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

The complexing properties of three hexaimino cryptands bearing furan (L(1)), pyridine (L(2)) and meta-xylyl (L(3)) 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, L(1) and L(2) form predominantly 1 : 1 species, However, additional 2 : 1 complexes are found between Cu+ and both ligands and between Ag+ and L(1), whereas only 2 : 1 species are formed with Mg2+, With L(3), 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 L(2) and the disilver complex with L(1) 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) Angstrom]. 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 L(2) 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 L(1) and L(2), With regards to the formation of binuclear complexes, no positive cooperative effect has been observed, except for the Mg2+ complexes with L(1) and L(2) and the Cu+ complex with L(3), The calorimetric study of the disilver complexes of L(1) shows that there is actually a strongly positive enthalpic cooperativity which is completely offset by the entropic change.