Acidity Constants of the Thienyl- and Phenyl-Pyridines and Stability Constants of the Corresponding Copper (II) 1:1 Complexes

The acidity constants of the six thienyl- and the three phenyl-pyridines, and the stability constants of the corresponding Cu²⁺ 1:1 complexes were determined in aqueous solution (I = 0.1, NaCIO₄; 25°). The basicity of the pyridyl group is increased by a shift of a thienyl or phenyl group in the order 2 < 3 < 4 and the influence of the 2′-thienyl group is greater than that of the 3′-thienyl group. The stability of the Cu²⁺ 1:1 complexes formed with 3- and 4-substituted pyridines is only dependent on their basicity, as is commonly observed for structurally related ligands and their complexes. Due to steric hindrance the complexes of the 2-substituted derivatives are less stable by about 1.5 log units. There is no evidence that the aromatic bonded sulfur of 2-(2′-thienyl)-pyridine participates in complex formation with Cu²⁺, in opposition to an earlier claim [1].