Weakly Coupled Biologically Relevant CuII2(μ‑η1:η1‑O2) cis-Peroxo Adduct that Binds Side-On to Additional Metal Ions

The ability of many copper metalloenzymes to activate O2 and transfer it to organic substrates has motivated extensive attention in the literature. Investigations focusing on synthetic analogues have provided a detailed understanding of the structures of potential intermediates, thereby helping to guide mechanistic studies. We report herein a crystallographically characterized synthetic CuII2(μ-η1:η1-O2) complex exhibiting cis-peroxo bonding geometry, known in iron chemistry but previously unobserved for copper. Detailed investigation by UV–vis, resonance Raman, and infrared spectroscopies provides evidence for a significantly diminished copper–oxygen interaction (ε ≈ 3000 M–1 cm–1, νCu–O = 437 cm–1, νO–O = 799 cm–1) relative to those in known ’coupled’ Cu2O2 species, consistent with magnetic measurements which show that the peroxide mediates only weak antiferromagnetic coupling (−2J = 144 cm–1). These characteristics are comparable with those of a computationally predicted transition state for O2 binding to type 3 copper centers, providing experimental evidence for the proposed mechanism of O2 activation and supporting the biological relevance of the CuII2(μ-η1:η1-O2) cis-species. The peroxide bonding arrangement also allows binding of sodium cations, observed both in the solid state and in solution. Binding induces changes on an electronic level, as monitored by UV–vis spectroscopy (Ka = 1700 M–1), reminiscent of redox-inactive metal binding by iron–oxygen species. The results presented highlight the analogous chemistry these reactive oxygen species undergo, with respect to both their mechanism of formation, and the molecular interactions in which they participate.