Intra- and Intermolecular NMR Studies on the Activation of Arylcyclometallated Hafnium Pyridyl-Amido Olefin Polymerization Precatalysts

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.

Unusual Hafnium–Pyridylamido/ERn Heterobimetallic Adducts (ERn=ZnR2 or AlR3)

“…[21] 1 H ROESY NMR spectroscopy provided information about the 3D structures of 2-5 in solution. [14] As far as H36 is concerned, the chemical shift in complex 2 is only 0.2 ppm higher than in the dimethyl precursor, whereas the change observed for 3 is much more pronounced. This was clearly indicated by the presence of NOE between H8 and H12 (Figure 1 b) and between H2 and H35 (and H36, which undergoes an exchange process with H35 (see below); Figure 1 c).…”

“…[7][8][9][10][11] A key feature of the ancillary ligand framework is the ortho-metalation of the aryl moiety bound to the pyridine fragment (Scheme 1). [12,13] Activation with Brønsted acids such as [HNMe 2 Ph][B(C 6 F 5 ) 4 ], [14] on the other hand, first leads to the protonation of the Hf IV À aryl bond (Scheme 1 b), which opens the metallacycle and results in a poorly active cation as long as ortho-metalation (with liberation of CH 4 ) is not restored. [12,13] Activation with Brønsted acids such as [HNMe 2 Ph][B(C 6 F 5 ) 4 ], [14] on the other hand, first leads to the protonation of the Hf IV À aryl bond (Scheme 1 b), which opens the metallacycle and results in a poorly active cation as long as ortho-metalation (with liberation of CH 4 ) is not restored.…”

“…[8,16,17] Despite the importance of this chemistry, to our knowledge, very limited molecular information (if any) is available on the ER n /Hf IV -pyridylamido interaction(s). [14] 1 a/1 b undergo ligand exchange with ER n to afford unusual heterobimetallic adducts 2-5 (Scheme 1 c) with yields approaching 70-80 %, as estimated by means of multinuclear and multidimensional low-temperature NMR experiments (see the Supporting Information). [18,19] Upon 4 ] (1 a/1 b, see the Supporting Information), which differ in the relative orientation of the Hf-Me and 2-(Me 2 CH)C 6 H 4 moieties.…”

Polystyrene Chain Growth from Di-End-Functional Polyolefins for Polystyrene-Polyolefin-Polystyrene Block Copolymers

“…Although the α-methylstyrene moiety remained intact during the CCTP performed using rac-[Me 2 Si(2-methylindenyl) 2 ]ZrCl 2 , it was sluggishly involved in the subsequent anionic styrene polymerization. Thus, we herein attempted the use of a dialkylzinc species bearing a styrene moiety instead of an α-methylstyrene moiety (i.e., 1 versus 3) in addition to the use of pyridylaminohafnium catalyst 4 instead of rac-[Me 2 Si(2-methylindenyl) 2 ]ZrCl 2 , as the former is reportedly a more efficient catalyst for the CCTP and prevents the undesirable β-elimination process [1,7,8,42,46,47]. It should also be noted that the styrene moiety did not interfere in the CCTP performed using 4.…”

Dimethyl-Aluminium Complexes Bearing Naphthyl-Substituted Pyridine-Alkylamides as Pro-Initiators for the Efficient ROP of ε-Caprolactone

“…Moreover, functionalizing the 6-position of the pyridine moiety of the pyridylimine with an aryl or 1-naphthyl group presents a versatile set of substrates for studying ortho vs. peri-palladation reactions during the formation of N,N,C-chelates [53,54]. Likewise, N,N-pyridyl-alkylamides and their 6-aryl and -naphthyl derivatives have also been attracting attention due, in large measure, to their emergence as supports for exceptionally active N,N- [45,55,56] and N,N,C-bound group 4 olefin polymerization catalysts [57][58][59][60].…”