Direct synthesis of cis-dihalido-bis(NHC) complex of nickel(ii) and catalytic application in olefin addition polymerization: Effect of halogen co-ligands and density functional theory study

Abstract

Two novel amine-containing N-heterocyclic carbene ligand precursors [H(1a–b)]Br have been prepared in good yield and fully characterized. Direct syntheses of cis- and trans-dihalido-bis(NHC) nickel complexes [Ni(NHC)₂X₂] (X = Cl, Br) are reported. The solid structures of trans-[Ni(1a–b)₂Br₂] (2a–b) and cis-[Ni(1a)₂Cl₂] (3) were determined by single-crystal X-ray analysis and 3 was found to be the first example of cis-configuration coordination of monodentate NHC ligands to a metal center for dihalido-bis(NHC) nickel complexes. DFT calculations were conducted to determine the energy difference between cis- and trans-isomers of complexes 2a and 3 bearing bromide and chloride co-ligands. The cis-[Ni(1a)₂Cl₂] (cis-3) is 1.77–1.55 kcal mol⁻¹ lower in energy than its trans-isomer in polar solvents including CH₂Cl₂ and THF, while the trans-[Ni(1a)₂Br₂] (trans-2a) is more stable than the cis-isomer similarly in the gas phase. The cis nickel complex 3 with two coordinated monodentate NHCs was tested for olefin addition polymerization at standard conditions. It was found that cis-3 was inactive in ethylene polymerization but showed moderate catalytic activities (0.5–3.0 × 10⁶ g of PNB (mol of Ni)⁻¹ h⁻¹) in the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst.