Dicopper-dioxygen

A dramatic discovery in this area was made in 1996 when a dicopper-dioxygen adduct was found to have two isomeric forms which featured either a side-on bridging unit Cu(/r ), ) -02)-Cu) " or a cyclic (Cu(/r-0)2Cu) " core depending on whether it was crystallized from CH2CI2 or thf, respectively. The two forms could be readily interconverted by reversible 0-0 bond cleavage and reformation, the 0-0 distance being 141 pm and 229 pm in the two isomers.The... 

Figure 5.3 Schematic representations of two different dicopper-dioxygen cores side-on p-t 2 r 2 (a) and end-on trara-p-1,2 (b).

Figure 17.10 Thermo-kinetic relation for oxidation of various phenols of Table 17.3 by a set of dicopper-dioxygen complexes (e and f, see Table 17.1). Figure adapted from Ref [75].

The 0-0 bond in side-on dicopper-dioxygen complexes can be easily cleaved, affording dicopper(III) bis-p-oxo diamond core-type structures.40,85,106 The first crystallographically crystallized complex of this type, described by Tolman and coworkers, contained /V-alkylated triazacyclononane ligands.106 Subsequently, a large number of bis-p-oxo dicopper(ffl) intermediates were observed in various chemical systems.97 In many cases, dicopper(II) X-T 2 T 2 peroxo species and dicopper... 

Oxidative transformations carried out by the reaction of copper(I) and O2 in Nature are widespread in terms of the variety of substrates and in copper active-site structures. The archetype for dicopper-dioxygen activation has been the dioxygen carrier for arthropods and mollusks, hemocyanin (He), which has a dinuclear copper(I) active site where the copper centers are ca. 

Early copper(T)/dioxygen reactivity studies were done with the classic tripodal pyridyl-amine ligand TMPA (Fig. 8) (see above). The low-temperature reaction of [Cu(TMPA)(RCN)]PF6 (R = Me or Et) with O2 was found to produce a dicopper-dioxygen adduct where the peroxide is bound in a [Pg.145]

The reactions of neutral phenols with two distinct dicopper-dioxygen complexes, a p-q q -peroxodicopper(II) complex and a bis(p-oxo)dicopper(III) complex, have been recently compared in a systematic study using the copper(I) complexes with the two related ligands LPy2 and LPyl (141). The Cu(I) complex with LPy2 yields predominantly the -peroxo... 

In 1996 Stack and co-workers reported an unusual 3 1 (copper 02 stoichiometry) reaction between a mononuclear copper(I) complex of a A-permethylated (lR,2R)-cyclohexanediamine ligand with dioxygen. The end product of this reaction, stable at only low temperatures (X-ray structure at —40 °C) is a discrete, mixed-valence trinuclear copper cluster (1), with two Cu11 and a Cu111 center (Cu-Cu 2.641 and 2.704 A).27 Its spectroscopic and magnetic behavior were also investigated in detail. The relevance of this synthetic complex to the reduction of 02 at the trinuclear active sites of multicopper oxidases4-8 was discussed. Once formed, it exhibits moderate thermal stability, decomposed by a non-first-order process in about 3h at —10 °C. In the presence of trace water, the major isolated product was the bis(/i-hydroxo)dicopper(II) dimer (2). 

Continuing their efforts with similar ligands, they prepared a thermally sensitive crystal of a bis(/i-qxo)dicopper(II I) compound (3).28 Average Cu O bond distance and Cu-Cu distance are 1.806 A and 2.743 A, respectively. Spectroscopic and kinetic parameters for this compound were also investigated. They also studied the reactivity properties of the copper-dioxygen complexes.25... 

During the course of modeling copper dioxygen chemistry, Kitajima et al. reported the synthesis of a yu-peroxo dinuclear complex with a 3,5-dimethyl-substituted tris(pyrazolyl)borate ligand, which showed remarkable physicochemical similarities to oxy-Hc and oxy-Tyr. Using a 3,5-di-isopropyl-substituted terminal ligand, they provided the first structural proof of the existence of peroxo dicopper(II) core (108) (copper geometry distorted square pyrami-... 

The second class of dioxygen carriers is that of haemocyanins. These proteins, which contain a binuclear Cu(I) site (thus in the oxidized Cu(II) met form they belong to the so-called Type 3 copper proteins , which contain an EPR-silent dicopper active site), regulate dioxygen transport in the respiration of arthropods and molluscs. Figures 7 and 8 show... 

It is interesting to underline that there is another (plant) enzyme which possesses a coordinatively similar dicopper environment catechol oxidase.11 As already mentioned in Chapter 6, Section 3, such an ubiquitous enzyme catalyses the two-electron oxidation by molecular dioxygen of catechols to the corresponding quinones (the so-generated quinones in turn polymerize to form brown polyphenolic catechol melanins, which protect damaged plants from pathogens or insects). 

As previously discussed, the binuclear Cu(I) haemocyanins constitute another important class of dioxygen carriers. The first model compound whose structure in the oxygenated form resembles most closely that of oxyhaemocyanin is the dicopper(II) complex with the tris(3,5-diisopro-pylpyrazolyl)borate ligand shown in Figure 26.29... 

Reversible Binding and Activation of Dioxygen and the Reactivity of Peroxo and Hydroperoxo Dicopper(II) Complexes... 

At present, we do not completely understand why only some of these very similar m-xylyl dicopper(I) complexes systems described above undergo ligand oxygenation reactions. However, based on the results outlined above, we can speculate on a number of aspects of this 02-activation process. Our studies implicate the presence of a copper-dioxygen (peroxo dicopper(II)) adduct as an intermediate in the oxygenation reaction and more recent kinetic studies (51) further support this conclusion. This adduct then either directly or via some further intermediate undergoes an electrophilic attack of the arene. The unique nature of this very fast reaction 2->3, and the observed inability to intercept the active... 

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