1 research outputs found
Unsymmetrical <i>N</i>‑Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2′-Bipyridine and <i>N</i>,<i>N</i>‑Diaryl-α-diimines?
The
new organonickel complexes [(R-PyMA)ÂNiÂ(Mes)ÂX] [R-PyMA = <i>N</i>-aryl-1-(pyridin-2-yl)Âmethanimine; aryl = phenyl, 2,6-Me<sub>2</sub>-, 3,5-Me<sub>2</sub>-, 2,4,6-Me<sub>3</sub>-, 2,6-<sup><i>i</i></sup>Pr<sub>2</sub>-, 3,5-(OMe)<sub>2</sub>-, 2-NO<sub>2</sub>-4-Me-,
4-NO<sub>2</sub>-, 2-CF<sub>3</sub>-, and 2-CF<sub>3</sub>-6-F-phenyl;
Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as
approximate 1/1 cis and trans isomeric mixtures or pure cis isomers
depending on the PyMA ligand and X. The [(R-PyMA)ÂNiÂ(Mes)ÂX] complexes
with X = Br or Cl were directly synthesized from the precursors <i>trans</i>-[(PPh<sub>3</sub>)<sub>2</sub>NiÂ(Mes)ÂX], while [(PyMA)ÂNiÂ(Mes)ÂX]
derivatives with X = F or I were obtained from [(PyMA)ÂNiÂ(Mes)ÂBr] through
X exchange reactions. Although density functional theory (DFT) calculations
show a preference for the sterically favored cis isomers, both isomers
could be observed in many cases; in three cases, even single crystals
for X-ray diffraction could be obtained for the trans isomers. Possible
intermediates for the isomerization were investigated by DFT calculations.
All complexes were studied by multiple spectroscopic means, electrochemistry,
and spectroelectrochemistry (for the reduction processes). The long-wavelength
metal-to-ligand charge-transfer (MLCT) absorptions vary markedly with
the R substituent of the ligand and the cathodic electrochemical potentials
to a far smaller degree. Both are almost invariable upon variation
of X. All of this is in line with Ni-based and π*-based lowest
unoccupied molecular orbitals (LUMOs). In line with the unsymmetric
character of the N<sub>Py</sub>^N<sub>methanimine</sub> ligand, electrochemistry
and MLCT transitions seem to not correspond to the same type of π*
LUMO, making these PyMA ligands more interesting than the symmetric
heteroaromatic polypyridine ligands such as 2,2′-bipyridine
(bpy; N<sub>Py</sub>^N<sub>Py</sub>) and <i>N</i>,<i>N</i>-diaryl-substituted aliphatic α-diimines (N<sub>methanimine</sub>^N<sub>methanimine</sub>) such as the diaza-1,3-butadienes (DAB).
First attempts to use these complexes in Negishi-type cross-coupling
reactions were successful