Mechanism of Thermal Decomposition of trans-Diethylbis(tertiary phosphine)palladium(II). Steric Effects of Tertiary Phosphine Ligands on the Stability of Diethylpalladium Complexes

Article abstract of DOI:10.1021/ja00541a013

trans- with a series of alkyl groups and tertiary phosphine ligands of various steric bulkiness (R' = Et, PR3 = PMe2Ph (1), PEt3 (2), PEt2Ph (3), PmePh2 (4), PEtPh2 (5); R'= Pr (6), Bu (7), PR3 = PMe2Ph) are thermolyzed in solution by β-elimination processes liberating alkane and alkene in a 1:1 ratio.Kinetic studies on thermolysis of trans- revealed that they decompose obeying the first-order rate law with respect to the concentration of the palladium complexes.Thermolysis predominantly proceeds from the four-coordinated complex without dissociation of the tertiary phosphine ligand.The minor parallel thermolysis pathway involving the dissociation of the phosphine is completely blocked by addition of the phosphine.Activation enthalpies for thermolysis of the ethylpalladium complexes having various steric bulkiness were virtually constant in the range of 26.0 +/- 1.0 kcal/mol whereas activation entropies showed considerable variation with increasing bulkiness of the phosphine.Thermolysis of trans- (8) cleanly liberated CH2=CD2 and CH2DCD3 with a small isotope effect (k_H/k_D = 1.4 +/- 0.1).The kinetic results together with examination of the molecular model suggest that the interaction between the bulky phosphine ligands and the ethyl groups causes the destabilization of the Pd-Et bonds.A thermolysis mechanism consistent with the kinetic result assuming an activation state distorted from the square-planar ground state is proposed.