THE COMPLEXATION OF DIVALENT METAL-IONS BY THE CRYPTANDS 4,7,13,16-TETRAOXA-1,10-DIAZABICYCLO[8.8.2]EICOSANE, -[8.8.5]TRICOSANE AND -[8.8.8]HEXACOSANE IN AQUEOUS-SOLUTION

The complexation of divalent metal ions (M(2+)) by the cryptands 4,7,13,16-tetraoxa-1,10-diazabicyclo[8.8.2]eicosane (C22C(2)), 4,7,13,16-tetraoxa-1,10-diazabicyclo[8.8.5]tricosane (C22C(5)) and 4,7,13,16-tetraoxa-1,10-diazabicyclo[8.8.8]hexacosane (C22C(8)) to form the cryptates [MC22C(n)](2+), where n = 2, 5 and 8, respectively, has been studied by potentiometric titration in aqueous solution at 298.2 K and I = 0.10 mol dm(-3) (NEt(4)ClO(4)). The stability constants, expressed as log K (dm(3) mol(-1)) for n = 2, 5 and 8, respectively, after each metal ion, are for Mg2+ = 3.5, 2.1 and 2.8; Ca2+ = 4.68, 2.0 and 2.6; Sr2+ = 4.3, 3.0 and 2.8; Ba2+ = 5.28, 2.8 and <2; Co2+ = 6.8, 5.0 and <6; Ni2+ = -, 5.5 and 5.4; Cu2+ = 9.4, 9.0 and 8.7; Zn2+ = 7.1, 6.4 and 6.3; Cd2+ = 9.0, 5.18 and 4.5; Hg2+ = 13.4, - and 16.5; and Pb2+ = 11.1, 8.1 and 8.02. In some cases the formation of [M(C22C(n))(2)](2+), [M(HC22C(n))](3+), [M(OH)C22C(n)](+) and [M(OH)C22C(n)](2+)(2) is also observed. The mono and doubly protonated cryptands, HC22C(n)(+) and H(2)C22C(n)(2+), are characterised by successive stability constants expressed as log K-1 (dm(3) mol(-1)) = 10.92, 11.43 and 11.06; and log K-2 (dm(3) mol(-1)) = 3.42, 8.10 and 8.41 for n = 2, 5 and 8, respectively. A comparison of these data with those for related systems is made, and the effects of the structural characteristics of the cryptand and cryptate are examined.