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Zuccaccia C, Macchioni A, Busico V, et al. Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts. J Am Chem Soc. 2008;130(31):10354-68doi: 10.1021/ja802072n.
Zuccaccia, C., Macchioni, A., Busico, V., Cipullo, R., Talarico, G., Alfano, F., Boone, H. W., Frazier, K. A., Hustad, P. D., Stevens, J. C., Vosejpka, P. C., & Abboud, K. A. (2008). Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts. Journal of the American Chemical Society, 130(31), 10354-68. https://doi.org/10.1021/ja802072n
Zuccaccia, Cristiano, et al. "Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts." Journal of the American Chemical Society vol. 130,31 (2008): 10354-68. doi: https://doi.org/10.1021/ja802072n
Zuccaccia C, Macchioni A, Busico V, Cipullo R, Talarico G, Alfano F, Boone HW, Frazier KA, Hustad PD, Stevens JC, Vosejpka PC, Abboud KA. Intra- and intermolecular NMR studies on the activation of arylcyclometallated hafnium pyridyl-amido olefin polymerization precatalysts. J Am Chem Soc. 2008 Aug 06;130(31):10354-68. doi: 10.1021/ja802072n. Epub 2008 Jul 10. PMID: 18613668.
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J Am Chem Soc. 2008 Aug 06;130(31):10354-68. doi: 10.1021/ja802072n. Epub 2008 Jul 10.

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

Journal of the American Chemical Society

Cristiano Zuccaccia, Alceo Macchioni, Vincenzo Busico, Roberta Cipullo, Giovanni Talarico, Francesca Alfano, Harold W Boone, Kevin A Frazier, Phillip D Hustad, James C Stevens, Paul C Vosejpka, Khalil A Abboud

Affiliations

  1. Department of Chemistry, University of Perugia, Via Elce di Sotto, 8, 06123 Perugia, Italy.

PMID: 18613668 DOI: 10.1021/ja802072n

Abstract

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 Brønsted ([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 Brønsted 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.

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