Cobaltocenium complexes

The known cobaltocenium complexes 48 and 49 were proposed as ligands for rhodium (Table 1.5). They conferred a lower activity to the catalyst, but Rh leaching was less than 0.2% for both ionic ligands, and recycling experiments showed unchanged activity and selectivity for several runs. 

Takahashi eta/, obtained a chiral version of a cobaltocenium complex. This was achieved via what appeared to be the synthesis of the first examples of planar-chiral ionic metallocenes." The amide cobaltocenium 384 was obtained as enantiomerically pure with the help of a (-)-menthyl group as the removable chiral auxiliary (Equation (64)). 

A heterobimetallic ferrocene-cobaltocenium complex shown as 4 in 41 has been prepared in the reaction of diaminobutane and the chlorocarbonyls of ferrocene and cobaltocenium. The electrochemical behaviour of this and related compounds including dendrimers as shown has been studied in this workd ... 

As an extension of this work,Kaifer et al. prepared a series of PPI dendrimers (G-l-G-4) functionalized with cobaltocenium at the periphery [48] and studied their electrochemical behavior and binding interactions with P-CD. While the positively charged cobaltocenium-terminated dendrimer 27G-1 is not com-plexed by P-CD in aqueous media, electrochemical reduction of the dendrimer in the presence of excess P-CD triggers the formation of a multisite inclusion complex with this host to provide pseudorotaxane-terminated dendrimer 28G-1 (Fig. 10). Similar electrochemical and binding behavior toward P-CD was observed for the G-2 and G-3 dendrimers. However, the resulting multisite inclu-... 

To test this hypothesis further, simple acyclic cobaltocenium derivatives (Fig. 38) containing tertiary amide groups ([72] and [73]) were prepared and complexation with Cl- and Br anions was investigated in solution by nmr spectroscopy. No shifts of the host s proton resonances were observed, emphasizing again the importance of the amide N—H group in anion binding. 

As seen the complex displays either a single ferrocenyl-centred oxidation (which however looks like it is partially chemically reversible) or a single cobaltocenium-centred reduction (which is affected by electrode adsorption), thus testifying that no interaction exists among the different metallocene units. 

Dendrimers Terminated with Cobaltocenium and Ferrocene-Cobaltocenium Units Like ferrocene, cobaltocenium is an excellent organometallic moiety to incorporate in or functionalize dendritic systems. As already discussed, it is indeed isoelectronic with ferrocene, highly stable, positively charged complex, which undergoes a reversible monoelectronic reduction to yield the neutral cobaltocene. 

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... 

Bis(cyclopentadienyl)zirconocenes, cationic complexes, 4,896 l,l -Bis(dialkylboryl)cobaltocenium cations, complexation behavior, 7, 88... 

Polyphosphazenes, with metallocene side-groups, 12, 308 Polyphosphorus ring compounds, with iron carbonyls, 6, 41 Polypropenes, via mono-Cp Ti(IV) complexes, 4, 494 Poly(propyleneimine) dendrimers, cobaltocenium, 7, 89... 

Hall, C. D., Djedovic, N., The synthesis and complexation of a cobaltocenium-based redox-active cryptand containing the phenanthroline unit. J. Organomet. Chem. 2002, 648, 8-13. 

As already mentioned, in contrast to the cobaltocenium moiety, the ferrocene derivatives represent neutral redox-active receptors for anions. As we are losing coulombic interactions, the complexation is mediated solely via hydrogen bonding. Consequently, the corresponding association constants can be evaluated only in HB non-competitive solvents and they are usually much lower when compared with those of cobaltocenium derivatives. This apparent disadvantage is compensated by the much higher synthetic potential of ferrocene (commercial availability of many derivatives, much easier synthetic handling) and the excellent electrochemical properties of this compound. 

Anion receptors incorporating cobaltocenium have been studied extensively due to the combination of an accessible redox couple and potential favourable electrostatic interactions of the cationic organometallic metallocene complex with anions. The first anion receptor based on this species was reported by Beer and co-workers in 1989 [6]. The macrocyclic bis-cobaltocenium receptor 1 was shown to bind (via electrostatic interaction) and to electro chemically sense bromide in acetonitrile solvent media. 

The accessible ferrocene/ferrocenium redox couple of ferrocene has led to its frequent use in electrochemical anion sensors. The chemical and structural similarity between ferrocene and cobaltocenium has meant that receptors based on these complexes often share the same design. The most relevant difference is that the ferrocene derivatives are neutral (until oxidised to ferrocenium),have no inherent electrostatic interaction with anions and therefore their complexes with anions exhibit lower stability constants. 

In the most obvious way, a metal complex can be added to the N terminus of the final peptide on the resin (bottom path in Fig. 3). This requires very similar chemistry as discussed above for solution metallation. The metal complex is treated more or less as another amino acid. HBTU and TBTU are the preferred coupling reagents in this case, also HATU might have advantages in difficult cases. We find that for less reactive metal complexes, e.g., cobaltocenium carboxylic acid, longer activation and coupling times of up to 24 h may be needed. 

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