Interplay of Steric and Electronic Effects on the Bonding Components in Aromatic Ring Agostic Interactions

Density functional theory (DFT) calculations on the effect of steric size adjacent to an agostic interaction in ligand assisted Pd–C bond formation involving aromatic rings gives insight into why the synthetic reaction can fail as the size of an alkyl group is increased. In [PdCl2(1-tetralone oxime)] agostic complexes, changing the C(7)-substituent on the ligand through the series H, Me, CHMe2, and CMe3 progressively reduces agostic and syndetic donations. For (N)–CMe3 imine complexes, CMe3 steric pressure at C(7) switches off agostic donation and increases syndetic donation significantly, especially where the aromatic ring can rotate. Electron withdrawal from the aromatic ring in this type of system has little effect, but electron donation into the ring invokes η1-covalency, especially with strong π-donation. This covalency can be switched off by further π-donation and the syndetic donation restored. These steric effects can be expected to impact the success of C–C bond formation chemistry derived from Pd–C cyclometalation reactions involving agostic and syndetic donations.