Cobalt complex compounds with pyridine

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

The chromium carbonyl linkers 1.40 (98) and 1.41 (99) were prepared from commercial triphenylphospine resin and respectively from pre-formed p-arene chromium carbenes and Fischer chromium amino carbenes. Their SP elaboration is followed by cleavage with pyridine at reflux for 2 h (1.40) and with iodine in DCM for 1 h at rt (1.41) both linkers produce the desired compounds in good yields. A similar cobalt carbonyl linker 1.42 (100) was prepared as a mixmre of mono- (1.42a) and bis- (1.42b) phosphine complex, either from pre-formed alkyne complexes on triphenylphosphine resin or by direct alkyne loading on the bisphosphine cobalt complex traceless cleavage was obtained after SP transformations by aerial oxidation (DCM, O2, hp, 72 h, rt) and modified alkynes were released with good yields and... 

Likewise when two alkyne molecules coordinate to a transition metal such as Co(I) with subsequent coupling of the C-C bond, oxidative cyclization takes place to give a metallacyclopentadiene. Further reaction of another alkyne molecule with the metallacyclopentadiene followed by reductive elimination liberates benzene derivatives. Thus cyclotrimerization of three alkyne molecules catalyzed by a cobalt complex [40,41] can be performed. If a nitrile is used as the second component, pyridine derivatives are obtained catalytically as shown in Scheme 1.13 [42]. The catalytic cyclotrimerization and cyclodimerization of alkynes and conjugated enynes have found extensive applications in synthesis of complex cyclic compounds such as steroid derivatives [43]. 

Matsui and coworkers reported the use of cobalt ion MIPs for chromatography based recognition studies on imprinted compounds. The authors chose to utilize an imprinting system described previously for the catalysis of aldol condensations (vide supra). This system was shown to be amenable to the study of MIP-metal ion mediated recognition. Preliminary studies were conducted to provide evidence for the complex formation between cobalt, polymerizable ligands, and dibenzoyl-methane, 28. Compleximetric titration of 28 in a model prepolymerization reaction mixture containing cobalt (II) acetate and pyridine in chloroform/methanol (5 1) showed formation of a complex with 1 1 stoichiometry between 28 and Co(II) (Fig. 19). 

The influenee of peripheral substituents at the phthalocyaninato moiety on the properties of the corresponding compounds was studied with a number of octa-substituted derivatives R PcM, R PcMLj and [R PcM(pyz)] (e.g. R = CH3, m = 8, M = Fe, L=py) [83]. Substituted mononuclear and bridged phthalocyaninatoiron pyrazine complexes R PcFe(pyz)2 and [R PcFe(pyz)] are accessible according to the procedure for the corresponding unsubstituted complexes [65]. With cobalt as central metal the tetra-substituted macrocycles R PcCo (m = 4 R = f-bu, NO 2) can be prepared. They coordinate with pyridine and substituted pyridines to form the adducts R PcCoL2 [84]. With pyrazine the binuclear complex (t-bu)4PcCo(pyz)CoPc(t-bu)4 and the oligonuclear [(N02)PcCo(pyz)]n can be isolated and characterized [84]. 

Werner proved that some complexes react very slowly,rhodium(III) approaching,and iridium(III) sometimes exceeding,the robustness of typical organic compounds. Jannik Bjerrum (17) found active charcoal to be an excellent catalyst,establishing within hours many cobalt(III) equilibria,and Deiepine found that the presence of ethanol allows oth RhCl and RhBr to react with pyridine to form trans-RhX (py) millions of times more rapidly than in the absence of alcohols. 

Complexes related to 57 described above were prepared with the diimino pyridine ligands with the benefit that a larger set of R groups could be linked to Co. The compounds 58 were made by reaction between the dihalide or monohalide cobalt complexes with magnesium or lithium reagents (Figure... 

Among transition-metal complex catalyzed reactions of alkynes with carbon-heteroatom unsaturated compounds the most studied is co-cyclotrimer-ization of alkynes with nitriles to pyridines. For this process the same complexes can be used as for the cyclotrimerization of alkynes. The first report of a cyclopentadienylcobalt complex catalyzed co-cyclotrimerization of alkynes with nitriles appeared in 1973 [92] and was soon followed by other papers [93]. Co-cyclotrimerization of alkynes and nitriles with all its aspects has been recently reviewed [94] and because of that we will focus only on recent developments in this area. In this regard, advances have been made in simple co-cy-clotrimerization of ethyne with various nitriles [95], combinatorial synthesis of substituted pyridines [96], and co-cyclotrimerization of hydroxyalkynes with nitriles in aqueous media catalyzed by cobalt complex with hydrophobic chain attached to the cyclopentadienyl ring [97]. 

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