INSERTION REACTIONS INTO PALLADIUM-CARBON BONDS OF COMPLEXES CONTAINING TERDENTATE NITROGEN LIGANDS - EXPERIMENTAL AND AB-INITIO MO STUDIES

Novel methyl complexes [Pd(Me)(N-N-N)]X (N-N-N = flexible or rigid ter dentate nitrogen Ligand, X = Cl, SO3CF3, BAr'(4)) have been synthesize d and fully characterized. All complexes readily underwent insertion o f carbon monoxide resulting in the quantitative formation of complexes [Pd{C(O)Me}(N-N-N)]X [X=Cl (1d-6d), BAr'(4) (1e-6e)]. Subsequently, c omplexes 2e-6e underwent quantitative insertion of norbornadiene, resu lting in complexes [Pd{C7H8C(O)Me}(N-N-N)]BAr'(4) (2f-6f). Unexpectedl y, these complexes, including even those containing rigid terdentate n itrogen ligands, possess a structure in which the nitrogen Ligand is c oordinated in a bidentate fashion. A kinetic study of the reaction of norbornadiene with complexes 1e-6e revealed that the reactivity of com plexes 1e-6e toward norbornadiene increases with increasing rigidity o f the terdentate Ligand, i.e., with increasing strain in the PdN3, moi ety, which indicates that insertion very Likely occurs via a mechanism involving nitrogen dissociation. This is fully supported by ab initio MO calculations on CO and ethylene insertion into carbon-palladium bo nds of cationic model systems containing a rigid terdentate nitrogen L igand, which showed that the lowest-energy pathway for both insertion reactions consists of substitution of one of the distal nitrogen atoms of the rigid terdentate nitrogen ligand by the substrate, followed by a rate-determining migratory insertion of the substrate into the carb on-palladium bond.