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

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 [NEt4]­[Ni­(PPh3)­X3] (X = Cl and Br) can act as a good leaving
group in reactions of [NEt4]­[Ni­(PPh3)­X3] 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/PPh3 Ni­(II) complexes
Ni­(PPh3)­(ItBu)­X2 (X = Cl, 1; X
= Br, 2) or Ni­(PPh3)­(IPr)­Br2 (3) in high yields. The PPh3 ligand in these obtained
mixed NHC/PPh3 Ni­(II) complexes can be easily substituted
by a more electron-donating phosphine ligand, i.e., PCy3, resulting in the corresponding mixed NHC/PCy3 Ni­(II)
complexes Ni­(PCy3)­(ItBu)­Br2 (4)
and Ni­(PCy3)­(IPr)­Br2 (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 (PPh3 vs PCy3) 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.