Kinetic and Mechanistic Studies of Geometrical Isomerism in Neutral Square-Planar Methylpalladium Complexes Bearing Unsymmetrical Bidentate Ligands of α-Aminoaldimines

A series of hemilabile ligands of α-aminoaldimines and their methylpalladium complexes have been prepared and characterized. Neutral square-planar methylpalladium complexes in the form of [R¹R²NCMe₂CHNR]Pd(Me)Cl (R = Me, R¹ = R² = Me (3a); R = Me, R¹ = R² = Et (3b); R = Et, R¹ = R² = Me (4a); R = ⁿPr, R¹ = R² = Me (5a); R = ⁱPr, R¹ = R² = Me (6a); R = ⁱPr, R¹ = R² = Et (6b); R = ⁱPr, (R¹, R²) = c-C₄H₈ (6c); R = ⁱPr, R¹ = ⁱPr, R² = H (6d); R = ⁱPr, R¹ = ^tBu, R² = H (6e); R = ^tBu, R¹ = R² = Me (7a); R = ^tBu, R¹ = R² = Et (7b); R = ^tBu, (R¹, R²) = c-C₄H₈ (7c); R = ^tBu, R¹ = ⁱPr, R² = H (7d); R = ^tBu, R¹ = ^tBu, R² = H (7e); R = Ph, R¹ = R² = Me (8a); R = Ph, R¹ = R² = Et (8b)) show geometrical isomerism. The relative ratios of trans/cis isomers appear to be predominated by the steric hindrance between the Pd-bound methyl group and imino or amino substituents (R and R¹ and R²). The NMR studies for the substitution reaction of (COD)Pd(Me)Cl with Et₂NCMe₂CHNⁱPr at −20 °C indicate that cis-6b is the major kinetic product, which isomerizes to the thermodynamic product in trans form quantitatively above −5 °C. Kinetic results show that the ligand substitution reaction likely undergoes an associative pathway, and the isomerization reaction proceeds via an intramolecular process that comprises imine dissociation and recoordination.