Chiral Nickel(II) and Palladium(II) NCN-Pincer Complexes Based on Substituted Benzene: Synthesis, Structure, and Lewis Acidity

Air- and moisture-stable nickel(II) and palladium(II) complexes
from phebox pincer ligands 1−5 have
been synthesized, and their Lewis activity was investigated. 2,6-Bis(oxazolinyl)phenylnickel
halide complexes [NiX(phebox)] and 2,6-bis(oxazolinyl)phenylpalladium
halide complexes [PdX(phebox)] were synthesized via oxidative addition
with [Ni(COD)2] or Pd2dba3, respectively,
followed by halide abstraction using silver(I) salts to form complexes
such as [Ni(phebox)][ClO4] (41). Herein, X-ray
crystal structures are reported for several pincer complexes having
the expected square-planar geometries with the terdentate NCN pincer
system. Complexes with general structure [M(phebox)]X where X is a
halogen showed no relative Lewis acidity. On the other hand, complexes
where X was exchanged for a less coordinating counterion showed increased
Lewis acidity. The relative Lewis acidity varies depending on the
substituents on the benzene core of the pincer ligands, due to the
electronic effects of the ligand on the metal center.