Metal-superoxo complexes

If low-valent M" + compounds are used, peroxides form instead (see below). Most transition metal ions catalyze, to some degree, catalytic decomposition of superoxides (display superoxide dismutase activity), thus limiting applicability of reaction (4.32) for the synthesis or in situ generation of metal superoxo complexes. 

Examples of both peroxo and superoxo complexes have been identified in which oxygen is coordinated either to one metal center (1,3) or to two metal centers (2,4) [3]. Examples of peroxo complexes are far more numerous than are those of transition metal superoxo complexes. Of the four different types of bonding shown in Figure 1, peroxo complexes of type 1 are the most common while superoxo complexes of type 4 are exhibited only in the case of some Co(III) complexes. In fact, cobalt is the only metal for which examples of all four bonding modes have been observed to date [3]. 

ESTABLISHED STRUCTURES OF SOME TRANSITION METAL SUPEROXO COMPLEXES... 

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... 

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 ( ). 

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. 

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. 

Infrared spectroscopy can be used to classify metal-dioxygen complexes as either superoxo species (p(O-O) from 1200 to 1070 cm-1) or as peroxo species (p(O-O) from 930 to 740 cm-1) (49). However, this system fails to accurately define the type of dioxygen species present in 8 and 15, as these complexes exhibit v(O-O) absorptions at 961 and 941 cm-1, respectively. Preparation of the complexes with 180-enriched dioxygen confirmed that the dioxygen was bound side-on (if) in these complexes Complex 8 exhibited isotopically shifted vibrations indicative of a side-on bound dioxygen 0(160-160) absorption at 961 cm-1, p(160-180) at 937 cm-1, and /(180-180) at 908 cm-1), as did complex... 

Coordination of dioxygen can occur in four ways, two of which may be considered to correspond to one-electron reduction (superoxo complexes) and the others to two-electron reduction (peroxo complexes). The four possible coordination modes are illustrated in Figure 2. As can be seen, each type of complex may be further categorized as to whether it is a mono- or bi-nuclear complex. Whether a peroxo- or a superoxo-type complex is formed is dependent upon the metal involved. For mononuclear complexes, superoxo complexes would be expected with metals which readily undergo one-electron oxidation, e.g. Fe11 or Co", while peroxo complexes would be expected for metals with a preference for two-electron oxidation, e.g. Ir1, Rh1, Pt°. 

This strategy has deliberately avoided the superoxo/peroxo dichotomy. The extent of electron transfer to dioxygen in these adducts is difficult to assess and is a subject which remains controversial. However, it is convenient to divide the wealth of metal-dioxygen complexes into superoxo-type and peroxo-type upon the basis of structural data from the point of view of ease of discussion. We have done this purely on the basis of bond lengths and IR data and do not intend this to be interpreted as a measurement of the extent of electron transfer from the metal to the dioxygen ligand. 

Although there are no binuclear superoxo complexes formed simply by the interaction of a metal complex and dioxygen, they are worthy of discussion here if only to complement mononuclear... 

Sulfur ligands, 633-655 coordination ability, 516 Sulfur monoxide metal complexes instability, 636 Superoxide dismutase, 773 copper complexes, 772 Superoxo complexes, 315-330 binuclear, 323, 325 reactions, 330 bridged... 

The first step consists of the formation of the dioxygen adduct which can have either a superoxo structure (1) if the metal is a potential one-electron donor, or a peroxo structure (2) if the metal is a potential two-electron donor. These superoxo or peroxo complexes can be considered as the formal, but not chemical, analogs of the superoxide 02 and peroxide 022- anions. The superoxo complex (1) can further react with a second reduced metal atom to give the /x-peroxo species (3), which can cleave itself into the oxo species (4), which may be hydrolyzed to give the hydroxo species (6) or react with a second metal atom to give the p.-oxo species (5). The alkylperoxo (7) and hydroperoxo (8) species can result from the alkylation or protonation of the peroxo species (2), or from anion exchange from metal salts by alkyl hydroperoxides or hydrogen peroxide. 

The complexes can be obtained from the reaction of a Rh1 complex with 02 (equation 5),37 from the reaction of a Com-superoxo complex with a reduced metal (equation 6),38 from the acid hydrolysis of a platinum-peroxo complex3 (equation 7) or from the reaction of potassium superoxide with rhodium (equation 8)40 or palladium complexes.41... 

These complexes can exist in a triangular peroxo form (7a) for early d° transition metals, or in a bridged (7b) or linear (7c) form for Group VIII metals. They can be obtained from the reaction of alkyl hydroperoxides with transition metal complexes (equations 9 and 10),42-46 from the insertion of 02 into a cobalt-carbon bond (equation ll),43 from the alkylation of a platinum-peroxo complex (equation 12),44 or from the reaction of a cobalt-superoxo complex with a substituted phenol (equation 13).45 Some well-characterized alkylperoxo complexes are shown (22-24). 

Recent reviews on the chemistry of metal-dioxygen complexes with particular relevance to cobalt systems include a number dealing with general properties,636,634 1 binuclear superoxo and peroxo complexes,642,643 reversible oxygenation,644-646 complex stability,647,448 catalytic oxidation649,650 and electronic651 and EPR652 spectral properties. 

The reaction of a superoxo complex with a second metal can lead to a bridging /x-peroxo species... 

Hydroperoxo complexes are prepared75 by protonation of peroxo complexes, by insertion of dioxygen into metal-hydrogen bonds, by hydrogen abstraction by metal dioxygen complexes, by reduction of superoxo complexes or by reaction of the metal ion with hydrogen peroxide. Well-defined stable species have been characterized for Cu,76 Ir, Pt, and other metals, for example, by syntheses of the type ... 

The synthesis of transition metal-dioxygen complexes is often carried out by the addition of dioxygen to a precursor complex. Binding involves some shift of electron density from the metal to the O2 ligand, generally regarded as resulting in either a superoxo (one-electron reduced or superoxide-hke) or... 

Hyla-Krispin, Natakaniec and Jezowska-Trzebiatowska have carried out EHMO type calculations on the ions [(H3N)5Co02Co(NH3)s] and [(NC)5Co02Co(CN)5] The 3 Og and orbitals of dioxygen lie well below the metal d-orbitals, the in-plane orbital just below the t2g metal d-orbitals and the Jr (l) orbitals (the HOMO) between the t2g and eg metal d-orbitals. In agreement with the EPR spectrum the unpaired electron in the superoxo complex is localised on dioxygen. 

---