Cobaltocenium anion receptors

In extension to the cobaltocenium anion receptor protocol, experiments in our laboratory led to the report of a series of receptors with anion-binding ability (107-109) (227). These hosts combined positively charged and neutral or-... 

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. 

We reported the first transition metal centered anion receptor to operate solely through electrostatic attraction in 1989 [9, 186]. Receptor 74 contains two positively charged, 18-electron, air stable, redox active cobaltocenium moieties. The reversible reduction potential of these redox active centers was observed to shift cathodically (up to 45 mV) on the addition of excess bromide ions. This... 

The first ester functionalized cobaltocenium receptor (74) developed by us in 1989, was based solely on electrostatic interactions (as discussed earlier) (9, 186). It was therefore decided to append the cobaltocenium moiety with secondary amide functionalized arms . In this manner, the receptors became more resistant to hydrolysis (a recurrent problem with ester-based systems), and also incorporated neutral hydrogen-bond donors capable of coordinating anions. [An interaction between a secondary amide (acetamide) and the bromide anion was physically investigated (by IR spectroscopy) as long ago as 1961 (211)]. We made the first report of this novel type of receptor (91 and 92) in 1992 (212), which was, in fact, the first class of inorganic anion receptor that incorporated hydrogen-bonding functionalities. 

We recently incorporated the ruthenium(II) bipyridyl moiety into acyclic, macrocyclic, and lower rim caUx[4Jarene structural frameworks to produce a new class of anion receptor capable of optical and electrochemical sensing (226, 253. 254). Stability constant determinations in DMSO using H NMR titration techniques demonstrated that these acyclic receptors (131 and 132) form strong complexes with chloride and dihydrogen phosphate anions (stronger than with analogous monopositive cobaltocenium based receptors). The ruthenium ion is dipositive and hence the electrostatic interactions are particularly favorable. The 4,4 -substituted ruthenium bipyridyls were observed to bind anions more... 

Another class of mixed-metal anion receptors has been investigated which possess redox reporter groups based on two different metal complexes. This enables the quahtative comparison of their comparative anion-sensing abih-ties. Macrocycles 35 and 36 combine the Ru (bpy)3 moiety with a bridging ferrocene or cobaltocenium imit [29]. Electrochemical experiments in acetonitrile solution revealed that the Ru VRu redox potential was insensitive to anion binding, whereas the ferrocene/ferrocenium (in 35) and cobal-tocene/cobaltocenium (in 36) redox couples were shifted cathodically (by 60 mV and 110 mV respectively with chloride). However, the first reduction of Ru°(bpy)3, a Hgand-centred process based on the amide substituted bipyridyl, was also found to imdergo an anion induced cathodic shift (40 mV and 90 mV with chloride for 35 and 36, respectively). 

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... 

The cobaltocenium moiety was studied extensively in the context of redox-responsive anion receptors. " The first class of anion receptor based on this system was reported by Beer and Keefe in 1989. The ester functionalized hi 5-cobaltocenium maeroeyelie Reeeptor 1 bound and eleetroehemieally sensed bromide in acetonitrile via favorable electrostatic interactions. 

Studies of synthetic porphyrin-based anion receptors should form the basis for more effective sensors. Metallocene-substituted porphyrins examined by Beer and coworkers have proven successful in the solution-phase binding of ions such as chloride, bromide and nitrate." The cobaltocenium-substituted and ferrocene-substituted porphyrins (Figure 134) bind ions in solution, as shown by H NMR and electrochemical studies. The latter measurements reveal that the porphyrin and ferrocene redox... 

A great variety of molecular hosts, mostly devised as anion receptors, contain cobaltocenium moieties. These can be combined with polyoxo macrocyclic ethers to give redox responsive coronands, 69, with electrochemical sensitivity to anion complexation [121]. 

Polyaza-oxa macrocycles can also be attached to cobaltocenium moieties, and function as both cation and anion receptors [123, 124]. 

In addition to inorganic anions, organic anions, including chiral species," " can be targeted. An early example of a car-boxylate anion receptor was the cobaltocenium-containing calrxarene derivative 17," which bound these guests selectivity over halide ions, due to two rigidly held amide units on the upper rim of the calixarene that create an ideal site... 

With the aim of developing electroactive systems capable of electrochemically and specifically recognizing anions [370], some anionic receptors have been bound to redox-active groups such as cobaltocenium, ferrocene, and transition-metal bipyridyl moieties [370-373]. Although such compounds were suitable for the electrochemical recognition of some anionic species in solution, their immobilization on an electrode surface to achieve sensitive layers has been investigated with very few examples. 

During the last 15 years, there have been several studies dealing with the use of cobaltocenium derivatives as anion receptors. Indeed, the molecular recognition by abiotic receptors is an area of intense current interest. This is due to the known essential role played by anions in chemical and biochemical processes. Thus the design of abiotic and allosteric receptor molecules that can show binding cooperativity of guest species is a research field to which a large number of... 

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). ... 

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). 

The first redox-active receptor for anions was the bis-cobaltocenium macrocylic ester 17 reported by Beer and Keefe in 1989.25 FAB-MS and FT-IR evidence indicates that 17 can bind anions in the oxidized state. Reduction of the cobaltoceniums to the zero state would be expected to weaken this interaction, and indeed a modest 45 mV j/2 shift is observed in acetonitrile upon addition of 4 equivalents of Br. ... 

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. 

A tetra-cobaltocenium receptor 4 has been synthesised that is supported by a porphyrin skeleton [9]. This exhibits parallel photo- and electrochemical sensing of anions with a trend in selectivity of Cl >Br NOg. Proton NMR titrations in CD3CN showed chloride and bromide to be bound in 1 1 stoichiometry with stability constants of 860 and 820 M"1, respectively, whereas nitrate exhibited weaker binding with K=190 M"1. In the UV-vis spectrum of 4 in acetonitrile solution the Soret band (Amax=425 nm) of the porphyrin was signif-... 

The incorporation of crown ether units into a cobaltocenium receptor has been shown to allow the switchable sensing of anions. Proton NMR titrations of receptor 8 in CD3CN solution gave log K values of 3.1 for chloride and 3.0 for... 

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