R. Ruiz et al., STABILIZATION OF COPPER(III) COMPLEXES BY DISUBSTITUTED OXAMIDES AND RELATED LIGANDS, Journal of the Chemical Society. Dalton transactions, (5), 1997, pp. 745-751
The electrochemical behaviour of a family of monomeric copper(II) comp
lexes of the related tetraanionic chelating ligands N,N'-o-phenylenebi
s(oxamate) (L(1)) and its methylamide (L(2)) and bis(methylamide) (LS)
has been investigated by cyclic voltammetry in acetonitrile at 25 deg
rees C and 0.1 mol dm(-3) NEt(4)ClO(4) as supporting electrolyte. The
copper(III)-copper(II) reduction potentials have been found to span a
potential range from +0.41 to -0.02 V (vs. saturated calomel electrode
), being reversible for all cases except the copper(II)-L(1) complex.
The trend in formal potentials along this series is explained in terms
of the stronger donor properties of the deprotonated-amido nitrogen a
toms than those of the carboxylate oxygen ones. Hence, the stabilizati
on of the trivalent oxidation state of copper is attributed to the inc
reasing number of deprotonated-amido donor groups. A perfect correlati
on has been observed within this family between the Cu-III-Cu-II poten
tials and the visible absorption maxima of the copper(II) complexes. T
he relative gain in crystal-field stabilization energy for the change
from the d(9) (Cu-II, square planar) to the low-spin d(8) (Cu-III, squ
are-planar) electronic configuration is the main factor in the overall
thermodynamic stability of the copper((III)) complexes. The molecular
structure of the stable copper(III) complex [PPh(4)l[CuL(3)]. MeCN ha
s been determined by single-crystal X-ray analysis. The metal is in a
nearly square-planar environment formed by the four amido nitrogen ato
ms of the chelating ligand, with short Cu-N bond distances (1.84-1.88
Angstrom) typical of trivalent copper.