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Structure and Isotope Effects of the β‐H Agostic (α‐Diimine)Nickel Cation as a Polymerization Intermediate
Author(s) -
Xu Hongwei,
White Paul B.,
Hu Chunhua,
Diao Tianning
Publication year - 2017
Publication title -
angewandte chemie international edition
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.831
H-Index - 550
eISSN - 1521-3773
pISSN - 1433-7851
DOI - 10.1002/anie.201611282
Subject(s) - agostic interaction , chemistry , diimine , kinetic isotope effect , cationic polymerization , polymerization , hydride , crystallography , ligand (biochemistry) , elimination reaction , bond cleavage , photochemistry , polymer chemistry , medicinal chemistry , organic chemistry , metal , catalysis , polymer , biochemistry , physics , receptor , deuterium , quantum mechanics
Single‐crystal X‐ray characterization of cationic (α‐diimine)Ni‐ethyl and isopropyl β‐agostic complexes, which are key intermediates in olefin polymerization and oligomerization, are presented. The sharp Ni‐C α ‐C β angles (75.0(3)° and 74.57(18)°) and short C α −C β distances (1.468(7) and 1.487(5) Å) provide unambiguous evidence for a β‐agostic interaction. An inverse equilibrium isotope effect (EIE) for ligand coordination upon cleavage of the agostic bond highlights the weaker bond strength of Ni−H relative to the C−H bond. An Eyring plot for β‐hydride elimination–olefin rotation–reinsertion is constructed from variable‐temperature NMR spectra with 13 C‐labeled agostic complexes. The enthalpy of activation (Δ H ≠ ) for β‐H elimination is 13.2 kcal mol −1 . These results offer important mechanistic insight into two critical steps in polymerization: ligand association upon cleavage of the β‐agostic bonds and chain‐migration via β‐H elimination.