Cryptand binding constant

Although the principal application for 9 has been in the synthesis of cryptands (see Chap. 8), this material has also served as precursor to a number of nitrogen based lariat ethers , sometimes referred to as crown complexanes . Binding constants for such compounds have been measured for a few examples in a few cases , but... 

The cation affinity of aza-crown ethers depends on the type of substituent attached to the nitrogen. Wester and Vogtle (1978) have determined the cation binding constants for substituted [2.2]-cryptands 138] and [89]—191 ] in... 

Rates of decomplexation (kJ2) of cation complexes can also be determined by nmr spectroscopy on the cation. Rates of complex formation are then calculated from kn and the binding constant. The results for several ligands, cations, and solvents are given in Table 20. Despite the wide variations, the rates of complex formation are all in the range 2 x 107 to 8 x 10 M 1 s 1. In contrast, rates of decomplexation for crown-ether complexes span a much broader range 6.1 x 102 to 2 x 105 s 1. Comparison of crown-ether data with data for [2.2.2]-cryptand [37] and the linear polyether [92] also shows that the... 

Thus a large binding constant corresponds to a high equilibrium concentration of bound metal, and hence a more stable metal-macrocycle complex. Typical binding constants for crown ethers and alkali metal cations in water are in the range 101 102. In methanol, this increases up to 106 for [K([18] crown-6)]+. The binding constant for K+ and [2.2.2] cryptand is about 1010. Some other examples are given in Table 1.3. 

Figure 3.36 Binding constants for various cryptands with alkali metal cations in methanol (dotted lines are estimates).

Figure 3.37 Comparison of the K+ binding constant for cryptand and corand hosts (MeOH, 25°C).

Menif, R. Reibenspies, J. Martell, A. E. Synthesis, protonation constants, and copper(II) and cobalt(II) binding constants of a new octaaza macrobicylic cryptand (MX)3crystal structures of the cryptand and of the carbonato-bridged dinuclear copper(II) cryptate, Inorg. Chem. 1991, 30, 3446-3454. 

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