Synthesis, characterization, and norbornene polymerization of η3-benzylnickel(II) complexes of N-heterocyclic carbenes

Neutral η1-benzylnickel carbene complexes, [Ni(η1-CH2C6H5)(IiPr)(PMe3)(Cl)] (3) (IiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) and [Ni(η1-CH2C6H5)(SIiPr)(PMe3)(Cl)] (4) (SIiPr = 1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-ylidene), were prepared by the reaction between [Ni(η3-CH2C6H5)(PMe3)(Cl)] and an equivalent amount of the corresponding free N-heterocyclic carbene. The preparation of η3-benzylnickel carbene complexes, [Ni(η3-CH2C6H5)(IiPr)(Cl)] (5) and [Ni(η3-CH2C6H5)(SIiPr)(Cl)] (6) were carried out by the abstraction of PMe3 from 3 and 4 by the treatment of B(C6F5)3. The treatment of AgX on 5 and 6 produced the anion-exchanged complexes, [Ni(η3-CH2C6H5)(NHC)(X)] (7, NHC = IiPr, X = O2CCF3; 8, NHC = IiPr, X = O3SCF3; 9, NHC = SIiPr, X = O2CCF3; 10, NHC = SIiPr, X = O3SCF3). The solid state structures of 3 and 10 were determined by X-ray crystallography. The η3-benzyl complexes of IiPr (5, 7, and 8) alone, in the absence of any activators such as borate and MAO, showed good catalytic activity towards the vinyl-type norbornene polymerization. The catalyst was thermally robust and the activity increases as the temperature rises to 130 °C.

Graphical abstractη3-benzylnickel complexes of N-heterocyclic carbenes are prepared and characterized in this study. While the imidazolinylidene complexes are inactive, the imidazolylidene-based η3-benzyl complexes exhibit good activity for norbornene homopolymerization without the requirement of any activator.Figure optionsDownload full-size imageDownload as PowerPoint slide