High-pressure NMR and ab initio computational studies on the insertion mechanism of carbon monoxide into cationic monoorganopalladium(II) complexes bearing tridentate nitrogen donor ligands

Article abstract of DOI:10.1021/om00012a032

A number of methyl and arylpalladium(II) cations bearing tridentate nitrogen-donor ligands has been prepared, and their reactivity in the insertion of carbon monoxide has been studied. The resulting acetyl and aroylpalladium complexes of the ligands N-(2-picolyl)-N,N′,N′-trimethylethylenediamine (pico) and N,N,N′,N″,N″-pentamethyldiethylenetriamine (pmdeta) were isolated and characterized. In the case of the 4-nitrophenylpalladium and 2,4,6-trimethylphenylpalladium pico- and pmdeta complexes, no carbonylation products could be isolated. In these cases, either the equilibrium of the carbonylation reaction did not lie fully to the side of the insertion product even at 10 atm of CO or the insertion product decarbonylated upon attempted isolation. The methylpalladium complex of the 2,6-bis[(dimethylamino)methyl]pyridine (NN′N) ligand reacted quantitatively with CO in CD₃-COCD₃ to give acetic anhydride, palladium metal, and the protonated ligand; a mechanism for the anhydride formation is proposed. With the exception of the 1-naphthoyl derivative, the aroyl complexes of the NN'N ligand could not be isolated due to decarbonylation upon attempted isolation. Most of the unstable insertion products could, however, be characterized by IR and high-pressure NMR. Crystals of the 1-naphthoylpalladium complex with the NN′N ligand were obtained from acetone/pentane under a CO atmosphere. This complex is the first example of an aroylpalladium(II) cation. Two reaction pathways for the carbonyl insertion reaction, i.e., dissociative and associative, have been evaluated using ¹H NMR studies and ab initio calculations. The insertion reaction at 10 atm of CO pressure in CD₃-COCD₃ is complete within 2.5 min for most complexes, with the exception of those bearing strongly electron-withdrawing para substituents (e.g., NO₂) or sterically demanding ortho substituents (e.g., 2,4,6-trimethyl) on the aryl ring. Ab initio calculations at the RHF, MP2/ /SCF, and CAS-SCF/CI levels on the cationic model system [Pd(CH₃)(NH₃)₃]⁺ + CO and the neutral system [Pd(CH₃)₂(NH₃)₂] + CO show that the carbonylation reaction follows a hybrid pathway, i.e., a concerted replacement of NH₃ by CO followed by migratory insertion of CO into the Pd-C bond instead of a purely dissociative or associative mechanism. For both the neutral and the cationic systems the rate-determining step is the migratory insertion. The insertion process is enhanced by coordination of the dissociated amine and is slightly more favorable in the neutral system. Together with the low-energy replacement of NH₃ by CO, this implies that in both systems the rate of carbonyl insertion should be independent of the applied CO pressure.