Abstract
2, 3-Dimethyl-5-hydroxyquinoxaline (5-OH-DMQx) and 1-hydroxy-6, 7, 8, 9-tetrahydrophenazine (1-OH-Tphz) have been synthesized. The acid dissociation constants of these ligands and the stability constants of their metal chelates have been measured in 50% (v/v) water-ethanol solution. The values obtained have been compared with those of 5-hydroxyquinoxaline and 2-substituted oxine metal chelates. The pKOH values of 5-OH-DMQx and 1-OH-Tphz are 9.98 and 10.01, respectively. These values are considerably larger than the corresponding constant of 5-hydroxyquinoxaline, 9.29.
As to the metal chelate stability constants, (a) the metal chelate stability order is as follows : 5-OH-DMQx; log K1 Cu>Zn>Co>Ni>Cd and logβ2 Cu>Co>Zn>Ni>Cd. 1-OH-Tphz; logK1 Cu>Zn>Co≈Ni>Cd and logβ2 Cu>Co>Ni≈Zn>Cd. (b) the stability constants of these metal chelates are smaller than the corresponding constants of 5-hydroxy-quinoxaline, regardless of the value of pKOH with the exception of the Zn-chelates. (c) The difference of the stability constants among Co-, Ni- and Zn-chelates is less than 0.4 log unit. The behavior of the stability constants of 5-OH-DMQx and 1-OH-Tphz indicates the similar tendency to that of 2-substituted oxine metal chelates. Accordingly, the stability constants of these metal chelates have been explained in terms of steric hindrance.
