A R T I C L E S
Berkefeld et al.
The aforementioned studies have prompted extensive studies
of novel neutral nickel(II) olefin polymerization catalysts.10 By
comparison to the aforementioned catalysts, they are much more
stable to protic and even aqueous media.10f,11-13,15b At the same
time, polymer microstructures and resulting crystallinities and
thermal behavior can be varied in a controlled fashion over a
wide range via the catalyst. However, despite intense studies
of their polymerization properties, to date the copolymerization
of ethylene or 1-olefins with polar vinyl monomers, like MA,
to high-molecular-weight copolymers employing neutral Ni(II)
precatalysts has suffered from either low effectivity or rapid
catalyst deactivation.14 An understanding of the problems
associated with this reaction is desirable. By comparison to the
aforementioned cationic Pd(II) complexes, neutral Ni(II) com-
plexes are, unfortunately, less ammenable to mechanistic studies
via direct observation with NMR spectroscopy.15 Neutral Ni(II)
salicylaldimine catalysts were found to be subject to rapid
deactivation in attempted copolymerizations of 1-olefins with
polar vinyl monomers. In a study of the organic decomposition
products formed by reaction of a salicylaldiminato phenyl
complex, [(N∧O)Ni(C6H5)(PPh3)] (N∧O
)
κ2-{(2,6-
(iPr)2C6H3)-N)C(H)-(3-anthryl-2-O-C6H2)}), with MA in tolu-
ene solution, methyl trans-cinnamate and methyl 3-phenyl-
butanoate were formed over the course of 1 day at 80 °C.
Carrying out the reaction in the presence of 2,3,3-d3-MA allowed
to trace back the origin of the methylene-hydrogen atoms in
the saturated reaction product, which are provided by the
substrate itself. Protonation of an intermittent metal alkyl species
by free ligand, (N∧O)H, formed by reductive elimination from
[(N∧O)NiH] species, was suggested as a most likely pathway.
In addition, hydrolysis of insertion products was identified as a
decomposition route.16
(6) (a) Johnson, L.; Bennett, A.; Dobbs, K.; Hauptman, E.; Ionkin, A.;
Ittel, S.; McCord, E.; McLain, S.; Radzewich, C.; Yin, Z.; Wang, L.;
Wang, Y.; Brookhart, M. Polym. Mater. Sci. Eng. 2002, 86, 319. (b)
Wang, L.; Hauptman, E.; Johnson, L. K.; Marshall, W. J.; McCord,
E. F.; Wang, Y.; Ittel, S. D.; Radzewich, C. E.; Kunitsky, K.; Ionkin,
A. S. Polym. Mater. Sci. Eng. 2002, 86, 322. (c) Johnson, L.; et al.
ACS Symp. Ser. 2003, 857, 131–142.
(7) (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 744–745. Review: (b) Berkefeld, A.; Mecking,
S. Angew. Chem., Int. Ed. 2008, 47, 2538–2542.
We now report a comprehensive study of the nature and
reactivity of neutral Ni(II) alkyl insertion products of acrylate
andvinylacetate,basedondirectNMRspectroscopicobservations.
(8) (a) Guironnet, D.; Roesle, P.; Ru¨nzi, T.; Go¨ttker-Schnetmann, I.;
Mecking, S. J. Am. Chem. Soc. 2009, 131, 422–423, Also see. (b)
Chen, C.; Luo, S.; Jordan, R. F. J. Am. Chem. Soc. 2008, 130, 12892–
12893.
Results and Discussion
(9) Kochi, T.; Noda, S.; Yoshimura, K.; Nozaki, K. J. Am. Chem. Soc.
2007, 129, 8948–8949.
General Considerations. Highly reactive Ni(II) hydride and
higher Ni(II) alkyl species [(N∧O)NiR(L)] (R ) H, CH3, C2H5;
L ) PMe3, dmso) of a salicylaldiminato ligand based on a
terphenylamine (N∧O; eq 1) were employed as precursors for
mechanistic studies. This particular salicylaldimine was chosen
because Ni(II) complexes of this ligand are long-lived, robust,
and very active catalysts for ethylene polymerization.10m,q The
Ni(II) hydride complex 1 was either synthesized on a preparative
scale (eq 1) from the corresponding Ni(II) chloride complex
1-Cl (Figure S1 and Table S1, Supporting Information),15b or
generated in situ in thf-d8 solution at low temperatures directly
in an NMR tube.
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