Binuclear cryptand

Besides (166), other binucleating cryptands have been synthesized (Lehn, 1980). The availability of systems such as these once again provides a means for fixing a pair of metal ions in close proximity in a well-defined environment. Such species are ideal for the types of studies involving binuclear systems outlined in Section 3.4. 

An exclusion complex of this type has been identified by X-ray crystallography for the KNCS complex of the smaller [2.2.1]cryptand,454 ([2.2.1]cryptand = 73, n = 1, m = 1, / = 0). The cation occupies a site in the 18-membered ring provided by the ligand rather than in the central cavity and so resembles the coordination of potassium by 18-crown-6 (Figure 21a). In contrast, the smaller Na+ cation is of optimal size for the [2.2.1]cryptand and so an inclusion complex occurs with the cation inside the cage (Figure 21b).454 Binuclear inclusion complexes have not yet been isolated for bicyclic cryptands. 

Figure 22 The molecular structure of the binuclear sodium complex of the tricyclic cryptand (110) (reproduced with...

Cadmium complexes of the cryptands (156) and (159)-(162) have been investigated, and both [CdL]2+ and the binuclear complexes [Cd2L]4+ are formed.1114,1115... 

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... 

Macrobicyclic ligands incorporating heterocyclic subunits. Vogtle (58) and Newkome (59) have successfully prepared cryptands containing respectively one (58) and three (59) pyridine subunits in the macrobicyclic framework (Figure 2.18). The ligand (58) forms complexes with many metal cations, and binuclear cobalt and copper complexes of (59) were isolated. 

The synthesis of binuclear copper(II) and manganese(II) complexes with L681 has been performed by treating the diprotonated cryptand L691, obtained by (2 -b 3) cyclocondensation of dap and L685 in the presence of hydrochloric acid [65], with Cu(Bp4)2 and Mn(CH3COO)2 in methanol and methanol acetonitrile, respectively (Eq. 3.26) [66, 67]. 

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