Insight into Oxide-Bridged Heterobimetallic Al/Zr Olefin Polymerization Catalysts

Reaction of (TBBP)AlMe⋅THF with [Cp*2Zr(Me)OH] gave [(TBBP)Al(THF)−O−Zr(Me)Cp*2] (TBBP=3,3’,5,5’‐tetra‐tBu‐2,2'‐biphenolato). Reaction of [DIPPnacnacAl(Me)−O−Zr(Me)Cp2] with [PhMe2NH]+[B(C6F5)4]− gave a cationic Al/Zr complex that could be structurally characterized as its THF adduct [(DIPPnacnac)Al(Me)−O−Zr(THF)Cp2]+[B(C6F5)4]− (DIPPnacnac=HC[(Me)C=N(2,6‐iPr2−C6H3)]2). The first complex polymerizes ethene in the presence of an alkylaluminum scavenger but in the absence of methylalumoxane (MAO). The adduct cation is inactive under these conditions. Theoretical calculations show very high energy barriers (ΔG=40–47 kcal mol−1) for ethene insertion with a bridged AlOZr catalyst. This is due to an unfavorable six‐membered‐ring transition state, in which the methyl group bridges the metal and ethene with an obtuse metal‐Me‐C angle that prevents synchronized bond‐breaking and making. A more‐likely pathway is dissociation of the Al‐O‐Zr complex into an aluminate and the active polymerization catalyst [Cp*2ZrMe]+. Bimetallic or not? Heterobimetallic Al/Zr catalysts with a potential free coordination site have been prepared (see figure). Methylalumoxane (MAO) free ethene polymerization could be achieved, but addition of 30 equivalents of an alkylaluminum scavenger was necessary. DFT calculations disproved a heterobimetallic mechanism, and are instead in favor of Zr−O bond breaking to generate the classical monometallic catalyst [Cp*2ZrMe]+.