Anagostic Interactions and Catalytic Activities of Sterically Bulky Benzannulated N-Heterocyclic Carbene Complexes of Nickel(II)

Nickel(II) bis(benzimidazolin-2-ylidene) complexes of the general formula [NiBr2(NHC)2] (NHC = 1,3-dibenzylbenzimidazolin-2-ylidene, 7; NHC = 1,3-diisopropylbenzimidazolin-2-ylidene, 8; NHC = 1,3-dibenzhydrylbenzimidazolin-2-ylidene, 9; NHC = 1,3-diisobutylbenzimidazolin-2-ylidene, 10; NHC = 1-isopropyl-3-benzylbenzimidazolin-2-ylidene, 11; NHC = 1-benzhydryl-3-benzylbenzimidazolin-2-ylidene, 12) have been prepared and fully characterized by spectroscopic methods and single-crystal X-ray structure analyses. All complexes adopt a square-planar geometry with nickel as the crystallographic inversion center and a trans arrangement of the carbene ligands. For complexes 11 and 12, bearing unsymmetrically substituted ligands, only the trans-anti configuration was found in the solid state. In addition, the structures of 8, 9, 11, and 12 reveal a fixed orientation of the N-isopropyl and N-benzhydryl substituents with the C−H groups pointing to the nickel(II) center to maximize rare intramolecular C−H···Ni anagostic or preagostic interactions. The large downfield shift of these C−H protons in the 1H NMR spectrum compared to their precursor salts indicates that these interactions are retained in solution. Preliminary catalytic studies show that complexes 712 are active in the Ullmann coupling of bromobenzene and 4-bromoanisole. In particular, complexes 8, 9, and 12, with sterically more demanding ligands, exhibit the best catalytic activities. The coupling reaction was found to be successful when carried out in neat [Bu4N]Br as ionic liquid, but not in dry DMF or in DMF with [Bu4N]Br as an additive.