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[11.7.5]pentacosane (L¹) and 4,7,10,23-tetramethyl-17-oxa-1,4,7,10,13,23-hexaazabicyclo[11.7.5]pentacosane (L²), respectively comprised of a tetraaza and a monooxatriaza macrocycle overstructured by a triazabridge, formed mononuclear complexes with Cu^II, Zn^II and Cd^II in aqueous solutions. The co-ordination of these metal ions by L² has been studied by means of potentiometric and NMR techniques. Thermodynamic and NMR data indicate that the metal ion is accommodated in the N₄ and in the N₃O fourteen-membered cyclic moiety in the cases of L¹ and L², 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 lowvalent 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 L¹, binuclear assemblies are formed in aqueous solution, allowing for the determination of their stability constants. Mono- and di-hydroxo complexes of the type [M₂L¹(OH)]⁺ and [M₂L¹(OH)₂] are formed in aqueous media. The macrobicycle 4,7,10,23-tetramethyl-14-oxo-17-oxa-1,4,7,10,13,23-hexaazabicyclo[11.7.5] pentacosane (L³) has been isolated as its cadmium(II) complex [Cd(HL³)Cl₂]ClO₄·H₂O the crystal structure of which was determined.