Cobalt chelate complexes

As expected, azide addition to dimethyl maleate is very sluggish, and a triazoline is not isolated32 because the LUMOfumarale < LUMOmaleale.304a Recently, however, it has been found that azide coordinated in a cobalt chelate complex [N3Co(DH)2NH3] is sufficiently reactive to add to diethyl maleate, and 25% of the pure triazoline has been obtained (Scheme 80).30 5... 

Considerable work has been expended on elucidating the mechanism of catalytic oxidation of hindered phenols. Reactions of this kind proved to be amenable to kinetic studies and structural work on isolated intermediates. Systems involving cobalt chelate complexes as catalysts have received much attention. In this section we discuss in some detail the most important mechanistic versions proposed thus far. 

Diggruber H, Plath PJ, Schulz-Ekloff G, Mohl M. Study on the structure, stability and propene oxidation capability of faujasite-encaged cobalt chelate complexes. J Mol Catal 1984 24 115-26. 

The neutral radical 11.12 is an interesting heterocyclic analogue of 2,2 -bipyridyl. This paramagnetic (spin-bearing) ligand forms an N, -chelated complex with bis(hexafluoroacetylacetonato)cobalt(ir). ... 

When, however, the ligand molecule or ion has two atoms, each of which has a lone pair of electrons, then the molecule has two donor atoms and it may be possible to form two coordinate bonds with the same metal ion such a ligand is said to be bidentate and may be exemplified by consideration of the tris(ethylenediamine)cobalt(III) complex, [Co(en)3]3+. In this six-coordinate octahedral complex of cobalt(III), each of the bidentate ethylenediamine molecules is bound to the metal ion through the lone pair electrons of the two nitrogen atoms. This results in the formation of three five-membered rings, each including the metal ion the process of ring formation is called chelation. 

Rate law and activation energy. The oxidation of nitrosobenzene by terf-butyl hydroperoxide is catalyzed by di-terf-butyl peroxyoxalate (TBO) and by a cobalt(II) chelate complex. 

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. 

The given structure shows two molecules of TTA to have reacted with a cobalt ion to form the cobalt-TTA complex, in which the cobalt atom forms a valence bond solid lines) with one, and a coordinate bond (broken lines) with the other, oxygen atom of each TTA molecule. Thus, in the cobalt-TTA complex there is a six-membered ring formed by each TTA molecule with the cobalt atom. Metal chelate complexes of this type have good stability, they are nonpolar and soluble in the organic phase. The usefulness of the chelating extractants in solvent extraction is therefore obvious. 

Chelation of ixo-maleonitriledithiolate (imdt) has been structurally characterized in the octahedral cobalt(III) complex trmw-[Co(imdt)2(P(ra-Bu)3)2], formed via reaction of cobalt(II) ion with K2(imdt) in the presence of the phosphine.1037 Simple chelating thiolates such as SCH2CH2S not only form mononuclear compounds, but participate in bridging in clusters such as [Co3(SCH2CH2S)3(PEt3)3]3+ (243).1038... 

Examples in organometallic systems are known. Reaction of thiuram disulfides, (R2NCS2)2, with Co(Cp)(CO)2 produces dithiocarbamato pseudo-octahedral cobalt(III) complexes Co(Cp)(dtc)2 with one chelated and one monodentate dtc, also accessible via Co(Cp)I(dtc).1050 Fluxional behavior, including monodentate chelate exchange, was observed for some complexes in temperature-dependent NMR studies. The Co(Cp)I(dtc) complex was defined in a crystal structure. 

The N,S-chelate 2-aminoethanethiolate (aet-) forms stable cobalt(III) complexes, including clusters where the S can take on a bridging role. Reaction of [Co(NH3)5C1]2+ with Ni(aet)2 in water for several hours affords the black tetranuclear compound Co4(aet)8, which features act in simple chelate and bridging roles (245).1083 The simple monomeric complex [Co(aet)2(en)]+ has been reported 1084 when heated in water at 50 °C, the trimer Co Co(aet)3 2 is one product, with the central Co surrounded by six bridging S atoms. 

A comparison of the initial rates obtained with various cobalt complexes (Table I) reveals that the chelate complexes of Co(II) are more efficient than the simple salts, the catalytic activity of Co(III) is lower than that of Co(II) and the reaction becomes slower by increasing the number of N atoms in the coordination spheres in both oxidation states. In general, the addition of amine derivatives increased the activity of the catalysts. 

The structures of metal-complex dyes, which must exhibit a high degree of stability during synthesis and application, is limited to certain elements in the first transition series, notably copper, chromium, iron, cobalt and nickel. The remaining members of the transition series form relatively unstable chelated complexes. The following description of the influence of electronic structure, however, is applicable to all members of the series. 

The cobalt(II) complexes with R-dtp and related ligands are readily oxidized by air and other oxidizing agents to yield the corresponding cobalt III) tm-chelates. Electronic spectra and Co NMR spectra data 334,33s) hayg shown that the spectrochemical series of sulfur ligands is ... 

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