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Phosphine and Thiophene Cyclopalladated Complexes: Hydrolysis Reactions in Strong Acidic Media
Author(s) -
García Begoña,
Hoyuelos Francisco J.,
Ibeas Saturnino,
Muñoz María S.,
Navarro Ana M.,
Peñacoba Indalecio A.,
Leal José M.
Publication year - 2010
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201000526
Subject(s) - chemistry , protonation , hydrolysis , reaction rate constant , medicinal chemistry , organopalladium , ligand (biochemistry) , phosphine , absorbance , sulfuric acid , stereochemistry , inorganic chemistry , palladium , organic chemistry , kinetics , catalysis , chromatography , ion , biochemistry , physics , receptor , quantum mechanics
The mechanisms for the hydrolysis of organopalladium complexes [Pd(CNN)R]BF 4 (R=P(OPh) 3 , PPh 3 , and SC 4 H 8 ) were investigated at 25 °C by using UV/Vis absorbance measurements in 10 % v/v ethanol/water mixtures containing different sulphuric acid concentrations in the 1.3–11.7 M range. In all cases, a biphasic behavior was observed with rate constants k 1obs , which corresponds to the initial step of the hydrolysis reaction, and k 2ob s , where k 1obs > k 2obs . The plots of k 1obs and k 2obs versus sulfuric acid concentration suggest a change in the reaction mechanism. The change with respect to the k 1obs value corresponds to 35 %, 2 %, and 99 % of the protonated complexes for R=PPh 3 , P(OPh) 3 , and SC 4 H 8 , respectively. Regarding k 2obs , the change occurred in all cases at about 6.5 M H 2 SO 4 and matched up with the results reported for the hydrolysis of the 2‐acetylpyridinephenylhydrazone (CNN) ligand. By using the excess acidity method, the mechanisms were elucidated by carefully looking at the variation of k i , obs ( i =1,2) versus ${c_{{\rm{H}}^ + } }$ . The rate‐determining constants, k 0,A‐1 , k 0,A‐2 , and k 0,A‐SE2 were evaluated in all cases. The R=P(OPh) 3 complex was most reactive due to its π‐acid character, which favors the rupture of the trans nitrogen–palladium bond in the A‐2 mechanism and also that of the pyridine nitrogen–palladium bond in the A‐1 mechanism. The organometallic bond exerts no effect on the relative basicity of the complexes, which are strongly reliant on the substituent.