Chemistry and structural studies on the dioxygen-binding copper-1,2-dimethylimidazole system

Article abstract of DOI:10.1021/ja00077a027

Studies of copper complexes with the 1,2-dimethylimidazole (Me₂im) system have provided insights into the factors which control dioxygen (O₂) binding and activation in imidazole (histidine) ligated copper complexes and proteins. A two-coordinate complex [Cu(Me₂im)₂](PF₆) (1(PF₆)) is formed by the reaction of 1,2-dimethylimidazole with [Cu(CH₃CN)₄](PF₆). Although 1 is unreactive toward O₂ or CO, reaction with one additional molar equivalent of Me₂im yields a three-coordinate complex [Cu(Me₂im)₃] (PF₆) (2(PF₆)) which reacts with O₂ (Cu/O₂ = 2:1, manometry), producing the EPR silent dioxygen adduct, formulated as [Cu₂(Me₂im)₆(O₂)]²⁺ (3). The structure of 1 has been studied by X-ray crystallography; it crystallizes in the monoclinic space group C2/c with Z = 4, a = 14.877 (2) ?, b = 15.950 (4) ?, c = 6.931 (4) ?, and β= 108.54 (2)°. The linear two-coordinate Cu(I) structure is typical and contains crystallographically equivalent Cu-N(imid) distances of 1.865 ?. The structures of 2 and 3 have been studied by X-ray absorption spectroscopy, using imidazole group-fitting and full curved-wave multiple scattering analysis. Complex 2 is best fit by a T-shaped structure involving two short (1.89 ?) and one longer (2.08 ?) Cu-N(imid) distances. Absorption edge data confirm that the dioxygen complex 3 should be formulated as a Cu(II)-peroxo species. The EXAFS of 3 can be fit by either of two models, A and B. Model A involves a four-coordinate species having a trans-μ-1,2-peroxo bridge, but the edge data do not fully support the presence of square planar coordination. Model B, which is more consistent with the edge data, involves a five-coordinate structure with a bent η²-η²-peroxo bridging between two coppers 2.84 ? apart. XAS studies on the crystallographically characterized complex [{Cu(TMPA)}₂-(O₂)]²⁺ (4) (TMPA = tris[(2-pyridyl)methyl]amine) were also used to provide insight into the XAS studies of 3. The reactivity of 3 (-90 °C) has been probed by exposure to a variety of reagents. TMPA causes displacement of the unidentate Me₂im ligands producing 4, while H⁺ liberates H₂O₂ (74%), CO₂ results in the formation of a percarbonato complex (λ_max = 350 nm) which thermally degrades to a carbonate species [Cu₂(Me₂im)₆(CO₃)]²⁺ (5), and tertiary phosphines effect the liberation of O₂, yielding [Cu(Me₂im)₃(PR₃)]⁺ (R = Ph (6a); R = Me (6b)). The UV-vis spectroscopic properties of 3 and its reactivity suggest that structure A is more likely, but considerable additional efforts in the area of Cu₂O₂ structure-spectroscopy-reactivity correlations are needed.