Mixed NHC/Phosphine Ni(II) Complexes: Synthesis and Their Applications as Versatile Catalysts for Selective Cross-Couplings of ArMgX with Aryl Chlorides, Fluorides, and Methyl Ethers

Abstract

New methods for the preparation of mixed NHC/phosphine Ni(II) complexes have been developed. It was shown that the quaternary ammonium cation in the easily available Ni(II) complexes [NEt 4 ][Ni(PPh 3 )X 3 ] (X = Cl and Br) can act as a good leaving group in reactions of [NEt 4 ][Ni(PPh 3 )X 3 ] with the bulky ItBu (ItBu = 1,3-ditertbutylimidazol-2-ylidene) or IPr [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] ligand, resulting in the corresponding mixed NHC/PPh 3 Ni(II) complexes Ni(PPh 3 )(ItBu)X 2 (X = Cl, 1; X = Br, 2 ) or Ni(PPh 3 )(IPr)Br 2 ( 3 ) in high yields. The PPh 3 ligand in these obtained mixed NHC/PPh 3 Ni(II) complexes can be easily substituted by a more electron-donating phosphine ligand, i.e., PCy 3, resulting in the corresponding mixed NHC/PCy 3 Ni(II) complexes Ni(PCy 3 )(ItBu)Br 2 ( 4 ) and Ni(PCy 3 )(IPr)Br 2 ( 5 ) in high yields. The crystal structures of these Ni(II) complexes have been characterized, which revealed a trans disposition of the NHC ligand to the phosphine ligand. The catalytic behaviors of them on varying the carbene ligand (ItBu vs IPr) as well as the phosphine ligand (PPh 3 vs PCy 3 ) were investigated in the cross-coupling of aryl Grignard reagents with a wide range of electrophiles. In addition to a significant synergic effect on their catalytic activities, high selectivity for the activation and transformation of C–Cl, C–F and C–O bonds was achieved based on the rational structural design. Complex 2 showed the highest catalytic activity for the cross-coupling of aryl chlorides and fluorides with aryl Grignard reagents, but exhibit little activity for the cross-coupling of aryl methyl ethers with aryl Grignard reagents. On contrast, complex 4 showed great potential for the aryl methyl ethers involved cross-coupling reactions, although its reactivity for the activation of the C–X bond is very poor. The difference in catalytic activity between 2 and 4 has been successfully employed to construct oligoarenes by selective cross-coupling reactions.