedral

New Polymer Inclusion Membranes in the Separation of Palladium, Zinc and Nickel Ions from Aqueous Solutions

“…On the basis on SX studies by a liquid–liquid partition method, both the partition constants (P n ) and the stability constants (β n ) of Pd(II), Ni(II) and Zn(II) complexes with 1-alkyl-1,2,4-triazoles ( 1 – 9 ) were determined. This method was previously used to determine the extraction constants of Cu(II) [ 41 , 42 , 43 ], Co(II) [ 44 , 46 ], Ni [ 47 ] and Zn [ 48 , 49 ] complexes with alkyl-imidazoles and Zn(II), Ni(II), Cu(II), Co(II) and Cd(II) with ethylenodiamino-bis-acetylacetone [ 45 ]. A detailed method of determining the extraction constants is described in the papers [ 41 , 42 , 43 , 44 , 46 , 47 , 48 , 49 ].…”

Studies on the separation of some transition metals using trialkylimidazole as selective extractant

“…It is assumed that the n value is equal to the number of ligand molecules attached to the central ion in the first complex being extracted. The method was used previously [6,7,[23][24][25][32][33][34][35][36][37]. The stability constants, βn, obtained in this way are collected in Table 2 together with stability constants for 1,2-dimethylimidazole [31], and 1,4dimethylimidazole [40].…”

“…They can be used for metal separation as extractants in hydrometallurgy or as a carrier in membrane techniques. So far, the extraction properties of 1-alkylimidazoles [6,32,33], 1-alkyl-2-methylimidazoles [7,34,35], 1-alkyl-2-ethylimidazoles [36], 1,2-dialkylimidazoles [23,25] and 1-alkyl-4-methylimidazoles [25,37] have been studied. On the other hand, our knowledge of donor and extraction properties of tri-substituted alkylimidazoles is very small.…”

Studies on the separation of some transition metals using trialkylimidazole as selective extractant

“…It is assumed that the n value is equal to the number of ligand molecules attached to the central ion in the first complex being extracted. The method was used previously [6,7,[23][24][25][32][33][34][35][36][37]. The stability constants, βn, obtained in this way are collected in Table 2 together with stability constants for 1,2-dimethylimidazole [31], and 1,4-dimethylimidazole [40].…”