Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and CPh3]B(C6F5)4]) and Br?nsted (HNR3]B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair MeHf{N(-),N,C(-)}]MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs MeHf{N(-),N,C(-)}]B(C6F5)4] (4) is obtained when CPh3]B(C6F5)4] is used. HNR3]B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable Me2Hf{N(-),N,C2}]B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, Me2Hf{N(-),N,C2}]B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of MeHf{N(-),N,C(-)}NR3]B(C6F5)4] (11). This species transforms over time into the trianionic-ligated Hf{N(-),C(-),N,C(-)}NR3]B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to MeHf{N(-),N}NMe2C6H4]B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to Hf{N(-),C(-),N,C(-)}NMe2Ph]B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair MeHf{N(-),N,C(-)}NR3]B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with HNMe3Ph]B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Br?nsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.