Structural Characterization of β-Agostic Bonds in Pd-Catalyzed Polymerization...

β-agostic Pd complexes are critical intermediates in catalytic reactions, such as olefin polymerization and Heck reactions. Pd β-agostic complexes, however, have eluded structural characterization, due to the fact that these highly unstable molecules are difficult to isolate. Herein, we report the single-crystal X-ray and neutron diffraction characterization of β-agostic (α-diimine)Pd–ethyl intermediates in polymerization. Short Cα–Cβ distances and acute Pd–Cα–Cβ bond angles combined serve as unambiguous evidence for the β-agostic interaction. Characterization of the agostic structure and the kinetic barrier for β-H elimination offer important insight into the fundamental understanding of agostic bonds and the mechanism of polymerization.