Metallocene receptors

It is also possible to affect H-bond strength electrochemically without the H-bonding site being an integral part of the redox couple. Good examples of this are found in the many redox-dependent receptors that utilize metallocenes, primarily ferrocene and cobaltocenium, as the redox couple. These are primarily used in ion receptors, but examples of metallocene receptors that show a significant redox dependence with neutral guests are the cobaltocenium and ferrocene diamides,... 

Beer, P. D., Szemes, F., Remarkable chloride over dihydrogen phosphate anion selectivity exhibited by novel macrocyclic bis [ruthenium(II) bipyridyl] and ruthenium(II) bipyridyl-metallocene receptors. J. Chem. Soc., Chem. Commun. 1995, 2245-2247. 

It is noteworthy that few of the receptors discussed so far exhibit specific binding and sensing of the chloride anion, yet this substrate is crucial for a large number of biological processes (261). The novel macrocyclic bis[ruthenium(II)bipyridyl] and ruthenium(II) bipyridyl-metallocene receptors (140-142) have been prepared. The H NMR titrations indicated that each re-... 

Figure 3 Metallocene receptors for inorganic and organic anions.

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

Electroionic signals are generated by electrochemical interconversion of a selective receptor molecule containing a redox-active group (metallocene, quinone, etc.) between states of low and high affinity for a given substrate (see Sections 6.4.1 and 8.3.1). 

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. 

Introduction of another ruthenium bipyridine moiety or bridging metallocene (ferrocene, cobaltocenium) results in sensors that specifically bind chloride anions (Figure 16.15). The structural modification of the amide receptor results in a decease in cavity dimensions and significant rigidity of the macrocycle. Therefore it cannot accommodate hydrogen phosphate anions, but only much smaller CF [45, 46]. 

A notable substitution product of ruthenocene is ethynyl-ruthenocene, which can be bound to estradiol, a form of estrogen studied for breast cancer treatment. With the rigid alkyne spacer, the steroid s affinity to the estrogen receptor was surprisingly unchanged upon addition of the metallocene. The addition of organometallic complexes to biomolecules may allow for the selective tuning of properties of the molecule. ... 

Mixed Metal Metallocene-Lewis Acid Anion Receptors... 

Very recently, Gmeiner and co-workers prepared metallocene-derived receptor ligands for G-protein-coupled receptors (GPGRs) such as dopamine and serotonin receptor subtypes. They used ruthenocene and ferrocene derivatives, in which the metallocenes replaced cyclophanes as so-called fancy bioisosters. In particular, compound 31 (Scheme 32) showed sub-nanomolar affinity and high specificity for the dopamine D4 and serotonin HTia receptor subtypes, and may thus be a suitable lead structure for the further development of selective organometallic GPCR ligands. 

There are now numerous reports of metallocene derivatives that bind and sense anions. Early work in this area was carried out by Beer, who first reported a redox-active anion receptor in 1989. " Beer published a number of cobaltocenium and fen-ocene anion... 

Fig. 5 Redox-active receptors with non-metallocene-based inorganic reporter groups.

---