Cryptates transition metal complexes

Some molecular transition-metal complexes of P , As3-, and Sb3 have been isolated as salts of cryptated alkali metal ions. The structures of the complex anions are shown in Fig. 15.3.9. The bond valence b and bond number of these complex anions are as follows ... 

Finally, Sargeson and co-workers have prepared a series of thioether-containing cage ligands, typically using template reactions. Transition metal complexes of these compounds exhibit interesting properties and the extremely robust nature of the complexes is attributed to the cryptate effect. 50,51... 

Perhaps the most apparent similarities with transition metal complexes are the main group metal ions and their supramolecular complexes. Here, the progression from the chelate to macrocyclic to cryptate effect can be readily recognized and the thermodynamics are, for the most-part, established. The term ionophore is often used for this class of hosts, originating from its use in biology for lipid-soluble molecules that serve as vehicles for transporting ions across membranes. 

Modem work on these and related bare post-transition element clusters began in the 1960s after Corbett and coworkers found ways to obtain crystalline derivatives of these post-transition element clusters by the use of suitable counterions. Thus, crystalline derivatives of the cluster anions had cryptate or polyamine complexed alkali metals as countercations [8]. Similarly, crystalline derivatives of the cluster cations had counteractions, such as AlCLj, derived from metal halide strong Lewis acids [9]. With crystalhne derivatives of these clusters available, their structures could be determined definitively using X-ray diffraction methods. 

Of particular significance in this respect has been the ability to prepare, characterize and study most intriguing species, the alkalides [2.79, 2.80] and the electrides [2.80, 2.81] containing an alkali metal anion and an electron, respectively, as counterion of the complexed cation. Thus, cryptates are able to stabilize species such as the sodide [Na+ c 9]Na- and the electride [K+ c 9]e-. They have also allowed the isolation of anionic clusters of the heavy post-transition metals, as in ([K+ c cryp-tand]2 Pb52-) [2.82]. 

Crown ethers have particularly large complexing constants for alkali metals equilibrium constants for cyclohexyl-crown-6, for example, are in the order K+ > Rb+ > Cs+ > Na+ > Li+. The cryptates have high complexing ability especially for M2+ ions and will render even BaS04 soluble. They also have good complexing ability for transition metal ions (e.g., for lanthanides). 

The macrobicyclic cryptand BISTREN (3), as discussed earlier, binds anions when hexaprotonated. This compound is also capable of binding pairs of transition metal ions [e.g. the bis-copper(Il) Complex (55)] when unprotonated (39, 138). The bis-copper(II) based receptor (55) was observed to bind a chloride ion cascaded between the two metal ions. In fact, receptor 55 bound chloride more strongly (log K = 3.55) than the hexaprotonated form of 3 (log K = 2.36), and this was attributed to the formation of strong coordinate bonds. Hydroxide was also cascade bound forming a thermodynamically very stable complex (log K = 11.56), the resulting complex being of a different type from that formed by non-cryptate complexes of the Cu(II) ion. 

In the beginning of the increased cognizance of the different types of chemical influences that mnltiple binding sites can impart, the focus was totally on transition metal coordination complexes. Indeed, for the majority of these effects (with the exception perhaps of the cryptate effect), supramolecular chemistry was not even on the radar screen. Now, however, decades after the term snpramolecular was coined by the now Nobel Laureate Jean-Marie Lehn, ... 

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