Cobalticinium ion

Fig. 8.7 Cyclic voltammograms of cobalticinium ion in 1 mM (tj-C5H5)2CoPF6-0.1 M Bu4NBF4-AN [30a]. Recorded using a hanging mercury drop electrode (a) and a glassy carbon electrode (b) at 100 mVs-1 and at 25 °C.

Ferrocene is only one of a large number of compounds of transition metals with the cyclopentadienyl anion. Other metals that form sandwich-type structures similar to ferrocene include nickel, titanium, cobalt, ruthenium, zirconium, and osmium. The stability of metallocenes varies greatly with the metal and its oxidation state ferrocene, ruthenocene, and osmocene are particularly stable because in each the metal achieves the electronic configuration of an inert gas. Almost the ultimate in resistance to oxidative attack is reached in (C5H5)2Co , cobalticinium ion, which can be recovered from boiling aqua regia (a mixture of concentrated nitric and hydrochloric acids named for its ability to dissolve platinum and gold). In cobalticinium ion, the metal has the 18 outer-shell electrons characteristic of krypton. 

This idea was later applied to redox systems formed by large complex ions with a small charge where the solvation energy varies little with the state of oxidation. Three such electrodes are now in frequent use. One is formed by ferrocene (Foe) and its oxidation product - ferricinium ion (Foe ). Another also proposed by Strehlow and coworkers [55] is formed by its cobalt analogue cobaltocene (Coc) and cobalticinium ion (Coc ). The third electrode system, formed by neutral bis(biphenyl)chromium(0) (BCr) and its first oxidation product (BCr ), was proposed by Schroer and Vlcek [56]. 

A completely different approach involves organometallic complexes with a relatively rigid structure that can be used as templates for the synthesis of zeolites. Balkus et al. showed that permethylated cobalticinium ion (CoCp2 ) can be employed to synthesize a new zeolite UTD-1, which was the first zeolite with 14-membered ring channels. The (CoCp2 ) complex was also used to synthesize ZSM-51 and AlPO-16. This zeolite and zeotype, respectively, exhibit cages into which this template nicely fits. Another example is the employment of 1 -aminoadamantane for the synthesis of chabazite. 

The ruthenium compound also has been prepared and displays similar properties, although it has not yet been the subject of an x-ray study. Bis-indenylcobalt, the first member of the series to be prepared, has a structure similar to the iron analogue on the basis of a comparison of x-ray powder photographs. This latter compound can be readily oxidized to give the quite stable Co(III) compound which, like the cobalticinium ion, may be precipitated as the triiodide, tribromide picrate, or perchlorate. The perchlorate exhibits a well-defined polarographic reduction wave at —0.6 V vs SCE. The corresponding half-wave potential for cobalticinium perchlorate is — 1.16 V. Thus the annulation of a benzene ring produces a very marked decrease in the reduction potential and, conversely, the oxidation of the new compound to the ion should become more difficult. 

On the other hand, the reduction of cobalticinium ion or rhodenocinium ion with sodium borohydride or lithium aluminum hydride gives cyclopen-tadienylcobalt-cyclopentadiene and 71-cyclopentadienyl rhodium-cyclopen-tadiene, respectively. 

Some interesting experiments on organo-metallic cobalt compounds have been reported by Page and Wilkinson (60). These authors were able to reduce cobalticinium ion polarographically at —1.16 V vs. SCE in 0.1 m sodium perchlorate at pH 6.2, but attempts to produce cobaltocene from cobalticinium ion by... 

Whereas the hexafluorophosphate anion was typically employed, products containing a number of other anions were synthesized. Anions included the bromide and nitrate ions. The use of these other anions resulted in greatly reduced yields on the order of less than 10%. It is believed that the hexafluorophosphate moiety is closely associated with that of cobalticinium and, because of size, makes it more difficulty for water to approach. The water likely promotes hydrolysis of the polymer and possibly of the organoantimony chloride, either during reaction or as a growing end group. 

This formation of cobaltous ion may, however, be avoided by oxidizing cobaltocene with hydrogen peroxide to give nearly quantitative yields of cobalticinium hydroxide, CpaCoOH (8, 85). 

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