Sandwich complex, infinite

The enniatins and beauveracin are cyclic hexadepsipeptides and so are 18-membered macrocycles built up of alternating amino acid and carboxylic acid residues. They exhibit lower selectivity than (137) and consequently are more broad spectrum ionophores. The structure of (138)KI has been solved544 but it was not possible to conclude, with certainty, whether the metal is entrapped in the central cavity, as in related cyclic polyether structures (Figure 14a), or whether it occupi.es a site between two adjacent ligand molecules to give an infinite sandwich (147) as has been found in the RbNCS complex of a synthetic ldlldl isomer of (138).545 Adducts of 1 2 sandwich and 2 3 club sandwich stoichiometry have been proposed for Cs+ complexes of (138),546a and the 1 2 K+ sandwich complex of (138) has been proposed as the species which transports K+ across lipid layers as it shields the metal effectively from solvent interactions.5461 ... 

Using tridentate ligands with various functionalities and coordinating abilities, a series of neutral and cationic tri-, tetra- and hexanuclear metalla-cycles have been synthesised [34]. Cyclic tetramers composed of Cp Ir, Cp Rh or (arene)Ru half-sandwich complexes and 6-purinethione derivatives have been isolated as triflate salts [351. The cationic complex (Cp Ir)4(Ll)4 (where L1 = 2-amino-6-purinethione), presented in Fig. 9a, forms in the solid state an infinite channel-like structure with, S symmetry. Replacing the 6-purinethione with pyridine-4-thiolato bridging ligands (L2), the trinuclear metalla-cycle [(Cp Ir)3(L2)3] was obtained (Fig. 9b) [36]. 

Figure 16. Infinite sandwich complex of the LDLLDL stereoisomer of enniatin B with Rb. Six Rb —O ligands are formed (Tishchenko et al., 1977).

The structure of the enniatin B complex with potassium iodide has been studied by X-ray crystallography Unfortunately, from this investigation it could not be concluded with certainty whether the metal ion is entrapped in the central cavity or, instead, occupies a site between two adjacent ligand molecules. An arrangement of the latter type has been observed in the crystal structure of the 1 1 complex between RbNCS and the synthetic LDLLDL isomer of enniatin B In this case, Rb" ions are coordinated by five carbonyl oxygens (three of the upper and two of the lower depsipeptide molecules) and the nitrogen atom of the isocyanate anion, thus forming infinite sandwiches. 

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