Mono- and poly-nuclear cryptate complexes of cage-like azamacrocyclic compounds: a thermodynamic and electrochemical approach

The compounds 4,7,10,17,23-pentamethyl-1,4,7,10,13,17,23-heptaazabicyclo[1 1.7.5]pentacosane (L(1)) and 4.7,10,23-tetramethyl-17-era-1,4.7,1 0,1 3.23-hexaazabicyclo[11.7.5]pentacosane (L(2)), respectively comprised of a tetraaza and a monooxatriaza macrocycle overstructured by a triazabridge, formed mononuclear complexes with Cu-II, Zn-II and Cd-III in aqueous solutions. The co-ordination of these metal ions by L(2) has been studied by means of potentiometric and NMR techniques. Thermodynamic and NMR data indicate that the metal ion is accommodated in the N-4 and in the N3O fourteen-membered cyclic moiety in the cases of L(1) and L(2), respectively, while the triaza bridge does not participate in the co-ordination. The complexation of Cu-I by these macrocycles has been studied by means of electrochemical measurements (cyclic voltammetry); both compounds stabilize the low-valent copper complexes. This behaviour can be ascribed to the presence of only tertiary nitrogen donors, which leads to a hydrophobic macrocyclic cavity in which the metal ion is encapsulated. Both compounds also form binuclear complexes with Cu-II in methanol solution. In the case of L(1), binuclear assemblies are formed in aqueous solution, allowing for the determination of their stability constants. Mono- and di-hydroxo complexes of the type [M(2)L(1)(OH)](+) and [M(2)L(1)(OH)(2)] are formed in aqueous media. The macrobicycle 4,7,10,23-tetramethyl-14-oxo-17-era-1,4,7,10,13,23-hexaazabicyclo[11.7.5]pentacosane (L(3)) h,, been isolated as its cadmium(ll) complex [Cd(HL(3))Cl2]ClO4 . H2O the crystal structure of which was determined.