Complexation of Polyfunctional Organic Guest Molecules by Macrocyclic Polyethers. Complexes of S-tert -Butylisothiuronium Salts with 18-Crown-6 and 1,3-Xylyl-18-crown-5

Crystalline complexes of S-tert-butylisothiuronium perchlorate ( t-BuSC(NH2),C104) and 18-crown-6 (18C6) and 1,3-xylyl-18-crown-5 (1,3X18C5) with a 1:l stoichiometry were obtained via extraction. X-ray diffraction analysis showed that in the solid state the guest molecule is coordinated via one of the two NH, groups to the crown ether. The second NH2 group is bound to the anion. 'H NMR spectroscopy of solutions of these complexes has been interpreted in a similar way. The relative association constant of t-BuSC(NH2),C104 with 18C6 compared to 1,3X18C5 in CDCl, at T = 293 K is KmI = 9.5 & 0.5. Lowering the temperature shows two different resonances for the NH2 hydrogen atoms; the low-field signal corresponds to the NH, group coordinated to the crown ether and the high-field signal to the NH, group bound to the anion. The change of free energy of activation for the mutual exchange of NH2 groups in CDC1, at the coalescence temperature T, ranges from AG = 13-16 kcalsmol-' for the studied complexes. Since the rates of complexation-decomplexation of the NH2+-anion and NH,-crown ether are fast, the observed slow mutual exchange of NH2 groups between an anion and crown ether site is interpreted by the presence of contact NH2+-anion ion pairs. Within such an ion pair the anion-NH2+ exchange may be fast, because it proceeds via a bimolecular reaction mechanism.