Computational study of the cyclopalladation mechanism of azobenzene with PdCl2 in N,N-dimethylformamide

The mechanism of cyclopalladation of azobenzene (L) with PdCl2 in N,N-dimethylformamide (dmf) was studied computationally, using DFT (B3LYP) methods supplemented with a continuum solvation model. Since the exact nature of the reacting complex is unknown, several candidates were considered. These were: (1) a mononuclear adduct with two ligand molecules, L–PdCl2–L, (2) a mononuclear adduct with one ligand and one solvent molecule, L–PdCl2–dmf, (3) a dinuclear adduct with a double chloride bridge, [L–PdCl–(μ-Cl)]2, and (4) a coordinatively unsaturated complex with an agostic interaction, L–PdCl2. The reaction profile initiating from L–PdCl2–dmf, which displays an agostic intermediate produced after displacement of the dmf molecule by the activating C–H bond, has the lowest barrier (20.4 kcal/mol in the step with the proton transfer to the O(dmf) atom). In all other reaction pathways, the proton migration is to a chlorine atom and is associated with remarkably high barriers. The results are related to previous experimental and other computational findings. While none of the reaction profiles includes explicit dissociation of the ligand, the proton transfer was found to occur only after the ligand is almost completely displaced from the coordinating shell. It was concluded that the transition state corresponds to 14-electron coordination of Pd and that ease of a ligand dissociation is an important, but not necessarily decisive, factor for cyclopalladation.