INFLUENCE OF PENDANT CYANOALKYL SIDE-ARMS ON THE STABILITIES OF METAL-ION COMPLEXES OF MONOAZA-18-CROWN-6 AND DIAZA-18-CROWN-6 ETHERS IN METHANOL

Pendant cyanomethyl and 2-cyanoethyl arms attached to the nitrogen ato ms of aza-crown ligands can influence complex stabilities with metal i ons in methanol in two distinct ways: by reducing the basicities of th e ring nitrogen atoms, and by disrupting the solvation of the ligands and complexes. In Ag+ complexes, and probably also in Zn2+, Cd2+ and P b2+ complexes, the dominant effect is the reduced nitrogen basicity, w ith a resulting net decrease in stability in the presence of the cyano alkyl arms. The stability constants of the alkali metal ion complexes, however, are increased, with the main factor being the poorer solvati on in methanol of the uncomplexed substituted macrocycles relative to the parent ligands, A18C6 and A218C6. For the alkaline-earth complexes , effects on free ligand solvation are overcompensated by a decreased solvation of the metal complexes; the side-arms prevent coordination o f the complexed cations by methanol molecules, with a consequent reduc tion in complex stability. In no cases is there any evidence of direct interactions between the metal ions and the pendant cyanoalkyl arms.