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.