Transformation of (µ-η 2 :η 2 -peroxo)dicopper(II) complexes bearing sterically bulky tridentate N,N′,N′′-trisubstituted tacn 2 to square pyramidal bis(µ-oxo)dicopper(III) complexes has been reported by Tolman et al. 3 In certain instances, they have observed a monooxygenase activity of the bis(µ-oxo)dicopper(III) complexes for the coordinated ligand as substrate. A different type of square planar bis(µ-oxo)dicopper(III) complexes having peralkylated-1,2-cyclohexanediamine ligands have been also prepared by Stack et al. 4 Very recently, partial formation of a bis(µ-oxo)dicopper(III) complex with a tridentate ligand containing two pyridyl sidearms 5 and a bis(µ-oxo)dicopper(III) complex with a bidentate ligand containing a pyridyl group have been reported. 6 However, there is no crystallographically characterized bis(µ-oxo)dicopper(III) complex having aromatic nitrogen donors. Thus, it is important to explore how the nature of the donor atoms and the stereochemistry of supporting ligands influence the formation, structure, and reactivity of bis(µ-oxo)dicopper(III) complexes.Karlin et al. have demonstrated that a copper(I) complex having a tetradentate tripodal tpa ligand, [Cu(tpa)(NCCH 3 )] + , reacts with O 2 to form a trans-(µ-1,2-peroxo)dicopper(II) complex ([Cu 2 (O 2 )-(tpa) 2 ] 2+ ) in a trigonal bipyramidal structure (λ max ( , M -1 cm -1 ) ) ∼440 nm (4000), 525 nm (11500), and ∼590 nm (7600)). 7 Previously we found that [Cu(Me-tpa)] + in acetone at -70 °C generates a trans-(µ-1,2-peroxo)dicopper(II) species, whereas the reaction of [Cu(Me 2 -tpa)] + (1a) with O 2 (Cu:O 2 ) 2:1) in acetone at -70 °C does not form a trans-(µ-1,2-peroxo)dicopper(II) species, but produces a brown species (1b, λ max ( , M -1 cm -1 ) ) 378 nm ( ∼22 000, 0.1 mM), 494 nm (330, 10 mM)). 8 Thus,
