Ancillary ligand and ketone substituent effects on the rate of ketone insertion into Zr-C bonds of zirconocene-l-Aza-l,3-diene complexes

Zirconocene-l-aza-l,3-diene complexes, [Me ₂Si(C ₅H ₄) ₂]Zr[N(Ar)CH=CHCH(Ph)] (Ar = Ph, 2a; Ar = /J-MeOC ₅H ₄, 2b) and Cp ₂Zr[N(Ar)CH=CHCH(Ph)] (Ar = Ph, 3a; Ar = p-MeOC ₆H ₄, 3b), have been synthesized and characterized by NMR spectroscopy. X-ray crystal structure determinations of compounds 2a and 3a,b reveal folded five-membered-ring moieties for the zirconacycles. DFT calculations and variable-temperature NMR experiments for complex 2a establish a rapid ring-flipping process at room temperature, with the conformation bearing a pseudoequatorial Ph group more stable by 5.6 kcal/mol. Kinetic studies on ketone insertion into these zirconocene complexes show second-order reactions, and the insertion is more favorable in the presence of a [Me ₂Si] ansa bridge, a less electron-rich substituent on the nitrogen, and a more basic ketone. One of the insertion products, namely Cp ₂Zr[N(Ph)CH=CHCH(Ph)CPh ₂O] (5a), has also been characterized by X-ray crystallography.