Complexation of Alkali Metal Ions by the Cryptand 4,7,13,16-Tetraoxa-1,10-diazabicyclo[8.8.5]tricosane in Several Solvents

A potentiometric titration study of the complexation of metal ions (M+) by 4,7,13,16- tetraoxa-1,10-diazabicyclo[8.8.5] tricosane (C22C5) at 298.2 K and I = 0.05 (NEt4ClO4) yields log(K/dm3 mol-1) =6.07 and 5.36, 7.55 and 5.95, 6.26 and 7.56, 5.5 and 6.66, 4.57 and 5.16, and 8.14 and 14.51 when M+ = Li+, Na+, K+, Rb +, Cs+ and Ag+, respectively, where the first and second magnitudes of each pair refer to the stability constant (K) for [M(C22C5)]+ in acetonitrile and ropylene carbonate solvent, respectively. A 7Li n.m.r. tudy yields kd (298.2 K) = 428 s-1, ∆ Hd ‡ = 24.7 kJ mol-1 and ∆ Sd ‡ = -111 J K-1 mol-1 for the monomolecular decomplexation of Li+ from [Li(C22C5)]+, and kc(298.2 K) = 9.83×107 dm3 mol-1 s-1 for the complexation process in propylene carbonate. These data are compared with those for related systems and are discussed in terms of the factors affecting cryptate stability and lability.