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Cu/O 2 intermediates in biological, homogeneous, and heterogeneous catalysts exhibit unique spectral features that reflect novel geometric and electronic structures that make significant contributions to reactivity. This review considers how the respective intermediate electronic structures overcome the spin forbidden nature of O 2 binding, activate O 2 for electrophillic aromatic attack and H-atom abstraction, catalyze the 4 e-reduction of O 2 to H 2 O, and discusses the role of exchange coupling between Cu ions in determining reactivity.Our focus has been on the use of spectroscopic methods to elucidate active sites in catalysis. In the area of Cu/O 2 chemistry, this has mostly involved studies on metalloenzymes, however these have led to parallel studies in Cu coordination chemistry and now to studies on Cu sites in zeolites. There are five main topics in Cu/O 2 biological, homogeneous and heterogeneous reactivity that will be the scope of this overview. First is the spin-forbidden, reversible binding of dioxygen by the coupled binuclear Cu site in hemocyanin. Next, we will consider O 2 activation by coupled binuclear copper sites for electrophilic attack on phenolic substrates in tyrosinase and related model complexes. We will then consider Hatom abstraction from relatively weak C-H bonds (~85 kcal/mol) by the "non-coupled" binuclear Cu enzymes and how the difference in magnetic "exchange" coupling can control reactivity. We will then move to the four e-reduction of O 2 to H 2 O by the multi-copper oxidases at a trinuclear Cu cluster, a structural motif originally defined to be present by MCD spectroscopy. 1, 2 Finally, we will focus on O 2 activation for H-atom abstraction from the strong C-H bond of methane (~105 kcal/mol) which in biology is accomplished by methane monooxygenases (MMO) but can now be achieved in the active sites of zeolites . The copper-oxygen intermediates in these systems have unique spectroscopic features that we have shown to reflect novel geometric and electronic structures that make key contributions to reactivity. I. Reversible O 2 Binding: Coupled binuclear Cu SitesHemocyanin (Hc) functions as an extracellular oxygen transport protein in arthropods and mollusks. 3 Deoxy-Hc contains 2 Cu(I) ions that reversibly bind O 2 to form the binuclear cupric site in oxy-Hc. Thus, 2e − are transferred to O 2 reducing it to the peroxide level. As will be discussed below, oxy-Hc has unique spectral features, and to understand these we first consider "normal" peroxide-Cu(II) bonding. 4 O 2 is a triplet that has two unpaired electrons in the doubly degenerate π * orbitals. Reduction of O 2 to peroxide leads to a fully occupied π * HOMO. As shown in Fig. 1A, upon binding O 2 2− end-on to a Cu(II), one π * orbital is stabilized due to σ bonding with the d 9 Cu(II) half occupied d orbital, which is in turn destabilized. This leads to the characteristic EPR Correspondence to: Edward I. Solomon. NIH Public Access Author ManuscriptFaraday Discuss. Author manuscript; available in PMC 2012 Januar...

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