Ligand cryptands

Fig. 18. Plot of change in complex stability, Alogf , that occurs for the pairs of ligands cryptand-2,2,2 and EN ( ), and 18-aneN204 and EN (O), as a function of metal ion radius (20). The diagram shows how neutral oxygen donors tend to stabilize the complexes of large relative to small metal ions in macrocyclic ligands. Data from Ref. (11).

Nickel(ll) complexes with macrobicyclic ligands (cryptands) 270... 

Ion transport by macrobicyclic ligands (cryptands) has been investigated using a bulk liquid membrane149. It was found that the relative transport rates are not proportional to complex stability. E.g. cryptand (Fig. 67 m = n = 1) which forms a very stable K+ complex, is an inefficient K+ carrier, because of the slow exchange rate of the complex167. On the other hand, cryptand (Fig. 68 3) which forms a more lipophilic and less stable complex, is quite an efficient K+ carrier. It appears that the cryptands display efficient carrier properties for those cations which form complexes with stability constants of about 10s in methanol. 

Fig. 13.2. Crystal structures of an oligodentate ligand (cryptand) which accommodates anions of different sizes and shapes. The complex with F (CIDMUX) shows only four short hydrogen bonds, whereas with Cl" and Br (CIDNAE, CIDNEI) the ion receptor shows six short contacts. The cylindrical azide ion appears to possess a shape well adapted to the binding pattern of the receptor molecule...

Early synthetic routes to Zintl ions (see Section 13.7) involved reduction of Ge, Sn or Pb in solutions of Na in liquid NH3. The method has been developed with the addition of the macrocyclic ligand cryptand-222 (crypt-222) (see Section 10.8) which encapsulates the Na" " ion and allows the isolation of salts of the type [Na(crypt-222)]2[Sn5] (equation 8.35). Zintl ions produced in this way include [805] (Figure 8.3), [Pb5], [Pb2Sb2] , [Bi2Sn2] , [Geg]2-, [Geg]" - and [SngTl] -. 

SO.5.9.7 NidteUU) complexes with macrobicyelic ligands (cryptands)... 

The 18-crown-6-cyclic polyether-KBr complex catalyses the homogeneous liquid phase molecular oxygen oxidation of ethylbenzene to the hydroperoxide. The macrobicyclic ligands (cryptands) are strong bases but the rates of proton transfer from ethyl nitroacetate to the free base cryptand and to the mono-protonated cryptand are ca, 10 and 100-fold smaller, respectively, than that of transfer to a normal base of similar basicity. This is attributed either to steric hindrance to proton transfer or to proton transfer occurring only to the thermodynamically unfavourable exo-nitrogen conformation. However, a large kinetic isotope effect = 3.9) is observed for the protonation of cryptand... 

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