Alteration in the binding property of a laterally nonsymmetric Aza cryptand toward Cu^II, Ag^I, and Tl^I ions upon derivatization with a cyanomethyl group

Abstract

The laterally nonsymmetric aza cryptand L_o readily forms mononuclear inclusion complexes with metal ions like Cu^II, Ni^II, Ag^|, Tl^I, and so forth. However, when the secondary amines present in the three bridges are substituted by a cyanomethyl group to yield a new cryptand L, its metal-binding properties are drastically altered. Thus, while L_o binds Ag^I at the tren end, showing AgN₄ coordination, in L, the bridgehead nitrogen atoms are pulled inside to form a two-coordinate complex of Ag^I and none of the O or N atoms present in the three bridges are involved in coordination. This is probably the first example where the two bridgehead atoms are pulled inside to the maximum to bind the metal ion. Likewise, the Tl^I ion shows TlN₄ coordination with L_o, while in L it is loosely bound at the tren end, showing TlN₃ coordination. Upon treatment with a Ag^I salt, the Tl^I can be replaced almost instantaneously. On the other hand, with copper(II) tetrafluoroborate salt in moist DMF, two of the cyanide groups in L are converted to carboxylic acid groups and bind the metal ion from outside. The Cu^II ion in this complex shows distorted octahedral coordination where the two O atoms from the two carboxylic acid groups and the two corresponding N atoms in the two bridges are bound equatorially. The two axial sites are occupied by the nearby bridgehead N atom and the O atom of a DMF molecule. All five complexes and the free ligands are characterized by spectroscopic and X-ray crystallographic studies.