title: Structure and Isotope Effects of the β-H Agostic (α-Diimine)Nickel Cation as a Polymerization Intermediate.
abstract: 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-Cα-Cβangles (75.0(3)° and 74.57(18)°) and short Cα-Cβdistances (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 with13C-labeled agostic complexes. The enthalpy of activation (ΔH≠) 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.
authors.affiliation: Chemistry, New York University, 100 Washington Square E., New York, NY, USA.
authors.forename: Hongwei
authors.initials: H
authors.lastname: Xu
authors.affiliation: Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands.
authors.forename: Paul B
authors.initials: PB
authors.lastname: White
authors.affiliation: Chemistry, New York University, 100 Washington Square E., New York, NY, USA.
authors.forename: Chunhua
authors.initials: C
authors.lastname: Hu
authors.affiliation: Chemistry, New York University, 100 Washington Square E., New York, NY, USA.
authors.forename: Tianning
authors.initials: T
authors.lastname: Diao
keywords: inverse kinetic isotope effect; nickel; polymerization intermediates; β-agostic interactions