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

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posted on 17.08.2009, 00:00 by Feng-Zhao Yang, Yu-Heng Wang, Mu-Chieh Chang, Kuo-Hsuan Yu, Shou-Ling Huang, Yi-Hung Liu, Yu Wang, Shiuh-Tzung Liu, Jwu-Ting Chen
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.