Schiff base complexes of cobalt

Floriani, C., and F. Calderazzo (1969). Oxygen adducts of Schiffs base complexes of cobalt prepared in solution. J. Chem. Soc. A 6, 946-953. 

Table II, Some Schiff Base Complexes of Cobalt(II)...

The resolution of racemic amino acid mixtures via coordination to a metal ion has been a popular field of study. [Cu(L-aa)2] complexes can be used to resolve DL-Asp, dl-G1u and DL-His.58,59 (—)-[Co(EDTA)] has been used to resolve DL-His having first resolved the racemic [Co(EDTA)] ion using the L-histidinium cation.60 Schiff base complexes of both Co111 and Ni11 have also been used to resolve amino acids.61,62 A more esoteric finding is that the bacterium Enterobacter cloacae prefers to metabolize the A-( —) isomer of/ac-[Co(GlyO)3] rather than the A-(+) form,63 an observation reminiscent of that made by Bailar using tris(ethylenediamine)cobalt(III) salts. 

Homogeneous catalyst systems include transition metals, for example, Schiff base complexes with cobalt [59]. Unfortunately, there is no protocol known to recover and recycle the expensive Schiff base. Many copper complexes have been employed for the oxidation of phenols to 1,4-benzoquinones [60]. Copper salts such as CUCI2 and CuBr2 are widely used for the oxidation of trimethylphenol with molecular oxygen at larger scale [61]. In addition, applications of the halides of Cr, Mn, Fe, Ni, and Zn as catalysts have been described [58]. Copper halides have also been combined with earth metal halides, for example, MgCl2 [62] or LiCl, in the presence of a phase transfer catalyst [63]. 

The oxidation of Ti " by Fe" and the reaction between TF" and Sn" have been studied using flow and potential-time dependences. An investigation has also been made of the Ag -Ce " reaction. Polarographic studies of planar Schiff base complexes of Co , Ni", and Cu have shown well-defined oxidation waves and in some chelates the two-electron oxidation Co" Co " is postulated. Kinetic studies have also been made on the oxidation of cobalt(ii) dipyridyl complexes by copper(n) and iron(m) perchlorates in anhydrous acetonitrile. Solvent effects have also been investigated in redox reactions of tetrahalogenoplatinum(iv) complexes of the type PtLaX4. ... 

Five-coordinate cobalt(II) Schiff base complexes of the Co(salen)B... 

We have prepared the parent and several substituted Schiff s base complexes of cobalt and reacted them with nitric oxide. The mononitrosyls so formed give non-conducting solutions in nitrobenzene and molecular weight determinations, where they are possible, show them to be monomeric in nitrobenzene. Thus, unless the solvent is coordinated, they may be considered to be five coordinate with a structure as shown ... 

An X-ray photoelectron spectroscopic study (132) of some cobalt(II) Schiff base complexes and their dioxygen adducts finds that the cobalt core electron binding energies in... 

In this section, we present material dealing with the direct oxidation and reduction of a variety of organocobalt species, including complexes with more than one cobalt center, electrodes functionalized with cobalt complexes, cobalt-containing SchifF-base complexes, cobalt porphyrins and corroles, and macrocyclic tetraamines. 

These radical species may (slowly) combine to form the ju-peroxodico-balt species or, as in the preparation of [CH3Co(NH3)5]2+, react (more rapidly) with the alkyl hydrazine to form the alkylated cobalt(III) species and dinitrogen. Schiff base complexes such as these are discussed in more detail later in this article. 

A more complete article (58b) further details the properties of faujasite EMT-entrapped cobalt(II)-Schiff base complexes as compared to the same complexes entrapped in zeolite Y. The Schiff bases used to... 

A Co(II) Schiff-base complex converts 1- and 2-alkenes into methyl ketones and the corresponding secondary alcohols in the presence of oxygen or H2O2 in primary alcohol solvent.543 A radical oxidation with cobalt hydroperoxide through the formation and subsequent decomposition of alkyl hydroperoxide was suggested.543 An efficient conversion of alkenylarenes to ketones was achieved by the use of molecular oxygen and EtjSiH in the presence of a catalytic amount of Co(II) porphyrin in 2-propanol.544... 

Nishinaga and co-workers isolated a series of stable cobalt(III)-alkyl peroxide complexes such as (170) and (171) in high yields from the reaction of the pentacoordinated Co"-Schiff base complex with the corresponding phenol and 02 in CH2C12. Complex (170 R=Bu ) has been characterized by an X-ray structure. These alkyl peroxide complexes presumably result from the homolytic addition of the superoxo complex Co111—02 to the phenoxide radical obtained by hydrogen abstraction from the phenolic substrate by the CoUI-superoxo complex. The quinone product results from / -hydride elimination from the alkyl peroxide complex (172)561,56,565,566 The quinol (169) produced by equation (245) has been shown to result from the reduction of the CoIU-alkyl peroxide complex (170) by the solvent alcohol which is transformed into the corresponding carbonyl compound (equation 248).561... 

Cobaltn-Schiff base complexes, e.g. Co(salen),567 Co(acacen)568 and cobalt(II) porphyrins,569 e.g. Co(TPP), are effective catalysts for the selective oxygenation of 3-substituted indoles to keto amides (equation 249), a reaction which can be considered as a model for the heme-containing enzyme tryptophan-2,3-dioxygenase (equation 21).66 This reaction has been shown to proceed via a ternary complex, Co-02-indole, with probable structure (175), which is converted into indolenyl hydroperoxide (176). Decomposition of (176) to the keto amide (174) readily occurs in the presence of Co(TPP), presumably via formation of a dioxetane intermediate (177).569,56 Catalytic oxygenolysis of flavonols readily occurs in the presence of Co(salen) and involves a loss of one mole of CO (equation 251).570... 

Bozell, J. J., Hames, B. R., and Dimmel, D. R., Cobalt-Schiff base complex-catalyzed oxidation of parasubsti-tuted phenolics - Preparation of benzoquinones. J Organic Chem 1995, 60 (8), 2398-2404. 

The precatalyst used in these water-based kinetic resolution reactions is the cobalt Schiff-base complex 9.40. Its structural similarity to the asymmetric epoxidation catalysts 9.38A and 9.38B is to be noted. In the actual catalytic system 9.40 is activated with small amounts of acetic acid and air to give a cobalt(III) complex where CH3C02 and H20 are additional ligands. The mechanistic details of this reaction are as yet unknown. 

Ruthenium compounds are widely used as catalysts for hydrogen transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. These systems demonstrate high activity. For example, Backvall and coworkers [146] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt-Schiffs base complex. Optimization of the electron-rich quinone, combined with the so-called Shvo Ru-cata-lyst, led to one of the fastest catalytic systems reported for the oxidation of secondary alcohols (Fig. 4.59). 

The aerobic oxidation of phenols in the presence of cobalt-Schiffs base complexes as catalysts is facilitated by (electron-donating) alkyl substituents in the ring and affords the corresponding p-quinones, e.g. the Vitamin E intermediate drawn in Fig. 4.87. When the para-position is occupied the reaction may be directed to the ortho-position [252, 253]. Copper compounds also mediate this type of oxidation, e.g. the Mitsubishi Gas process for the Vitamin E intermediate... 

PdCl2 in the presence of a base, and cobalt-Schiff base complexes have also been proposed as catalysts. 

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