Reaction mechanism of decomposition of phosphine-bridged Pd(I)-Pd(I) dimers into Pd(II) monomers by molecular iodine

Abstract

Mechanistic studies on decomposition of Pd(I)-Pd(I) dimers via Pd(II)-Pd(II) intermediates into Pd(II) monomers were performed by the reaction of [Pd2I2(dppm)2] (1) (dppm = bis(diphenylphosphino)methane) and [Pd2(p3)2](BF4)2 (2) (p3 = bis(diphenylphosphinoethyl)phenylphosphine) with molecular iodine, where dppm and p3 are bridging “short-bite” bidentate and “regular-bite” tridentate phosphine ligands, respectively. It was previously reported that the decomposition rate constant for the Pd(II)-Pd(II) intermediate of 1, [Pd2I4(dppm)2], to give the monomeric [PdI2(dppm)] was independent of the I2 concentration under the pseudo-first-order conditions with excess I2. On the other hand, we have found that the decomposition was retarded by an addition of I2, which reduces the nucleophilicity and donating ability of the iodido ligand by association of I2 to give the I-–I2 interaction. The activation parameters of the decomposition with a large negative ΔS‡ value similar to substitution reactions of usual four-coordinate squarer-planar palladium(II) complexes suggested the associative intramolecular substitution mechanism. The oxidation of 2 to the Pd(II)-Pd(II) intermediate, [Pd2I2(p3)2]2+, and its decomposition into the monomeric [PdI(p3)]+ by I2 were also investigated kinetically and both reactions were also retarded with an increase in the I2 concentration. From the activation parameters of the intramolecular decomposition, it was indicated that the associative character was reduced because of breakdown of the sterically restricted framework. Such different properties in solution of 1 and 2 were applied complementarily to Heck-type C–C coupling reaction and dehalogenation reaction of aryl halide, respectively.