Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 92

equilibrium between side-on peroxo 5b and dicopper compound 5
is sensitive to the nature of the counter-anion and solvent mixture. Species 5a
and have been shown to be interconvertible, an important finding, since the

reaction represents a unique O-O bond forming reaction. 81 The
structure of 5 (Chart 2, R = benzyl) reveals O...O = 2.287 Å, with a short
value of Cu...Cu = 2.794 Å. Complex 5 is best described as a
dicopper(III) species. 82 The relationship of this structure to
side-on species has been studied. 83
Stack and co-workers 84,85 have also demonstrated O-O bond breaking
reactions in copper(I)-dioxygen reactivity studies utilizing ligands possessing
the simple ethylenediamine core. Three-coordinate copper(I) complexes,
cyclohexanediamine), react with dioxygen
at -80 °C in to afford diamagnetic species ( 6 ). An
x-ray structure of one derivative (Chart 2; reveals a square-
planar core surrounding copper, with very short Cu-O bonds (Ave. =
1.81 Å), Cu...Cu = 2.743 (1) Å, and O...O = 2.344(1) Å. Cu absorption K-
edge data corroborate the +3 copper oxidation state in

5

( 6a , L = (1 R ,2 R )-

these

complexes. 86
The same research group also discovered a significant new
reaction stoichiometry. 87
reacts with to give a trinuclear cluster compound
( 7 ) with two
ligands (O...O = 2.37 Å) and three copper ions. An x-ray structure (Chart 2)
reveals one unique Cu atom, with short Cu-O bond distances (~ 1.83 Å,
compared to the distances of ~ 2.00 ave. Å). X-ray absorption
measurements and comparisons 86 indicate this is a copper(III) ion, affirming
the overall reaction to be a four-electron reduction and O-O cleavage of
dioxygen.
A novel peroxocopper complex with is formed when
copper(II) perchlorate is reacted with tridentate ligand 4-methyl-2,6-
bis(pyrrolidinomethyl)phenol (HL) in methanol solution, in the presence of
air, triethylamine base and 3,5-di-tert-butylcatechol. 88 The four copper(II)
ions in ( 8 , Chart 2) form a near-planar rectangle,
with alternate methoxo (Cu...Cu = 3.03 Å) and phenolato (Cu...Cu = 2.99 Å)
bridging. Weaker axial ligation is completed by bridging perchlorato oxygen
atoms. The peroxide ligand has a typical O-O bond length of 1.453(4) Å.
The nature of any chemistry in the generation of 8 is not clear.
A mononuclear copper(II)-hydroperoxo complex was characterized
by Masuda and co-workers, 89 using a ligand similar to TMPA, bis(6-
pivalamine-2-pyridylmethyl)-2-pyridylmethyl)amine (complex 4 ; Chart 2).
The compound forms from reaction of with a copper(II) complex of this
ligands and N-H hydrogen bonding provides stabilization of the bound
ligand.