No abstract

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...

Oxygenation of [Cu2(UN-O−)(DMF)]2+ (1), a structurally characterized dicopper Robin–Day class I mixed-valent Cu(II)Cu(I) complex, with UN-O− as a binucleating ligand and where dimethylformamide (DMF) binds to the Cu(II) ion, leads to a superoxo-dicopper(II) species [CuII2(UN-O−)(O2•−)]2+ (2). The formation kinetics provide that kon = 9 × 10−2 M−1 s−1 (−80 °C), ΔH‡ = 31.1 kJ mol−1 and ΔS‡ = −99.4 J K−1 mol−1 (from −60 to −90 °C data). Complex 2 can be reversibly reduced to the peroxide species [CuII2(UN-O−)(O22−)]+ (3), using varying outer-sphere ferrocene or ferrocenium redox reagents. A Nernstian analysis could be performed by utilizing a monodiphenylamine substituted ferrocenium salt to oxidize 3, leading to an equilibrium mixture with Ket = 5.3 (−80 °C); a standard reduction potential for the superoxo–peroxo pair is calculated to be E° = +130 mV vs SCE. A literature survey shows that this value falls into the range of biologically relevant redox reagents, e.g., cytochrome c and an organic solvent solubilized ascorbate anion. Using mixed-isotope resonance Raman (rRaman) spectroscopic characterization, accompanied by DFT calculations, it is shown that the superoxo complex consists of a mixture of μ-1,2- (21,2) and μ-1,1- (21,1) isomers, which are in rapid equilibrium. The electron transfer process involves only the μ-1,2-superoxo complex [CuII2(UN-O−)(μ-1,2-O2•−)]2+ (21,2) and μ-1,2-peroxo structures [CuII2(UN-O−)(O22−)]+ (3) having a small bond reorganization energy of 0.4 eV (λin). A stopped-flow kinetic study results reveal an outer-sphere electron transfer process with a total reorganization energy (λ) of 1.1 eV between 21,2 and 3 calculated in the context of Marcus theory.

Copper(II)-hydroperoxide species are significant intermediates in processes such as fuel cells and (bio)chemical oxidations, all involving stepwise reduction of molecular oxygen. We previously reported a CuII-OOH species that performs oxidative N-dealkylation on a dibenzylamino group that is appended to the 6-position of a pyridyl donor of a tripodal tetradentate ligand. To obtain insights into the mechanism of this process, reaction kinetics and products were determined employing ligand substrates with various para- substituent dibenzyl pairs (-H,-H; -H,-Cl; -H,-OMe and -Cl,-OMe), or with partially or fully deuterated dibenzyl N-(CH2Ph)2 moieties. A series of ligand-copper(II) bis-perchlorate complexes were synthesized, characterized, and the X-ray structures of the -H, -OMe analog was were determined. The corresponding metastable CuII-OOH species were generated by addition of H2O2/base in acetone at –90 °C. These convert (t1/2 ~ 53 s) to oxidatively N-dealkylated products, producing para-substituted benzaldehydes. Based on the experimental observations and supporting DFT calculations, a reaction mechanism involving dibenzylamine H-atom abstraction or electron-transfer oxidation by the CuII-OOH entity could be ruled out. It is concluded that the chemistry proceeds by rate limiting Cu–O homolytic cleavage of the CuII–(OOH) species, followed by site-specific copper Fenton chemistry. As a process of broad interest in copper as well as iron oxidative (bio)chemistries, a detailed computational analysis was performed, indicating that a CuIOOH species undergoes O–O homolytic cleavage to yield a hydroxyl radical and CuIIOH rather than heterolytic cleavage to yield water and a CuII-O•−.

Previous efforts to synthesize a cupric superoxide complex possessing a thioether donor have resulted in the formation of an end-on trans-peroxodicopper(II) species, [{(Ligand)CuII}2(μ-1,2-O22−)]2+. Redesign/modification of previous N3S tetradentate ligands has now allowed for the stabilization of the monomeric, superoxide product possessing a S(thioether)-ligation, [(DMAN3S)CuII(O2•−)]+ (2S), as characterized by UV-vis and resonance Raman (rR) spectroscopies. This complex mimics the putative CuII(O2•−) active species of the copper monooxygenase PHM and exhibits enhanced reactivity towards both O-H and C-H substrates in comparison to close analogues [(L)CuII(O2•−)]+, where L contains only nitrogen donor atoms. Cu-S(thioether) ligation with its weaker donor ability (relative to an N-donor) are demonstrated by comparisons to the chemistry of analogue compounds.