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Kinetic study of the insertion of norbornadiene into palladium-carbon bonds of complexes containing the rigid bidentate nitrogen ligand bis(arylimino)acenaphthene

The ionic [Pd(C7H8C(O)R)(Ar-BIAN)]X (R = Me, Et, iPr, Ph; X = Cl, Br, I; Ar = p-An, P-FC6H4, P-BrC6H4, p-Tol, Ph, o,o′-Me2C6H3,o,o′-iPr 2C6H3) complexes (1b-12b), bearing the bidentate nitrogen ligand bis(arylimino)acenaphthene (Ar-BIAN), have been synthesized via reaction of the corresponding neutral acylpalladium complexes Pd(C(O)R)X(Ar-BIAN) (1a-12a) with norbornadiene (nbd). For the first time, an extensive kinetic study of this migratory alkene insertion into acyl-palladium bonds of neutral complexes containing α-diimine ligands has been carried out. It has been found that under pseudo-first-order circumstances these reactions follow the rate law kobsd = k1 + k2[nbd], which shows that these reactions proceed via a pathway independent of alkene concentration (k1 pathway) and a pathway dependent on alkene concentration (k2 pathway). The dramatic decrease of the rate constants k1 and k2 upon increasing the steric bulk of the BIAN ligand and the large negative entropy of activation and low enthalpy of activation for both pathways indicate that the k1 and k2 pathways are closely related and involve associative processes. From the influence of solvent, X and C(O)R ligand, steric and electronic properties of the BIAN ligand, the presence of free halide and free BIAN, and the parameters of activation, mechanisms have been proposed for both pathways. The k1 pathway may proceed via a rate-determining solvent-assisted halide or nitrogen dissociation, followed by alkene association and migratory insertion, while the k2 pathway may occur via a rate-determining migratory alkene insertion in a contact ion pair intermediate. This species may be formed via alkene association followed by either halide or nitrogen dissociation.