Synthesis and structure of indenylnickel(II) chlorides bearing free N-heterocyclic carbene ligands and their catalysis for styrene polymerization

The reaction of (R-Ind)2Ni (Ind = C9H7, indenyl) with an equivalent of a bulky aryl-substituted imidazolium salt in CH2Cl2/THF at 45 °C results in the corresponding N-heterocyclic carbene (NHC) indenylnickel(II) chloride of the type (R-Ind)Ni(L)Cl [R = 1-H, L = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), 1; R = 1-Me, L = IPr, 2] in high yield. Complexes 1 and 2 were characterized by elemental analysis, NMR spectroscopy and X-ray crystallography. The resulted NHC indenylnickel(II) complexes are capable of polymerizing styrene in the presence of NaBPh4 to give atactic polystyrene with Mn values in the range of 104. The present studies show a close relationship between the structure and catalytic activity of the NHC indenylnickel(II) halides (including the previously reported indenylnickel(II) halides bearing alkyl-substituted NHC ligands), and complex 2 shows the highest catalytic activity. In comparison with its phosphine analogue (1-Me-Ind)Ni(PPh3)Cl, complex 2 shows significant improvements in stability and catalytic performance.

Graphical abstractNeutral indenylnickel(II) halides bearing a free N-heterocyclic carbene (NHC) ligand (1 and 2) have been synthesized and characterized by X-ray structural analysis. A close relationship between the structure and catalytic activity of NHC indenylnickel(II) halides is observed for the first time.Figure optionsDownload full-size imageDownload as PowerPoint slide