Cobaltocene oxidation

In the presence of oxygen, cobaltocene oxidizes a-diketones and o-quinones ... 

A special type of oxidative addition takes place between cobaltocene and boron halides (MeBBr2, PhBCl2, BCI3, BBr3) . Treatment of cobaltocene in hexane or... 

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. 

In a similar manner to manganocene, cobaltocene is easily oxidized to the corresponding cobaltocenium ion [Co(f/5-C5H5)2]+, which, as illustrated in Figure 48, exhibits two distinct chemically reversible reductions corresponding to Coni/Con and Con/CoI.88... 

Because of their reversible electrochemical properties, ferrocene [biscyclopentadie-nyl-iron(II), FeCp2 and cobaltocenium [biscyclopentadienyl-cobalt(III), CoC p2 1 I are the most common electroactive units used to functionalize dendrimers. Both metallocene residues are stable, 18-electron systems, which differ on the charge of their most accessible oxidation states zero for ferrocene and + 1 for cobaltocenium. Ferrocene undergoes electrochemically reversible one-electron oxidation to the positively charged ferrocenium form, whereas cobaltocenium exhibits electrochemically reversible one-electron reduction to produce the neutral cobaltocene. Both electrochemical processes take place at accessible potentials in ferrocene- and cobaltocenium-containing compounds. 

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).

The 18-electron rale is not obeyed as consistently by these types of oiganome-tank compounds a by the carbonyl and nitrosyl complexes and their derivatives. For example, in addition to ferrocene. M(i 5-CsHs)2 compounds are known for most of the other elements of the first transition series (M — V, Cr, Mn.Co, Ni) and these cannot obey ihe 18-electron rule. However, only ferrocene shows exceptional thermal stability (stable to 500 C) and is not oxidized by air. Furthermore, cobaltocene, a 19-electron species, is readily oxidized to the 18-electron cobaltocenium ion. (Co(ip-CsH )3)4 , which reflects much of the thermal stability of ferrocene. Mixed cyclopentadienyl carbonyl complexes are common K -CjHjMCO) ]. [(if-CjH )-Cr(CO), . [( -CjHOMnCCOjJ, [(>r-C,H,>Fe(CO ,, . [fo -CjiyCoCoy. and (ip-CsH,)Ni(CO) 2. Of interest is the fact that among these compounds, the odd-atomic-number elements (V. Mn, and Co) form monomers and the even-atomic-number elements (Cr. Fe. and Ni) Ibrm dimers, which is in direct contrast to the behavior shown by the simple carbonyl complexes. Cyclopentadienyl derivatives are now known for every main group and transition metal of the periodic table and for most of the -block metals.89... 

Asymmetrical, Mixed Bisporphyrinates - The synthetic paths (paths — f, — j, k) preclude an easy synthesis of heterobimetallic complexes like RuOs(OEP)2. Such complexes were obtained in pure forms by stepwise metallation of H2(DPB) to RuOs(DPB) [232]. Separation of a mixture of [Ru(OEP)]2, [Ru(OETAP)]2, and Ru2(OETAPXOEP) was achieved by stepwise oxidation of these bis-metalloporphyrins using AgBF4 in toluene when first [Ru(OEP)]2BF4 and then [Ru2(OETAP)(OEP)]BF4 precipitated. The latter was then reduced with cobaltocene to give pure Ru2(OETAP)(OEP) [233]. 

An interesting case of reductively induced hydride migration is summarized in Scheme 9.156 Chemical oxidation of [Cp Fe(dppe)H] affords the 17-electron monocation, which reversibly binds CO at -80°C. Spectroscopic studies established that [Cp Fe(dppe)(CO)H]+ is a genuine 19-electron complex, which undergoes hydride migration to an endo site of the C5Me5 ring when reduced with cobaltocene. 

Cyclopentadienyl complexes of cobalt exist mainly in three oxidation states Co Co, and Co. Co is represented by complexes of the type CpCoL2, where L is CO, alkene, alkyne, or phosphine. Apart from derivatives of cobaltocene, half-sandwich complexes (see Half-sandwich Complexes) (CpCoX)2 or CpCoLX with cobalt in oxidation state II are known. CpCo occurs in various CpCoL Xm compositions (Section 9.2). 

Complexes of the type CpCo(PR3)2 are alkylated at the metal with small alkyl halides to give CpCo(PR3)2R (Scheme 25). Bulky halides produce ring-substituted hydrido cations instead, explained by attack of the electrophile from the exo site followed by ring-to-metal proton transfer. This reaction could be electrophilic addition (see Electrophilic Reaction), 5ei, or more probably radical addition initiated by electron transfer similar to the RX reaction of cobaltocene (Section 7.1). Since the oxidation potential of CpCo(P(alkyl)3)2 is more negative than that of cobaltocene, this latter mechanism is very plausible. 

Redox potentials of cobaltocene and substituted cobaltocenes (Section 7.3) have been determined for the reduction of cobaltocenium to cobaltocene, a potential of —0.95 V (vs. SCE) in acetonitrile or —0.86 V (vs. SCE) in CH2CI2 was measured (—1.35 V vs. the ferrocene/ferrocenium couple in aprotic solvents). The large potential difference between oxidation of ferrocene and cobaltocene is intriguing, since it is related in a simple fashion to the difference in the HOMCULUMO gap... 

Cobaltocene is stable as a monomer under all conditions, showing no tendency to dimerize, as does the rhodium congener rhodocene. Cobaltocenium salts are easily prepared by several routes, including mild oxidation of cobaltocene, and are stable as cations. However, neutral electron-rich monomers are no longer stable if the aromaticity of one of the Cp rings is perturbed by interposed CH2 groups (e.g. CpCo(cyclohexadienyl)). Dimerization of a C-C bond adjacent to the jr-system frequently occurs (Scheme 28). The process can be followed electrochemically on reduction of the CpCo()] -E)+ cation to the neutral sandwich, which then dimerizes. Logically, the same dimerization is observed with so-called half-open cobaltocene, that is, a bis(pentadienyl)cobalt, which has one pentadienyl see... 

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