Mechanistic Studies on Halo-Ligand Substitution of Five-Coordinate Trigonal-Bipyramidal Palladium(II) Complexes of Tris(2-(diphenylphosphino)ethyl)phosphine with Trimethyl Phosphite in Chloroform at Various Temperatures and Pressures

Abstract

Five-coordinate trigonal-bipyramidal palladium(II) complexes [Pd(pp3)X]X (pp3 = tris(2-(diphenylphosphino)ethyl)phosphine, X- = Cl-, Br-, I-) have been synthesized, and their structures in the solid state and in solution have been confirmed by X-ray crystal structure analysis and 31P NMR spectroscopy, respectively. The 31P NMR chemical shifts of the axial and equatorial phosphorus atoms indicate that the σ- and π-donating abilities of the axial monodentate ligands are in the order Cl- > Br- > I- ≫ P(OCH3)3. The thermodynamic parameters for the equilibria between the halo complexes, [Pd(pp3)Cl]+ + X- ⇄ [Pd(pp3)X]+ + Cl- (X- = Br-, I-), have been determined as follows: K298 = 1.70, ΔH° = −5.66 ± 0.07 kJ mol-1, and ΔS° = −14.6 ± 0.2 J K-1 mol-1 for Br-; K298 = 16.8, ΔH° = −16 ± 1 kJ mol-1, and ΔS° = −30 ± 3 J K-1 mol-1 for I-. It is revealed that the relative stability of the halo complexes, I- > Br- > Cl- for the axial ligands, is determined by the difference in enthalpy. The second-order rate constants at 25 °C and activation parameters for the halo-ligand substitution with trimethyl phosphite, [Pd(pp3)X]+ + P(OCH3)3 → [Pd(pp3)(P(OCH3)3)]2+ + X- (X- = Cl-, Br-, I-), have been obtained as follows: k298 = 1.19 × 10-1 mol-1 kg s-1, ΔH⧧ = 19 ± 1 kJ mol-1, ΔS⧧ = −198 ± 3 J K-1 mol-1, and ΔV⧧ = −25.5 ± 0.5 cm3 mol-1 at 302.5 K for X- = Cl-; k298 = 6.20 × 10-2 mol-1 kg s-1, ΔH⧧ = 23 ± 3 kJ mol-1, ΔS⧧ = −191 ± 3 J K-1 mol-1, and ΔV⧧ = −24.5 ± 0.7 cm3 mol-1 at 302.5 K for X- = Br-; k298 = 1.09 × 10-2 mol-1 kg s-1, ΔH⧧ = 57 ± 4 kJ mol-1, ΔS⧧ = −89 ± 15 J K-1 mol-1, and ΔV⧧ = −22.6 ± 0.8 cm3 mol-1 at 302.9 K for X- = I-. The associative mechanism is proposed on the basis of the kinetic behavior and the activation parameters. The kinetic properties are discussed in terms of the electronic properties of the axial and entering ligands.