KINETICS OF FORMATION OF CA2- BIMOLECULAR RATE CONSTANTS FOR THE FULLY-DEPROTONATED LIGANDS REVEAL THE EFFECT OF MACROCYCLIC LIGAND CONSTRAINTS ON THE RATE-DETERMINING CONVERSIONS OF RAPIDLY-FORMED INTERMEDIATES TO THE FINAL COMPLEXES( COMPLEXES OF ACYCLIC AND MACROCYCLIC POLY(AMINO CARBOXYLATE) LIGANDS )
Sl. Wu et al., KINETICS OF FORMATION OF CA2- BIMOLECULAR RATE CONSTANTS FOR THE FULLY-DEPROTONATED LIGANDS REVEAL THE EFFECT OF MACROCYCLIC LIGAND CONSTRAINTS ON THE RATE-DETERMINING CONVERSIONS OF RAPIDLY-FORMED INTERMEDIATES TO THE FINAL COMPLEXES( COMPLEXES OF ACYCLIC AND MACROCYCLIC POLY(AMINO CARBOXYLATE) LIGANDS ), Inorganic chemistry, 36(9), 1997, pp. 1884-1889
The apparent bimolecular rate constants, k(1) (M-1 s(-1)), for the for
mation of Ca2+ complexes of a series of acyclic (edta, egta, cdta) and
macrocyclic (dota, teta, do3a) poly(amino carboxylate) ligands were d
etermined in the pH range 7-13 using the fluorescent ligand quin2 in a
stopped-flow apparatus to monitor the ligand competition reaction. Th
e k(1) values are observed to reach maximum constant values at high pH
, characteristic of reactions involving the fully-deprotonated ligand
species. Bimolecular formation constants kCa(L), k(CaHL), and k(Ca)(H2
L), characteristic of the reaction of the fully-deprotonated and mono-
and diprotonated ligands, respectively, were derived from the pH depe
ndence of the k(1) values. The k(Ca)(L) values of the acyclic ligands
are edta, 4.1 x 10(9) M-1 s(-1); egta, 2.1 x 10(9) M-1 s(-1); and cdta
, 2.3 x 10(9) M-1 s(-1), while the corresponding values for the macroc
yclic ligands are dota, 4.7 x 10(7) M-1 s(-1); teta, 1.1 x 10(7) M-1 s
(-1); and do3a, 1.0 x 10(8) M-1 s(-1). The smaller values for the macr
ocyclic ligands are consistent with ligand-dictated constraints impose
d on the conversion of a stable intermediate to the final complex, a p
rocess which involves the simultaneous stripping of several water mole
cules from the first-coordination sphere of the Ca2+ ion.