Cobaltocenium unit

The anion coordination properties of receptors such as compound [93] (Fig. 51) are currently under investigation. This molecule contains both a redox-active ruthenium bipyridyl moiety and also a cobaltocenium unit. This type of host has already been shown by 1H nmr and fluorescence emission spectroscopy to exhibit remarkable selectivity for the chloride anion in preference to dihydrogenphosphate (Beer and Szemes, 1995). 

My group also studied the binding interactions between (3-CD and dendrimers containing multiple cobaltocenium units in their peripheries.32 Furthermore, we have... 

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. 

For example, Kaifer et al. decorated the periphery of a POPAM dendrimer with ferrocene and cobaltocenium units. Such a fourth-generation dendrimer (Fig. 4.4) proved to be a suitable guest system - with numerous docking sites -for /9-cyclodextrin as host compound (see Section 8.3.6) [8]. 

Fig. 4.4 Fourth-generation POPAM dendrimer with terminal cobaltocenium units...

The last three compounds (30-32) all comprise three macrocyclic rings with cobaltocenium units attached to a terminal ring and in one case (32) with ferrocene at the other end. 

Sobransingh D, Kaifer AE (2006) New dendrimers containing a single cobaltocenium unit covalently attached to the apical position of newkome dendrons electrochemistry and guest binding interactions with cucurbit[7]uril. Langmuir 22 10540-10544... 

Cyclic voltammetric experiments indicated that not only was the anion bound by the host, but that the redox potential of the reversibly reducible cobaltocenium fragment was cathodically perturbed (Fig. 11) (the presence of a bound anion making the cobaltocenium unit more difficult to reduce). Tertiary amides showed no electrochemical effect on the addition of anions, ruling out the op-... 

Cobaltocenium based systems have therefore proved very versatile in the field of anion coordination chemistry. The role of the cobaltocenium unit is to enhance interaction with the bound anion and to function as a sensing unit. The structure of the receptor can then be controlled by simple organic manipulations of the Cp groups. In this manner, novel anion selectivity can readily be incorporated into these receptors. 

A similar effect is seen in macrocycle 36 which incorporates the Ru (bpy)3 moiety with a bridging cobaltocenium unit [29]. In acetoni-... 

Scheme 4 Electrochemically triggered self-assembly of P-CD hosts around a dendrimer with four peripheral cobaltocenium units.

The synthesis of polymers containing main-chain cobaltocenium units has also been studied as this 18-electron organometallic moiety is isoelectronic with ferrocene. The synthesis of partially soluble polyesters (4.6, E = 0, R=(CH2)4, CH2C6H4CH2) was reported using solution condensation polymerization techniques, and the soluble fractions were of low molecular weight (M 2500-4000,... 

Comparison of cyclic, 75, and non-cyclic hosts revealed that the former is much more effective in chloride ion complexation [125]. Ditopic macrocyclic hosts containing two cobaltocenium units, 76, can be yet more efficient. 

Among ditopic cobaltocenium receptors, 81, those containing alkyl spacers form 1 1 host-guest complexes with halide anions, whereas those containing larger, aromatic spacers form 2 1 complexes. The selectivity and complexing capacity of the receptors decreases with increasing distance between the cobaltocenium units... 

Like their isoelectronic ferrocene counterparts, cobaltocenium units are resistant to strong oxidizing agents and possess interesting electrochemical behavior. The synthesis of polyesters and polyamides containing cationic cobalt units in the main chain has been reported (32,74-76). Sheats and Carraher reported the synthesis of cobaltocenium polymers containing tin, antimony, titanium, and zirconium atoms in their backbone via the reaction of the dicarboxylic... 

A further generation of anion receptors with related principles was also synthesized by Bcci eta/. They combined calrx[4]-arene with cobaltocenium units 388-390 (Figure 11). ... 

A series of poly(propyleneimine) dendrimers functionalized with 4, 8, 16, 32 peripheral cobaltcenium subunits was prepared. The compound with four Co nuclei, 393, and the one with 32 cobalt atoms, 394, are presented below (Figures 80 and 81). The reduced form of these complexes precipitated on the electrode surface. In the presence of a fourfold excess of /3-dextrin, the reduction of the cobaltocenium units was fully reversible, this being an indication of the formation of inclusion complexes between the cobaltocene units and /3-dextrin." ... 

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