Evaluation of the Agostic and Syndetic Donations in Aromatic Ring Agostic Interactions Involved in Heteroatom Ligand-Directed C–H Bond Activation

Natural bond orbital (NBO) analysis obtained from density functional theory (DFT) calculations was used to show whether syndetic donation (π or σ-electron density in the vicinity of the agostic carbon interacting with a metal center) is present in a variety of ligand-directed aromatic ring C–H bond activations by palladium acetate. Benzyl alcohol gave a complex with reduced agostic donation compared to N,N-dimethylbenzylamine and 2-phenylpyridine and similar syndetic π-donation except for 2-phenylpyridine, which is much lower. Phenethyl alcohol gave a simple coordination complex without interactions and 3-phenylpropan-1-ol gave an agostic interaction with a benzylic C–H bond and no syndetic donation. The ketones 1-tetralone, 1-indanone, and acetophenone gave agostic complexes with well-developed syndetic π-donation; PhCO2H gave well-developed agostic and syndetic π-donation, PhCH2CO2H gave an η2-complex, and PhCH2CH2CO2H gave a coordination complex with no interactions. Amide functional groups next to an aromatic ring gave agostic and η2-complexes, the commonly used oxazolalone ligand gave η2-coordination, and the cyclic ether benzofuran gave a coordination complex with no interactions. The results provide further evidence for syndetic donation in C–H bond activation of aromatic rings and reinforce the multicomponent nature of agostic interactions in a variety of ligand types.