Copper-dioxygen species

Biomimetic copper-dioxygen chemistry has advanced considerably since the first structurally-characterized copper-dioxgygen adduct. However, it has been difficult to simulate the room-temperature stability of hemocyanin in these model complexes due to the fact that unlike the enzyme active sites, these models usually do not possess protective environments which can help stabilize potentially reactive copper-dioxygen species. Recently, two room-temperature stable copper-dioxygen complexes have been synthesized which come closer to the goal of mimicking the dioxygen carrier hemocyanin. 

The above considered reactions model the reductive half cycle of GO where a primary alcohol is oxidized to an aldehyde with concomitant reduction of a (phe-noxyl)copper(II) complex to the reduced (phenol)copper(I) species. In the first two cases, reoxidation of the reduced catalyst was achieved by an external oxidant such as tris(4-bromophenyl)aminium or an electrode but not dioxygen. 

Interaction of dioxygen species with Fe aq and with Fe " aq has been very briefly reviewed. In relation to 0x0-, peroxo-, and superoxo-complexes as models for intermediates in oxygenase activity, a brief report on a 2000 symposium on activation of oxygen summarizes the then-current situation in the search for a mechanism common to mono- and dinuclear iron sites, mono- and dinuclear copper sites, and copper-iron sites. The outline proposals comprise ... 

A side-on p,-Tq2 Tq2-peroxo dicopper(II) complex. A very important development in copper-dioxygen chemistry occurred in 1989 with the report by Kitajima et al. [10,108] that another Cu202 species could be prepared and structurally characterized by using copper complexes with a substituted anionic tris(pyrazolyl)borate ligand. This intensely purple compound, Cu[HB(3,5-iPr2pz)3] 2(02) (5), was prepared either by reaction of Cu[HB(3,5-iPr2pz)3] (4) with 02 or by careful addition of aqueous hydrogen peroxide to the p-dihydroxo... 

Continued research efforts in Cu 02 chemistry are clearly required, as new insights concerning current systems are likely, spectroscopic/structural correlations are incomplete, and alternate structural types are likely to be found. Room-temperature stable copper-dioxygen complexes have been recently generated. However elusive, unstable Cu 02 species have proved amenable to kinetic and spectroscopic characterization through the use of in situ low-temperature manipulations. 

For tricoordinate complexes [Cu2(Nn)]2+ (4a) or tetracoordinate nitrile adducts [Cu2(Nn)(CH3CN)2]2+ (4b), reversible binding can be demonstrated (33, 35). These react with 02 at —80 °C in CH2Cl2 solution, producing deep brown or purple species [Cu2(Nn)(02)]2+ (5) (33, 35). As found for our other copper-dioxygen complexes, reversible 02 or... 

As stated previously, early work into the direct observation of unstable Cuf-02 species met with little success. These species were often postulated based on both chemical logic, as well as limited kinetic information, but direct evidence pointing to the presence of such complexes was lacking. This situation changed in the mid-1980s in large part due to the utilization of low-temperature (-40 to -105 °C) stopped-flow spectroscopy where direct observation of spectroscopically identifiable copper-dioxygen intermediates was possible. This has allowed for the direct observation of a number of elementary steps in the formation and subsequent reactions of these complexes. 

In contrast to iron and cobalt, end-on superoxo-copper(II) species do not dominate the field of copper-oxygen chemistry. In 1 1 copper-dioxygen adducts, an alternative side-on, t 2 coordination mode is sometimes observed these [(L)Cu11 (t 2-(02 ")] or [(L)Cum-(ri2-(022 )] complexes are discussed below. Mononuclear copper-dioxygen complexes easily react with the second molecule of the Cu(I) complex, forming peroxo- or dioxo-bridged dinuclear species (Section 4.4). For sterically unhindered... 

The net result of this reaction is a complete cleavage of 0-0 bond accompanied by an increase of the oxidation state of each copper center by 2. This 0-0 bond breaking occurs seamlessly and often reversibly. Dicopper systems offer fascinating opportunity for dioxygen activation via 0-0 bond breaking resulting copper(III) species can act as competent oxidants for a variety of substrates.40... 

Tolman and co-workers reported dioxygen activation at a single copper site (Equation 11.53) [116]. The existence of the copper peroxide species was supported by both experimental and computational studies. 

Two structural types of tetranuclear copper-dioxygen compounds exist where the peroxide is bridged by four Cu(II) ions in either a cis (56,57) or trans (58) environment (Fig. 5). However, the mechanism of formation of these species from Cu /02 chemistry is not well understood considering that reduction to peroxide only requires two electron equivalents yet the compound provides 4 electrons from Cu(I). 

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