Modeling Copper-Dioxygen Reactivity in Proteins: Aliphatic C-H Bond Activation by a New Dicopper(II)-Peroxo Complex

Abstract

The central role of the binding and/or activation of dioxygen by copper(1) ions in a wide rangeof important processes in biology and catalysis has led to intense interest in the synthesis, characterization, and examination of the reactivity of Cu,02 complexes.' Relatively few such complexes have been definitively identified, however, and only two, [((TMPA)Cu)2(02)] (PF&233 and [ (TpiPr2Cu)2(O,)] ,2v4 have been characterized structurally by X-ray crystallography. The former complex contains a transp-1,2-peroxo ligand, while the latter has a planar dicopper(I1)p-$$-peroxo core that accurately models the dioxygen adduct of hemocyanin ( H c ) . ~ Despite extensive studies of the reaction pathways followed by these and other less well-defined Cu/02 adducts in synthetic and biological systems, our understanding of the relationship between their structural features and their reactivity with substrates remains rudimentary.Iv6 In particular, insight into the mechanistic details of aliphatic hydrocarbon oxidations that involve Cu/02 species in proteins, such as those catalyzed by dopamine 6-monooxygenase (DPM),7 peptidylglycine a-amidating enzyme (PAM)! and particulate methane monooxygenase (pMM0),9 has been limited by the relatively small number of synthetic copper-dioxygen complexes available for study and their general lack of reactivity with aliphatic C-H bonds. Here we report the preparation of a new dicopper(I1)p-g2:q2-peroxo complex and demonstrate unequivocally the cleavage of an sp3 C-H(D)bond by the Cu202 unit.1° Treatment of solutionsof [ L C U ( C H ~ C N ) ] C F ~ S O ~ (L = 1,4,7triisopropyl1,4,7-tria~acyclononane)~~J~ in CH2Cl2 or acetone with dry O2 at -78 OC resulted in the formation of red-brown [ ( L C U ) ~ ( O ~ ) ] (CF3S03)2 (Scheme 1). The adduct was not perturbed by an N2 purge or application of vacuum at -78 OC, suggesting that 0 2 binding is essentially irreversible a t this temperature. Assignment of a dicopper(II)-p-$:q2-peroxo structure to the product is based on (i) manometric data [Cu:02 = 2.2(2):1]; (ii) its UV-vis spectrum, which contains 0z2+ Cu(11) charge transfer bands [Figure 1; A, , = 365 (t l l 000 M-i