Phosphine-Phenoxide Nickel Catalysts for Ethylene/Acrylate Copolymerization: Olefin Coordination and Complex Isomerization Studies Relevant to the Mechanism of Catalysis

The insertion copolymerization of ethylene and acrylate
remains
a challenge in polymer synthesis due to decreased activities upon
incorporation of the polar monomer. Toward gaining mechanistic insight,
two elusive four-membered chelated intermediates generated after acrylate
insertion were prepared (1-CCO and 2-CCO), and their ligand coordination and substitution behavior were studied.
Specifically, an ethylene-coordinated species was characterized by
NMR spectroscopy upon exposing 2-CCO to ethylene at low
temperatures, a rare observation for neutral late-transition metal
polymerization catalysts. Thermodynamics of chelate-opening and monomer
coordination from 2-CCO were determined at −90
°C (ΔG of 0.4 kcal/mol for ethylene and
1.9 kcal/mol for 1-hexene). The Gibbs energy barrier of ligand exchange
from pyridine to ethylene, a prerequisite for ethylene insertion in
catalysis, was determined to be 3.3 kcal/mol. Ligand-binding studies
reveal that compared to NiMe and Ni­(CH2SiMe3) complexes, acrylate inserted species 1L-CCO and 2L-CCO produce compressed thermodynamic binding scales for
both electronically and sterically differentiating ligands, potentially
related to their more electron-deficient nickel centers as suggested
by computational studies. Triethylphosphine complexes 1P, 2P, and 2P–Me were observed as
both cis and trans isomers in solution. 31P­{1H} EXSY NMR studies of 2P reveal conversion between
a cis and trans isomers that does not involve exchange with free PEt3, supporting the mechanism of intramolecular isomerization. 2-CCO, a neutral Ni­(II) precatalyst that does not display
an auxiliary ligand, serves as a highly active catalyst for copolymerization.