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, al kaline earth, Co2+, Ni2+, Zn2+, Cu2+, Cu+, Cd2+ and Ag+ cations have b een investigated in acetonitrile, by means of UV-VIS spectrophotometry and/or potentiometry, The formation of both mononuclear (1 : 1) and b inuclear (2 : 1) complexes is ligand and cation dependent, L(1) and L( 2) form predominantly 1 : 1 species, However, additional 2 : 1 complex es are found between Cu+ and both ligands and between Ag+ and L(1), wh ereas only 2 : 1 species are formed with Mg2+, With L(3), there is gen erally formation of both the mono- and the bi-nuclear species, The sto ichiometries detected in solution are in agreement with those of the s olid complexes when they could be isolated, The structures of the mono silver complex with L(2) and the disilver complex with L(1) are presen ted, 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 th e two silver cations are in close proximity [d(Ag-Ag) = 3.115(2) Angst rom]. The stability of the complexes, which increases when going from alkali to alkaline earth, transition and heavy metal cations, is alway s lower than that found for the corresponding diazapolyoxa cryptates i n the same medium, Although L(2) shows a marked preference for Mg2+ ov er Ca2+ within the alkaline earths, a lack of selectivity is observed in each series of cations and even between alkaline earth and heavy me tal ions, resulting from a remarkable enthalpy-entropy compensation ef fect demonstrated by the results of the calorimetric study of the comp lexation 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 coo perative 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 strongl y positive enthalpic cooperativity which is completely offset by the e ntropic change.