Rapid geometrical equilibrium of palladium(II) complexes with tris[2-(diphenylphosphino)ethyl]phosphine disulfide and diselenide and their catalytic activity for Suzuki coupling reaction

Palladium(II) complexes with a tetradentate pseudo-tripodal ligand having two phosphino groups and two phosphine sulfide or selenide groups, pp3X2 (pp3 = tris[2-(diphenylphosphino)ethyl]phosphine, X = S (1) or Se (2)), were prepared from [PdCl(pp3)]Cl. Both of these phosphine chalcogenide complexes 1 and 2 showed rapid equilibrium between the five-coordinate [PdCl(pp3X2)]Cl with two bound phosphine chalcogenide groups and four-coordinate [PdCl2(pp3X2)] with two dissociated pendant ones in chloroform. The thermodynamic parameters for the reaction, [PdCl(pp3X2)]+ + Cl−⇄[PdCl2(pp3X2)], were obtained by low-temperature 31P NMR as follows: K298 = 3.7 × 103 and 5.4 × 102 mol−1, ΔH∘ = 11.3 ± 0.3 and 13.4 ± 0.4 kJ mol−1, and ΔS∘ = 106 ± 2 and 97 ± 2 J mol−1 K−1 for 1 and 2, respectively. The rate for the geometrical change at 246.7 K for 1 was appreciably faster than that for 2. These thermodynamic and kinetic results indicate that the phosphine selenide Se atoms can stabilize the five-coordinate structure by effective π-back donation from Pd(II) compared with the phosphine sulfide S atoms. Difference in retention of the catalytic activity for Suzuki coupling, 2 > 1 > [PdCl(pp3 or p3)]Cl, was explained by difference in the π-accepting ability that stabilizes the catalytically active Pd(0) species. Considering the rapid dissociation-coordination equilibrium of the phosphine chalcogenide groups on Pd(II), it is probable that the oxidative addition and the subsequent transmetallation of the Pd(II) species are hardly blocked by the phosphine chalcogenide groups.