Dinuclear peroxo dioxygen complex

In Fig. 1 some of the most common metal-dioxygen binding modes found for mononuclear and dinuclear transition metal complexes are diagrammed. Note that the terms superoxo and peroxo ultimately... 

It has long been known (93) that cobalt(II) complexes of phthalocyanines interact with molecular oxygen. The water-soluble tetrasulfonato derivative of the parent phthalocyanine selectively and catalytically oxidizes 2,6-di-tert-butylphenol to the benzoquinone and the dipheno-quinone in both homogeneous solution (94) and when polymer-supported (95). The active intermediate in the catalytic cycle is proposed to be the (as expected) mononuclear dioxygen complex of the cobalt-tetrasulfonatophthalocyanine system (92). It has been proposed that the formation of a peroxo-bridged dinuclear complex is responsible for the deactivation of the cobalt(II)-tetrasulfonatophthalocyanine system, since such a dinuclear system would be unable to further bind and activate dioxygen (96). Such deactivation results, ultimately, in loss of the catalyst and low turnover ratios. 

A trans-n- 1,2-Peroxo Dicopper(II) Complex. Our own efforts have resulted in the structural and spectroscopic characterization of five types of copper-dioxygen complexes (6), distinguished on the basis of the ligands used for their synthesis and on their distinctive structures or physical properties. Thus, the manner in which hemocyanin binds 02 is not the only one possible, and it is of considerable interest to deduce the structures, along with associated spectroscopy and reactivity of a variety of types. Dioxygen can bind to dinuclear transition metals in a variety of structural modes, shown in Figure 2. As mentioned, mode C is present in oxy-Hc and Kitajima s model complex (Scheme 1), whereas we have structural and spectroscopic evidence for types A (30-32), B (33-35), and F (36-38) for peroxo 022- binding, and mode D (39, 40) in the case of hydroperoxo (OOH ) complexes. 

Our knowledge of dioxygen complexes has increased considerably since Vaska s review of 1976. Theoretical and spectroscopic studies have given a description of the bonding in these complexes which rationalises the structures observed experimentally, and which provides a useful basis for the discussion of the reactivity. Very few theoretical studies on dinuclear complexes have appeared however, and we know of only one investigation of the relative merits of the and rf bridging peroxo structures . ... 

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... 

Two-electron reduction of dioxygen into coordinated peroxide can be easily performed by two metal centers undergoing concomitant one-electron oxidations, as shown in Equation 4.4 (Section 4.2.2). A variety of transition metal ions (cobalt, nickel, iron, manganese, copper, etc.) can form dinuclear peroxides. These complexes differ in structure (cA-p-1,2-peroxides, trans- l- 1,2-peroxides, p-r 2 r 2-peroxides), in stability and subsequent reactivity modes, and in the protonation state of the peroxo ligands (Figure 4.3). In certain cases, dinuclear p-r 2 r 2-peroxides and bis-p-oxo diamond core complexes interconvert, as discussed below for copper-dioxygen adducts. 

The chemistry of cobalt-dioxygen complexes of hydrotiis(p5 azolyl)-borate ligands (Tp(R)) has been reviewed with special reference to the formation of low-valent metal-peroxo and high-valent metal-oxo spedes, such as Co(ll)-superoxo, alkylperoxo and dinuclear Co(in)-bis(proxo) complexes. In the hydrotris(3,5-dnsopropyl-l-pyrazolyl)borate ligand system (TpCPr )), oxygenation of the proximal isopropyl substituents on Tp Pr3) is mediated by the p,-peroxodicobaIt(III) and the alkylperoxocobalt species. 

Dioxygen and its ions can bind in mononuclear and dinuclear structures in a number of ways,962 as illustrated in Scheme 1. The typical reaction of dioxygen with Co compounds involves a number of these binding forms, outlined in Scheme 2. Mononuclear Co111—peroxo complexes are relatively rare, but yellow trigonal bipyramidal complexes [Co(02)L2]+ (L = chelating phosphines dppe or dppp) have been characterized structurally where the 022 is bonded to the Co in the side-on r]2 form (Co—O 1.858(7) 1.881(4) A), with O—O stretching frequencies ( 870 cm-1) consistent with Coin-peroxo speciation.963... 

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-... 

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 ... 

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