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

Abstract

Complex formation between Pd(H(2)O)(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 </= [H(2)A](tot) </= 0.50 M and 0.40 </= [H(+)] </= 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 right arrow over left 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(+)]. Multiwavelength global analysis of the kinetic traces and the UV-vis spectral changes suggest that a monodentate oxygen-bonded hydrogen maleate complex, [Pd(H(2)O)(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(H(2)O)(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 </=2 mM. Global multiwavelength 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 preequilibrium 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.