Synthesis and Oxidative Reactivity of 2,2′-Pyridylpyrrolide Complexes of Ni(II)

Synthesis
and characterization of divalent nickel complexed by 2-pyridylpyrrolide
bidentate ligands are reported, as possible precursors to complexes
with redox active ligands. Varied substituents on the pyrrolide, two
CF3 (L2), two tBu (L0),
and one of each type (L1) are employed and the resulting
Ni­(Ln)2 complexes show different
Lewis acidity toward CO, H2O, tetrahydrofuran (THF), or
MeCN, the L2 case being the most acidic. Density functional
theory calculations show that the frontier orbitals of all three Ni­(Ln)2 species are localized at the
pyrrolide groups of both ligands and Ni­(Ln)2+ can be detected by mass spectrometry and
in cyclic voltammograms (CVs). Following cyclic voltammetry studies,
which show electroactivity primarily in the oxidative direction, reactions
with pyridine N-oxide or Br2 are reported. The former yield
simple bis adducts, Ni­(L2)2(pyNO)2 and the latter effects electrophilic aromatic
substitution of the one pyrrolide ring hydrogen for both chelates,
leaving it brominated.