Receptors redox-active center

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... 

In this section, freely diffusing receptors in solution will be discussed, in which the redox-active center or reporter group is, or is an integral part of an inorganic or an organic group. 

A number of other metal-based redox-active centers have been incorporated into supramolecular receptors, representative examples of which are displayed in Fig. 5 (Compounds 24-28). Many of these receptors electro-chemically respond to cations. but species that respond to anions and neutral molecules are also known. A number of the cation binders are organometallic crown ether and metallocrown or metallothiacrown derivatives, for example. Compound 24. Flow-ever, in many cases, the redox processes are not particularly reversible, and relatively small anodic shifts in the metal-centered redox couples are observed. A series of self-assembled [12]metallocrown-3 complexes, two of which are 25 and 26, were found by Severin to bind halide salts of small Group 1 metals strongly in organic solvents, with affinities similar to those of the cryptands. X-ray crystal structures revealed that the metal cation was... 

There are numerous examples of organic redox-active reporter groups that have been incorporated into supra-molecular receptors over the past 20 years. Many of these date to the 1980s and early 1990s. when Gokel and coworkers reported a number of crown ether and cryptand derivatives containing redox-active centeres. Much of this... 

Beer. P.D. Crowe. D.B. Ogden, M.I. Drew, M.G.B. Main. B. Ammonium redox-responsive receptors containing multiple ferrocene and quinone redox-active centers attached to di-aza and tri-aza crown-ether macrocycles. J. Chern. Soc., Dalton Trans. 1993. 2107-2116. 

Fabbrizzi s S[36] is unusual in using a nonmetallic, redox-active guest with a metal center serving as the receptor. An anionic nitrobenzoate guest is held by coordination of the carboxylate to the free apical position in Zn2+. The electron-deficient nitrobenzoate engages in PET with the anthracene fluorophore to switch the fluorescence OFF . 

When the redox active guest anthraquinone-2-carboxylic acid is added to compound 47, a partial quenching of about 20 % of the MLCT emission of the ruthenium center of the complex is observed, while no quenching is observed when the same substrate is added to the metal complex in absence of the cyclodextrin receptor. The partial quenching can be then ascribed to the occurrence of an electron transfer process between the guest complexed into the CD cavity and the appended metal center [88]. 

Transferrin binds the iron as an Fe center with a coordinating bicarbonate anion thought to be a necessary adjunct. It is possible that this bicarbonate has a buffer function on release of the iron in the form of iron(II). The considerations presented in Section 8.7.4.3 on iron redox chemistry and core formation examine such aspects in greater depth. The uptake is probably via receptor-mediated endocytosis and the majority view is now that ferroxidase activity is the next step in the uptake process with the overall process being an oxidative one starting from Fe. ... 

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