Superoxo complex

Superoxo complexes having a nonlinear M-O-O configuration are known at present only for Fe, Co, Rh and perhaps a few other transition metals, whereas the Vaska-type (Ila) complexes are known for almost all the transition metals... 

In this new type of coordination mode d(O-O) is long (147 pm) and the weakness of the 0-0 linkage is also shown by the very low value of 851 cm for v(O-O), both values being more characteristic of peroxo than of superoxo complexes/ ... 

Figure 14.5 (a) Reaction of Al,Al -ethylenebis(3-Bu -salicylideniminato)cobalt(II) with dioxygen and pyridine to form the superoxo complex [Co(3-Bu Salen)2(02)py] the py ligand is almost coplanar with the Co-O-O plane, the angle between the two being 18°.< (b) Reversible formation of the peroxo complex [Ir(C0)Cl(02)(PPh3)2]. The more densely shaded part of the complex is accurately coplanar. ... 

Figure 14.17 Structures of (a) the tetraperoxochromate(V) ion [Cr (02)4] , (b) the pyridine oxodiperoxo-chromium(VI) complex [Cr 0(02)2py], and (c) the triamminodiperoxochromium(IV) complex [Cr" (NH3)3(02)2] showing important interatomic distances and angles. (This last compound was originally described as a chromium(II) superoxo complex [Ci (NH3)3(02)2] on the basis of an apparent 0-0 distance of 131 pm/ and is a salutary example of the factual and interpretative errors that can arise even in X-ray diffraction studies. " ...

Collman JP, Hutchison JE, Lopez MA, Tabard A, Guilard R, Seok WK, Ibers JA, L Her M. 1992. Synthesis and characterization of a superoxo complex of the dicobalt cofacial diporphyrin [(/u,-02)Co2(DPB)(l,5-diphenylimidazole)2][PF6], the structure of the parent dicobalt diporphyrin Co2(DPB), and a new synthesis of the free-base cofacial diporphyrin H4(DPB). J AmChemSoc 114 9869. 

Examples are tending to be more sophisticated and complex in form. For example, a dinuclear complex featuring a bridging phosphinate and phenolate in addition to peroxide (221) has been reported,966 as a model for phosphodiester systems. Apart from dicobalt(III) systems, a mixed-valence CoII,ni di-/i-superoxo complex (222) has been prepared.967 Transition between the three redox states CoII,n, Co11,111, and Co111111 is electrochemically reversible. 

The majority of the titanium ions in titanosilicate molecular sieves in the dehydrated state are present in two types of structures, the framework tetrapodal and tripodal structures. The tetrapodal species dominate in TS-1 and Ti-beta, and the tripodals are more prevalent in Ti-MCM-41 and other mesoporous materials. The coordinatively unsaturated Ti ions in these structures exhibit Lewis acidity and strongly adsorb molecules such as H2O, NH3, H2O2, alkenes, etc. On interaction with H2O2, H2 + O2, or alkyl hydroperoxides, the Ti ions expand their coordination number to 5 or 6 and form side-on Ti-peroxo and superoxo complexes which catalyze the many oxidation reactions of NH3 and organic molecules. 

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 ... 

Synthesis of bimetallic /r-peroxo complexes has been described by Mimoun. In particular, for Co species reaction between a superoxo complex with a reduced metal is a feasible method, for Pt species acid catalyzed hydrolysis of peroxo complex may be used and for Rh or Pd the protocol implies reaction of potassium superoxide with appropriate precursors. 

The structure of the active component, manganese pyrophosphate, has been reported in the literature (24). It is layer like with planes of octahedrally coordinated Hn ions being separated by planes of pyrophosphate anions (P20y ). Examination of models of this compound gave calculated Hn-Hn thru space distances of 3.26 and 3.45 angstroms, a metal-metal distance close to that found for binuclear dibridged peroxo- and superoxo- complexes of cobalt ( ). 

While most superoxo complexes—in contrast to peroxo compounds— have been assigned a bent, end-on coordination mode [9], the superoxide ligand of Tp Cr(02)Ph was suggested to exhibit the more unusual side-on (r] ) coordination [10]. The reactivity of the complex did not allow for the determination of its molecular structure however, close analogs could be isolated, crystallized and structurally characterized by X-ray diffraction. For example, the reaction of [Tp Cr(pz H)]BARF (pz H = 3-tert-butyl-5-methylpyrazole, BARF = tetrakis(3,5-bis(trifiuoromethyl)phenyl)borate) with O2 produced the stable dioxygen complex [Tp Cr(pz H)( ] -02)]BARF (Scheme 3, bottom), which featured a side-on bound superoxide ligand (do-o = 1.327(5) A, vo-o = 1072 cm ) [11]. Other structurally characterized... 

Finally, a well-characterized 02-insertion transforms Tp Cr(02)Ph into the paramagnetic 0x0 alkoxide Tp Cr(0)0Ph, see Scheme 12 [4]. This reaction proceeds below room temperature, and the starting material has only been characterized by in-situ IR spectroscopy. However, analogous O2 complexes were isolated and characterized by X-ray crystallography, so there can be little doubt about its assignment as a side-on bonded Cr(IH) superoxo complex. 

These are typically prepared from low concentrations of chemically or photochemically generated low-valent metal complex (Cr2q, L(H20)2Co2 +, or L(H20)Rh2 +) and a large excess of 02 in slightly acidic aqueous solutions according to the chemistry in Eq. (1), where L = N4-macrocycle, (H20)4 or (NH3)4. The rate of formation of the superoxo complexes is mostly limited by the rate of water substitution at the metal centers, except in the case of L(H20)Rh2+ ions, which are pentacoordinate in solution (44). Selected kinetic data are shown in Table I. 

The corresponding bands in other superoxo complexes in Table II are obscured by ligand-based transitions. 

Another general method is based on oxygen insertion into metal-hydrogen bonds (50,72,79-81) by any of several known mechanisms. Hydrogen abstraction by superoxo complexes followed by oxygenation of the reduced metal, as in the catalytic reaction of Eqs. (3)-(4) (50,72), works well but is limited by the low availability of water-soluble transition metal hydrides and slow hydrogen transfer (equivalent of reaction (3)) for sterically crowded complexes. 

Crystal structure analysis has been carried out for several hydroperoxo complexes. The 0-0 bond length (1.40-1.48 A) (79,87,90,100,103,105) is significantly longer than that in superoxo complexes and close to the 1.49 A value for hydrogen peroxide (54). 

The UV spectral data for several hydroperoxo complexes in aqueous solution are shown in Table II. Intense transitions appear for all the compounds at wavelengths that are well below the 270-290 nm maxima for the superoxo complexes. This feature is particularly useful in mechanistic studies of the complex reactions involving several forms of activated oxygen simultaneously (58). 

Prior to the work described below (50), hydrogen transfer to superoxometal complexes has been proposed by some (124 126) and questioned by others (127) who introduced plausible alternative mechanistic pathways. The work with rhodium hydrides (50) sought to establish whether hydrogen abstraction by superoxo complexes is a feasible and reasonable mechanism for thermodynamically favorable cases. 

All the superoxo complexes and rhodium hydrides in this work can be handled under both aerobic and anaerobic conditions. The ability to work with superoxides in the absence of 02 and with the hydrides in the presence of 02 provides an exceptionally large range of reaction conditions and an opportunity to detect and identify various intermediates, and thus establish the mechanism with reasonable confidence. 

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