Copper-dioxygen complexes and their roles in biomimetic oxidation reactions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 94

non-fluorinated substituted pyrazolyl borate systems originally studied by
Kitajima and co-workers. The UV-vis spectrum of the is similar to
that of other crystallographically-characterized peroxo
species, with absorptions at 334 and 510 nm, and resonance Raman studies
have confirmed its peroxo-binding mode. Compound is
stable indefinitely at 25 °C, probably due to the fact that the dicopper(II)-
peroxo center is protected by robust trifluoromethyl groups. Unfortunately,
has not yet been characterized crystallographically.

Besides the two room-temperature stable dicopper(II) peroxo
compounds mentioned above, a new dicopper(III) complex has
recently been structurally-characterized by Suzuki and co-workers. 92
Although we have previously reported on the partial formation of a
dicopper(III) compound with two pyridyl side arms, 66 no
dicopper(III) complex containing aromatic nitrogen donors had ever been
structurally-characterized until the work of Suzuki and co-workers. They
showed that by introducing two 6-methylpyridyl groups into the TMPA ligand
to form bis-(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine,

a

dicopper(III) complex is formed instead of a trans
peroxo)dicopper(II) complex generated from TMPA (see Sect. 2.1). This
complex can be reversibly converted between the copper-dioxygen adduct and
the copper(I) precursor, and it shows an intense absorption band at 378 nm
~19,000 ) in dichloromethane, which is higher in energy than other
known complexes. This formulation has been
confirmed by both resonance Raman spectroscopy (with an isotope-sensitive
band at 590 ) and by x-ray crystallography. The distance of
2.758 Å is comparable to other structurally characterized
dicopper(III) complexes. The distances between the copper centers and
the nitrogens of the 6-methylpyridyl groups are 2.55 and 2.48 Å, much longer
than the other two copper nitrogen distances of 1.97 and 1.91 Å. In this way,
the formally copper(III) centers can adopt a square-planar geometry favored
by d 8 metal centers and converts the traditionally tetradentate, tripodal TMPA
ligand to a quasi-bidentate ligand capable of supporting a
dicopper(III) core. It should be noted that in the copper(I) oxidation state