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

Studies of copper complexes with the 1,2-dimethylimidazole (Me[sub 2]im) system have provided insights into the factors which control dioxygen (O[sub 2]) binding and activation in imidazole (histidine) ligated copper complexes and proteins. A two-coordinate complex [Cu(Me[sub 2]im)[sub 2]](PF[sub 6]) (1(PF[sub 6])) is formed by the reaction of 1,2-dimethylimidazole with [Cu(CH[sub 3]CN)[sub 4]](PF[sub 6]). Although 1 is unreactive toward O[sub 2] or CO, reaction with one additional molar equivalent of Me[sub 2]im yields a three-coordinate complex [Cu(Me[sub 2]im)[sub 3]](PF[sub 6])(2(PF[sub 6])) which reacts with O[sub 2](Cu/O[sub 2] = 2:1, manometry), producing the EPR silent dioxygen adduct, formulated as [Cu[sub 2](Me[sub 2]im)[sub 6](O[sub 2])][sup 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 onemore » 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. 50 refs., 9 figs., 3 tabs.« less