Dicobalt complexes

Configurationally labile cations, as varied as [Fe(Me2bpy)3] 72 (Fig. 26), [Fe(phen)3] and [Co(Me2bpy)3] " complexes, dicobalt(II) triple helicates, di-... 

Although there is a striking similarity between the iron complexes, V and VI, and the corresponding cobalt complexes (dicobalt octacarbonyl and IV), there is one important difference the iron complexes are anions, while the cobalt complexes are imcharged. With this in mind, it is possible to explain the course of the hydroxymethylation reaction in a manner analogous to that postulated for the hydroformylation reaction. 

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

On the way to cydo-Cig, l,6-bis(triisopropylsilyl)-l,3,5-hexatriyne (2) [50] was subjected to reaction with [Co2(CO)8], followed by ligand exchange with the bridging bis(diphenyphosphino)methane (dppm) ligand, whereupon smooth deprotection yielded the stable dark-red dicobalt complex 3 (Scheme 1). Oxida-... 

It is of incidental interest that a little work has been done on dicobalt systems. Doyle and Sykes have made a study of the reduction of decammine-//-amidodi-cobalt(Ill), (NH3)5Co NH2-Co(NH3)5 , by V(II). Since the rate is independent of hydrogen-ion concentration the mechanism cannot involve an amide bridge and must be outer-sphere, as it is in the case of the reduction of Co(NH3)6 by V(1I) . Both the binuclear complex and Co(NH3)6 are inert to substitution but the former is capable of functioning as a two-equivalent oxidant. Thus the two likely mechanisms are... 

Collman JP, Hutchison JE, Lopez MA, Tabard A, Guilard R, Seok WK, Ibers JA, L Her M. 1992. Synthesis and characterization of a superoxo complex of the dicobalt cofacial diporphyrin [(/u,-02)Co2(DPB)(l,5-diphenylimidazole)2][PF6], the structure of the parent dicobalt diporphyrin Co2(DPB), and a new synthesis of the free-base cofacial diporphyrin H4(DPB). J AmChemSoc 114 9869. 

Le Mest Y, L Her M. 1995. Electrochemical generation of a new type of dioxygen carrier complex. Reversible fixation of dioxygen by the highly electron-deficient two-electron oxidized derivative of a dicobalt face-to-face diporphyrin. J Chem Soc Chem Commun 1441. 

Two S/P ligands derived from camphor, CamPHOS and MeCamPHOS were also developed by these authors for the diastereoselective coordination to alkyne-hexacarbonyldicobalt complexes (Scheme 10.68). These two ligands were converted in good yields into their borane-protected forms. The influence of the alkyne group (R) on their coordination to dicobalt-hexacarbonyl-alkyne complexes was evaluated. It was shown that MeCamPHOS ligand provided a... 

Figure 4 Mononuclear quinquepyridine (4, 4" -bis(4-chlorophenyl analogue) Co11 complex (left, reproduced with permission of the Royal Society of Chemistry from J. Chem. Soc., Chem. Commun., 1992, 768-771) and dinuclear septipyridine (4,4"" -bis(4-mercaptomethylphenyl)-4"",4"" -bis(4-mercaptopropylphenyl) derivative) dicobalt(II) complex (right, reproduced with permission of the American Chemical Society from Inorg. Chem., 1993, 32, 5477-5484). Space filling representations also shown.

Table 3 Some structurally characterized dicobalt dioxygen complexes.

Examples are tending to be more sophisticated and complex in form. For example, a dinuclear complex featuring a bridging phosphinate and phenolate in addition to peroxide (221) has been reported,966 as a model for phosphodiester systems. Apart from dicobalt(III) systems, a mixed-valence CoII,ni di-/i-superoxo complex (222) has been prepared.967 Transition between the three redox states CoII,n, Co11,111, and Co111111 is electrochemically reversible. 

A combination of Co-mediated amino-carbonylation and a Pauson-Khand reaction was described by Pericas and colleagues [286], with the formation of five new bonds in a single operation. Reaction of l-chloro-2-phenylacetylene 6/4-34 and dicobalt octacarbonyl gave the two cobalt complexes 6/4-36 and 6/4-37 via 6/4-35, which were treated with an amine 6/4-38. The final products of this domino process are azadi- and azatriquinanes 6/4-40 with 6/4-39 as an intermediate, which can also be isolated and separately transformed into 6/4-40 (Scheme 6/4.11). 

The basis for the toxicological activity of this substance is the reaction of cobalt ion with cyanide ion to form a relatively nontoxic and stable ion complex. The hexacyanocobaltate ion contains a Co2+ central metal ion with six cyanide ions as ligands. This coordination complex involves six coordinate covalent bonds whereby each cyanide ion supplies a pair of electrons to form each covalent bond with the central cobalt ion. The formation constant for the hexacyanocobaltate ion is even larger than for dicobalt EDTA,3 and thus the cobalt ion preferentially exchanges an EDTA ligand for six cyano ligands ... 

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. 

A more complicated type of reaction leading to 2-styrylindoles is observed when 2-arylazirines are treated with the rhodium complexes,70 [(Ph3P)2 Rh(CO)Cl] or [Rh(CO)2Cl]2, or with dicobalt octacarbonyl71 (Scheme 42). In contrast, 2-arylazirines with molybdenum hexacarbonyl give pyrazines and dihydropyrazines, and with diiron enneacarbonyl give pyrroles (see Sections V,C,2 and IV,A,1, respectively). The use of relatively low molar ratios of 2-arylazirine to rhodium catalyst (2 1) causes the formation of 2,5-diarylpyrroles. 

The indazoline products can also be made directly from the palladium complexes 78 by heating them with the isonitrile in toluene at 120CC.162 They are also formed in dicobalt octacarbonyl-catalyzed reactions of azo-arenes with isocyanides but in this case an alternative reaction pathway leading to indazolo[2,l- ]indazoles (79) is observed (Scheme 96).163 Products of the latter type are formed from sterically hindered isocyanides hence it is likely that in these cases a double metallation is favored over isocyanide insertion into a monometallated species (Scheme 97). 

A direct synthesis of triarylimidazoles and triarylimidazolines has been accomplished by the dicobalt octacarbonyl-catalyzed reaction of benzyla-mine derivatives with carbon tetrachloride. When the reaction temperature is 150°C a complex product is formed and yields of heterocyclic products are poor. By lowering the reaction temperature to 120°C or reducing the reaction time, or by using [Mo(CO)6] and [f)5-C5H5Mo(CO)3]2 as the... 

Subtle differences in the behavior of azoarenes toward cobalt carbonyl derivatives are observed in regard to metal-complex formation. Azobenzene is transformed by dicobalt octacarbonyl in processes of orthometallation and carbonyl insertion into 2-phenylindazolin-3-one (see Section IV,D,2). In contrast, cyclopentadienylcobalt dicarbonyl effects N—N bond cleavage, and carbonylation of the isolable complex 88a provides 1 -phenylbenzimid-azolin-2-one (Scheme 106).171... 

The [2+2+1] cycloaddition of an alkene, an alkyne, and carbon monoxide is known as the Pauson-Khand reaction and is often the method of choice for the preparation of complex cyclopentenones [155]. Groth and coworkers have demonstrated that Pauson-Khand reactions can be carried out very efficiently under microwave heating conditions (Scheme 6.75 a) [156]. Taking advantage of sealed-vessel technology, 20 mol% of dicobalt octacarbonyl was found to be sufficient to drive all of the studied Pauson-Khand reactions to completion, without the need for additional carbon monoxide. The carefully optimized reaction conditions utilized 1.2 equivalents of... 

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