Equilibrium and Formation/Dissociation Kinetics of Some $Ln^{III}$PCTA Complexes

Content Provider	Scilit
Author	Tircsó, Gyula Kovács, Zoltán Sherry, A. Dean
Copyright Year	2006
Abstract	The protonation constants ( ) of 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-triacetic acid (PCTA) and stability constants of complexes formed between this pyridine-containing macrocycle and several different metal ions have been determined in 1.0 M KCl at 25 °C and compared to previous literature values. The first protonation constant was found to be 0.5−0.6 log units higher than the value reported previously, and a total of five protonation steps were detected (log = 11.36, 7.35, 3.83, 2.12, and 1.29). The stability constants of complexes formed between PCTA and $Mg^{2+}$, $Ca^{2+}$, $Cu^{2+}$, and $Zn^{2+}$ were also somewhat higher than those previously reported, but this difference could be largely attributed to the higher first protonation constant of the ligand. Stability constants of complexes formed between PCTA and the $Ln^{3+}$ series of ions and $Y^{3+}$ were determined by using an “out-of-cell” potentiometric method. These values ranged from log K = 18.15 for Ce(PCTA) to log K = 20.63 for Yb(PCTA), increasing along the Ln series in proportion to decreasing $Ln^{3+}$ cation size. The rates of complex formation for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA) were followed by conventional UV−vis spectroscopy in the pH range 3.5−4.4. First-order rate constants (saturation kinetics) obtained for different ligand-to-metal ion ratios were consistent with the rapid formation of a diprotonated intermediate, Ln(H_{2}$PCTA)^{2+}$. The stabilities of the intermediates as determined from the kinetic data were 2.81, 3.12, 2.97, and 2.69 log K units for $Ce(H_{2}$PCTA), $Eu(H_{2}$PCTA), $Y(H_{2}$PCTA), and $Yb(H_{2}$PCTA), respectively. Rearrangement of these intermediates to the fully chelated complexes was the rate-determining step, and the rate constant $(k_{r}$) for this process was found to be inversely proportional to the proton concentration. The formation rates $(k_{OH}$) increased with a decrease in the lanthanide ion size [9.68 × $10^{7}$, 1.74 × $10^{8}$, 1.13 × $10^{8}$, and 1.11 × $10^{9}$ $M^{-}$1 $s^{-}$1 for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA), respectively]. These data indicate that the Ln(PCTA) complexes exhibit the fastest formation rates among all lanthanide macrocyclic ligand complexes studied to date. The acid-catalyzed dissociation rates $(k_{1}$) varied with the cation from 9.61 × $10^{-}$4, 5.08 × $10^{-}$4, 1.07 × $10^{-}$3, and 2.80 × $10^{-}$4 $M^{-}$1 $s^{-}$1 for Ce(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA), respectively.
Related Links	http://europepmc.org/articles/pmc2597434?pdf=render https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2597434/pdf
Ending Page	9280
Page Count	12
Starting Page	9269
ISSN	00201669
e-ISSN	1520510X
DOI	10.1021/ic0608750
Journal	Inorganic Chemistry
Issue Number	23
Volume Number	45
Language	English
Publisher	American Chemical Society (ACS)
Publisher Date	2006-10-18
Access Restriction	Open
Subject Keyword	Journal: Inorganic Chemistry Inorganic Chemistry Metal Ion
Content Type	Text
Resource Type	Article
Subject	Physical and Theoretical Chemistry Inorganic Chemistry