Cobalticinium

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. 

Even more intriguing was the observation that the same products were formed upon pyrolysis of ionic cobalticinium salts of commo-metallocar-... 

The discovery of the thermal metal transfer reaction (27) afforded a third preparative route to bimetallic twelve-vertex metallocarboranes. This method involves the pyrolysis of eleven-vertex cfoso-metallocarboranes or cobalticinium salts of eleven-vertex cowimo-metallocarboranes and results in the production of several isomeric, closed, neutral, twelve-vertex bimetallocarboranes [Eqs. (9) and (10) Fig. 7]. Yields are reasonable in... 

An interesting reducing agent is cobaltocene (37), although its use is somewhat limited by the low solubility of the cobalticinium salts. 

The condensation of l,l -bis(chlorocarbonyl)-cobalticinium chloride (82) with various phenols and alcohols followed by chromatographic purification and excess ammonium hexafluorophosphate gave the ester-linked polycobalticinium macrocycles (83)-(86) (Scheme 28). The first... 

The general poor solubility of these macrocyclic polycobalticinium systems coupled with their lability to ester hydrolysis has very recently led us to a new synthetic strategy which utilizes the stable amide linkage to construct novel acyclic cobalticinium anion receptors (132). 

Only in the case where the cobalticinium receptor contains at least one amide N-H linkage are significant one-wave cathodic shifts pro-... 

An alternative to light-related detection is an electrochemical response. If the sensor and analyte are in solution then cyclic voltammetry can be used to detect changes in redox potential between the free sensor and its complex with the analyte. Supramolecular applications of this approach were pioneered by Beer who linked crown ethers to electrochemically responsive ferrocenium [1] and cobalticinium [14] groups. In the former case a response was detected when cations complementary to the crown ether cavity were added to acetonitrile solutions of the sensors in the latter, anions were detected by an acyclic receptor. 

Beer PD et al (1992) Acyclic redox responsive anion receptors containing amide linked cobalticinium moieties. J Chem Soc Chem Commun 270-272... 

Penta(isopropyl)- and penta(neopentyl)-Cp cobalticinium salts, on the other hand, have a high enough barrier to interconversion (AG% 17.1 and 19.4 kcal/mol, respectively) with coalescence temperatures of 65° and 100°C, respectively, in the NMR (14). Thus, both enantiomers can coexist at 20°C with only a slow interconversion between them. In tricarbonyl-methyl-penta(isopropyl)cyclopentadienyl molybdenum the activation energy has been estimated to 13 1 kcal/mol (211) (cf. comment in Section IV, C, 2). 

G. Wilkinson, The Preparation and Some Properties of the Cobalticinium Salts, J. Am. 

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]. 

Chemical Shifts (ppm) of the Various Ring Protons OF Substituted Cobalticinium Salts"... 

All four organometallic cyclopentadienyl systems discussed heretofore have had their homoannular electronic effects examined by PMR spectroscopy. Of the four, only the cobalticinium salts exhibited shielding for the... 

More recently, UV data for a related system, substituted cobalticinium salts (77) have been tabulated. A similarity in the positions of the absorption bands in this system and the analogous substituted ferrocenes was observed. A decrease in the absorbance of the band at 260-275 m/x with both donating and withdrawing groups was the only electronic effect of note. 

Another approach developed by Balkus and co-workers involves the use of or-ganometallic complexes such as phthalocyanines and cobalticinium cations as tern-... 

Cobalticinium reacts with hydride to give a neutral, 18-electron sandwich compound in which one cyclopentadienyl ligand has been modified into as... 

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