Cobalt complexes chelating ligands

Complexes of cobalt(III) with O-donor ligands are generally less stable than those with Af-donors although the dark-green [Co(acac)3] and M iCo-( 204)3] complexes, formed from the chelating ligands acetylacetonate and oxalate, are stable. Other carboxylato complexes such as those of... 

Cobalt(III) complexes containing mixed chelating ligands have been produced. Reaction of potassium bis[biuretocobaltate(III)], K2[Co(bi)2] with R2dtc or Rxant at 0° produces (313) the blue-violet [Co(bi)2(S—S)] ion (S—S = R2dtc or RXant). If the reaction is performed above 0° in the presence of water, the products are [Co(bi)2(S—S>2] and biuret. 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

It has long been known that, when bound to cobalt(II), the pyridine-based chelate ligands 2,2 -bipyridine (bipy), 1,10-phenanthroline (phen), and 2,2 6, 2"-terpyridine (terpy) form complexes that react with dioxygen in aqueous solution (32-34). The mixed-ligand complexes [Co(terpy)(bipy)]2+ and [Co(terpy)(phen)]2+ can act as oxygen carriers in aqueous solutions at pH values as low as 3.0 (34b), and the superoxo species thus formed are apparently dinuclear. In addition, the dinuclear bipyridine complex [(bipy)2Coin(/ 2-0 )(/ 2-02 )CoIn(bipy)2 ]3+ has been shown to catalyze the oxidation of 2,6-di-ter -butylphenol to the feri-butyl-substituted diphenoquinone and quinone (35). 

The most common hydrox y oxime ligands are salicylaldoximes, complexes of which with copper(II), nickel(II) amd palladium(II), cobalt(II), iron(II), iron(HI) and manganese(II) have been investigated extensively including crystal and molecular structures13 of the first three. In an interesting study,71 the reactions of cobalt bis chelates of this type have been studied with aluminum isopropoxide. 

Several complexes of bivalent cobalt and zinc with imidazoline-(l//,3i/)-2-thione (34 R = H) of the forms ML2X2 (X = halide ion) and ML4X2 (X = N03 or C104) have been isolated and characterized by physicochemical techniques.247 In contrast with the S monodentate behaviour concluded for the earlier-reported Ni11 complexes, the ligands are N—S bidentate in these metal salts to form four-membered chelate rings. Similar coordination behaviour has been observed for the... 

The rate of hydrolysis of bidentate triphosphate in [Co(NH3)4HnP3Oi0] has been studied by phosphomolybdate analysis and 31P NMR.297,298 Both the X-ray crystal structure299 and the 31P NMR spectrum of [Co(NH3)4H2P3O 0] are consistent with structure (91) in which one terminal phosphate residue is not bonded to the metal centre. Kinetic studies establish that hydrolysis of the chelated ligand occurs at some two thirds of the rate for the free ligand. Hubner and Milbum300 have noted large rate increases of ca. 10s for cobalt(III) complexes with a 3 1 metal pyrophosphate stoichiometry. 

Metal binding in procarboxypeptidase A is weaker than in the active enzyme ( 107), Table 7). It was proposed that the bonding involves sulfur and a weaker ligand than N (107). In view of the present concept of the chelating site in carboxypeptidase, further studies of the zymogen are necessary. In that connection, the cobalt complex should be valuable. 

Pyridine ligands 186 are monodentate, forming complexes of the kind presented by 189. Pyridine is coordinated in the same way in copper acetate 194 [314] and iron rhodanide complexes [315], adducts of cobalt complex with fe(salicylidene)ethyle-nediamine 195 [316] and nickel chelate, formed by tridentate N,S-donor azomethine ligand 196 [317] ... 

The use of Cp rings with pendant phosphines in CpCoL2 complexes has also been reviewed. Carbonyl complexes of this type (Cp Co(CO)2) lose CO at room temperature to afford pendant phosphane adducts (equation 46). The chelated phosphane can then be uncoordinated with ligands such as cod substitution of cod (Section 5.1.4) with alkynes allows the cobalt complex to participate in cychzation reactions (Scheme 26). 

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