CHEMISTRY AND STRUCTURAL STUDIES ON THE DIOXYGEN-BINDING COPPER-1,2-DIMETHYLIMIDAZOLE SYSTEM

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) angstrom, b = 15.950 (4) angstrom, c = 6.931 (4) angstrom, and beta = 108.54 (2)degrees. The linear two-coordinate Cu(I) structure is typical and contains crystallographically equivalent Cu-N(imid) distances of 1.865 angstrom. 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 angstrom) and one longer (2.08 angstrom) 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-mu-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 eta2-eta2-peroxo bridging between two coppers 2.84 angstrom 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-degrees-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 (lambda(max) = 350 nm) which thermally degrades to a carbonato 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.