Abstract
Stopped-flow has been used to determine rate and stability constants for complex formation of tungstate with catechol and catechol derivatives. Pseudo-first-order and close-to-equilibrium kinetics were applied at 25°, μ = 0.1 M (KN03), and 7.3 < pH < 9.0. The overall formation (klit M~' sec"1) and dissociation (Zr_ir, sec'1) rate constants, respectively for the 1:1 com- plexes are as follows: catechol, (1.10 ± 0.10) X 102 3, (3.8 ± 0.5) X 10"1; 3,4-dihydroxybenzoic acid, (2.40 ± 0.09) X 102, (2.9 ± 1.0) X 10"1; L-dopa, (2.41 ± 0.08) X 102, (3.0 ± 1.0) X 10'1; gallic acid, (6.05 + 0.32) X 102, (7.3 ± 2.0) X 10"1; pyrogallol, (6.92 ± 0.39) X 102, (6.2 ± 2.0) X 10"1. For catechol the apparent rate constants for formation of the bis com- plex are k2f = 4.0 ± 0.1 M'1 sec-1 and L_2r - 4 X 10"4 sec"1. The apparent overall stability constants may be calculated from the relation Knapp = knf/k.m, where «=1,2. Only one species of complex is formed; only the fully or partially pro- tonated phenolic moieties correspond to reactive forms of the ligand. Comparison with other systems shows an approx- imately 100-fold variation in formation rate constant with different ligands. For catechol and its derivatives benzene ring substitution affects the formation rate constant. Pyrogallol and gallic acid, both of which have three hydroxy groups on
the benzene ring, are approximately 6 times more reactive than catechol. Statistically, adding one hydroxy group at a nieghboring position to the two hydroxy groups of catechol cannot account for an enhancement factor greater than 2, indi- cating that the nature of the substituent plays a role in the complexation mechanism. The fact that K2¡ > K,¡ for tungsto- catechol complexes is due primarily to the fact that k_l¡ > k_2f. This difference in dissociation rate constant may be due to differences in the inner coordination shell, for in the mono complex hydroxy groups may be present, whereas they almost certainly absent from the bis complex.