On the First Insertion of α-Olefins in Hafnium Pyridyl-Amido Polymerization Catalysts

Abstract

Reactions of [{N -,N,C naphthyl-}HfMe][MeB(C 6 F 5 ) 3 ] (2) precatalyst with a series of R-olefins have been investigated in order to intercept the active polymerization species generated by an in situ modification of the precursor by insertion of a single monomer unit into the Hf-C Aryl bond. In all cases the first migratory insertion of monomer occurs into the Hf-C Aryl bond rather than the Hf-C Alkyl bond. A low-temperature polymerization with 170 equiv of 1-hexene activated with tris-(pentafluorophenyl)borane (FAB) allows for the complete NMR characterization of a Hf-C Alkylaryl methyl cation. This structure agrees with DFT studies of the kinetically favored diastereomer, although a number of other structures are more stable thermodynamically. Attempts to trap an inserted catalyst through the stoichiometric addition of 2-vinylpyridine or 3-ethoxy-1-propylene led to complicated reactions, but in both cases, experimental and computational data suggest that both processes initiate through insertion of the substrate into the Hf-C Aryl bond. In addition, an activated Hf-dibutyl complex is studied in an attempt to minimize differences in the rates of initiation and propagation, as a butyl group is a reasonable mimic for a propagating polymer chain. Tritylborate activation of this complex cleanly generates 1 equiv of 1-butene in close proximity to the Hf-butyl cation via β-hydride abstraction. This reaction results in formation of isotactic poly(1-butene) with a fairly high molecular weight rather than butene oligomers. The observed molecular weight is consistent with a small fraction of active species, and quenching studies show a similar fraction of butene-modified ligand. These results are consistent with slow insertion into the Hf-C Aryl bond followed by fast polymerization kinetics, with the latter rate constant being 3 orders of magnitude faster than the former.

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

“…Adducts 2 and 3 are unusual and represent the first heterobimetallic adducts with a bridging aryl, although aryl-bridged homobimetallic Me 3 Al(m-Ph)AlMe 2 has been reported. The same phenomenon has been reported for the mono-inserted product (Scheme 1 a, d H = 0.06 ppm) [12] and the product derived from activation with ammonium salts (Scheme 1 b, d H = À0.25 and d H = À0.49 ppm). It was found that after the cleavage of the HfÀC1 bond subsequent to the addition of ER nÀ1 , C1 still remained in proximity to Hf.…”

“…[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.…”

Facing Unexpected Reactivity Paths with ZrIV–Pyridylamido Polymerization Catalysts

“…In recent papers, Macchioni et al. have developed a detailed understanding of the polymerization mechanisms with a series of arylcyclometallated Hf IV –pyridylamido complexes 2b,c. These studies provide clear evidence of the fundamental role played by the unusual HfC Ar bonds, emphasizing the importance of the true identification of the active species for the study of new catalytic processes or the design of improved catalytic structures.…”

“…All poly(1‐hexene)s produced by 6 b showed moderate isotacticities ( m 4 =0.47) with approximately 5–6 mol % of regioerrors as evidenced by their 13 C{ 1 H} NMR spectra (see the Supporting Information). Although the observed isotacticity can be rationalized on the basis of an in‐situ ligand modification resulting from the monomer insertion into the Zr IV Ar bond (that translates into a lowering of the symmetry of the active species),1b, 2a–e there is no apparent reason to explain the dramatic activity decrease observed in the case of the heteroaryl‐containing systems 7 b and 8 b 2a,c. Experimental evidence for the olefin insertion into the Zr IV Ar bond was gathered by treating the two representative activated species 6 b and 7 b with a stoichiometric amount of 1‐hexene at room temperature.…”

“…This class of compounds has been successfully applied to the production of highly isotactic and high molecular weight polypropylenes in high‐temperature solution processes2 and olefin‐based block copolymers through chain‐shuttling polymerization 3. Due to the utility of these complexes in the production of specialty polyolefins, many research efforts have been devoted to ascertain the identity of the true active species involved in the polymerization process, as well as to provide reliable mechanistic understandings of the polymerization processes 2b,c. Overall, these studies have provided fundamental insights into the wealth of organometallic chemistry of nitrogen‐based early transition‐metal complexes, highlighting at the same time their unique polymerization characteristics.…”