Cobalt complexes, with bromides

Cobalt complexes with square planar tetradentate ligands, including salen, cor-rin, and porphyrin types, all catalyse the reduction of alkyl bromides and iodides. Most preparative and mechanistic work with these reactions has used cobalamines, including vitamin-B,. A generalised catalytic cycle is depicted in Scheme 4.10 [219]. At potentials around -0.9 V vs. see, the parent ligated Co(lll) compound un-... 

Rate constants for zinc as the hydrated ion and in complexes with bromide and iodide ions are given in Fig. (4). Those for zinc in m. KCl and KCNS are 6 x lo and 17 x iO . Thus k increases in the order (NO a) < A(C1") < ft(Br-) < ft(CNS ) < k I ). Preliminary results for nickel and cobalt indicate a similar order for A Br ), A(CNS ) and k I ) these reactions are a great deal slower than for zinc. It appears possible that increasing covalency of bonding between the metal ion and its addenda, lowers the activation energy for discharge. 

The reaction was successfully carried out with various aryl(hetaryl) iodides and bromides involving different aryl thiols and alkyl thiols. A plausible catalytic cyde includes reduction of Co(II) complexes to Co(I), substitution of iodide ligand by SR leading to the formation of ArSCo(I), oxidative addition of ArX to this cobalt complex with formation of Co(III) derivative, followed by reductive elimination (Scheme 3.57) resulted in the product formation and regeneration of Co(I) catalyst. 

One-phase titration methods have also been developed. These methods are not truly one-phase titrations but the term is used to indicate the absence of a second organic phase. One of these methods, applied to the analysis of sodium and triethanolamine lauryl sulfates and lauryl ether sulfates, use a quaternary amine as a titrant and cobalt(II) thiocyanate as indicator. Centrimide was found to avoid the use of chloroform which was not possible with other titrants examined, such as domiphen bromide and oxyphenonium bromide. The pink color of the indicator changes to violet as an excess of titrant forms a complex with the indicator [238]. 

A warmed alcoholic solution of cobalt(Il) nitrate and 2-formylpyridine S-methyldithiocarbazate, 6, yielded diamagnetic [Co(6-H)2]N03 [126]. However, cobalt(II) chloride, bromide and thiocyanate yielded complexes with cobalt(III) cations and cobalt(II) anions, [Co(9-H)2]2 [C0A4]. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Aryl methyl ketones have been obtained [4, 5] by a modification of the cobalt-catalysed procedure for the synthesis of aryl carboxylic acids (8.3.1). The cobalt tetracarbonyl anion is converted initially by iodomethane into the methyltetra-carbonyl cobalt complex, which reacts with the haloarene (Scheme 8.13). Carboxylic acids are generally obtained as by-products of the reaction and, in several cases, it is the carboxylic acid which predominates. Unlike the carbonylation of haloarenes to produce exclusively the carboxylic acids [6, 7], the reaction does not need photoinitiation. Replacement of the iodomethane with benzyl bromide leads to aryl benzyl ketones in low yield, e.g. 1-bromonaphthalene produces the benzyl ketone (15%), together with the 1-naphthoic acid (5%), phenylacetic acid (15%), 1,2-diphenylethane (15%), dibenzyl ketone (1%), and 56% unchanged starting material [4,5]. a-Bromomethyl ketones dimerize in the presence of cobalt octacarbonyl and... 

As an active initiator for a co-polymerization, acyl-cobalt complexes also work well. As demonstrated by Alper and Lee, an equimolar mixture of Go2(GO)s, benzyl bromide (BnBr), and dihydro-1,10-phenanthroline 17, possibly generating BnGOGo(GO)4 under the reaction conditions, co-polymerized PO or 1,2-butene oxide with GO, and the... 

The chlorides of the other polyhasic ammine complexes with cobalt described in (II) may easily be prepared in solution by a similar procedure, but only the ethylenedi-amine compound can be directly isolated as a solid. The other chlorides must be made indirectly from the nitrates, bromides, or iodides of the respective series,... 

