Homoleptic Nickel(II) Complexes of Redox-Tunable Pincer-type
Ligands
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Abstract
Different synthetic methods have
been developed to prepare eight new redox-active pincer-type ligands,
H(X,Y), that have pyrazol-1-yl flanking donors attached to an <i>ortho</i>-position of each ring of a diarylamine anchor and
that have different groups, X and Y, at the <i>para</i>-aryl
positions. Together with four previously known H(X,Y) ligands, a series
of 12 Ni(X,Y)<sub>2</sub> complexes were prepared in high yields by
a simple one-pot reaction. Six of the 12 derivatives were characterized
by single-crystal X-ray diffraction, which showed tetragonally distorted
hexacoordinate nickel(II) centers. The nickel(II) complexes exhibit
two quasi-reversible one-electron oxidation waves in their cyclic
voltammograms, with half-wave potentials that varied over a remarkable
700 mV range with the average of the Hammett σ<sub>p</sub> parameters
of the <i>para</i>-aryl X, Y groups. The one- and two-electron
oxidized derivatives [Ni(Me,Me)<sub>2</sub>](BF<sub>4</sub>)<sub><i>n</i></sub> (<i>n</i> = 1, 2) were prepared synthetically,
were characterized by X-band EPR, electronic spectroscopy, and single-crystal
X-ray diffraction (for <i>n</i> = 2), and were studied computationally
by DFT methods. The dioxidized complex, [Ni(Me,Me)<sub>2</sub>](BF<sub>4</sub>)<sub>2</sub>, is an <i>S</i> = 2 species, with
nickel(II) bound to two ligand radicals. The mono-oxidized complex
[Ni(Me,Me)<sub>2</sub>](BF<sub>4</sub>), prepared by comproportionation,
is best described as nickel(II) with one ligand centered radical.
Neither the mono- nor the dioxidized derivative shows any substantial
electronic coupling between the metal and their bound ligand radicals
because of the orthogonal nature of their magnetic orbitals. On the
other hand, weak electronic communication occurs between ligands in
the mono-oxidized complex as evident from the intervalence charge
transfer (IVCT) transition found in the near-IR absorption spectrum.
Band shape analysis of the IVCT transition allowed comparisons of
the strength of the electronic interaction with that in the related,
previously known, Robin–Day class II mixed valence complex,
[Ga(Me,Me)<sub>2</sub>]<sup>2+</sup>