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 M_n 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.