Thio derivatives of β-diketones and their metal chelates. V. Acid dissociation constants of some Monothio-β-diektones and stability constants of anickel(II) and a copper(II) chelate

Abstract

The acid dissociation constants (PK_D) of the monothio derivatives of acetyl-acetone, dibenzoylmethane, and 3,3,3-trifluoro-1-(2- thenoyl)acetone, viz. 4-mercap-topent-3-en-2-one (I), 3-mercapto-1,3- diphenylprop-2-en-1-one (II), and 1,1,1-trifluoro-4-mercapto-4-(2- thienyl)but-3-en-2-one (III), were determined in dioxan/water solutions with varying mole fraction (n₂) of dioxan. Over a range of n₂ from 0.18 to 0.44 the values of pK_D were found to vary linearly with n₂ as given by the equations: (I) pK_D = 5.7+12.0n₂; (II) pK_D = 7.0+10.9n₂; (III) pK_D = 3.55 + 9.2n₂. The thio derivatives (I), (II), and (III) have pK_D values 2.0-2.7 log units lower than their oxygen analogues. The logarithm of the stability constants (log β₂) for the nickel(II) and copper(II) complexes of (II) were found to be 21.7 and 22.2, respectively. For both complexes K₂ > K₁. Comparison with the stability constants of the dibenzoylmethane complexes shows that for nickel(II) the complex of the monothio-β-diketone is more stable than that of the β-diketone while the converse is true for copper(II).