Redox responsive anion receptors

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

Ferrocene has also been extensively exploited in redox responsive anion receptor design. One advantage of ferrocene is that its synthetic chemistry... 

In a step towards the fabrication of prototype sensory devices organisation of redox-active anion receptors on to electrode surfaces is being exploited. Importantly, self-assembled monolayers or thin polymer films of metal-based receptors can generate an amplified response to anion binding akin to the dendritic effect and could potentially become the basis of robust anionsensing devices. 

Costa. J. Delgado, R. Drew. M.G.B. Felix, V. Saint-Maurice, A. A new redox-responsive 14-membered tetraazamacrocycle with ferrocenylmethyl arms as receptor for sensing transition-metal ions. J. Chem. Soc., Dalton Trans. 2000. 1907-1916 and references therein. Lloris. J.M. Martinez-Miner. R. Soto, J. Pardo, T.J. An electrochemical study in acetonitrile of macrocyclic or open-chain ferrocene-containing oxa-aza or polyaza receptors in the presence of protons, metal cations and anions. J. Organomet. Chem. 2001. 637-639. 151-158 and references therein. 

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. 

Beer. P.D. Hesek, D. Kingston. J.E. Smith, D.K. Stokes, S.E. Drew, M.G.B. Anion recognition by redox-responsive ditopic bis-cobaltocenium receptor molecules including a noVel calix[4]arene derivative that binds a dicarboxylate dianion. Organometallics 1995. 14. 3288. 

Beer, P.D. Hazlewood, C. Hesek, D. Hodacova. J. Stokes. S.E. Anion recognition by acyclic redox-responsive amide-linked cobaltocenium receptors. J. Chern. Soc. Dalton Trans. 1993, 8, 1327. 

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]. 

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. 

Another interesting development has been the self-assembly of metallo-dithiocarbamate macrocychc receptors for electrochemical anion sensing. The naphthyl-based Cu(II) macrocycle 48 displays a cathodic shift of 85 mV in the Cu(II)/(III) redox couple in the presence of dihydrogenphosphate or perrhenate, but gives no response to hahdes in acetonitrile solution [35]. It is proposed that this selectivity is controlled by cavity size. A related receptor incorporating thiourea and hydrogen-bond donor groups, 49, revealed... 

In contrast to the recognition of cations or anions, the electrochemical sensing of neutral molecules by redox-active receptors is relatively rare, with few systems undergoing a significant electrochemical response to complexation. Some ferrocene compounds that electrochemically respond to neutral molecules and charge-neutral species are shown in Figure 4. 

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