Equilibrium and Formation/Dissociation Kinetics of Some LnIIIPCTA Complexes
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- 作者：Gyula Tircsó ; Zoltá ; n Ková ; cs ; A. Dean Sherry
- 刊名：Inorganic Chemistry
- 出版年：2006
- 出版时间：November 13, 2006
- 年：2006
- 卷：45
- 期：23
- 页码：9269 - 9280
- 全文大小：188K
- 年卷期：v.45,no.23(November 13, 2006)
- ISSN：1520-510X
文摘
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 differentmetal ions have been determined in 1.0 M KCl at 25 
C and compared to previous literature values. The firstprotonation constant was found to be 0.5-0.6 log units higher than the value reported previously, and a total of fiveprotonation steps were detected (log 
= 11.36, 7.35, 3.83, 2.12, and 1.29). The stability constants of complexesformed between PCTA and Mg2+, Ca2+, Cu2+, and Zn2+ 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 constantsof complexes formed between PCTA and the Ln3+ series of ions and Y3+ 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 Ln3+ cation size. The rates of complex formation forCe(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA) were followed by conventional UV-vis spectroscopy in the pHrange 3.5-4.4. First-order rate constants (saturation kinetics) obtained for different ligand-to-metal ion ratios wereconsistent with the rapid formation of a diprotonated intermediate, Ln(H2PCTA)2+. The stabilities of the intermediatesas determined from the kinetic data were 2.81, 3.12, 2.97, and 2.69 log K units for Ce(H2PCTA), Eu(H2PCTA),Y(H2PCTA), and Yb(H2PCTA), respectively. Rearrangement of these intermediates to the fully chelated complexeswas the rate-determining step, and the rate constant (kr) for this process was found to be inversely proportional tothe proton concentration. The formation rates (kOH) increased with a decrease in the lanthanide ion size [9.68 ×107, 1.74 × 108, 1.13 × 108, and 1.11 × 109 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 macrocyclicligand complexes studied to date. The acid-catalyzed dissociation rates (k1) 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.
) 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 differentmetal ions have been determined in 1.0 M KCl at 25 C and compared to previous literature values. The firstprotonation constant was found to be 0.5-0.6 log units higher than the value reported previously, and a total of fiveprotonation steps were detected (log = 11.36, 7.35, 3.83, 2.12, and 1.29). The stability constants of complexesformed between PCTA and Mg2+, Ca2+, Cu2+, and Zn2+ 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 constantsof complexes formed between PCTA and the Ln3+ series of ions and Y3+ 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 Ln3+ cation size. The rates of complex formation forCe(PCTA), Eu(PCTA), Y(PCTA), and Yb(PCTA) were followed by conventional UV-vis spectroscopy in the pHrange 3.5-4.4. First-order rate constants (saturation kinetics) obtained for different ligand-to-metal ion ratios wereconsistent with the rapid formation of a diprotonated intermediate, Ln(H2PCTA)2+. The stabilities of the intermediatesas determined from the kinetic data were 2.81, 3.12, 2.97, and 2.69 log K units for Ce(H2PCTA), Eu(H2PCTA),Y(H2PCTA), and Yb(H2PCTA), respectively. Rearrangement of these intermediates to the fully chelated complexeswas the rate-determining step, and the rate constant (kr) for this process was found to be inversely proportional tothe proton concentration. The formation rates (kOH) increased with a decrease in the lanthanide ion size [9.68 ×107, 1.74 × 108, 1.13 × 108, and 1.11 × 109 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 macrocyclicligand complexes studied to date. The acid-catalyzed dissociation rates (k1) 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.