Metal-containing receptors

In recent years, the development of metalated container molecules has also become an attractive research goal (35-38). This is mainly due to the fact that such compounds allow for an interplay of molecular recognition and transition-metal catalysis (39-41). Consequently, several research groups are involved in the development of new receptor molecules that create confined environments about active metal coordination sites. The aim of this review is to highlight recent advances in this area. 

Many aspects of anion recognition by metal-based receptors have been covered in previous reviews [1-5]. This review is not intended to be comprehensive rather, it provides an overview of the subject with particular reference to the specific properties that metal-containing sub-units impart to this class of anion sensor. The most recent advances in the field are also detailed including the use of metallo-receptors in dendrimer, functionalised nanoparticle and surface-bound anion sensors. 

Palladium-carbon-catalyzed hydrogenation of (16) gave (24), and NaBH4 reduction of (22) gave (25), both receptors now containing a saturated linkage between the aza crown system and the ferrocenyl redox center. This methylene-ferrocene linkage was found to serve as an insulator both compounds (24) and (25) are totally electrochemically insensitive to the presence of any alkali metal cation. 

Pioneering work in this area was carried out by the groups of Lehn and Martell [34, 35]. One example of a metal containing cryptand is dicopper(II) complex 14 which was shown to interact with various anions such as N3 , OCX. SCN. SO)2. HCOCT, CH3COO, HC03-, and 03 [36]. Complex formation can easily be detected by the color change of an aqueous solution of the receptor from blue in the absence of suitable anionic substrates to green in their presence. 

The highest stability of the sensor-analyte complex is achieved when the substrate fits perfectly in the hole within the receptor. The analyte does not have to fit the receptor cavity perfectly efficient binding can be achieved by careful design of the receptor. In the case of metal ion sensors the receptor must contain a proper type and number of donor atoms angular orientation and directionality of lone electron pairs are also of crucial importance [5]. 

Fruit tissues respond to ethylene by exhibiting increases in the activities of enzymes that catalyze ripening reactions, and in some cases, the increases in enzyme activity probably are the result of de novo synthesis, rather than activation of preexisting enzymes. Other target tissues respond similarly to ethylene. But it is not known whether ethylene acts directly to evoke new enzyme production. Interpretation of results with inhibitors of RNA and protein synthesis is inconclusive, because it could be merely that RNA and protein synthesis are essential to maintain the cells in a state competent to respond to ethylene. Moreover, there are some responses to ethylene, besides fruit ripening, which occur under conditions which apparently do not directly involve RNA and protein synthesis (e.g., membrane permeability changes). It has been proposed that the in vivo ethylene receptor site contains a metal such as copper (34,35). 

Similarly, nickel(ll) and copper(ll) transition metal dithiocarbamate ion-pair receptors 21, containing amide-and crown ether-recognition sites, bind alkali metal cations and various anions. The sandwich K+ complex of the nickel(ll) receptor cooperatively enhanced the binding of acetate anion, while the copper(ll) receptor electrochemi-cally can sense anions and cations via perturbation of the copper(n)/copper(m) dithiocarbamate redox couple <2002JSU89>. 

Leonard Undoy is Professor of Inorganic Oiemistry at the University of Sydney. He has had wide experience in Host-Cuest and Supramoiecular Chemistry. His research has focused on rational receptor design for metal ion and small molecule recognitioa macrocydic chemistry and the application of self-assembly processes for the construction of metal-containing / ... 

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... 

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