Ancillary Ligand and Ketone Substituent Effects on the Rate of Ketone Insertion into Zr−C Bonds of Zirconocene−1-Aza-1,3-diene Complexes

Zirconocene−1-aza-1,3-diene complexes, [Me2Si(C5H4)2]Zr[N(Ar)CHCHCH(Ph)] (Ar = Ph, 2a; Ar = p-MeOC6H4, 2b) and Cp2Zr[N(Ar)CHCHCH(Ph)] (Ar = Ph, 3a; Ar = p-MeOC6H4, 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 [Me2Si] ansa bridge, a less electron-rich substituent on the nitrogen, and a more basic ketone. One of the insertion products, namely Cp2Zr[N(Ph)CHCHCH(Ph)CPh2O] (5a), has also been characterized by X-ray crystallography.