Formation of mixed-ligand complexes of zinc(II) with chloride and substituted pyridines in 1,2-dichloroethane as solvent

Abstract

The formation of mixed-ligand complexes of zinc(II) chloride with pyridine (py) and substituted pyridines R-py [R-py = 4-methyl-(4Me-py), 3-methyl-(3Me-py), 2-methyl-(2Me-py), 2-ethyl-(2Et-py), 4-acetyl-(4MeCO-py), and 4-cyano-pyridine (4CN-py)] in 1,2-dichloroethane has been investigated at 25.0 °C by means of solvent extraction. The extraction equilibrium of zinc(II) between the NaCl aqueous phase and the NBu^t₄Cl–1,2-dichloroethane phase is given by equation (I), where the Zn²⁺+ 2 Cl^–+ 2 NBu^t₄Cl (org) [graphic omitted] (NBu^t₄)₂ZnCI₄(org)(i), thermodyanamic extraction constant log K_ex,0= 5.61 ± 0.03 and ‘org’ denotes the species in the organic phase. The addition of substituted pyridines in this extraction system gives rise to the equilibria (ii) and (iii). (NBu^t₄)₂ZnCI₄(org)+ R-py (org) [graphic omitted] (NBu^t₄)ZnCl₃(R-py)(org)+ NBu^t₄Cl (org)(ii), (NBu^t₄)ZnCl₃(R-py)(org)+ R-py (org) [graphic omitted] ZnCl₂(R-py)₂(org)+ NBu^t₄Cl (org)(iii), The successive formation constants, K₁ and K₂, were determined as log K₁= 0.45, 0.25, –0.07, –0.47, 0.07, 0.13, and –0.48 ± 0.03, and log K₂=–1.09, –1.34, –3.21, –3.33, –1.70, –3.56, and –4.86 ± 0.03 for the 4Me-py, 3Me-py, 2Me-py, 2Et-py, py, 4MeCO-py, and 4CN-py complexes, respectively. Except for the 2Me-py and 2Et-py complexes, the values of log K₁ and log K₂ show linear correlations with the pK_a of the conjugate acid of the substituted pyridines.