Hydrazine forms68 a 2 1 complex with eobalt(ii) bromide Co(N2H4)Br2,2H20 and phenylhydrazine forms69 2 1 complexes with all cobalt(n) halides. The latter seem to be octahedral in the solid state but dissolve in acetone to give blue solutions with characteristic pseudotetrahedral electronic spectra. 

Radical additions of primary, secondary, and tertiary alkyl bromides 249 to diethyl mesaconate 248 catalyzed by 5 mol% vitamin B12a 247 (X=OH2) proceeded in yields of 63-90% [301]. Deuteration experiments and comparison to similar addition processes support that 247 is initially reduced to cobalt complex 253A. This reacts with 249 giving an alkylcobalamin(III) intermediate... 

The reaction of a Co(I) nucleophile with an appropriate alkyl donor is used most frequently for the formation of a Co-C bond, which also can be formed readily by addition of a Co(I) complex to an acetylenic compound or an electron-deficient olefin (5). The nu-cleophilicity of Co(I) in Co(I)(BDHC) is expected to be similar to that in the corrinoid complex, as indicated by their redox potentials. The formation of Co-C a-bond is the attractive criterion for vitamin Bi2 models. Sodium hydroborate (NaBH4) was used for the reduction of Co(III)(CN)2(BDHC) in tetrahydrofuran-water (1 1 or 2 1 v/v). The univalent cobalt complex thus obtained, Co(I)(BDHC), was converted readily to an organometallic derivative in which the axial position of cobalt was alkylated on treatment with an alkyl iodide or bromide. As expected for organo-cobalt derivatives, the resulting alkylated complexes were photolabile (17). 

The second-order rate constants for reactions of Co(I)(BDHC) with alkyl halides were determined spectrophotometrically at 400 nm (17). These rate constants are listed in Table VII along with those for Co(I)(corrinoid)(vitamin Bi2s) in methanol at 25°C (35). These data indicate that the SN2 mechanism is operative in the reaction of Co(I)(BDHC) the iodides are more reactive with the cobalt complex than the bromides, and the rate decreases with increasing bulkiness of the alkyl donor. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B12s, which is confirmed by the rate ratios for... 

It was her thesis work with Alfred Werner34 that was to enter her in the annals of the history of chemistry. Among the compounds she made was cis-bis(ethylenediamine)dinitro-cobalt(III) bromide. This was the very first synthesis of a chiral octahedral cobalt complex, though at the time the significance of her synthesis was overlooked.31(a) Werner was so impressed with her work that, for her last year, he took her on as his personal assistant, the first women to be chosen for this prestigious post.35 More important for the impoverished Humphrey, she at last had some income in very expensive Switzerland. 

A large majority of diacidobis(l, 2-ethanediamine)cobalt(III) complexes can be synthesized from the (carbonato)bis(l, 2-ethanediamine)cobalt(III) ion. This is now readily available in high yield and purity.1 cis- and trans-Dichlorobis(l, 2-ethanediamine)cobalt(III) chlorides are obtained from the carbonato complex with hydrochloric acid using appropriate conditions. These isomeric dichloro salts are widely used starting materials for further syntheses. In this respect, the analogous dibromo complexes can be more convenient starting materials because bromide is substituted from the cobalt center more readily than is chloride ion. The milder conditions minimize the production of cobalt(II), a common side reaction. 

The optical purity of the recovered hydrosilane indicated 70% retention of configuration in the overall process. In a similar way the reaction of methylmagnesium bromide with the silyl-cobalt complex afforded a chiral silyl-Grignard reagent (equation 27). 

When the phase transfer is carried out in the presence of excess THABr (twice the amount required by stoichiometry), only two P NMR signals are observed, one at 240 ppm and another at 450 ppm, with an integration ratio of 30 1, respectively. Based on these data we assign the peak at 450 ppm to the [PWn039Co(H20)] species and the peak at 240 ppm to [PWiiOsgCoBr]. The latter species contains the bromide ion coordinated to cobalt. This is an important discovery, since up to date it was believed that as PWnCo is dried, the only two species that exist are aqua ( [PWn039Co(H20)] ) and pentacoordinated ([PWn039Co( )] ) cobalt complexes, the latter with an empty coordination site on cobalt. 

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