Yl. Shi et al., KINETICS AND MECHANISMS OF THE COMPLEXATION OF AQUEOUS LANTHANIDE IONS BY 4-(2-PYRIDYLAZO)RESORCINOL, Journal of the Chemical Society. Dalton transactions (Print), (21), 1998, pp. 3565-3576
Three kinetic steps were observed for the complexations of Eu3+ and UO
22+ by PAR [4-(2-pyridylazo)resorcinol] or PAN [1-(2-pyridylazo)-2-nap
hthol] in different buffered solutions. The first step can be assigned
to the co-ordination of the nitrogen donor from the pyridine moiety o
f the ligand based on the dependencies of [Eu3+], pH, pressure, nature
and concentration of the buffer. The rate-determining step is the rel
ease of water molecules from the co-ordination sphere of the lanthanid
e ion. Variations in the rate of the first step with different lanthan
ide ions indicated that a co-ordination number changeover is involved
in this lanthanide series. For the second step the formation of a ''hy
droazone-Ln(3+) chelate'' intermediate accounts for all of the observe
d kinetic behaviors. The kinetic investigations of the third step show
that there is a deprotonation preequilibrium preceding the transition
state of the final product with two chelated 5-membered rings involve
d. Surprisingly, the rate constants of the three steps for the complex
ation of UO22+ by PAR are very close to those for 18-crown-6 and diaza
-18-crown-6 reacting with uranyl ion. The differences in the kinetics
between PAR and PAN can be related to the difference in their structur
es.