Peroxo complexes, with cobalt

The complex is green (e = 306 M-1 cm.-1 at 700 nm.) and paramagnetic.4 7 From an x-ray crystallographic study,8 the 02 bridge is known to be cobalt atoms (Co—O—O angles 120°), and the 0—O distance is 1.32 A. The five atoms in the ring are very nearly coplanar. The complex is stable in acidic solutions, but in neutral and alkaline solutions it is reduced to the corresponding peroxo complex, with subsequent decomposition of the latter. Ferrous ion also reduces the complex, in the first place to the peroxo complex. 

There is no firm evidence for the structure of the initial protonated species, but, bearing in mind that the HOMO of a /<-peroxo complex is essentially a jTg orbital of dioxygen (Sect. D), and on the basis of other structural evidence, it seems probable that the proton is bound to one oxygen only. The structure of the final isomerised product has been determined by X-ray crystallography Protonation of a /<-peroxo complex of cobalt with tertiary arsine ligands has also been reported ... 

The only jU-superoxo complexes to have been characterized are those which contain cobalt. They are readily prepared by treatment of the corresponding /i-peroxo complexes with strong oxidants. Thompson and Wilmarth observed that MnO, HOCl, Br2, BrOf and NOf are all effective oxidants toward [(en) Co(/r-NH2,02)Co(en)2] in acidic solution whereas Fe ", HjOj, Ag" " and CtjO are not. In other systems CI2, Pb02, persulfate and Ce in nitric acid solution have also effected oxidation. A number of well-characterized //-superoxo cobalt(III) complexes are listed in Table 55. There are no reports of /i-superoxo complexes containing Schiffbase ligands and all that are known contain either terminal amine or cyano groups. 

Another group of cobalt(11) complexes that show catalytic activity in 2,6-DTBP oxidation to the corresponding BQ and DPQ has been described by Bedell and Martell [56]. They are 1 1 Co(II) polyamine chelates (with the ligands listed below), some of which form monobridged p-peroxo complexes with dioxygen, whereas others afford dibridged (p-peroxo)(p-hydroxo) derivatives with the ligands listed in Table II. 

In general, reactivity toward molecular oxygen is lower for complexes of the second row transition metals than for third row complexes [3-7]. It has been found, however, that a cobalt(I) phosphine complex is far more reactive toward O2 than either the rhodium(I) or iridium(I) compounds having the same ligand system [70]. This leads, in at least one instance, to the rather unexpected reactivity sequence Co > Ir > Rh (Table 6). As in the iridum case, rhodium(I) forms side-bonded peroxo complexes with dioxygen. Reaction products, however, can be quite different as in the oxygenation of /nj(triphenylphosphine)chlororhodium(I), equation (23) [31]. 

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. ... 

The hexamine cobalt (II) complex is used as a coordinative catalyst, which can coordinate NO to form a nitrosyl ammine cobalt complex, and O2 to form a u -peroxo binuclear bridge complex with an oxidability equal to hydrogen peroxide, thus catalyze oxidation of NO by O2 in ammoniac aqueous solution. Experimental results under typical coal combusted flue gas treatment conditions on a laboratory packed absorber- regenerator setup show a NO removal of more than 85% can be maitained constant. 

CoCl(PPh3)3], Reaction of [Co(TIMEN )]Cl 9 with oxygen in the presence of NaBPh leads to the formation of the peroxo-complex [Co(r -02)(TIMEN 5 )] BPh 10, which is a rare example of a side-on r -peroxo cobalt complex (the majority of Co-O -adducts are rj -O -complexes, i.e. end-on). The authors also showed that 10 is capable of converting molecular oxygen to benzoylchloride. 

Diketonate cobalt(III) complexes with alkyl peroxo adducts have been prepared recently and characterized structurally, and their value in hydrocarbon oxidation and olefin epoxidation examined.980 Compounds Co(acac) 2(L) (O O / - B u) with L = py, 4-Mepy and 1-Meim, as well as the analog of the first with dibenzoylmethane as the diketone, were prepared. A distorted octahedral geometry with the monodentates cis is consistently observed, and the Co—O bond distance for the peroxo ligand lies between 1.860(3) A and 1.879(2) A. 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

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 ( ). 

Cobalt(II) porphyrins bind dioxygen as would be expected by analogy with the wealth of cobalt(II) complexes which display this property. The initial addition product is invariably a superoxo-type species, confirmed by ESR, IR and X-ray studies.154 Subsequent reaction of the oxygenated complex with more Co11 porphyrin leads to a peroxo-bridged dimer. No X-ray data are available for cobalt porphyrin peroxo-bridged dimers but the formation of such dimers is well established in cobalt chemistry. 

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). 

The complex has a green color with a characteristic absorption peak at 670 nm. ( = 890 M-1 cm.-1). It is paramagnetic (one unpaired electron), and e.p.r. studies indicate the equivalence of the cobalt atoms and delocalization of the odd electron.7 Recent x-ray crystallographic studies 6 have shown that the bridging oxygen atoms are bond angle 118°) and that the 0—0 bond distance is 1.31 A. This is shorter than that normally found in peroxides (1.48 A.) and is close to that found for the superoxide ion in K02. Whereas in the peroxo complex there is a torsion angle of 146° about the O—O bond, the Co—0—O—Co atoms are coplanar in the superoxo complex. 6... 

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