Coordination compound synthesis, cobalt

Classical methods for the synthesis of frans-dinitro-bis(ethylenediamine)cobalt(III) nitrate involve conversion of the cis isomer by heat1 or action of sodium nitrite on frans-chloronitrobis(othylenediamine)cobalt(III) nitrate.2 These methods involve isolation of intermediate coordination compounds and consequently are subject to low yields, based upon original cobalt(II) salt. The following procedure for the preparation of the trans compound gives an 84% yield it depends upon oxidation of cobalt(II) to cobalt(III) in the presence of sodium nitrite, ethylene-diamine, and nitric acid. 

In Volume V of this series, syntheses were presented for coordination compounds of 2,2 -iminodipyridine [di-2-pyridylamine, NH(C6H4N)2] with copper(II) and cobalt-(II). Nickel(II) has also been shown to coordinate with this ligand. A similar behavior is exhibited by zinc salts, which form 1 1 derivatives if a 1 1 mol ratio of reactants is used. The specific choice of a solvent medium for synthesis is dictated by the solubility of the zinc compound. Either acetone or methanol may be used with zinc chloride, methanol with zinc acetate, and pyridine with zinc cyanide. 

EXPERIMENT 2.2 SYNTHESIS OF A COBALT AMMINE COORDINATION COMPOUND 29... 

Many optically-active organic molecules are present in plants and animals, and they can often be isolated and obtained in a pure form. In recent years, considerable success has been achieved in the selective synthesis of individual isomers. However, laboratory preparations of compounds that can exhibit optical activity normally yield 50-50 (racemic) mixtures of the two optical isomers and hence produce an optically-inactive material (Section 3.4). Therefore, the basic step in the laboratory preparation of an optically-active coordination compound is separation from its optical isomer. For example, racemic [Co(en)3] is readily prepared by the air oxidation of a cobalt(II) salt... 

Synthesis of the compound [26] was straightforward, by reaction of cyclodextrin 6-bromide or iodide with the Co(I) B12 anion. The compound IZ could be handled at low pH, since protonation of the benzimidazole ligand prevents its coordination to the cobalt, a coordination that promotes carbon-cobalt bond fission. It had the expected optical spectrum, and the correct mass spectrum as well. However, on neutralization to pH 7.0 it underwent homolytic fission of the carbon-cobalt bond at 25 °C with a half-life of 150 minutes or so. Thus it readily generates the cyclodextrinyl radical under physiological conditions. 

The reaction occurs with some enantioselectivity and requires the presence of pivaldehyde (which is also oxidized)29,30. The reaction occurs for many other alkenes using transition metals coordinated to 1,3-diketone type ligands31-34. Use of a cobalt(II) complex and aldoacetal in place of the Mn(III) compound and pivaldehyde gives a novel method for the synthesis of acid-sensitive epoxides35. 

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