Unusual Hafnium-Pyridylamido/ER_n Heterobimetallic Adducts (ER_n = ZnR2 or AlR3)

摘要

NMR spectroscopy and DFT studies indicate that the Symyx/Dow Hf(IV)-pyridylamido catalytic system for olefin polymerization, [{N-,N,C_(Nph)~-}HfMe][B(C6Fs)4] (1, Nph = naphthyl), interacts with ER_n (E=Al or Zn, R = alkyl group) to afford unusual heterobimetallic adducts [{N~-,N}HfMe(μ-C_(Nph))(μ-R)ER_(n-1)][B(C6F5)4] in which the cyclometalated Nph acts as a bridge between Hf and E. ~1H VT (variable-temperature) EXSY NMR spectroscopy provides direct evidence of reversible alkyl exchanges in heterobimetallic adducts, with ZnR2 showing a higher tendency to participate in this exchange than AlR3. l-Hexene/ER_n competitive reactions with 1 at 240 K reveal that the formation of adducts is strongly favored over 1-hexene polymerization. Nevertheless, a slight increase in the temperature (to > 265 K) initiates 1-hexene polymerization.