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Structural and Electronic Properties of Old and New A<sub>2</sub>[M(pin<sup>F</sup>)<sub>2</sub>] Complexes
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new homoleptic complexes of the form A<sub>2</sub>[MÂ(pin<sup>F</sup>)<sub>2</sub>] have been synthesized with the dodecafluoropinacolate
(pin<sup>F</sup>)<sup>2–</sup> ligand, namely (Me<sub>4</sub>N)<sub>2</sub>[FeÂ(pin<sup>F</sup>)<sub>2</sub>], <b>1</b>;
(Me<sub>4</sub>N)<sub>2</sub>[CoÂ(pin<sup>F</sup>)<sub>2</sub>], <b>2</b>; (<sup>n</sup>Bu<sub>4</sub>N)<sub>2</sub>[CoÂ(pin<sup>F</sup>)<sub>2</sub>], <b>3</b>; {KÂ(DME)<sub>2</sub>}<sub>2</sub>[NiÂ(pin<sup>F</sup>)<sub>2</sub>], <b>4</b>; (Me<sub>4</sub>N)<sub>2</sub>[NiÂ(pin<sup>F</sup>)<sub>2</sub>], <b>5</b>; {KÂ(DME)<sub>2</sub>}<sub>2</sub>[CuÂ(pin<sup>F</sup>)<sub>2</sub>], <b>7</b>; and
(Me<sub>4</sub>N)<sub>2</sub>[CuÂ(pin<sup>F</sup>)<sub>2</sub>], <b>8</b>. In addition, the previously reported complexes K<sub>2</sub>[CuÂ(pin<sup>F</sup>)<sub>2</sub>], <b>6</b>, and K<sub>2</sub>[ZnÂ(pin<sup>F</sup>)<sub>2</sub>], <b>9</b>, are characterized
in much greater detail in this work. These nine compounds have been
characterized by UV–vis spectroscopy, cyclic voltammetry, elemental
analysis, and for paramagnetic compounds, Evans method magnetic susceptibility.
Single-crystal X-ray crystallographic data were obtained for all complexes
except <b>5</b>. The crystallographic data show a square-planar
geometry about the metal center in all Fe (<b>1</b>), Ni (<b>4</b>), and Cu (<b>6</b>, <b>7</b>, <b>8</b>) complexes independent of countercation. The Co species exhibit
square-planar (<b>3</b>) or distorted square-planar geometries
(<b>2</b>), and the Zn species (<b>9</b>) is tetrahedral.
No evidence for solvent binding to any Cu or Zn complex was observed.
Solvent binding in Ni can be tuned by the countercation, whereas in
Co only strongly donating Lewis solvents bind independent of the countercation.
Indirect evidence (diffuse reflectance spectra and conductivity data)
suggest that <b>5</b> is not a square-planar compound, unlike <b>4</b> or the literature K<sub>2</sub>[NiÂ(pin<sup>F</sup>)<sub>2</sub>]. Cyclic voltammetry studies reveal reversible redox couples
for NiÂ(III)/NiÂ(II) in <b>5</b> and for CuÂ(III)/CuÂ(II) in <b>8</b> but quasi-reversible couples for the FeÂ(III)/FeÂ(II) couple
in <b>1</b> and the CoÂ(III)/CoÂ(II) couple in <b>2</b>.
Perfluorination of the pinacolate ligand results in an increase in
the central C–C bond length due to steric clashes between CF<sub>3</sub> groups, relative to perhydropinacolate complexes. Both types
of pinacolate complexes exhibit O–C–C–O torsion
angles around 40°. Together, these data demonstrate that perfluorination
of the pinacolate ligand makes possible highly unusual and coordinatively
unsaturated high-spin metal centers with ready thermodynamic access
to rare oxidation states such as NiÂ(III) and CuÂ(III)
