Lead cryptand metal complexation

Gyclodextrin cavities form the early models of host molecules involved in supramolecular assemblies. There are many other molecules known as cryptands which can be designed to offer a cavity of fairly precise dimensions to accommodate various ions or metal complexes. It may be possible to locate not just one, but two, guest molecules inside a cryptand cavity, and this may lead to new electron transfer reactions in restricted environments another step towards synthetic photoinduced biochemical reactions. 

There are many examples of platinum(II) interacting with metals such as lead(II) or thallium(I) but few where the same metals interact with platinum(O). Catalano et al. have reported a series of metallocryptands such as the one shown in (11) that act as hosts for thallium(I)72 and lead(II).73 They have also reported an unsupported thallium(I) interaction with the platinum in [Pt(PR3)3] (R = Ph or R3 = Ph2py).74 The Pt Tl separations in the cryptands (2.791-2.795 A) are slightly shorter than those in the unsupported complexes (2.865-2.889 A).72,74... 

The larger-ringed macrocycles of (53a-d) form binuclear complexes with alkali metal, alkaline earth, silver(I) and lead(II) cations.68,192 The two 18-membered rings are large enough to allow for cations as large as Rb+ to be incorporated within their cavity, with a net result of increasing the metal-metal separation. Thus, crystal structure data for the disodium complex of (53a) indicate the sodium ions to be 6.40 A apart,193,194 compared to a 3.88 A separation found for the aforementioned disilver complex of (52a). Heteronuclear complex formation has also been observed, e.g. with both Ag+ and Pb2+ incorporated in the same cryptand.192... 

Polyaza-, polythia-ligands. Recognition of transition metal ions. Replacing the oxygen sites with nitrogen or sulphur yields macrocycles and cryptands that show marked preference for transition metal ions and may also allow highly selective complexation of toxic heavy metals such as cadmium, lead and mercury [2.41-2.44, A.14]. 

The ratio of the size of the metal ion and the radius of the internal cavity of the macrocyclic polyether determines the stoichiometry of these complexes. The stoichiometry of these complexes also depends on the coordinating ability of the anion associated with the lanthanide. For example, 12-crown-4 ether forms a bis complex with lanthanide perchlorate in acetonitrile while a 1 1 complex is formed when lanthanide nitrate is used in the synthesis [66]. Unusual stoichiometries of M L are observed when L = 12 crown-4 ether and M is lanthanide trifluoroacetate [67]. In the case of 18-crown-6 ligand and neodymium nitrate a 4 3 stoichiometry has been observed for M L. The composition of the complex [68] has been found to be two units of [Nd(18-crown-6)(N03)]2+ and [Nd(NCh)<--)]3. A similar situation is encountered [69] when L = 2.2.2 cryptand and one has [Eu(N03)5-H20]2- anions and [Eu(2.2.2)N03]+ cations. It is important to note that traces of moisture can lead to polynuclear macrocyclic complexes containing hydroxy lanthanide ions. Thus it is imperative that the synthesis of macrocyclic complexes be performed under anhydrous conditions. 

Macropolycyclic polyamine ligand (a form of cryptand leading to extremely inert metal ion complexes)... 

Alkali metal NMR in conjunction withcryptand complexation finally led to the sensational discovery of alkali metal anions by Dye and coworkers. (14) The complexing power of the cryptands is such that the ionophore is capable of extracting the cation from alkali metal in solutions of THF, methylamine, and ethylamine. The electron left behind is conclusively proved to form the anion M (M = Na, Rb, Cs) leading to a separate resonance line at low temperature. The corresponding shielding is close to the theoretical value computed for the metal anion. A most striking feature of this resonance is its solvent independence. The absence of solvent-induced chemical shifts for Na ... 

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