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 center dot center dot center dot Ni anagostic or preagostic interactions. The large downfield shift of these C-H protons in the H-1 NMR spectrum compared to their precursor salts indicates that these interactions are retained in solution. Preliminary catalytic studies show that complexes 7-12 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.