Cobalt complexes, with iodides

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-... 

Photoreduction of cobalt(III) complexes can occur under a variety of conditions. Irradition of the charge transfer bands of these systems results only in decomposition with production of cobaltous ion and oxidation of one of the ligands. In some instances photoreduction can be initiated by irradiation of the ligand field transitions. Irradiation of ion pairs formed by these complexes with iodide ion with ultraviolet light also leads to reduction of the complexes. Finally, irradiation of iodide ion in the presence of the complexes leads to reduction. 

Photolysis of ion pairs of cobalt(III) complexes with iodide ions leads to oxidation of iodide and reduction of the complex.55,63-86 Under the normal experimental conditions, however, most of the light is absorbed by free iodide and the reduction of the complex is effected by hydrated electrons produced as in reaction (36).86... 

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. 

There is also clear evidence of a change from predominantly class-a to class-b metal charactristics (p. 909) in passing down this group. Whereas cobalt(III) forms few complexes with the heavier donor atoms of Groups 15 and 16, rhodium(III), and more especially iridium (III), coordinate readily with P-, As- and S-donor ligands. Compounds with Se- and even Te- are also known. Thus infrared. X-ray and nmr studies show that, in complexes such as [Co(NH3)4(NCS)2]" ", the NCS acts as an A -donor ligand, whereas in [M(SCN)6] (M = Rh, Ir) it is an 5-donor. Likewise in the hexahalogeno complex anions, [MX ] ", cobalt forms only that with fluoride, whereas rhodium forms them with all the halides except iodide, and iridium forms them with all except fluoride. 

Formation of 772-complexes is known for both mono- and bis-phospho-nio-benzophospholides and has been observed (Scheme 18) in the reactions of the cation 23 with Jonas reagent to give the cobalt complex 49 [49], addition of the zwitterion 25 to a Mo-Mo triple bond to afford the dinuclear complex 50 [47], and finally, upon treatment of 26 with copper iodide to yield the complex 51 [46] which is peculiar because of the presence of the same ligand in two different coordination modes. Whereas it is clear that the metal atoms in all complexes supply inappropriate templates for the formation of 77 -complexes, the preference of rf-(,n)- over a possible a-coordina-tion is less well understood [49]. 

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,... 

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... 

Reductive coupling of allylic halides. This cobalt complex (1 equiv.) effects reductive coupling of allylic halides to form 1,5-dienes with preservation of the geometry of the double bonds/ The major product from coupling of terpenoid allylic halides is that formed by head-to-head coupling. The triphenylphosphine liberated during the reaction is removed as methyltriphenylphosphonium iodide, obtained by reaction with methyl... 

The oxidative addition of methyl iodide to an unsaturated cobalt carbonyl according to Equation (27) was proposed by Wender, CO insertion gives an acetyl species (28) which is thought to be hydrogenated by cobalt carbonyl hydride or H to yield acetaldehyde [4]. Numerous examples of the oxidative addition of methyl iodide to transition metal complexes with a electron configuration (e.g. Rh Ir ) ate known from the literature [66, 67]. For the carbonytaiion of methanol, the rate has been found to be the oxidative addition of methyl iodide to rhodium(l) [68]. 

In a search for a suitable catalyst we started with lanthanide iodides, monitoring by 1 B NMR analysis the stability of 1 M catecholborane solutions in tetrahydrofliran containing 10 molar % of SmL, f-BuOSmL, and Lat. Unfortunately, in the presence of these iodides a signal corresponding to borane-tetrahydrofiiran, and other signals, appeared in less than 1 h. In contrast, catecholborane in tetrahydrofliran was stable in the presence of nickel(II), cobalt(II) and iron(H) chloride complexes with dppe. 

Cobalt is determined spectrophotometrically using the absorption spectrum of CoCU " in 10 M hydrochloric acid. Iodine is determined by precipitation of silver iodide after reduction of the complex with sulfur dioxide. Titratable H" " is determined by a potentiometric pH titration. AnaL Calcd. for H3[Co4l3024Hi2] 3H20 Co, 22.04 I, 35.59 titratable H+, 0.282 ratio CorlrH" " = 1 0.75 0.75. Found Co, 22.0 I, 35.45 titratable H", 0.285 ratio Co.T H = 1 0.75 0.75. 

The high selectivity of the reaction of cobalt-carbene complexes with alkynes for furan products was taken advantage of in the synthesis of bovolide, a natural flavor constituent of butter. The carbene complex (230) was prepared in two steps from n-pentanal and was treated with 3 equiv. of 2-butyne. The crude reaction mixture, which presumably contained the furan (231), was treated directly with 3 equiv. of trimethylsilyl iodide to give bovolide in - 50% yield from carbene complex (230). 

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. 

This reaction is of interest as an alternative route to the formation of ethylene, since the subsequent dehydration step is well-established. Cobalt complexes are usually used for this reaction with various kinds of promoters such as iodides, phosphines, and other transition metal complexes [1]. 

Some insight into the mechanisms of the iodine-promoted carbonylation has been obtained by radioactive tracer techniques [17] and low-temperature NMR spectroscopy [18]. The mechanism involves the formation of HI, which in a series of reactions forms with rhodium a hydrido iodo complex which reacts with ethylene to give an ethyl complex. Carbonylation and reductive elimination yield propionic acid iodide. The acid itself is then obtained after hydrolysis. The rate of carboxylation was reported to be accelerated by the addition of minor amounts of iron, cobalt, or manganese iodide [19]. The rhodium catalyst can be stabilized by triphenyl phosphite [20]. However, it is doubtful whether the ligand itself would meet the requirements of an industrial-scale process. 

Methylbromoarsines, synthesis 26 Vanadium(III) fluoride, synthesis 27 Sulfur(IV) fluoride, synthesis 33 Peroxydisulfuryl difluoride, synthesis 34 Trichloro(tripyridine)chromium(III), synthesis 36 Tris(3-bromoacetylacetonato)chromium(III), synthesis 37 Trichloro(tripyridine)molybdenum(III), synthesis 39 Uranyl chloride 1-hydrate, synthesis 41 Rhenium(III) iodide, synthesis 50 Potassium hexachlororhenate(IV) and potassium hexa-bromorhenate(IV), synthesis 51 Iron-labeled cyclopentadienyl iron complexes, synthesis 54 Inner complexes of cobalt(III) with diethylenetriamine, synthesis 56... 

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