Hexa Schiff-base cryptands: solution thermodynamic and X-ray crystallographic studies of main group, transition and heavy metal ion complexes

Abstract

The complexing properties of three hexaimino cryptands bearing furan (L¹), pyridine (L²) and meta-xylyl (L³) groups towards alkali, alkaline earth, Co²⁺, Ni²⁺, Zn²⁺, Cu²⁺, Cu⁺, Cd²⁺ and Ag⁺ cations have been investigated in acetonitrile, by means of UV–VIS spectrophotometry and/or potentiometry. The formation of both mononuclear (1 : 1) and binuclear (2 : 1) complexes is ligand and cation dependent. L¹ and L² form predominantly 1 : 1 species. However, additional 2 : 1 complexes are found between Cu⁺ and both ligands and between Ag⁺ and L¹, whereas only 2 : 1 species are formed with Mg²⁺. With L³, there is generally formation of both the mono- and the bi-nuclear species. The stoichiometries detected in solution are in agreement with those of the solid complexes when they could be isolated. The structures of the monosilver complex with L² and the disilver complex with L¹ are presented. The former shows the silver cation in an unexpected position, as being bonded to three pyridine nitrogen atoms as well as to two imine nitrogen atoms of one side of the macrocycle. The latter shows that the two silver cations are in close proximity [d_Ag-Ag= 3.115(2)Å]. The stability of the complexes, which increases when going from alkali to alkaline earth, transition and heavy metal cations, is always lower than that found for the corresponding diazapolyoxa cryptates in the same medium. Although L² shows a marked preference for Mg²⁺ over Ca²⁺ within the alkaline earths, a lack of selectivity is observed in each series of cations and even between alkaline earth and heavy metal ions, resulting from a remarkable enthalpy–entropy compensation effect demonstrated by the results of the calorimetric study of the complexation of Ca²⁺, Sr²⁺, Ba²⁺ and Ag⁺ by the two ligands L¹ and L². With regards to the formation of binuclear complexes, no positive cooperative effect has been observed, except for the Mg²⁺ complexes with L¹ and L² and the Cu⁺ complex with L³. The calorimetric study of the disilver complexes of L¹ shows that there is actually a strongly positive enthalpic cooperatively which is completely offset by the entropic change.