Heme-Cu/O
2 adducts are of interest in the elucidation of the fundamental metal-O
2 chemistry occurring in heme-Cu enzymes which effect reductive O-O cleavage of dioxygen to water. In this report, the chemistry of four heme-peroxo-copper [Fe
III-(O
22-)-Cu
II]
+ complexes (
1-4), varying in their ligand architecture, copper-ligand denticity, orboth and thus their structures and physical properties are compared in their reactivity toward CO, PPh
3, acids,cobaltocene, and phenols. In
1 and
2, the copper(II) ligand is N
4-tetradentate, and the peroxo unit is bound side-onto iron(III) and end-on to the copper(II). In contrast,
3 and
4 contain a N
3-tridentate copper(II) ligand, and theperoxo unit is bound side-on to both metal ions. CO "displaces" the peroxo ligand from
2-
4 to form reducedCO-Fe
II and CO-Cu
I species. PPh
3 reacts with
3 and
4 displacing the peroxide ligand from copper, forming(porphyrinate)Fe
III-superoxide plus Cu
I-PPh
3 species. Complex
2 does not react with PPh
3, and surprisingly,
1reacts neither with PPh
3 nor CO, exhibiting remarkable stability toward these reagents. The behavior of
1 and
2compared to that of
3 and
4 correlates with the different denticity of the copper ligand (tetra vs tridentate). Complexes
1-
4 react with HCl releasing H
2O
2, de
monstrating the basic character of the peroxide ligand. Cobaltocene causesthe two-electron reduction of
1-
4 giving the corresponding
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-oxo [Fe
III-(O
2-)-Cu
II]
+ complexes, in contrast to thefindings for other heme-peroxo-copper species of different design. With
t-butyl-substituted phenols, no reactionoccurs with
1-
4. The results described here emphasize how ligand design and variations influence and control notonly the structure and physical properties but also the reactivity patterns for heme-Cu/O
2 adducts. Implications forfuture investigations of protonated heme/Cu-peroxo complexes, low-spin analogues, and ultimately O-O cleavagechemistry are discussed.