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Kinetics and Mechanism for Formation of Olefin Complexes in the Reaction between Palladium(II) and Maleic Acid

Shi, Tiesheng LU and Elding, Lars Ivar LU (1998) In Inorganic Chemistry 37(21). p.5544-5549
Abstract
Complex formation between Pd(H2O)(4)(2+) and maleic acid (H(2)A) has been studied at 25 degrees C and 2.00 M ionic strength in acidic aqueous solution. Reaction takes place with 1:1 stoichiometry. The kinetics has been followed by use of stopped-flow spectrophotometry under pseudo-first-order conditions with maleic acid in excess. In the concentration ranges 0.01 less than or equal to [H(2)A](tot) less than or equal to 0.50 M and 0.40 less than or equal to [H+] less than or equal to 2.00 M, kinetic traces are biphasic. The biphasic kinetics and the dependence of reaction rate on pH and maleic acid concentration are rationalized in terms of a complex reaction mechanism of the type A reversible arrow B --> C where, in addition, both steps... (More)
Complex formation between Pd(H2O)(4)(2+) and maleic acid (H(2)A) has been studied at 25 degrees C and 2.00 M ionic strength in acidic aqueous solution. Reaction takes place with 1:1 stoichiometry. The kinetics has been followed by use of stopped-flow spectrophotometry under pseudo-first-order conditions with maleic acid in excess. In the concentration ranges 0.01 less than or equal to [H(2)A](tot) less than or equal to 0.50 M and 0.40 less than or equal to [H+] less than or equal to 2.00 M, kinetic traces are biphasic. The biphasic kinetics and the dependence of reaction rate on pH and maleic acid concentration are rationalized in terms of a complex reaction mechanism of the type A reversible arrow B --> C where, in addition, both steps contain contributions from parallel reactions. The amplitude of the first phase increases with increasing [H(2)A](tot) and with decreasing [H+]. Multi wavelength global analysis of the kinetic traces and the UV-vis spectral changes suggest that a monodentate oxygen-bonded hydrogen maleate complex, [Pd(H2O)(3)OOCCH=CHCOOH](+), B, with stability constant K-2 = 205 +/- 40 M-1 is formed as an intermediate in this first step via two parallel reversible reactions in which Pd(H2O)(4)(2+) reacts with maleic acid and hydrogen maleate, respectively. In the following step, B --> C, slow intramolecular ring closure with a rate constant of 0.8 +/- 0.1 s(-1) at 25 degrees C gives the reaction product C, which is concluded to be a 4.5-membered olefin-carboxylato chelate complex on the basis of stoichiometry and UV-vis/NMR spectra. Parallel and irreversible attack by maleic acid and hydrogen maleate acting as olefins on the intermediate B also leads to formation of C. C is stable for at least 20 h for concentrations of less than or equal to 2 mM. Global multi wavelength analysis and simulations show that accumulation of the intermediate B is between ca. 2% and 60% depending on pH and concentration of maleic acid. Neither a steady-state approximation nor a rapid pre-equilibrium assumption can be used for the kinetics data treatment. Exact rate expressions for the fast and slow phases have been used to derive all rate constants involved. Olefins are inefficient nucleophiles toward palladium(II), even less efficient than carboxylic acids and carboxylates. (Less)
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organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Palladium(II), Maleic acid, Reaction Mechanism, Stopped-flow , Kinetics, Olefins
in
Inorganic Chemistry
volume
37
issue
21
pages
6 pages
publisher
The American Chemical Society
external identifiers
  • scopus:0542444635
ISSN
1520-510X
DOI
10.1021/ic980072f
language
English
LU publication?
yes
id
222cd5b7-b769-4130-b957-fa9adaa6bc64
date added to LUP
2016-12-17 19:27:18
date last changed
2017-07-30 05:19:12
@article{222cd5b7-b769-4130-b957-fa9adaa6bc64,
  abstract     = {Complex formation between Pd(H2O)(4)(2+) and maleic acid (H(2)A) has been studied at 25 degrees C and 2.00 M ionic strength in acidic aqueous solution. Reaction takes place with 1:1 stoichiometry. The kinetics has been followed by use of stopped-flow spectrophotometry under pseudo-first-order conditions with maleic acid in excess. In the concentration ranges 0.01 less than or equal to [H(2)A](tot) less than or equal to 0.50 M and 0.40 less than or equal to [H+] less than or equal to 2.00 M, kinetic traces are biphasic. The biphasic kinetics and the dependence of reaction rate on pH and maleic acid concentration are rationalized in terms of a complex reaction mechanism of the type A reversible arrow B --> C where, in addition, both steps contain contributions from parallel reactions. The amplitude of the first phase increases with increasing [H(2)A](tot) and with decreasing [H+]. Multi wavelength global analysis of the kinetic traces and the UV-vis spectral changes suggest that a monodentate oxygen-bonded hydrogen maleate complex, [Pd(H2O)(3)OOCCH=CHCOOH](+), B, with stability constant K-2 = 205 +/- 40 M-1 is formed as an intermediate in this first step via two parallel reversible reactions in which Pd(H2O)(4)(2+) reacts with maleic acid and hydrogen maleate, respectively. In the following step, B --> C, slow intramolecular ring closure with a rate constant of 0.8 +/- 0.1 s(-1) at 25 degrees C gives the reaction product C, which is concluded to be a 4.5-membered olefin-carboxylato chelate complex on the basis of stoichiometry and UV-vis/NMR spectra. Parallel and irreversible attack by maleic acid and hydrogen maleate acting as olefins on the intermediate B also leads to formation of C. C is stable for at least 20 h for concentrations of less than or equal to 2 mM. Global multi wavelength analysis and simulations show that accumulation of the intermediate B is between ca. 2% and 60% depending on pH and concentration of maleic acid. Neither a steady-state approximation nor a rapid pre-equilibrium assumption can be used for the kinetics data treatment. Exact rate expressions for the fast and slow phases have been used to derive all rate constants involved. Olefins are inefficient nucleophiles toward palladium(II), even less efficient than carboxylic acids and carboxylates.},
  author       = {Shi, Tiesheng and Elding, Lars Ivar},
  issn         = {1520-510X},
  keyword      = {Palladium(II),Maleic acid,Reaction Mechanism,Stopped-flow ,Kinetics,Olefins},
  language     = {eng},
  month        = {09},
  number       = {21},
  pages        = {5544--5549},
  publisher    = {The American Chemical Society},
  series       = {Inorganic Chemistry},
  title        = {Kinetics and Mechanism for Formation of Olefin Complexes in the Reaction between Palladium(II) and Maleic Acid},
  url          = {http://dx.doi.org/10.1021/ic980072f},
  volume       = {37},
  year         = {1998},
}