Cobaltocene reactions

Cobaltocene, [Co ()7 -C5H5)2], is a dark-purple air-sensitive material, prepared by the reactions of sodium cyclopentadiene and anhydrous C0CI2... 

Cobaltocene is partially oxidized and in part undergoes insertion of a borylene group, RB. The borinato ligands derive from the unknown borabenzene ( 6.5.3.4). Some porphyrinatoindium and thallium complexes ( 6.5.2.2) can also be synthesized via oxidative addition reactions (TPP)InCl is added oxidatively to Co2(CO)g and Mn2(CO) to give (TPP)In—Co(CO)4 and (TPP)ln — Mn(CO)j, respectively, and (oep)InCl is added to CojfCOg to yield (oep)ln— 0(00)4. 

The reaction of cobaltocene with organoboron dihalides RBX2 (R = Me, Ph and X = Cl, Br mainly) and boron trihalides (BC13, BBr3) leads essentially to three types of (boratabenzene) cobalt complexes, 19,20, and 21 (7,57). CoCp2 plays a dual role in part it acts as a reductant, in part it... 

This unique formation of a borabenzene ring skeleton is thought to proceed as delineated in Scheme 1. A more detailed explanation together with comments on stoichiometry, reaction conditions, and by-products may be found elsewhere (2,7). However, it should be noted that Scheme 1 is based on the well-known reducing power of cobaltocene and its ability to add radicals efficiently (80), both properties being intimately connected with the uncommon 19-e configuration of CoCp2. 

Bora-2,5-cyclohexadienes have a much greater synthetic potential than is apparent from the examples given so far. This may be exemplified by two recent reactions. Reductive complex formation in the system Co(acac)3/COD/25/Mg/THF affords complex 51 via the organotin route (77) while an earlier synthesis used the cobaltocene route (60). Ni(COD)2 very cleanly forms the (Tj3-l,4,5-cyclooctenyl)nickel complex 52 (29). 

The observation of radical anions has been confirmed by ESR measurements as illustrated by [Ir4(CO)12], (g = 2.002) 208). Similarly, a toluene solution of Co4(CO)i2 reacts with cobaltocene precipitating a brown compound which is extremely reactive and contains a cobaltocenium cation for each four cobalt atoms of the anion55. With excess cobaltocene (or alkali metals) in THF the reaction proceeds further as shown in Eq. (20),... 

Otherwise, reaction of 132 with a hydride source such as NaBH4 afforded the neutral allenylethynyl complex 140 by regioselective addition at the Cy atom (Fig. 23) [372]. In contrast, reduction with one equivalent of cobaltocene, and subsequent trapping of the resulting radical with Ph3SnH, yielded the isomeric 5,5-diphenylpenta-l,3-diynyl complex 141, suggesting that the unpaired electron of the pentatetraenylidene radical is predominantly localized on the Ce atom [372]. 

It is interesting to note that the complex of the composition [CioHio(CC13)Co] obtained previously (137) by reaction of carbon tetrachloride with cobaltocene is the 1-endo-trichloromethyl analogue of complex (XXIV) this complex can be reduced by lithium aluminium hydride to the l-endo-dichloro-methyl derivative (99). 

These compounds have been obtained indirectly by reactions of silylated acetylenes with metal carbonyls or olefin complexes. Thus, trimethylsilylphenylacetylene reacts with rj5-cyclopentadienylcobalt dicarbonyl, cobaltocene, or rjs-cyclopentadienyl-(l,3-cyclooctadiene) cobalt, in refluxing xylene, to give a mixture of cis- and trans-bis-(trimethylsilyl)cyclobutadiene complexes (R = Me, R = Ph) 68, 127, 137) ... 

Several preparations of silylated cyclopentadienylmetal complexes involve the formation of a triorganosilylcyclopentadienyl anion (by treatment of a silylated cyclopentadiene with an alkali metal in tetrahy-drofuran or by metalation with n-butyllithium), followed by reaction with metal chlorides. This type of reaction has been used for the synthesis of silylated ferrocenes (41, 43, 58, 83, 84, 103, 107, 116, 135, 142, 171, 172), cobaltocenes (135), nickelocene (135), titanium cyclopen-tadienyls (46, 145), and cyclopentadienylmanganese tricarbonyl (30) [Eqs. (19) and (20)]. It is remarkable that Si—C5H6 bonds are not... 

Reactions of the 19-valence-electron cobaltocene system are discussed first, followed by a brief discussion of an application to phase-transfer catalysis. Diene complexes are also considered. 

Fig. 12. Reactions of cobaltocene with organic compounds containing active hydrogen atoms in the presence of dioxygen.

Similar reactions have been reported (161) for cobaltocene with nitric oxide (NO). (See Scheme 13.) In this case, however, rather than producing the peroxide-bridged structure 63, the more stable ether-linked species 64 was produced. Complex 64 was crystallographically characterized, its reactions were studied, and a mechanism for its formation was proposed. 

Scheme 13. The reaction of cobaltocene with nitrous oxide to produce 64, an ether-linked system (compare to 63, a peroxo-linked system).

Reactions of the (Tj5-C5Hs)cobaIt-olefin complexes (26) prepared according to Eqs. (25) and (26) with alkali metals (Li, Na, K) in the presence of olefins lead to the elimination of the second C5H5 ligand from the cobalt (Scheme 5). Complexes 27a and 27b, or the mixed complex 27c are obtained in high yields [Eq. (28)]. The syntheses of the pure complexes 27a and 27b do not, of course, require the isolation of intermediates 26a and 26b. As mentioned previously, synthesis is readily achieved from cobaltocene (24) by reaction with either stoichiometric amounts or excess alkali metal in the presence of COD or ethylene [Eq. (24)]. The alkali metal cyclopentadienides which are formed are easily separated from the cobalt complexes and can be used for the synthesis of cobaltocene (51) [Scheme 5 Eq. (29)]. 

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